7 Vitamins in Health and Disease

7.1 Introduction to Vitamins

Overview

Did you know that your body is unable to manufacture sufficient quantities of vitamins, so you must consume them in your diet? Dispersed throughout the food supply, vitamins are essential, organic substances made by plants and animals. Most vitamins known today were first discovered based upon their ability to “cure” certain diseases such as scurvy, rickets, and beriberi. By simply putting the missing vitamin back in the diet, these diseases were cured.

Recognizing the link between vitamin deficiency and conditions such as rickets or blindness was an important scientific accomplishment of the twentieth century. Vitamins participated in a wide range of normal body functions, and an adequate supply is critical throughout life.

Standardized reference values for vitamins are recommended levels of routine intake that promote health. Dietary Reference Intakes include Estimated Average Requirement (EAR), Adequate Intake (AI), and Tolerable Upper Level (UL) for each vitamin.

Vitamins are categorized as fat soluble or water soluble based upon their ability to dissolve in water. This property determines vitamin characteristics and establishes how each behaves in the body. There are four fat-soluble and ten water-soluble vitamins. Discussion of any vitamin includes function, disease associated with deficiency, criticality for health, and good food sources.

Supplementing a diet with vitamins is commonplace in the US. Those wishing to consume additional vitamins beyond that acquired in food should consider the UL for each and avoid more than double the RDI. Unprocessed foods are the best source of vitamins. Fruits and vegetables are natural sources of vitamins with antioxidant properties.

Objectives

• Identify water-soluble and fat-soluble vitamins
• Describe vitamin functions
• Relate Dietary Reference Intakes to age group

7.2 What Is a Vitamin?

Overview

Your body is unable to synthesize or manufacture vitamins. You must consume them in your diet. Adequacy is determined by the amount that minimizes the risk of known deficiency diseases as well as certain chronic degenerative diseases such as dementia, cardiovascular disease, cancer, diabetes mellitus, some forms of renal disease, and arthritis.

Over time insufficient intake of a specific vitamin leads to serious health problems. For example, a vitamin A deficiency initially causes night blindness. At this stage, supplying the missing vitamin restores health. If it progresses to blindness, the damage is irreversible.

Where should we get our vitamins? Everyone should eat at least five fruits and vegetables a day. Fruits and vegetables are excellent sources of vitamins and carotenoids. These vitamins are important antioxidants that prevent or delay the onset of aging, certain cancers, cardiovascular disease, and cataracts.

Key Concepts

• Vitamin solubility
• Dietary recommendations
• Importance of water solubility

Implications of Vitamin Solubility

Vitamins are categorized based on their ability to dissolve in water. This property determines the following characteristics: how a vitamin is (1) absorbed from the digestive tract and (2) transported through the circulatory system. Solubility also determines (3) storage, (4) role in the body, and (5) susceptibility to excretion by the kidney. Fat-soluble vitamins, or those that will not go into solution in water but require a fat carrier for optimal absorption, are stored in fat tissue or the liver and cannot be excreted by the kidney.

Dietary Reference Intakes

In the Dietary Reference Intakes (DRIs), RDA and AI re provided for different life-stage groups, which may vary based on gender. The groups include ages zero to six months, seven to twelve months, one to three years, four to eight years, nine to thirteen years, fourteen to eighteen years, nineteen to thirty years, thirty-one to fifty years, fifty-one to seventy years, and over seventy years. Additional groups are provided for pregnant or lactating women.¹ A requirement is defined as the lowest continuing intake level of a nutrient that maintains health. Each type of DRI refers to the average daily nutrient intake viewed over time. Daily variation is not problematic.

Recommended Dietary Allowances

RDAs are a general recommendation that applies to most individuals who are in a state of good health. RDAs paint a broad brush to meet the needs of 98 percent of individuals in an age and gender group.

The AI is the mean intake needed for health. Recommendations for daily intake are listed either as an RDA or AI based upon the availability of data. An AI indicates that more research is needed. When this research is completed, AI estimates are replaced with EAR and RDA.

EAR is the intake at which the risk of inadequacy is 0.5 (50 percent). The RDA is the intake at which the risk of inadequacy is very small (2-3 percent). The AI is set without being able to estimate the average requirement.

It is assumed that the AI is at or above the RDA if one could be calculated. At intakes between the RDA and the UL, the risks of inadequacy and of excess are both close to zero. At intakes above the UL, the risk of adverse effects may increase.

Tolerable Upper Limit or UL is the highest level of daily nutrient intake that is likely to pose no risk of adverse health effects. The UL represents intake from water, food, and supplements. The UL is not a recommended level of intake, and there is no established benefit for consuming amounts exceeding recommendations (RDA or AI).

As intake extends above the UL, the risk of adverse effects grows. UL has not been established for vitamin K, riboflavin, thiamin, vitamin B12, pantothenic acid, biotin, or carotenoids.

Supplementation

If you are considering supplementing your diet with vitamins, a good rule of thumb is a maximum of 200 percent (or two times) of the RDA for a specific vitamin. Most good-quality multivitamin tablets do not exceed 100 percent, a more conservative approach.

Your body has a range within which it efficiently utilizes vitamins. Falling below the range (less than one-half the RDA) can lead to a deficiency, and going above twice the RDA can lead to an imbalance or toxicity. Vitamins often share the same pathway for absorption, and an excess of one can prevent the efficient absorption of another. Put another way, too much of one vitamin can cause a deficiency in another.

Many functions that require vitamins are rate limited. A minimum amount is required, but an excess does not accelerate or improve function. Amounts in excess of needs are stored (fat soluble) or excreted (water soluble). Consumption of fat-soluble vitamins in excess storage capacity damages tissue and leads to undesirable consequences. Think twice about supplementing. Consider this: except for vitamin D, supplements are not provided for men and women training for space flight at NASA; foods provide all the nutrients that they need!31

The Food Supply

Natural, unprocessed foods provide more vitamins than unfortified, refined counterparts. Whole grains are a significant source of thiamin, riboflavin, niacin, vitamin B6, folate, and vitamin E. As wheat, rice, and corn are milled and processed, however, the amount of each vitamin falls. To compensate for this loss, many grain-based foods are enriched.

B-complex vitamins are added to breakfast cereals to replace those lost during processing. Breads are enriched with folate due to the relationship between a maternal deficiency and risk of neural tube birth defects. Milk is enriched with vitamin D to forestall rickets.

Fruits and Vegetables

Many people living in the US do not consume enough fruits and vegetables. In addition, some lifestyle choices, such as smoking, cause antioxidant loss and increase need. Real foods offer the best source of antioxidants and vitamins. Learn more at the MyPlate website (https://www.myplate.gov). Check out fruit and vegetable groups and find ways to improve the vitamin content of your diet!

Summary

Knowing the water solubility of a vitamin tells you if it needs a carrier during digestion (fat soluble), if it is transported in the bloodstream (water soluble), if it needs a protein carrier for transport (fat soluble), and if the excess is stored in the liver and fat tissue (fat soluble). DRIs are standard recommendations for daily vitamin intake that includes RDA, AI, EAR, and UL. DRI values are evidence based and help people meet daily needs while avoiding oversupplementing any vitamin.

7.3 Vitamins A, D, E, and K

Overview

The fat-soluble vitamins include vitamins A, D, E, and K. These vitamins are best absorbed when your meal contains some fat. Seems obvious, but a glass of water and a vitamin tablet for breakfast does not optimize absorption; real food is needed.

The absorption of fat-soluble vitamins varies based upon the composition of the meal and the efficiency of fat absorption. When a meal contains foods that are high in viscous fiber, a nondigestible carbohydrate, absorption of fat and fat-soluble vitamins is slightly reduced. Manufactured foods containing fat-based fat replacers such as caprenin, salatrim, or olestra also reduce absorption, raising concerns about dietary adequacy.² To compensate for this reduction, products containing fat-based replacers are enriched with fat-soluble vitamins.

While it is impossible to overdose on fat-soluble vitamins by eating too many vitamin-rich foods, it is possible with supplements. Consuming more than two times the RDA can be toxic for some vitamins. The most toxic is vitamin A. Excess amounts damage the liver and other body organs. In the case of fat-soluble vitamins, the kidneys cannot assist in reducing the load by expelling it in urine. Due to the potential for toxicity, use caution when supplementing with fat-soluble vitamins.

Key Concepts

• Functions of the fat-soluble vitamins
• Deficiencies associated with inadequate intake
• Recommendations and food sources

Vitamin A

Fat-soluble vitamin A has a multitude of functions in the body, among them the maintenance of healthy teeth, skeletal and soft tissues, mucus membranes, and skin.³ It also is known for its importance in vision, especially night vision. Pigments in the retina of the eye require vitamin A, or retinol, to function and signal the brain that light has struck the retina. When vitamin A is inadequate, the eye cannot readily adjust to changes in light intensity, and night blindness results.

At this point, the condition can be corrected with preformed vitamin A from animal sources (such as milk, eggs, or fish oils) or with preformed vitamin A (beta-carotenoid) from plants.⁴ If allowed to progress, a vitamin A deficiency leads to permanent blindness due to a condition called xerophthalmia, or dry eye.⁵ Worldwide, this is a leading cause of new blindness.

Vitamin A is measured in either IUs or micrograms (mcg). Here’s how to convert: 1 IU = 0.3 mcg vitamin A (retinol); 1 IU = 0.6 mcg beta-carotene. When regularly consumed as supplements at levels of twice the recommended use or more, it is possible to get too much vitamin A. The UL for both men and women (>19 years old) is 3,000 mcg/day. Poisoning occurs with doses of 25,000 IU (7,500 mcg) or more.⁶ While large doses of beta-carotene don’t cause illness, excesses can turn skin color yellow and orange. Simply reducing intake returns skin color to normal.

Vitamin A is found in whole milk, fortified breakfast cereals, cod and halibut fish oils, and eggs. Carotenoids are found in plant foods, such as dark-green leafy vegetables and those that are bright orange/yellow, like summer squash and carrots. The deeper the color, the greater the amount of beta-carotene. Others include kale, mangoes, peas, sweet red peppers, sweet potatoes, turnip greens, apricots, broccoli, cantaloupe, cherries, chili peppers, nectarines, peaches (if not canned), tomatoes, and tomato juice.

Vitamin D

In some ways, vitamin D does not fit the definition of a vitamin. Why? Your body can make it! Exposure to sunlight activates a cholesterol-like substance in skin oils, leading to the synthesis of vitamin D.⁷ While the body is capable of manufacturing vitamin D, for optimal health, an external source is often necessary. Lack of adequate sunshine in winter months, too much time spent indoors, and head-to-toe clothing all contribute to insufficient endogenous (in the body) production.⁸

Vitamin D acts as a hormone in the regulation of blood calcium levels. To maintain calcium levels, vitamin D influences (1) the absorption of calcium and phosphorous from the gastrointestinal tract, (2) kidney excretion of calcium, and (3) the deposition of calcium in bones. Vitamin D conserves body calcium by optimizing absorption in the intestine and minimizing loss in urine.

Adequate vitamin D promotes proper calcium deposition and prevents rickets. Rickets develops during childhood when bones remain soft and weak, resulting in bowed legs or knock knees, thickened wrists and ankles, and breastbone projection.⁹ Common among children during the smoggy days of the early twentieth century, rickets is prevented by an adequate supply of vitamin D. A US initiative in the 1930s effectively reduced the incidence of rickets through fortification of milk with 100 IU vitamin D per cup.

Few foods are natural sources of vitamin D. Modest amounts are found in beef liver, cheese, and egg yolk, while fatty fish and fish liver oils are among the best sources. Adequate skin exposure to sunlight initiates endogenous production; however, sun avoidance, including the use of sunscreen, can limit it.

Vitamin E

Vitamin E is not one compound but a collection of eight different forms of tocopherol; however, only the alpha (α-) form is a human requirement. It is found in cell membranes, where it prevents the formation of free radicals. How? It has antioxidant properties that neutralize compounds that cause oxidation damage to body cells.¹⁰

In foods, vitamin E is found in association with plant oils, where it protects polyunsaturated fatty acids from oxidation. It is destroyed by oxygen, metals, light, and heat. Consequently, the vitamin E content of a given food is reduced by poor harvesting, storage, or food-preparation techniques. Vitamin E is found in vegetable oils, wheat germ, green leafy vegetables, egg yolk, soybean, and nuts. What about vitamin E supplements? High doses are associated with an increased risk of bleeding by reducing the body’s ability to form blood clots.

Vitamin E acetate is an oil derivative used as an additive in tetrahydrocannabinol (THC)–containing e-cigarettes or vaping products, where it is heated to very high temperatures. Inhaling this heated oil is associated with an increased risk of severe respiratory illness. Why? Preliminary research suggests that it interferes with lung functioning, causing shortness of breath, coughing, and chest pain.^11,12,13 The US Food and Drug Administration (FDA) doesn’t review or authorize vaping products. While recommending avoidance as the best approach, FDA advises that users do not need to modify or add substances to retail products.

Vitamin K

Vitamin K is the name for a family of compounds sharing a common chemical structure that includes phylloquinones (vitamin K1) and menaquinones (vitamin K2).1⁴ Vitamin K1 is found in dark-green leafy vegetables, such as kale, turnip greens, cabbage, spinach, broccoli, peas, and green beans. Meats, dairy foods, and eggs provide moderate amounts of K2, which also is manufactured by bacteria in the large intestine and is present in small amounts in fermented foods.

Vitamin K is found throughout body tissues, including the liver, brain, heart, pancreas, and bone. It participates in the synthesis of blood clotting factors and proteins in bone, muscle, and kidneys. Most people living in the US consume adequate amounts, and deficiencies are uncommon.

Long-term antibiotic use, however, can destroy vitamin K producing beneficial bacteria living in the bowel (large intestine), reducing endogenous production. In such situations, a multiple vitamin supplement is recommended. Those taking anticoagulants or with bleeding disorders should discuss vitamin K status with their health care providers before beginning supplementation.

Summary

Vitamins A, D, and K are fat-soluble vitamins that perform a number of essential functions. Adequate intake of vitamin A prevents blindness; vitamin D, rickets; and vitamin K, excessive bleeding. Although adequate intake of all three vitamins is possible with a balanced, varied meal pattern, some people benefit from supplementation. Caution is needed when supplementing with vitamin A because routinely consuming more than twice the RDA can lead to toxic effects.

7.4 Vitamin C, Biotin, Choline, and Folate

Overview

Except for vitamin B12, water-soluble vitamins are not stored in the body. They are either utilized or excreted. An individual who takes too many vitamin C tablets produces bright-yellow urine. Why? The kidneys clear the excess from the bloodstream. This is possible because water is a carrier for this group of vitamins. Consequently, it is less likely that supplementation will lead to toxicity with water-soluble vitamins than with fat-soluble vitamins.

Key Concepts

• Functions of four water-soluble vitamins
• Deficiencies associated with inadequate intake
• Recommendations and food sources

Vitamin C

Vitamin C, or ascorbic acid, is vital to the biosynthesis of collagen in connective tissue, bone, teeth, tendons, and blood vessels. It contributes to immune function, enhances absorption of nonheme iron in plant-based foods, and is an important antioxidant that limits the cell-damaging effects of free radicals. While vitamin C has been marketed as a treatment for cancer, research has failed to confirm this belief.¹⁵ Many animals, rats included, can make their own vitamin C, but humans and guinea pigs cannot and must consume it in foods.

Consuming a megadose through supplementation does not proportionally enhance benefits. Excess intake results in increased amounts in feces (less is absorbed) and urine (more is excreted), with side effects including diarrhea and cramping.¹⁶

Adequate vitamin C intake prevents scurvy, a deficiency disease that is rare in the US but still common in malnourished populations around the world.¹⁷ Early symptoms include weakness, fatigue, and aching limbs. Left untreated, scurvy progresses to gum disease, the failure to form scar tissue after an injury, the opening of previously healed wounds, and purple bruises (skin hemorrhages). Latent, or not apparent, scurvy is associated with chronic alcoholism and is characterized by weakness, difficulty walking, and leg pain.¹⁸ Miraculously, scurvy is readily cured with vitamin C supplementation.

Good food sources of vitamin C include citrus fruits, cranberries, broccoli, potatoes, tomatoes, Brussels sprouts, kale, cabbage, beet greens, and strawberries. Although not particularly high in vitamin C, potato products are a significant source for those consuming large amounts of these foods.

Biotin

Biotin is a cofactor for a group of five carboxylases (enzymes) required for the metabolism of fatty acids, glycogen, and amino acids.¹⁹ It also has a role in histone modification, gene regulation, and cell signaling. Food sources bound with protein include organ meats; some fruits, including bananas and apples; seeds; nuts; and some vegetables, including sweet potato, spinach, and broccoli. Dietary avidin, a protein in egg whites, binds biotin and would prevent absorption if foods are not cooked. Cooking denatures avidin and prevents it from binding with biotin in foods.

Deficiencies are rare and are not seen in healthy people eating a normal diet. Chronic alcoholism inhibits absorption, and biotin levels are low in 15 percent of alcoholics.¹⁹ While no adverse effects from excess supplementation are known, a lack of data suggests caution in exceeding the RDI.

Choline

All plants and animals need choline, a precursor for phospholipids, which are vital to cell membrane integrity.²⁰ It is critical in the production of acetylcholine, a neurotransmitter for brain and nerve function. A deficit alters liver function. While humans can produce choline, the amount is insufficient. Choline is available in many foods as a part of cell membranes. Meat, poultry, dairy, fish eggs, and soybean are high in this vitamin. Mushrooms, potatoes, beans, broccoli, and peanuts are examples of plant foods rich in choline.

Lecithin added as an emulsifier during food processing is an additional source. Excess supplementation precipitates hypotension, accompanied by sweating, diarrhea, and a fishy body odor. High intakes also increase the risk of liver toxicity.

Folate

Folate is the generic term for a water-soluble vitamin that is sometimes referred to as vitamin B9. It is also known as folacin or folic acid. It is important in the formation of DNA, proper cell division, and the maturation of red blood cells.²¹

In 1998 fortification of foods made from grains, like bread and pasta with folic acid, began when the FDA required fortification with 140 mcg folic acid per 100 grams of enriched bread. Subsequently, in 2016, voluntary addition of up to 154 mcg folic acid per 100 grams of corn masa flour was approved. Most adults living in the US have adequate intakes because folate is naturally present in a variety of foods, including vegetables, fruits, nuts, beans, seafood, eggs, dairy products, meat, poultry, and fortified grains.

Folate deficiency is often associated with an inadequate intake of other nutrients, resulting from poor diet, alcoholism, or disorders that affect nutrient absorption during digestion. Megaloblastic anemia, a condition where red blood cells cannot divide, is a clinical manifestation of a folate and/or vitamin B12 deficiency.

Some women develop deficiencies during pregnancy as a result of increased cell division. When this occurs, the fetus may suffer from neural tube defects. To reduce risk, women capable of becoming pregnant are advised to consume 600 mcg of dietary folate equivalents (DFT) daily. Adequate consumption before conception is important to meet demands during the early stages, when pregnancy is not yet confirmed.

What is a dietary folate equivalent?

1 mcg DFT = 1 mcg folate; 0.6 mcg folic acid from fortified foods; dietary supplements consumed with food; or 0.5 mcg folic acid supplement consumed without food.21

Summary

Although vitamin C adequacy is possible with a balanced, varied diet replete with fruits and vegetables, a chronic deficiency leads to night blindness, which left untreated results in permanent blindness. Biotin is an important cofactor for a group of enzymes and plays a role in gene regulation and cell signaling. Choline is involved in the production of phospholipids and neurotransmitters in the brain. Folate is critical to the formation of DNA, cell division, and red blood cell maturation. Women who might become pregnant are advised to take a folate supplement. Incidences of neural tube defects in newborn children raised concerns, and folate fortification of foods manufactured from grains was instituted.

7.5 Niacin, Pantothenic Acid, Riboflavin, Thiamin, B6, and B12

Overview

Sometimes referred to as the B vitamins, this group of water-soluble vitamins participated in a wide range of processes, including redox reactions, formation of coenzymes, energy metabolism, amino acid synthesis, neurological function, and DNA synthesis. It’s easy to see just how critical vitamin adequacy is for good health. Historically, deficiencies were related to poor access to vitamin-rich foods and food processing that unintentionally removed vitamins, like milling rice. Although today we don’t hear much about pellagra or beriberi, these deficiency diseases remind us of the importance of healthful food access and availability.

Key Concepts

• Functions of water-soluble B vitamins
• Deficiencies associated with inadequate intake
• Recommendations and food sources

Niacin

Niacin or vitamin B3 functions in biological redox reactions, including intracellular respiration.22 Requirements are met by nicotinic acid or nicotinamide in foods as well as the conversion of dietary protein containing tryptophan. Excess is excreted in the urine. A niacin deficiency is known as pellagra. Symptoms include dermatitis (particularly in areas exposed to the sun), dementia, and diarrhea. Lack of adequate pyridoxine and iron can precipitate a deficiency. Niacin found in grains is bound, or difficult for the gastrointestinal tract to digest, with 70 percent expelled in feces.

Treatment of grains with alkali increases bioavailability and improves absorption. Major sources of niacin in the US food supply are mixed dishes containing meat, fish, or poultry. Fortified breakfast cereals and grain products also are sources. Pellagra is uncommon in industrialized populations but is still found where people live in poverty and food insecurity. Naturally occurring niacin in foods is not known to cause toxicity. Side effects of supplements containing nicotinic acid include a burning sensation with flushing, tingling, and itching of the skin accompanied by a reddened of the arms, face, and chest.²² This effect can be minimized by taking therapeutic niacin with food.

Supplements containing nicotinamide are not known to cause skin flushing. Reports suggest that high doses of slow-release niacin can cause liver dysfunction and damage. Medical supervision and strict adherence to dosage recommendations are advised.

Pantothenic Acid

Pantothenic acid, also known as vitamin B5, is a component in coenzyme A (CoA), which is involved in fatty acid metabolism. Many different foods are sources of phosphopantetheine, which is converted to pantothenic acid during digestion by enzymes in the intestine.²³ Deficiencies are rare, and status is not normally measured in healthy people. A lack of dietary pantothenic acid caused burning feet syndrome in prisoners of war during World War II. This condition, which included numbness and burning of the hands and feet, headache, fatigue, irritability, and gastrointestinal disturbances, was the result of nutritionally poor diets. It was reversed with pantothenic acid supplementation.

Riboflavin

Riboflavin or vitamin B2 is an essential part of coenzymes involved in energy production, cellular function, growth and development, and metabolism of fats, drugs, and steroids.²⁴ Food sources include eggs, organ meats, lean meats, and milk. Foods made with processed grains are fortified with riboflavin, which is also found in green vegetables.

Among healthy people in the US, a riboflavin deficiency is rare. Sometimes endocrine disorders, like thyroid hormone insufficiency, can precipitate a deficiency, which is characterized by dermatitis and disorders of skin and mouth tissues. The American College of Sports Medicine recommends that vegetarian athletes consult a sports dietitian to avoid a deficiency.²⁵ Why? Vigorous exercise increases needs, and a plant-based diet lacks animal sources that are rich in riboflavin.

The rate of absorption is proportional to intake and increases when consumed with food. Excess intake via supplementation is excreted in urine. Most people living in the US obtain riboflavin from milk, milk drinks, bread products, and fortified breakfast cereals. Milk packaged in opaque containers retains more riboflavin, as it is destroyed by light.

Thiamin

Thiamin, also known as B1, is critical to energy metabolism, cellular function, and growth and development. The most common food sources in the US are whole grains, meat, and fish, as well as fortified breads and cereals.²⁶ About half the thiamin in the US diet occurs naturally in foods, while the remainder is from fortification. Heating foods reduces thiamin content, as does discarding cooking in water. Why is discarding cooking water an issue? Thiamin dissolves in water and is lost when cooking water isn’t incorporated into the meal. While brown rice retains its natural supply of thiamin, polished or white rice provides only one-tenth that amount and must be enriched. Thiamin deficiencies are rare in societies where a varied, balanced diet is consumed.

Early stages of a deficiency are connoted by weight loss, apathy, irritability, and anorexia due to loss of appetite. A thiamin deficiency results in beriberi, which once was common in areas where unfortified polished rice was a staple. In industrialized countries, severe thiamin deficiencies are associated with alcohol abuse. Effects of exercise on thiamin needs have not been extensively examined.

Vitamin B6

Vitamin B6, commonly known as pyridoxine, is the generic name for a group of six compounds (vitamers) with B6 activity.²⁷ B6 is important in the synthesis of nonessential amino acids and neurotransmitters that allow cells to communicate. It is important to the body’s immune function and hemoglobin formation.

It is involved in cognitive development and maintenance of normal blood levels of homocysteine, an amino acid. Elevated homocysteine levels are associated with increased risk of cardiovascular disease, and plasma vitamin B6 status is inversely correlated with homocysteine concentration; however, clinical trials that tested the relationship between vitamin B6 status and risk of heart disease or stroke found no benefit to supplementation with this vitamin.²⁸

Food sources include fish, organ meats, potatoes and starchy vegetables, and non-citrus fruits as well as fortified cereals. A glycosylated form  in fruits, vegetables, and grains is less bioavailable (absorbed and circulating in the bloodstream) than that in fish, meats, and fortified cereals. Vitamin B6 from food sources is not associated with adverse effects; however, excess supplementation with oral pyridoxine can cause neuropathy, painful skin lesions, photosensitivity, nausea, and heartburn. The UL for vitamin B6 is 100 mg/day for adults, less for children and adolescents (based on body size).

A B6 deficiency is uncommon among healthy people. When it occurs, a deficiency is characterized by dermatitis, anemia, and depression and is found in people with impaired renal function and autoimmune diseases and those with alcohol dependence.

Vitamin B12

Vitamin B12 is a group of compounds containing cobalt that are collectively called cobalamins.²⁹ Red blood cell formation, neurological function, and DNA synthesis all required vitamin B12. Major food sources include meat, milk, poultry, seafood, and eggs. Fortified breakfast cereals are recommended for vegetarians as a high bioavailability source of vitamin B12, which is not present in unfortified plant foods.³⁰

Historically, people with no animal products in their diets consumed a fermented food that supplied B12, an example of which is tempe, a fermented soybean-based food. It also is synthesized by bacteria, fungi, and other microorganisms, and mushrooms are a modest source containing about one-fourth as much B12 per gram as eggs.

Adequate stomach acid is necessary for the effective absorption of protein-bound B12 in foods. Hydrochloric acid and proteases (enzymes that act on proteins) release B12 from binding proteins, making it available (free form) for further action. This free form subsequently combines with intrinsic factor, a glycoprotein secreted by stomach cells, in preparation for absorption in the small intestine. Intrinsic factor capacity or availability to combine with free B12 sets the upper limit on absorption. Once intrinsic factor capacity is exceeded, B12 absorption decreases.

Summary

The B vitamins play critical roles in metabolism, and vitamin-rich foods sources are essential to good health. Historically, patterns of vitamin deficiency diseases such as pellagra or beriberi were traced to restricted access to food sources or processing that removed vitamins.

References

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