11.3: The Missing Link of Nutrition—Vitamins

Although the vitamin pioneers could change the diet or use food extracts to heal, they understood little of either why the disease occurred, or why it was healed. In fact, those pioneering scientists were uncovering more scientific problems than they were solving.

Consider beriberi, what we now know is a thiamin-deficiency disease. Thiamin wasn’t purified until 1926, 14 years after Funk’s announcement that his crude extract of rice hulls cured beriberi. And it wasn’t until 1936 that the chemical structure of thiamin was known and synthesized. As for what thiamin had to do with nutrition, real understanding had to wait for the 1940s and the 1950s. Even today, some puzzles remain.

Regarding pellagra, Goldberger’s discoveries were for some years thought to be a matter of protein nutrition. He had cured pellagra, but he had no idea that the disease was really the result of a particular vitamin deficiency. He was never to know. It was 1937 before niacin was identified and recognized as the long-sought, anti-pellagra factor.

An Early Vision of Vitamin Subtlety

Soon after Goldberger’s first reports on pellagra, it was becoming apparent that the role of vitamins was complex, and that vitamins were intricately related to all the nutrients. Goldberger’s team wanted to know exactly why the corn-based diet led to pellagra, and why adding other protein-containing foods could cure and prevent the disease.

Several other researchers had been exploring the world of amino acids and showed that some were essential in the diet. They began giving individual amino acids to pellagra patients. In 1921, they thought they had proved that pellagra was caused by a deficiency of the amino acid tryptophan.

The solution seemed very tidy. The protein of corn has limited amounts of two amino acids, lysine and tryptophan. The reasoning ran that it was the low level of tryptophan in the corn-based diet that led to pellagra.

Then came disappointment and confusion. For other researchers reported that they, too, had cured pellagra, but with an amino-acid-free extract of yeast. Thus it seemed that tryptophan wasn’t the deficiency that caused pellagra. But then why should tryptophan and improved protein diets cure the disease? What made the yeast extract work? Was a vitamin involved? If so, what had the vitamin to do with tryptophan? Even later, when niacin was identified in 1937 as the substance that prevented pellagra, the puzzle of tryptophan was still unsolved.

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The body can convert the amino acid tryptophan to the B-vitamin niacin.

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It was finally solved in 1945, when it was found that the body can convert tryptophan to niacin. Pellagra had become prevalent in the South among the poor and institutionalized because of a double dietary failure of their corn-based diet: It was low in niacin, and the corn protein was also low in tryptophan.

But even if Goldberger had somehow known this, he wouldn’t have had a glimmer of how it all worked. An understanding of vitamin chemistry had to wait for the revelation of some of the basic chemistry of life—at the level of the cells and the molecules of which we are made.

Eijkman, the beriberi pioneer, was right in saying that “the truth need not necessarily be simple.” Despite what you see on the internet, in magazines, and advertisements, the truth of how the body uses the vitamins and other nutrients it takes from food isn’t simple.

Each vitamin performs an essential function in its own way. But the most common role of vitamins is to assist in the reactions of metabolism.

Vitamins as Coenzymes in Metabolism

The chemical reactions that occur in cells are collectively called metabolism. The very word comes from the Greek term for change. And even the early Greek physicians saw that change was the essence of life, the difference between life forms and the inanimate.

Metabolism includes both the energy-releasing reactions in which larger molecules are broken down, and the energy-requiring reactions in which larger molecules are made.

To understand what vitamins are and what they do, scientists need to know the fine details of energy metabolism. Here was the great stumbling block for the vitamin pioneers. Lacking this knowledge, they had no insight into how, for example, a shortage of thiamin could cause the weakness of beriberi victims. They didn’t know that thiamin serves as a coenzyme (“cooperates with enzymes”) in the chemical reactions that the body uses to release energy from carbohydrate, fat, and protein.

We’ve seen how the body breaks foods down to the simpler chemical fuels and building materials of life (e.g., starch into glucose). And we’ve seen how the body manufactures some of its components (e.g., amino acids into protein). Both of these processes—the breaking down (catabolism) and the building up (anabolism)—involve a series of intricate chemical reactions, each catalyzed by an enzyme.

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Catalysts are substances that markedly increase the speed of chemical reactions. Enzymes are biological catalysts. Without enzymes, very few of the body’s chemical reactions can take place.

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Coenzymes are crucial accessories to enzymes, usually acting as carriers—carrying a product of one chemical reaction to be used in another chemical reaction. Niacin, for example, is a part of a coenzyme that picks up the hydrogen released in one reaction, and ferries it to be used in another reaction.

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The eight B-vitamins are: thiamin, riboflavin, niacin, vitamin B₆, folate, vitamin B₁₂, biotin, pantothenic acid (see Table 1-1).

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Each of the eight B-vitamins is an essential component of different coenzymes. A single B-vitamin deficiency can have widespread effects for several reasons:

• One coenzyme is usually used in a variety of chemical reactions. (An enzyme, in contrast, is very specific and usually catalyzes only a single reaction.)
• One B-vitamin can be a component of more than one coenzyme. Thiamin, for example, is an essential component of several coenzymes.
• Coenzymes are needed in the chemical reactions that release energy from food—reactions that are basic to the function of cells throughout the body.