B Vitamins:

April 2001

B Vitamins:
The Bottom Line

By Karen Plawecki, M.S., R.D.
Contributing Editor

The B vitamins have received an update. The Institute of Medicine, Washington, D.C., recently reviewed the role of B vitamins in health: their sources, functions and current recommended intake levels. As a result, there is new information on the B vitamins and their role in maintaining health and preventing disease.

Understanding the acronyms
Dietary Reference Intakes (DRIs) are the revised and updated versions of the Recommended Dietary Allowances (RDA). The DRIs not only focus on how much of a nutrient is needed to prevent deficiency, but levels needed for disease prevention.

Values within a nutrient’s DRI consist of the Estimated Average Requirement (EAR), RDA or Adequate Intake (AI) and Tolerable Upper Intake Level (UL). The EAR is the amount of nutrient that meets 50% of the population’s needs. The RDA is based on this level plus an additional amount to cover most people’s needs. If not enough evidence exists to determine the EAR, then an AI is recommended for the nutrient, using an estimate that ensures needs are being met. The UL indicates the maximum safe-consumption level of a nutrient, not a goal of how much is to be consumed. Due to minimal research and limited adverse effects at high doses, a UL has not been set for all B vitamins.

The B basics
The B vitamins are water-soluble compounds with various sensitivities to light. Many diseases related to vitamin-B deficiencies, including beriberi (thiamin) and pellagra (niacin), have essentially disappeared — partly due to food fortification. Their functions fall primarily into two categories: energy production and the transfer of one-carbon units. The latter is at the center of much research on birth defects and vascular disease.

B-ing involved in energy
Several B vitamins function as coenzymes in metabolizing carbohydrates, proteins and/or fats.

Thiamin (B1): Thiamin’s RDA is 1.2 mg per day for men and 1.1 mg per day for women. Intake surveys show that U.S. adults meet their thiamin requirements through common foods, such as enriched grains, ready-to-eat cereals and pork products.

Riboflavin (B2): The RDA for adults is 1.3 mg per day and 1.1 mg per day for men and women, respectively. Significant food sources include dairy products, poultry and enriched/fortified grains.

Niacin (nicotinamide, nicotinic acid): Niacin in meat, fish, poultry and fortified/enriched cereal grains is highly bioavailable. The body also can convert tryptophan to niacin, but deficiencies of riboflavin and B6 reduce this conversion. The RDA is 16 mg per day and 14 mg per day of niacin equivalents (NE) for men and women, respectively. The UL for adults is 35 mg per day. High niacin doses (1.5 to 3.0 grams per day) via supplementation or fortification have been linked with adverse side effects, including flushing of skin, liver toxicity and glucose intolerance.

Vitamin B6 (pyridoxine): The RDA for B6 is 1.3 mg per day for young adults and increases with age. The UL is set at 100 mg per day, as high doses from supplements have led to sensory neuropathy. Vitamin B6 is very bioavailable, and good sources include fortified cereals, soy-based meat alternatives, meat, fish, poultry, organ meats, starchy vegetables and noncitrus fruits.

Biotin: The AI for biotin for adults is 30 mg per day. Various levels of biotin are found across food groups. Raw eggs contain avidin, a protein that, if consumed, renders biotin unavailable, which could lead to a biotin deficiency risk. Cooking, however, destroys avidin.

Pantothenic acid: This is found in a diverse set of sources, such as poultry, tomato-based products, egg yolk and whole grains. The AI for adults is 5 mg per day, and reported deficiencies are rare.

One-carbon transfers
High homocysteine levels serve as markers for problems occurring with one-carbon transfers. Formation of homocysteine is an intermediate step in the metabolism of the sulfur amino acids, cystathione and methionine. Folate, B12 and B6 are part of the enzymes in this cycle.

Folate: This vitamin is involved with DNA and amino-acid metabolism and carries the methyl group. The RDA for folate is 400 mg perday of dietary folate equivalents (DFEs) for both men and women. DFEs account for the approximately 50% reduced bioavailability of folate in food vs. the folic acid form found in fortified foods and supplements. To reduce the risk of neural-tube defects, the daily recommended consumption for women of child-bearing age is 400 mg from fortified foods or supplements in addition to folate found in food. The UL for folate from fortified foods or supplements is 1,000mg per day to help ensure detection of B12 deficiency even though the folate level is adequate. Common food sources include citrus fruits, green vegetables and fortified grains.

Vitamin B12 (cobalamine): B12 is used in the energy-utilization pathway. An adequate amount of B12 is needed for blood-cell formation and neurological function. The RDA for adults is 2.4 mg per day, and can be found in animal sources, or B12-fortified foods. Due to reduced absorption of B12 with aging, fortified foods or supplements are recommended for those over 50.

Choline: This new entry into the DRIs serves as part of the methyl-donor betaine, in addition to its use as a structural component of cell walls and its role in lipid and cholesterol metabolism. The AI is 550 mg per day for men and 425 mg per day for women. The UL is 3.5 grams per day for adults to reduce hypotension risk.

A full report on the updated DRIs of B vitamins, including age-specific requirements, can be viewed at www.nap.edu/catalog/6015.html.

Karen Plawecki, a registered dietitian, earned her master’s degree in nutrition with an emphasis in food science from Purdue University,West Lafayette, IN. Currently, she teaches a nutrition and food course at the University of Illinois, Urbana-Champaign.

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