13.5: How Vitamin Needs are Measured

How certain can we be that our judgments of the need for vitamins are sound? Are nutrition scientists really confident that most of us don’t need extraordinary amounts of vitamins— huge doses of niacin to prevent mental illness, or massive intakes of vitamin C to keep from catching cold? Let’s look at a few of the principles applied in determining vitamin needs and judging when they’re met in optimum ways.

Originally, the only measure of vitamin deficiency or adequacy was symptomatic deficiency. This served practical purposes, but it wasn’t a satisfactory measure. We can see why from the vitamin realities we’ve observed. Deficiencies don’t cause symptoms until they are severe and have persisted for some time. One wouldn’t want to wait until signs of beriberi developed before getting more thiamin.

But as science began to understand how vitamins play their chemical roles, it became possible to set more and more precise standards for vitamin adequacy, and to assess the stores in a given person’s body in terms of those standards.

In principle, the system isn’t very different from the baking of a cake. If we know the chemistry of cooking, we can both predict how much of each ingredient is needed, and we can study the resulting cake to determine whether there wasn’t enough or too much of something— or whether the ingredients were optimal for the best cake possible.

Suppose we used buttermilk to make a cake. Along with the buttermilk, we add a certain amount of baking soda. The object is to cause a reaction between the acid buttermilk and the alkaline baking soda, the product of which is carbon dioxide. The carbon dioxide forms bubbles, which are caught in the protein substance of the flour. And in baking, the gas bubbles form little balloons, surrounded by flour components, to make the cake rise, and to make it light and fluffy. One can roughly calculate the amount of buttermilk and baking soda needed to produce the desired reaction.

If we add too little baking soda, we don’t get the full potential of the reaction with the buttermilk. Not only will the cake taste acid because of the excess buttermilk, there won’t be enough gas bubbles, and the cake won’t rise well. It will be dense and heavy, and the compliments will be strained at best.

But going the other way and adding too much baking soda, beyond the chemical requirements for reacting with the buttermilk, would be just as bad. There’s just so much buttermilk to help yield carbon dioxide. The extra soda won’t only be wasted, it will be tasted, giving the cake the characteristic bitterness of alkalinity. Any compliments will come through pursed lips.

In a similar way, if we don’t get enough vitamins, our bodies’ chemical processes won’t take place as they should. The chemical reactions won’t be completed as they need to be for good health. And in the same way, an excess of vitamins will simply be a burden, perhaps needing to be cleaned away by excretion, or perhaps causing chemical problems.

So it is that by understanding the exact role of each vitamin in chemical reactions that we can predict, say, how much thiamin is needed in metabolism. And by looking at body wastes and products of reactions in the blood, we can tell whether the chemical processes have gone forward adequately, or whether they have been hampered by the lack of a vitamin. Body stores of a vitamin can also be measured.

Obviously, the problem of such assays is more difficult than it is with a cake, since so many more factors are involved. Body chemistry is infinitely more complex.

But the principles remain. And paramount among them is the concept that vitamin adequacy or inadequacy is measurable—not a matter of guesswork, faith, or subjective feeling.

As was pointed out earlier, the recommended vitamin intakes are amounts intended to assure that the body chemistry has what it needs, both immediately and for storage. Just as the bakery must continually make sure it always has ample supplies on hand, the body’s storage of ingredients is important. The baker can check the cupboards. The physician can measure bodily stores and circulating levels of vitamins.

But there’s no point in going beyond full. There will be no additional helpful reactions. There will be nothing but needless expense and the burden of excess.

Human needs can vary, just as cakes and their ingredients vary. But the variation isn’t so great as is popularly believed. Gross distortions of vitamin intakes are no more valuable, and no less risky, than gross distortions of your favorite cake recipe.