A biological approach.

Bernard G. Prusak seems to consider the issue of vegetarianism almost exclusively from an ethical perspective. While it is refreshing to see ethics considered in the choices of daily life, there is also biology to consider, and Prusak seems to relegate that side of the story to a matter of divergent opinion. Unquestionably, nutrition involves deeply held values and evokes powerful emotions, but biology is more than just a matter of opinion. There are many nutrients essential for human health that are found only in animal foods, particularly meat. Vitamin [B.sub.12] is one example, the branched chain amino acids are another. They are essential for optimizing insulin sensitivity, but cannot be ingested in sufficient quantity from plant food sources, because plant proteins have low quantities of these compounds. Our gastrointestinal tracts and their digestive secretions are those of an omnivore, not a herbivore. Countless other instances could be cited.

One of the important conclusions from natural selection is the understanding that, over the course of evolution, all organisms drop the ability to make biologically active compounds if the food they eat supplies those compounds. The fact that we need vitamin [B.sub.12] but can't make it for ourselves is simply a reflection of the fact that our physiologies evolved in the context of a diet that included meat. Millions of years ago, our hominid ancestors became dependent on meat. That trait is a part of our heritage.

The effects of inadequate nutrition are individually small, but cumulative over time. This is why we can ingest inadequate diets and not see immediately bad effects. No one doubts that drinking too much alcohol causes the next day's hangover, but effects of diet are more subtle. This is what permits us to hold scientifically unsound views on nutrition: their wrongness is not so personally and immediately apparent.

Can we build on what we know? Can we adopt a vegetarian diet and supplement those foods with the missing nutrients? Through modern technology and science, that is now possible, but it is still not natural. Moreover, the book of nutrition is not yet fully written. We don't today know the full extent of our dependencies.

Clearly we should avoid cruelty in our use of animals. Equally clearly, we need to eat animals--not for convenience, not because we have dominion over them, but because they provide what is essential for our health. We must confront the fact that all organisms eat other organisms--are utterly dependent upon doing so. That is one aspect of the arrangement of our world that caused George Ellis to see kenosis as the underlying principle of the cosmos.

ROBERT P. HEANEY, MD

Omaha, Neb.