14.4: Riboflavin

The history of riboflavin lacks the human drama of the stories of thiamin and vitamin C. And while the lack of riboflavin in the diet can certainly produce some appalling clinical effects—in some cases not unlike those of pellagra—the deficiency symptoms weren’t even recognized for humans until after they’d been observed in rats. This doesn’t mean that riboflavin deficiency wasn’t occurring among humans. Rather, it was probably masked, or at least blurred, by the effects of other forms of malnutrition.

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Using purified diets, a rat’s diet could be made deficient in just riboflavin. This identifies the specific effects of a riboflavin deficiency, confirmed by the relief from these effects when riboflavin is restored to the diet.

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Severe riboflavin deficiency is generally seen only among severely malnourished populations. People in these populations suffer from so many deficiencies—such as niacin and protein deficiencies—that specific signs of riboflavin deficiency are hard to separate out. But specific effects of riboflavin deficiency have been established in human studies.

Riboflavin and Body Chemistry

Riboflavin is an essential part of coenzymes that are needed in the chemical reactions that extract energy from food. Like thiamin, and for much the same basic reasons, riboflavin needs are tied to energy needs.

There are some specific symptoms of riboflavin deficiency (see Table 14-1), but some of the symptoms are shared with those of niacin and vitamin B₆ deficiencies. This is because riboflavin-containing coenzymes play an important role in the functioning of these vitamins, and thus a riboflavin deficiency can cause symptoms of niacin and vitamin B6 deficiencies as well.

Another reason for the overlap of vitamin-deficiency symptoms is that a dietary pattern that results in one deficiency also tends to result in others. We saw in Chapter 11 that those who suffered from the niacin-deficiency disease pellagra had diets that included very little milk or meat. Animal foods are a good source of protein, riboflavin, and several other B-vitamins, so those with pellagra tended to be deficient in several nutrients in addition to niacin.

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A riboflavin-containing coenzyme is needed for the conversion of the amino acid tryptophan to niacin.

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This fundamental lesson in nutrition—that signs of malnutrition don’t usually occur singly— is one of surest commonsense defenses against faddism. Consider the claim of some promoters that riboflavin is an answer to baldness.

The idea seems to derive from some rat experiments in which animals deprived of riboflavin lost quite a lot of hair. But there’s no evidence that hair loss is a sign of human riboflavin (or any other nutrient) deficiency. Moreover, if someone were to suffer from a vitamin deficiency as severe as that which causes hair loss in rats, one would expect to see also other symptoms of severe riboflavin deficiency (see Table 14-1).

Riboflavin Intake

Milk is a major source of riboflavin in the U.S. diet. Milk consumption has fallen, but it has been compensated for somewhat by increased consumption of other dairy foods such as cheese, yogurt, and ice cream. But when the dairy product is high in fat, as are cheese and ice cream, there’s less riboflavin.

This is logical since riboflavin is a water-soluble, rather than a fat-soluble, vitamin. A cup of low-fat (1%) milk provides 105 calories and about a third of the adult RDA for riboflavin. But those 105 calories eaten as ice cream (~⅓ cup) or cheese (~1 oz) provide less than a tenth of the riboflavin RDA.

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Severe riboflavin deficiency in rats eventually leads to cataracts and partial blindness. In the long run, there’s coma and death.

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Riboflavin is destroyed by ultraviolet light. This posed a problem in the days when milk was commonly bottled in clear bottles and delivered on home doorsteps. When the milk bottles sat too long on the doorstep—exposing them too long to daylight—the riboflavin content fell. Direct sun could wipe out half the milk’s riboflavin in a couple of hours. Today, of course, most of our milk comes in waxed cartons or opaque plastic containers and is purchased at grocery stores.

The liver, kidney, and heart of animals are very rich in riboflavin, but these aren’t commonly eaten. Enrichment of bread and other grain products with riboflavin has boosted riboflavin intake in this country. Other good sources of riboflavin include meat, eggs, green leafy vegetables, and nuts such as almonds.