Water-Soluble Vitamins

The water-soluble vitamins include ascorbic acid (vitamin C), thiamin, riboflavin, niacin, vitamin B6 (pyridoxine, pyridoxal, and pyridoxamine), folacin, vitamin B12, biotin, and pantothenic acid.

Dietary Sources, Patterns of Intake, and Levels of Water-Soluble Vitamins

Vitamin C

Vitamin C is active in the body either as L-ascorbic acid or as dehydroascorbic acid. Its best-known function is as a cofactor for the enzyme required to hydroxylate prolyl and lysyl residues for the synthesis of connective tissue proteins. The 1980 Recommended Dietary Allowance (RDA) for adults—60 mg/day—maintains a body pool of 1.5 g, whereas 10 mg/day is sufficient to prevent or cure scurvy (NRC, 1980).

Since the beginning of the century, the amount of vitamin C in the food supply has increased, partly because of the greater supplies of citrus fruits and dark-green vegetables and partly because of the fortification of some foods. According to the U.S. Department of Agriculture’s Continuing Survey of Food Intakes of Individuals (CSFII) in 1985 (USDA, 1987), the mean vitamin C intake for children 1 to 5 years of age was 187% of the RDA, and for women 19 to 50 years of age, it was 125% of the RDA, based on 4 nonconsecutive days of intake. For men 19 to 50 years of age, the mean intake was 207% of the RDA (USDA, 1986), based on a 1-day intake.

NHANES II—the National Health and Nutrition Examination Survey, conducted from 1976 to 1980—indicated that 3% of the respondents 3 to 74 years of age had low serum vitamin C levels. Therefore, the Joint Nutrition Monitoring Evaluation Committee concluded that this vitamin should be accorded high-priority status for future monitoring (DHHS-USDA, 1986).

The Nationwide Food Consumption Survey (NFCS) of 1977-1978 indicated that 73% of the vitamin C intake came from fruits and vegetables. Mean intakes of vegetables and fruits were decidedly less for women and children under 131% of the poverty level on the basis of 4 nonconsecutive days of intake in 1985 (USDA, 1987).

The highest levels of vitamin C are found in green peppers, broccoli, citrus fruits, strawberries, melons, tomatoes, raw cabbage, and leafy greens such as spinach, turnip, and mustard greens. Losses of vitamin C occur when foods are cooked in large amounts of water, exposed to extensive heating, or exposed to air.

Thiamin

Thiamin functions in the body in the form of thiamin pyrophosphate (TPP), the coenzyme for the transfer of active aldehyde in carbohydrate metabolism and decarboxylation of α-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).

Riboflavin

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.

Niacin

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

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 meet the RDA does not denote a deficient diet because actual requirements differ in a population (see Chapter 3). Women in lower income groups had lower intakes of vitamin B6 than did those of higher income brackets. The major food sources of vitamin B6 in those surveys were meats, poultry, fish, grain products, fruits, and vegetables. More research is needed on the adequacy of vitamin B6 intake and the nutritional status of the population for this vitamin (DHHS-USDA, 1986).

Vitamin B12

Vitamin B12 substances are physiologically active cobalamins. The coenzyme (5′-deoxyadenosyl) and methyl forms of this vitamin are essential for the recyling of the active folate coenzyme, for the methylation of homocysteine to form methionine, and for metabolism of propionate. Vitamin B12 is also essential in the metabolism of fatty acids and aliphatic amino acids through its role in the isomerization of methylmalonyl-CoA to succinyl-CoA.

Vitamin B12 is synthesized by bacteria and is found only in animal foods such as meats, milk and milk products, and eggs. The 1980 RDA is 3 µg/day for individuals 7 years of age and older. National surveys indicate that intakes are higher for males than for females and higher for those in higher economic groups. The 1985 CSFII indicated that 60% of women 19 to 50 years of age consumed 100% of the RDA or more (USDA, 1987).

Folacin (Folic Acid or Folate)

Folacin intakes have been studied very little, because values for this vitamin in food composition tables are imputed. In addition, present analytical methods for this vitamin are not very reliable. There is also some concern that the 1980 RDA for folacin is unrealistically high (DHHS-USDA, 1986).

Folacin coenzymes are essential in the body for the transfer of single carbon units. They are needed for the synthesis of purine, methionine, and thymidylate, for the catabolism of histidine, and for the conversion of serine to glycine. The metabolism of folacin and vitamin B12 is linked because normal activity of methyl vitamin B12 is needed to maintain the metabolically active form of folacin.

The 1980 RDA was set at 400 µg/day for persons 11 years of age and older. The first national survey to report folacin intakes was CSFII in 1985. Women 19 to 50 years of age had average intakes of 189 µg/day (USDA, 1987), whereas intakes for men 19 to 50 years of age averaged 305 µg/day (USDA, 1986). On the basis of limited data, women 20 to 44 years of age were reported in NHANES II to be at greatest risk for folacin deficiency (Senti and Pilch, 1984).

The Joint Nutrition Monitoring Evaluation Committee accorded high priority to monitoring the status of vitamin C, because of some low serum values reported in NHANES II and concluded that thiamin, riboflavin, and niacin warrant continued monitoring. That committee also recommended further investigation of the relationship between dietary intakes and nutritional status for both vitamin B6 and folacin (DHHS-USDA, 1986).

Following is the evidence relating water-soluble vitamins to chronic diseases.