Plant-Based Diet – what physicians need to know

Doctor holding tray of vegetables and a blood monitor
Posted by: Plant-Based Diet Team Post Date: January 6, 2021

Plant-Based Diet – what physicians need to know


The plant-based diet includes vegetables, legumes, whole grains, fruits, tree nuts and seeds.  The American Academy of Nutrition (formerly the American Dietetic Association) has formally endorsed the plant-based diet as both nutritionally adequate for all phases of the life cycle, and as having all necessary nutrients except Vitamin B12 in their position paper. Their full statement is well worth reading:

“It is the position of the Academy of Nutrition and Dietetics that appropriately planned vegetarian, including vegan, diets are healthful, nutritionally adequate, and may provide health benefits for the prevention and treatment of certain diseases. These diets are appropriate for all stages of the life cycle, including pregnancy, lactation, infancy, childhood, adolescence, older adulthood, and for athletes. Plant-based diets are more environmentally sustainable than diets rich in animal products because they use fewer natural resources and are associated with much less environmental damage. Vegetarians and vegans are at reduced risk of certain health conditions, including ischemic heart disease, type 2 diabetes, hypertension, certain types of cancer, and obesity. Low intake of saturated fat and high intakes of vegetables, fruits, whole grains, legumes, soy products, nuts, and seeds (all rich in fiber and phytochemicals) are characteristics of vegetarian and vegan diets that produce lower total and low-density lipoprotein cholesterol levels and better serum glucose control. These factors contribute to reduction of chronic disease. Vegans need reliable sources of vitamin B-12, such as fortified foods or supplements.” (1)
American Academy of Nutrition position statement

There may be a health advantage of not consuming large amounts of very highly processed foods. Americans in general currently consume large amounts of these foods. Data from NHANES 2009–2010 suggests that Americans currently consume 60% of their calories from ultra-processed foods such as candy, sugared beverages, cakes, cookies, pizza, French fries, salty and sweet snacks, and desserts, with only 5% of calories from fruits and less than 1% from vegetables. (2)

While a patient on a plant-based diet typically consumes less processed foods, it is still wise for the physician to make sure they are not consuming vegan processed foods in excess. Using the term “whole-food plant-based diet” encourages the patient to focus both on plant-foods and on whole foods as opposed to highly processed foods. However, meat and dairy alternatives, while often processed, are valuable transition foods which maintain the texture and flavor of foods they are familiar with and support the patient socially by giving foods such as a veggie burger, which can help them feel more comfortable.

Nutrients present in various foods play an important role in maintaining the normal functions of the human body. The major nutrients present in foods include the macronutrients: carbohydrates, proteins, and lipids, plus vitamins, minerals and fiber. Besides these, there are some bioactive food components known as “phytonutrients” that play an important role in the prevention and treatment of disease. (3)

The following chart illustrates the macronutrients typically consumed by following the current government recommendations, known as MyPlate, compared to the macronutrients consumed on a whole foods plant-based diet (4):

A whole food plant-based diet is commendable, and a well-planned vegan diet can be adequate to achieve proper nutrition. Individuals who adhere to WFBP meal plans have higher overall dietary quality as defined by the HEI-2015 score as compared to typical US intakes, with the exceptions of calcium for older women, and vitamins B12 and D without supplementation. (4) There are some nutrients of concern that the physician should take note of. These are Vitamin B12, vitamin D, calcium, iron, zinc and omega 3 fatty acids. These are addressed in more detail as separate articles in the nutrition chapter.

Adequate water is of course essential, and other water-based beverages such as tea are useful as well. Care should be taken to avoid heavily sugared beverages.


The concern that vegans, and particularly vegan athletes, may not consume an adequate amount and quality of protein is unsubstantiated. Vegetarian diets that include a variety of plant products provide the same protein quality as diets that include meat. (1)  The amounts and proportions of amino acids consumed by vegans are typically more than sufficient to meet and exceed individual daily requirements, provided that a reasonable variety of foods are consumed and energy intake needs are being met. (5) In developed countries, plant proteins are typically mixed, especially in vegan diets, and total intake of protein usually exceeds the requirement. This results in intakes of all 20 amino acids that are more than sufficient to cover requirements. (5)

There is very little evidence at present regarding a marked difference in protein digestibility in humans. The more precise data collected so far in humans, assessing real (specific) oro-ileal nitrogen digestibility, has shown that the differences in the digestibility between plant and animal protein sources are only a few percent different. (6)

Several studies have shown the adequacy of protein intake among vegans. The EPIC-Oxford study showed that vegans had 13.1% of their Calories from protein yielding 64g. (7, 8) A French study showed vegans consuming 12.8 percent of the calories as protein for a total of 62g. (9) An American study of vegans showed 14.1% of Calories as protein resulting in an intake of 71gm of protein (10) and a Belgian study showed that vegans consumed 14.0% of their Calories as protein resulting an intake 82gm. (11) All these studies exceeded the RDA of greater than 10% Calories from protein or approximately 50gm.

The myth that plant proteins must be combined at every meal to be of any use to the body was popularized in the early 70’s by the book Diet for a Small Planet by Frances Moore Lappé. The author has since been retracting the statement frequently. “In combating the myth that meat is the only way to get high-quality protein, I reinforced another myth,” she said. Unfortunately, the protein combining myth has taken root in the public and even among a few doctors.

Combining two or more incomplete protein foods (those low in one or more essential amino acids), such as rice and beans, peanut butter and whole grain bread, tortillas with beans, and cooked beans with cornbread is not required in every meal, as long as variety is present over a day or two. (1) The reason for this lies in the pool of indispensable amino acids (IAAs) maintained by the body, (12, 13) that can be used to complement dietary proteins. The pool of amino acids come from four sources (14, 15, 16):

  • Enzymes secreted into the intestine to digest proteins.
  • Intestinal cells sloughed off in the intestine.
  • Intracellular spaces of the skeletal musculature
  • Synthesis of amino acids by intestinal microflora.

Thus patients can consume, for example, beans at dinner and have a grain-based breakfast the next morning, and still have sufficient amino acids.


Carbohydrates play an important role in the human body. They act as an energy source, help control blood glucose and insulin metabolism, participate in cholesterol and triglyceride metabolism, and help with fermentation. The digestive tract breaks down carbohydrates upon consumption into monosaccharides including glucose. The polysaccharide glycogen represents the main storage form of glucose in the body. Glycogen is made and stored primarily in the cells of the liver and skeletal muscle, where it is stored until further energy is needed. (17)

In its 2002 report, the Institute of Medicine (IOM) established an RDA for carbohydrate of 130 g/d for adults and children aged ≥1 y. This value is based on the amount of sugars and starches required to provide the brain with an adequate supply of glucose. The IOM set an acceptable macronutrient distribution range (AMDR) for carbohydrates of 45–65% of total calories. (18)

The four classes of carbohydrates in food are:

  • Monosaccharides: The most basic, fundamental unit of a carbohydrate. These are simple sugars, such as glucose, galactose, fructose, with the general chemical structure of C6H12O6.
  • Disaccharides: Compound sugars, such as sucrose and lactose, containing two monosaccharides with the elimination of a water molecule with the general chemical structure C12H22O11
  • Oligosaccharides: Polymers, such as maltodextrins and raffinose, containing three to ten monosaccharides
  • Polysaccharides: Polymers, such as amylose and cellulose, containing long chains of monosaccharides connected through glycosidic bonds

Different carbohydrates have different effects on raising blood glucose:

  • Simple Carbohydrates: One or two sugars (monosaccharides or disaccharides) combined in a simple chemical structure. These are easily utilized for energy, and can cause a rapid rise in blood glucose and insulin secretion from the pancreas.
  • Complex Carbohydrates: Three or more sugars (oligosaccharides or polysaccharides) bonded together in a more complex chemical structure. These take longer to digest and therefore have a more gradual effect on the increase in blood sugar.

Glycemic Index

The Glycemic Index (GI) is a kinetic parameter which reflects the ability of carbohydrate contained in consumed foods to raise blood glucose in vivo. Considerable epidemiologic evidence links consuming lower glycemic index (GI) diets with good health.

The glycemic index (GI) concept was introduced by Jenkins et al in the early 1980s as a ranking system for carbohydrates based on their immediate impact on blood glucose levels. (19)  GI was originally designed for people with diabetes as a guide to food selection, advice being given to select foods with a low GI. (20) Lower GI foods were considered to confer benefit as a result of the relatively low glycemic response following ingestion compared with high GI foods.

The GI concept has been extended to also take into account the effect of the total amount of carbohydrate consumed. Thus glycemic load (GL), a product of GI and quantity of carbohydrate eaten, provides an indication of glucose available for energy or storage following a carbohydrate containing meal.

In addition to a role in the treatment of diabetes, low GI and GL diets have more recently been widely recommended for the prevention of chronic diseases including diabetes, obesity, cancer and heart disease and in the treatment of cardiovascular risk factors, especially dyslipidaemia. (21)

There are various research methods for assigning a GI value to food. In general, the number is based on how much a food item raises blood glucose levels compared with how much pure glucose (100) raises blood glucose. GI values are generally divided into three categories:

Low GI: 1 to 55

Medium GI: 56 to 69

High GI: 70 and higher

Comparing these values, therefore, can help guide healthier food choices. For example, an English muffin made with white wheat flour has a GI value of 77. A whole-wheat English muffin has a GI value of 45

Examples of foods with low, middle and high GI values include the following (22):

  • Low GI: Green vegetables, most fruits, raw carrots, kidney beans, chickpeas, lentils and bran breakfast cereals
  • Medium GI: Sweet corn, bananas, raw pineapple, raisins, oat breakfast cereals, and multigrain, oat bran or rye bread
  • High GI: White rice, white bread and potatoes

Appendix A: Glycemic Index (glucose = 100) for a range of foods (22)

Dietary fiber

Dietary fiber is a key component of a healthy diet as recommended by several nutritional guidelines. (23) Dietary fiber is defined as the edible parts of plants or analogous carbohydrates that are resistant to digestion and absorption in the human small intestine. (24)

Traditionally, dietary fiber was defined as the portions of plant foods that were resistant to digestion by human digestive enzymes; this included certain polysaccharides and lignin. More recently, the definition has been expanded to include oligosaccharides, such as inulin, and resistant starches. (25)  Soluble and insoluble dietary fibers make up the two basic categories of dietary fibers. Cellulose, hemicellulose and lignin are not soluble in water whereas pectins, gums and mucilages become gummy in water. (26)

Dietary fiber intake provides many health benefits. Individuals with high intakes of dietary fiber appear to be at significantly lower risk for developing coronary heart disease, stroke, hypertension, type 2 diabetes, obesity, and certain gastrointestinal diseases. Increasing fiber intake can lower blood pressure and serum cholesterol levels. Increased intake of soluble fiber improves glycemia and insulin sensitivity in non-diabetic and diabetic individuals. Fiber supplementation in obese individuals significantly enhances weight loss. Increased fiber intake benefits a number of gastrointestinal disorders including the following: gastroesophageal reflux disease, duodenal ulcer, diverticulitis, constipation, and hemorrhoids. Dietary fiber intake provides similar benefits for children as for adults. The recommended dietary fiber intakes for children and adults are 14 g/1000 kcal. (25) However, average fiber intakes for US children and adults are less than half of the recommended levels.

Prebiotic fibers appear to enhance immune function. A prebiotic is a non-absorbable compound that, through its metabolization by microorganisms in the gut, modulates composition and the activity of the gut microbiota, thus conferring a beneficial physiologic effect on the host. (27) The consumption of prebiotics is difficult to measure since they are found in very diverse food groups, in wide ranges of supplements, and there isn’t an analytic test or universally agreed-upon method. Estimated consumption in US and European diets is several grams a day for naturally occurring prebiotics: inulin and fructo-oligosaccharides (FOS). (28, 29)

Inulin is a prebiotic that occurs naturally in leeks, asparagus, onions, wheat, garlic, chicory, oats, soybeans, and Jerusalem artichokes. Fructo-oligosaccharides (FOS) are oligosaccharides that occur naturally in plants such as onion, chicory, garlic, asparagus, banana, artichoke, among many others. They are composed of linear chains of fructose units, linked by β (2-1) bonds. The number of fructose units ranges from 2 to 60 and often terminate in a glucose unit. Dietary FOS are not hydrolyzed by small intestinal glycosidases and reach the cecum structurally unchanged. There, they are metabolized by the intestinal microflora to form short-chain carboxylic acids, L -lactate, CO2, hydrogen and other metabolites. (30)


Dietary lipids are found in foods from both plants and animals. There are two types of fatty acids of nutritional concern:

• Saturated fatty acids, found in higher proportions in animal products.

• Unsaturated fatty acids — Monounsaturated and polyunsaturated fatty acids are found in higher proportions in plants.

Saturated fatty acid (SAFA) intake occurs to some degree from all fat-containing foods, with especially high amounts in dairy products, butter, and meats. (31) Most dietary SAFA have 12-18 carbon atoms, with foods varying in the relative amounts of individual SAFA. Palmitic (C16) and stearic acids (C18) are predominant in butter, dairy and meat products; lauric (C12) and myristic (C14) acids in butter, dairy foods, coconut, and palm kernel oils. (32) In many Western countries, in particular, SAFA intakes exceed 10% of Calories. (31, 33)

Randomized controlled trials that lowered intake of dietary saturated fat and replaced it with polyunsaturated vegetable oil reduced CVD by approximately 30%, similar to the reduction achieved by statin treatment. (34)  Similarly, consuming polyunsaturated and/or monounsaturated fats in preference to saturated fats and trans fatty acids has beneficial effects on insulin sensitivity and reduces the risk of Type 2 Diabetes Mellitus. (35)  Many of the other pathologies addressed in this book also demonstrate an adverse reaction to saturated fat.

Coconut oil contains greater than 80% SAFA. A common misconception is that SAFAs in coconut oil are metabolized differently from long-chain SAFA (≥C12). However, coconut oil contains approximately 50% lauric acid and 15% myristic acid. (36) Studies have shown that coconut oil generally raised total and low-density lipoprotein cholesterol to a greater extent than cis unsaturated plant oils, but to a lesser extent than butter. (37) Observational evidence suggests that consumption of coconut flesh or squeezed coconut in the context of traditional dietary patterns does not lead to adverse cardiovascular outcomes. However, due to large differences in dietary and lifestyle patterns, these findings cannot be applied to a typical Western diet. (37) Overall, the weight of the evidence from interventional studies to date suggests that replacing coconut oil with cis unsaturated fats would improve blood lipid profiles, leading to a reduction in risk factors for cardiovascular disease. (37)

Trans fat is an unsaturated fat found primarily in partially hydrogenated oils (and foods containing these oils) and in small amounts in some animal products. Trans unsaturated fatty acids are monounsaturated or polyunsaturated fatty acids containing at least one double bond in the trans configuration. (34) Clinical trials have consistently documented the adverse effects of trans fatty acids on the lipid risk factors for CVD. Replacement of calories from other types of fats with trans fatty acids raises LDL cholesterol, apolipoprotein B, triglycerides, and lipoprotein(a), as well as lowering HDL cholesterol and apolipoprotein A1. (38)

Essential fatty acids are addressed in a separate article.


The material in this section is adapted from the National Institutes of Health Fact Sheets. (39)

Vitamin A

Vitamin A is the name of a group of fat-soluble retinoids, including retinol, retinal, and retinyl esters. (40, 41, 42) Vitamin A is involved in immune function, vision, reproduction, and cellular communication. (40, 43, 44) Vitamin A is critical for vision as an essential component of rhodopsin, a protein that absorbs light in the retinal receptors, and because it supports the normal differentiation and functioning of the conjunctival membranes and cornea. (41, 43, 44) Vitamin A also supports cell growth and differentiation, playing a critical role in the normal formation and maintenance of the heart, lungs, kidneys, and other organs. (41)

Two forms of vitamin A are available in the human diet: preformed vitamin A (retinol and its esterified form, retinyl ester) and provitamin A carotenoids. (40, 41, 42, 43, 44) Preformed vitamin A is found in foods from animal sources, including dairy products, fish, and meat. The most important provitamin A carotenoid is beta-carotene. Other provitamin A carotenoids are alpha-carotene and beta-cryptoxanthin. The body converts these plant pigments into vitamin A. Both provitamin A and preformed vitamin A must be metabolized intracellularly to retinal and retinoic acid, the active forms of vitamin A, to support the vitamin’s important biological functions. (41, 42) Other carotenoids found in food, such as lycopene, lutein, and zeaxanthin, are valuable phytochemicals but are not converted into vitamin A.

RDAs for vitamin A are given as retinol activity equivalents (RAE) to account for the different bioactivities of retinol and provitamin A carotenoids, all of which are converted by the body into retinol. 1 mcg RAE is equivalent to the following amounts: 1 mcg of retinol, 2 mcg of supplemental beta-carotene, 12 mcg of dietary beta-carotene, and 24 mcg of dietary alpha-carotene or beta-cryptoxanthin. (44)

Vitamin A is listed on many food and supplement labels in international units (IUs) even though nutrition scientists rarely use this measure. Conversions between IU and mcg RAE are as follows: (45)

1 IU retinol = 0.3 mcg RAE

1 IU supplemental beta-carotene = 0.3 mcg RAE

1 IU dietary beta-carotene = 0.05 mcg RAE

1 IU dietary alpha-carotene or beta-cryptoxanthin = 0.025 mcg RAE

Table 1: Recommended Dietary Allowances (RDAs) for Vitamin A (46)

Age Male Female Pregnancy Lactation
0–6 months* 400 mcg RAE 400 mcg RAE    
7–12 months* 500 mcg RAE 500 mcg RAE    
1–3 years 300 mcg RAE 300 mcg RAE    
4–8 years 400 mcg RAE 400 mcg RAE    
9–13 years 600 mcg RAE 600 mcg RAE    
14–18 years 900 mcg RAE 700 mcg RAE 750 mcg RAE 1,200 mcg RAE
19–50 years 900 mcg RAE 700 mcg RAE 770 mcg RAE 1,300 mcg RAE
51+ years 900 mcg RAE 700 mcg RAE    

Because vitamin A is fat soluble, the body stores excess amounts, primarily in the liver, and these levels can accumulate. Although excess preformed vitamin A can have significant toxicity (known as hypervitaminosis A), large amounts of beta-carotene and other provitamin A carotenoids are not associated with major adverse effects, thus offering a safer route to vitamin A. (47)

Excess beta-carotene is predominantly stored in the fat tissues of the body. The most common side effect of excessive beta-carotene consumption is carotenodermia, a physically harmless condition that presents as a conspicuous orange skin tint arising from deposition of the carotenoid in the outermost layer of the epidermis. (48) Carotenodermia is quickly reversible upon cessation of excessive intakes.

Retinol and carotenoid levels are typically measured in plasma, and plasma retinol levels are useful for assessing vitamin A inadequacy. However, their value for assessing marginal vitamin A status is limited because they do not decline until vitamin A levels in the liver are almost depleted. (42) Frank vitamin A deficiency is rare in the United States.

Table 2: Plant food sources of provitamin A

Food mcg RAE per serving IU per serving Percent DV*
Sweet potato, baked in skin, 1 whole 1,403 28,058 561
Spinach, frozen, boiled, ½ cup 573 11,458 229
Carrots, raw, ½ cup 459 9,189 184
Pumpkin pie, commercially prepared, 1 piece 488 3,743 249
Cantaloupe, raw, ½ cup 135 2,706 54
Peppers, sweet, red, raw, ½ cup 117 2,332 47
Mangos, raw, 1 whole 112 2,240 45
Black-eyed peas (cowpeas), boiled, 1 cup 66 1,305 26
Apricots, dried, sulfured, 10 halves 63 1,261 25
Broccoli, boiled, ½ cup 60 1,208 24
Tomato juice, canned, ¾ cup 42 821 16
Ready-to-eat cereal, fortified with 10% of the DV for vitamin A, ¾–1 cup (more heavily fortified cereals might provide more of the DV) 127–149 500 10
Baked beans, canned, plain or vegetarian, 1 cup 13 274 5
Summer squash, all varieties, boiled, ½ cup 10 191 4
Pistachio nuts, dry roasted, 1 ounce 4 73 1

*DV = Daily Value.

Vitamin A can interact with certain medications, and some medications can have an adverse effect on vitamin A levels. A few examples are provided below. Individuals taking these and other medications on a regular basis should discuss their vitamin A status with their healthcare providers.

Orlistat (Alli®, Xenical®), a weight-loss treatment, can decrease the absorption of vitamin A, other fat-soluble vitamins, and beta-carotene, causing low plasma levels in some patients. (49) The manufacturers of Alli and Xenical recommend encouraging patients on orlistat to take a multivitamin supplement containing vitamin A and beta-carotene, as well as other fat-soluble vitamins. (50, 51)

Several synthetic retinoids derived from vitamin A are used orally as prescription medicines. Examples include the psoriasis treatment acitretin (Soriatane®) and bexarotene (Targretin®), used to treat the skin effects of T-cell lymphoma. Retinoids can increase the risk of hypervitaminosis A when taken in combination with vitamin A supplements. (49)

B Vitamins

The eight B vitamins: thiamine (B1), riboflavin (B2), niacin (B3), pantothenic acid (B5), vitamin B6, folate (B9) and vitamin B12, act as coenzymes in a substantial proportion of the enzymatic processes that underpin every aspect of cellular physiological functioning. As a coenzyme, the biologically active form of the vitamin binds within a protein “apoenzyme” creating a “holoenzyme”, thereby increasing the resultant enzyme’s competence in terms of the diversity of reactions that it can catalyze. (52)

The B vitamins themselves are not grouped on the basis of any chemical structural similarity, but rather with regards to their water solubility and the inter-related, cellular coenzyme functions that they play. (53) Overall, the plethora of functions undertaken by B vitamins can generally be subdivided into their roles in catabolic metabolism, leading to the generation of energy, and anabolic metabolism, resulting in the construction and transformation of bioactive molecules. (53)

Vitamin B1 – Thiamin plays a central role in the generation of energy from carbohydrates. It is involved in RNA and DNA production, as well as nerve function. Its active form is a coenzyme called thiamin pyrophosphate(TPP), which takes part in the conversion of pyruvate to acetyl coenzyme A(CoA) in metabolism. (54)

Vitamin B2 – Riboflavin is involved in the energy production for the electron transport chain, the citric acid cycle, as well as the catabolism of fatty acids (beta oxidation). (55)

Vitamin B3 – Niacin is composed of two structures: nicotinic acid and nicotinamide. There are two co-enzyme forms of niacin: nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP). Both play an important role in energy transfer reactions in the metabolism of glucose, fat and alcohol. (56) NAD carries hydrogens and their electrons during metabolic reactions, including the pathway from the citric acid cycle to the electron transport chain. NADP is a coenzyme in lipid and nucleic acid synthesis. (57)

Vitamin B5 – Pantothenic acid is involved in the oxidation of fatty acids and carbohydrates. Coenzyme A, which can be synthesized from pantothenic acid, is involved in the synthesis of amino acids, fatty acids, ketones, cholesterol, (58) phospholipids, steroid hormones, neurotransmitters (such as acetylcholine), and antibodies. (59)

Vitamin B6 – Pyridoxine is usually stored in the body as pyridoxal 5′-phosphate (PLP), which is the co-enzyme form of vitamin B6. Pyridoxine is involved in the metabolism of amino acids and lipids; in the synthesis of neurotransmitters (60) and hemoglobin, as well as in the production of nicotinic acid (61) Pyridoxine also plays an important role in gluconeogenesis.

Vitamin B7 – Biotin plays a key role in the metabolism of lipids, proteins and carbohydrates. It is a critical co-enzyme of four carboxylases: acetyl CoA carboxylase, which is involved in the synthesis of fatty acids from acetate; propionyl CoA carboxylase, involved in gluconeogenesis; β-methylcrotonyl CoA carboxylase, involved in the metabolism of leucin; and pyruvate CoA carboxylase, which is involved in the metabolism of energy, amino acids and cholesterol. (62)

Vitamin B9 – Folic Acid acts as a co-enzyme in the form of tetrahydrofolate (THF), which is involved in the transfer of single-carbon units in the metabolism of nucleic acids and amino acids. THF is involved in pyrimidine nucleotide synthesis, so is needed for normal cell division, especially during pregnancy and infancy, which are times of rapid growth. Folate also aids in erythropoiesis. (57)

Vitamin B12 is addressed in a separate article.

In terms of their origins, the B vitamins are typically synthesized by plants, with their synthesis in plant chloroplasts, mitochondria and the cytosol carefully regulated to the plant’s fluctuating requirements, with the exception of vitamin B12 which is synthesized by soil bacteria. (53) Therefore, a well-planned plant-based diet should allow the patient to get all the B vitamins they need. Supplements are readily available and may be indicated in patients following an overly restrictive plant-based diet.

Table 3: The B vitamins: nomenclature, dietary sources, coenzyme forms (roles) (53)

Vitamin Generally Known as Good Dietary Sources RDA 1 (mg) UL 2 Principal Bioactive Coenzymes (and Principal Coenzyme Role (52))
B1 Thiamin(e) Cereals (esp. whole grain), brown rice, green vegetables, potatoes, pasta 1.2/1.1 Thiamine pyrophosphate (Generation of leaving group potential)
B2 Riboflavin Leafy vegetables, legumes, yeast, mushrooms 1.3/1.1 Flavoproteins: flavin adenine dinucleotide (FAD) or flavin mononucleotide (FMN) (redox reactions)
B3 Niacin Whole grain cereal, legumes, mushrooms, nuts 16/14 35 mg Nicotinamide adenine dinucleotide (NAD) and its phosphate (NADP) (redox reactions)
B5 Pantothenic acid Whole grain cereals, broccoli 5 Co-enzyme A (CoA) (acyl activation and transfer)
B6 Vitamin B6 (referring to: pyridoxal, pyridoxamine, pyridoxine) Legumes, nuts, bananas, potatoes 1.3/1.3 (1.7/1.5 >50 year) 100 mg pyridoxal-5′-phosphate (PLP) and pyridoxamine-5′-phosphate (PMP) (Generation of leaving group potential)
B7 Biotin Leafy vegetables 30 (µg) biotin (carboxylation reactions)
B9 Folic acid/folate Leafy vegetables, legumes, citrus fruits 400 (µg) 1000 µg tetrahydrofolates inc. methyltetrahydrofolate (One carbon transfer)
B12 Vitamin B12 (referring to: the cobalamins) Supplemented plant-based foods, and supplements 2.4 (µg) Methylcobalamin, adenosylcobalamin (vicinal rearrangements)

1 Recommended Daily Allowance; 2 Upper limit — Food and Nutrition Board, Institute of Medicine, USA estimated “adequate intake” due to lack of data required to arrive at an RDA.

Table 4: The B Vitamins, symptoms of deficiency, and risk factors (over and above low consumption) (53)

Vitamin Symptoms of Deficiency Brain Specific Symptoms of Deficiency Specific Risk Factors for Deficiency
B1 Mild deficiency: general fatigue/weakness gastro-intestinal symptoms. (63) Deficiency: “Beri-beri”— Peripheral nerve damage and cardiovascular dysfunction leading to: pain, impaired sensory perception; swelling, weakness and pain in the limbs; shortness of breath, irregular heart rate, heart failure. (64) Mild deficiency: irritability, emotional disturbances, confusion, disturbed sleep, memory loss. (63) Deficiency: Wernicke-Korsakoff syndrome (neurodegeneration, within the medial thalamus and cerebellum). Ataxia, abnormal motor function and eye movement, amnesia, apathy, confabulation. (64) Alcohol abuse, obesity (63)
B2 Weakness, oral pain/tenderness, burning/itching of the eyes, dermatitis, anemia. (65) Fatigue, personality change, brain dysfunction. (65) inherited riboflavin malabsorption/utilization (10%–15% prevalence) (66)
B3 Pellagra: dermatitis/photo dermatitis, alopecia, muscle weakness, twitching/burning in the extremities, altered gait, diarrhea. (67) Depression, anxiety, progressing to vertigo, memory loss, paranoia, psychotic symptoms, aggression (Pellagrous insanity) (67) Alcohol abuse
B5 Numbness/burning sensations in extremities, dermatitis, diarrhea. (68) Encephalopathy, behavior change, demyelination (68)  
B6 Anemia Irritability, impaired alertness, depression, cognitive decline, dementia, autonomic dysfunction, convulsions. (69) Alcohol abuse, age-related malabsorption, contraceptive medications (70)
B7 Seborrheic eczematous rash, tingling/burning of the extremities. (71) Depression, lethargy, hallucinations, seizures. (71) Type II diabetes, poor gluco-regulation (72)
B9 Megaloblastic anaemia, peripheral neuropathy, spinal cord lesions, metabolic abnormalities. (73, 74) Affective disorders , behavior changes, psychosis, cognitive impairment/decline, dementia (inc Alzheimer’s disease and vascular dementia). (73) Common genetic polymorphisms (inc. MTHFR C667T) (75) Low Riboflavin and B12 (76)
B12 See separate article on Vitamin B12    

Vitamin C

The recognition of vitamin C is associated with the search for the cause of the hemorrhagic disease, scurvy. Isolated in 1928, vitamin C is essential for the development and maintenance of connective tissues. It plays an important role in bone formation, wound healing and the maintenance of healthy gums. Vitamin C plays an important role in a number of metabolic functions including the activation of the B vitamin, folic acid, the conversion of cholesterol to bile acids and the conversion of the amino acid, tryptophan, to the neurotransmitter, serotonin. It is an antioxidant that protects body from free radical damage. (77)

Deficiency of this vitamin is often associated with anemia, infections, bleeding gums, scurvy, poor wound healing, capillary hemorrhage, muscle degeneration, atherosclerotic plaques and neurotic disturbances. For the correction of deficiency, vitamin C is often supplemented in large doses and unlike fat soluble vitamins, toxicity is rare. (77) However, vitamin C deficiency and scurvy are rare in developed countries. (78)

A well-planned plant-based diet is unlikely to be deficient in in vitamin C as there are numerous plant sources. Vitamin C is found in citrus fruits, green peppers, red peppers, strawberries, tomatoes, broccoli, brussels sprouts, turnips and leafy vegetables. (77) Supplements typically contain vitamin C in the form of ascorbic acid, which has equivalent bioavailability to that of naturally occurring ascorbic acid in foods, such as orange juice and broccoli. (79, 80, 81) Other forms of vitamin C supplements include sodium ascorbate; calcium ascorbate; and other mineral ascorbates.

Table 5: Recommended Dietary Allowances for Vitamin C (78)

Age Male Female Pregnancy Lactation
0–6 months 40 mg* 40 mg*    
7–12 months 50 mg* 50 mg*    
1–3 years 15 mg 15 mg    
4–8 years 25 mg 25 mg    
9–13 years 45 mg 45 mg    
14–18 years 75 mg 65 mg 80 mg 115 mg
19+ years 90 mg 75 mg 85 mg 120 mg
Smokers Individuals who smoke require 35 mg/day more vitamin C than nonsmokers.

Table 6: Selected Food Sources of Vitamin C (32)

Food Milligrams (mg) per serving Percent (%) DV*
Red pepper, sweet, raw, ½ cup 95 158
Orange juice, ¾ cup 93 155
Orange, 1 medium 70 117
Grapefruit juice, ¾ cup 70 117
Kiwifruit, 1 medium 64 107
Green pepper, sweet, raw, ½ cup 60 100
Broccoli, cooked, ½ cup 51 85
Strawberries, fresh, sliced, ½ cup 49 82
Brussels sprouts, cooked, ½ cup 48 80
Grapefruit, ½ medium 39 65
Broccoli, raw, ½ cup 39 65
Tomato juice, ¾ cup 33 55
Cantaloupe, ½ cup 29 48
Cabbage, cooked, ½ cup 28 47
Cauliflower, raw, ½ cup 26 43
Potato, baked, 1 medium 17 28
Tomato, raw, 1 medium 17 28
Spinach, cooked, ½ cup 9 15
Green peas, frozen, cooked, ½ cup 8 13

Vitamin D is addressed in a separate article.

Vitamin E

Vitamin E is found naturally in some foods, added to others, and available as a dietary supplement.

Vitamin E is the collective name for a group of fat-soluble compounds with distinctive antioxidant activities, specifically that stop the production of reactive oxygen species formed when fat undergoes oxidation. (82)  Naturally occurring vitamin E exists in eight chemical forms (alpha-, beta-, gamma-, and delta-tocopherol and alpha-, beta-, gamma-, and delta-tocotrienol) that have varying levels of biological activity. (83) Alpha- (or α-) tocopherol is the only form that is recognized to meet human requirements.

In addition to its activities as an antioxidant, vitamin E is involved in immune function and, as shown primarily by in vitro studies of cells, cell signaling, regulation of gene expression, and other metabolic processes. (83) Alpha-tocopherol inhibits the activity of protein kinase C, an enzyme involved in cell proliferation and differentiation in smooth muscle cells, platelets, and monocytes. (78) Vitamin-E–replete endothelial cells lining the interior surface of blood vessels are better able to resist blood-cell components adhering to this surface. Vitamin E also increases the expression of two enzymes that suppress arachidonic acid metabolism, thereby increasing the release of prostacyclin from the endothelium, which, in turn, dilates blood vessels and inhibits platelet aggregation. (78)

Numerous foods provide vitamin E. Nuts, seeds, and vegetable oils are among the best sources of alpha-tocopherol, and significant amounts are available in green leafy vegetables and fortified cereals. Most vitamin E in American diets is in the form of gamma-tocopherol from soybean, canola, corn, and other vegetable oils and food products. (84) Frank vitamin E deficiency is rare and overt deficiency symptoms are unusual in otherwise healthy people. (78)

Table 7: Selected Food Sources of Vitamin E (Alpha-Tocopherol) (32)

Food Milligrams (mg) per serving Percent DV*
Wheat germ oil, 1 tablespoon 20.3 100
Sunflower seeds, dry roasted, 1 ounce 7.4 37
Almonds, dry roasted, 1 ounce 6.8 34
Sunflower oil, 1 tablespoon 5.6 28
Safflower oil, 1 tablespoon 4.6 25
Hazelnuts, dry roasted, 1 ounce 4.3 22
Peanut butter, 2 tablespoons 2.9 15
Peanuts, dry roasted, 1 ounce 2.2 11
Corn oil, 1 tablespoon 1.9 10
Spinach, boiled, ½ cup 1.9 10
Broccoli, chopped, boiled, ½ cup 1.2 6
Soybean oil, 1 tablespoon 1.1 6
Kiwifruit, 1 medium 1.1 6
Mango, sliced, ½ cup 0.7 4
Tomato, raw, 1 medium 0.7 4
Spinach, raw, 1 cup 0.6 3

DV = Daily Value

Table 8: Recommended Dietary Allowances for Vitamin E (Alpha-Tocopherol) (78)

Age Males Females Pregnancy Lactation
0–6 months* 4 mg (6 IU) 4 mg (6 IU)    
7–12 months* 5 mg (7.5 IU) 5 mg (7.5 IU)    
1–3 years 6 mg (9 IU) 6 mg (9 IU)    
4–8 years 7 mg (10.4 IU) 7 mg (10.4 IU)    
9–13 years 11 mg (16.4 IU) 11 mg (16.4 IU)    
14+ years 15 mg (22.4 IU) 15 mg (22.4 IU) 15 mg (22.4 IU) 19 mg (28.4 IU)

Because the digestive tract requires fat to absorb vitamin E, people with fat-malabsorption disorders are more likely to become deficient than people without such disorders. Deficiency symptoms include peripheral neuropathy, ataxia, skeletal myopathy, retinopathy, and impairment of the immune response. (78, 85) People with Crohn’s disease, cystic fibrosis, or an inability to secrete bile from the liver into the digestive tract, for example, often pass greasy stools or have chronic diarrhea; as a result, they sometimes require water-soluble forms of vitamin E, such as tocopheryl polyethylene glycol-1000 succinate. (83)

In general, clinical trials have not provided evidence that routine use of vitamin E supplements prevents cardiovascular disease or reduces its morbidity and mortality. Most research results do not support the use of vitamin E supplements by healthy or mildly impaired individuals to maintain cognitive performance or slow its decline with normal aging. Evidence to date is insufficient to support taking vitamin E to prevent cancer. Patients should not exceed the upper limits listed in Table 9.

Table 9: Tolerable Upper Intake Levels for Vitamin E (78)

Age Male Female Pregnancy Lactation
1–3 years 200 mg (300 IU) 200 mg (300 IU)    
4–8 years 300 mg (450 IU) 300 mg (450 IU)    
9–13 years 600 mg (900 IU) 600 mg (900 IU)    
14–18 years 800 mg (1,200 IU) 800 mg (1,200 IU) 800 mg (1,200 IU) 800 mg (1,200 IU)
19+ years 1,000 mg (1,500 IU) 1,000 mg (1,500 IU) 1,000 mg (1,500 IU) 1,000 mg (1,500 IU)

Vitamin K

“Vitamin K,” the generic name for a family of compounds with a common chemical structure of 2-methyl-1,4-naphthoquinone, is a fat-soluble vitamin that is naturally present in some foods and is available as a dietary supplement. (86) These compounds include phylloquinone (vitamin K1) and a series of menaquinones (vitamin K2). (87) Menaquinones have unsaturated isoprenyl side chains and are designated as MK-4 through MK-13, based on the length of their side chain. (86, 87) MK-4, MK-7, and MK-9 are the most well-studied menaquinones.

Phylloquinone is present primarily in plant foods and is the main dietary form of vitamin K. (44) Menaquinones, which are predominantly of bacterial origin, are present in modest amounts in various animal-based and fermented foods. (86) Almost all menaquinones, in particular the long-chain menaquinones, are also produced by bacteria in the human gut. (88, 89) MK-4 is unique in that it is produced by the body from phylloquinone via a conversion process that does not involve bacterial action. (90)

Vitamin K interacts with a few medications. In addition, certain medications can have an adverse effect on vitamin K levels.

Vitamin K functions as a coenzyme for vitamin K-dependent carboxylase, an enzyme required for the synthesis of proteins involved in hemostasis (blood clotting) and bone metabolism, and other diverse physiological functions. (44, 88) Prothrombin (clotting factor II) is a vitamin K-dependent protein in plasma that is directly involved in blood clotting. Warfarin (Coumadin®) and some anticoagulants used primarily in Europe, antagonize the activity of vitamin K and, in turn, prothrombin. (91) For this reason, individuals who are taking these anticoagulants need to maintain consistent vitamin K intakes. However, many plant foods are very high in vitamin K. Patients being treated with Warfarin might require more careful monitoring.

Antibiotics can destroy vitamin K-producing bacteria in the gut, potentially decreasing vitamin K status. This effect might be more pronounced with cephalosporin antibiotics, such as cefoperazone (Cefobid®), because these antibiotics might also inhibit the action of vitamin K in the body. (89, 92) Vitamin K supplements are usually not needed unless antibiotic use is prolonged (beyond several weeks) and accompanied by poor vitamin K intake. (92)

Bile acid sequestrants, such as cholestyramine (Questran®) and colestipol (Colestid®), are used to reduce cholesterol levels by preventing reabsorption of bile acids. They can also reduce the absorption of vitamin K and other fat-soluble vitamins, although the clinical significance of this effect is not clear. (92, 93) Vitamin K status should be monitored in people taking these medications, especially when the drugs are used for many years. (93)

Orlistat is a weight-loss drug that is available as both an over-the-counter (Alli®) and prescription (Xenical®) medication. It reduces the body’s absorption of dietary fat and in doing so, it can also reduce the absorption of fat-soluble vitamins, such as vitamin K. Combining orlistat with warfarin therapy might cause a significant increase in prothrombin time. (94) Otherwise, orlistat does not usually have a clinically significant effect on vitamin K status, although clinicians usually recommend that patients taking orlistat take a multivitamin supplement containing vitamin K. (95, 96, 97)

Osteocalcin is another vitamin K-dependent protein that is present in bone and may be involved in bone mineralization or turnover. (88)

People with malabsorption syndromes and other gastrointestinal disorders, such as cystic fibrosis, celiac disease, ulcerative colitis, and short bowel syndrome, might not absorb vitamin K properly. (44, 88, 98) Vitamin K status can also be low in patients who have undergone bariatric surgery, although clinical signs may not be present. (99) These individuals might need monitoring of vitamin K status and, in some cases, vitamin K supplementation.

In the circulation, vitamin K is carried mainly in lipoproteins. (100) Compared to the other fat-soluble vitamins, very small amounts of vitamin K circulate in the blood. Vitamin K is rapidly metabolized and excreted. Based on phylloquinone measurements, the body retains only about 30% to 40% of an oral physiological dose, while about 20% is excreted in the urine and 40% to 50% in the feces via bile. (100, 87) This rapid metabolism accounts for vitamin K’s relatively low blood levels and tissue stores compared to those of the other fat-soluble vitamins. (100)

The Food and Nutrition Board did not establish upper limits for vitamin K because of its low potential for toxicity. (44) In its report, the FNB stated that “no adverse effects associated with vitamin K consumption from food or supplements have been reported in humans or animals.”

Table 10: Adequate Intakes for Vitamin K (44)

Age Male Female Pregnancy Lactation
Birth to 6 months 2.0 mcg 2.0 mcg    
7–12 months 2.5 mcg 2.5 mcg    
1–3 years 30 mcg 30 mcg    
4–8 years 55 mcg 55 mcg    
9–13 years 60 mcg 60 mcg    
14–18 years 75 mcg 75 mcg 75 mcg 75 mcg
19+ years 120 mcg 90 mcg 90 mcg 90 mcg

Table 11: Selected Food Sources of Vitamin K (101, 32)

Food Micrograms (mcg) per serving Percent DV*
Collards, frozen, boiled, ½ cup 530 662
Turnip greens, frozen, boiled ½ cup 426 532
Spinach, raw, 1 cup 145 181
Kale, raw, 1 cup 113 141
Broccoli, chopped, boiled, ½ cup 110 138
Soybeans, roasted, ½ cup 43 54
Carrot juice, ¾ cup 28 34
Soybean oil, 1 tablespoon 25 31
Edamame, frozen, prepared, ½ cup 21 26
Pumpkin, canned, ½ cup 20 25
Pomegranate juice, ¾ cup 19 24
Okra, raw, ½ cup 16 20
Pine nuts, dried, 1 ounce 15 19
Blueberries, raw, ½ cup 14 18
Iceberg lettuce, raw, 1 cup 14 18
Grapes, ½ cup 11 14
Vegetable juice cocktail, ¾ cup 10 13
Canola oil, 1 tablespoon 10 13
Cashews, dry roasted, 1 ounce 10 13
Carrots, raw, 1 medium 8 10


Naturally occurring compounds found in foods of plant origin, known as phytonutrients or phytochemicals (phyto means plant in Greek), are thought to be partly responsible for the protective health benefits of plant-based foods and beverages, beyond those conferred by their vitamin and mineral contents. These phytonutrients, which are part of a large and varied group of chemical compounds, also are responsible for the color, flavor, and odor of plant foods, such as blueberries’ dark hue, broccoli’s bitter taste, and garlic’s pungent odor.

Research strongly suggests that consuming foods rich in phytonutrients provides health benefits. (102, 3) Phytonutrients are reported to have antioxidant and anti-carcinogenic properties, as well as a spectrum of potential tumor-blocking activities. (103, 104) Studies show that there are as many as 100 different phytonutrients in just one serving of vegetables. (104) Each phytonutrient comes from a variety of plant sources, and has different effects and benefits on the body. Some researchers estimate that there are up to 4000 phytonutrients in existence. (104)

Phytonutrients are found in fruits, vegetables, whole grains, legumes, herbs, spices, nuts, and seeds and are classified according to their chemical structures and functional properties. They include flavonoids, flavonols, flavanols, proanthocyanidins, and procyanidins. Phytonutrients also include compounds such as salicylates, phytosterols, saponins, glucosinolates, polyphenols, protease inhibitors, monoterpenes, carotenoids, phytoestrogens, sulfides, terpenes, lectins, and many more.

Though the broadest groups of phytonutrients, such as flavonoids, isoflavones, or anthocyanidins, often are referred to as if they were a homogenous group, the individual compounds within each group have different chemical structures, are metabolized differently by the body, and may have different health effects. (105) In addition, part of the health benefits of a plant-based diet may result from additive and synergistic combinations of phytonutrients present in whole and minimally processed foods, which may be responsible for their potent antioxidant and anticancer activities. (106)

Studies have found that phytonutrients have the potential to stimulate the immune system, prevent toxic substances in the diet from becoming carcinogenic, reduce inflammation, prevent DNA damage and aid DNA repair, reduce oxidative damage to cells, slow the growth rate of cancer cells, trigger damaged cells to apoptosis before they can reproduce, help regulate intracellular signaling of hormones and gene expression, and activate insulin receptors. (107, 108) In addition, there likely are health effects of phytonutrients that researchers haven’t yet recognized. (107)

Many other studies have focused on the antioxidant function of phytonutrients. For many of the phytonutrients in food, their antioxidant effects on cell signaling and gene expression may be more important for health benefits than direct antioxidant activity, effects that can be seen even with low concentrations of phytonutrients in plasma and tissues. (109)

There’s little information on the average intake of phytonutrients among Americans. However, it shouldn’t be surprising that the intake of phytonutrients is higher among people consuming a plant-based diet. (110)



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Appendix A: Glycemic Index of Some Foods (22)

Food Glycemic Index
High-Carbohydrate foods  
White wheat bread* 75 ± 2
Whole wheat/whole meal bread 74 ± 2
White rice, boiled* 73 ± 4
Unleavened wheat bread 70 ± 5
Brown rice, boiled 68 ± 4
Couscous† 65 ± 4
Wheat roti 62 ± 3
Rice noodles† 53 ± 7
Udon noodles 55 ± 7
Specialty grain bread 53 ± 2
Sweet corn 52 ± 5
Chapatti 52 ± 4
Spaghetti, white 49 ± 2
Spaghetti, whole meal 48 ± 5
Corn tortilla 46 ± 4
Barley 28 ± 2
Breakfast Cereals  
Cornflakes 81 ± 6
Instant oat porridge 79 ± 3
Rice porridge/congee 78 ± 9
Wheat flake biscuits 69 ± 2
Millet porridge 67 ± 5
Muesli 57 ± 2
Porridge, rolled oats 55 ± 2
Fruit and Fruit Products  
Watermelon, raw 76 ± 4
Pineapple, raw 59 ± 8
Mango, raw† 51 ± 5
Banana, raw† 51 ± 3
Orange juice 50 ± 2
Strawberry jam/jelly 49 ± 3
Peaches, canned† 43 ± 5
Orange, raw† 43 ± 3
Dates, raw 42 ± 4
Apple juice 41 ± 2
Apple, raw† 36 ± 2
Potato, instant mash 87 ± 3
Potato, boiled 78 ± 4
Pumpkin, boiled 64 ± 7
Sweet potato, boiled 63 ± 6
Potato, french fries 63 ± 5
Plantain/green banana 55 ± 6
Taro, boiled 53 ± 2
Vegetable soup 48 ± 5
Carrots, boiled 39 ± 4
Dairy Alternatives  
Rice milk 86 ± 7
Soy milk 34 ± 4
Lentils 32 ± 5
Chickpeas 28 ± 9
Kidney beans 24 ± 4
Soya beans 16 ± 1
Snack Products  
Rice crackers/crisps 87 ± 2
Popcorn 65 ± 5
Soft drink/soda 59 ± 3
Potato crisps 56 ± 3
Chocolate 40 ± 3
Glucose 103 ± 3
Sucrose 65 ± 4
Honey 61 ± 3
Fructose 15 ± 4

Data are means ± SEM.

* Low-GI varieties were also identified.

† Average of all available data.

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