the transfer of active aldehyde in carbohydrate metabolism and decarboxylation of a-keto acids such as pyruvate. The requirement for thiamin is directly correlated with carbohydrate intake and increases as the metabolic rate increases due to pregnancy, lactation, or increased physical exercise. The 1980 RDA of 0.5 mg/1,000 kcal was set to maintain normal levels of TPP-dependent erythrocyte transketolase activity and urinary excretion. For those whose total caloric intake is less than 2,000 kcal, at least 1.0 mg/day is recommended.
NFCS data for 1977-1978 indicate that 83% of all respondents consumed 70% or more of the thiamin RDA (USDA, 1984). Men and women 19 to 50 years of age in the 1985 survey averaged 0.70 mg/1,000 kcal, whereas children 1 to 5 years old averaged 0.79 mg/1,000 kcal (USDA, 1986, 1987).
In its coenzyme forms (flavin mononucleotide and flavin adenine dinucleotide), riboflavin functions in oxidation-reduction reactions in energy production, in the respiratory chain, and in many other metabolic pathways. Richest food sources of riboflavin include liver, milk, dark-green leafy vegetables, and enriched breads and cereals.
The 1980 RDA for riboflavin is 0.6 mg/1,000 kcal; a minimum of 1.2 mg/day is recommended for those whose caloric intake is less than 2,000 kcal/day. In 1985 the mean intake for men and women 19 to 50 years of age was 0.82 mg/1,000 kcal and 0.88 mg/1,000 kcal, respectively (USDA, 1986, 1987); for children 1 to 5 years of age, it was 1.12 mg/1,000 kcal (USDA, 1987). Food groups contributing the most riboflavin to diets of women and children in the 1985 and 1986 surveys were enriched grain products, milk and milk products, meat, poultry, and fish.
In nutrition literature, the term niacin is used generically to encompass the active forms of this vitamin, nicotinic acid and nicotinamide; however, estimates of niacin requirements take into account preformed niacin as well as that obtained as equivalent (NE) in the body from tryptophan metabolism. For this purpose, it is estimated that when 60 mg of tryptophan are consumed by an adult, enough is oxidized to produce 1 mg of niacin (NRC, 1980).
Hundreds of enzymes in the body require niacin in its coenzyme forms nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP). Many reactions utilizing these enzymes are involved in energy metabolism. Hence, the 1980 RDA is set at 6.6 niacin equivalents (NE) per 1,000 kcal, and an intake of not less than 13 NE is recommended when the caloric intake is less than 2,000 kcal. One NE is equal to either 1 mg of niacin or 60 mg of tryptophan.
The 1985 CSFII indicated that the mean intake of preformed niacin for women (USDA, 1987) and men (USDA, 1986) 19 to 50 years was 10.8 NE/1,000 kcal, whereas for children 1 to 5 years it was 9.6 (USDA, 1987). Average diets in the United States have been estimated to furnish 500 to 1,000 mg or more of tryptophan per day, providing 8 to 17 NE (NRC, 1980). Grain products, meat, poultry, and fish were the most important sources of preformed niacin reported in the 1985 and 1986 CSFII; nuts and legumes were identified as good sources.
Vitamin B6 is the generic term used for pyridoxine, pyridoxal, and pyridoxamine, the coenzyme forms of which are pyridoxal phosphate and pyridoxamine phosphate. Vitamin B6-dependent enzymes are needed in a wide range of reactions, most of which involve amino acid metabolism. The 1980 RDAs were based on a ratio of 0.02 mg of vitamin B6 per gram of protein consumed. The allowance for adult females was therefore set at 2.0 mg/day, assuming a protein intake of 100 g/day; for adult males, it was set at 2.2 mg/day, assuming a protein intake of 110 g/day. A lower allowance presumably would be appropriate for those with lower protein intakes.
A major difficulty in assessing vitamin B6 intake is that values for the B6 content of food are unreliable. In the 1977-1978 NFCS, on the 3 days studied, 72% of the respondents consumed at least 80% of the desired ratio of vitamin B6 to protein, whereas only 39% had at least 80% of the vitamin B6 RDA. The 1985 CSFII indicated that on the 1 day surveyed, men 19 to 50 years of age on average consumed 85% of the B6 RDA, but that only 27% of women consumed 70% or more of their B6 RDA (USDA, 1986, 1987). However, the mean ratio of vitamin B6 to protein was 0.019 for women, despite the fact that 43% of them consumed less than 50% of the RDA (USDA, 1987). Failure to