On the average, thin experimental animals always outlive their chubby brothers and maintain vigorous vitality more abundantly. Insurance company statistics conclusively prove that after the age of 30 or 35, persons who remain slightly underweight are much better bets for a long life than those who are only a little overweight.

So inconceivably complex are the relations of food to the human body that it would be a grievous mistake to be overly dogmatic in prescribing foods to make one live longer. But enough experimental work has been done to point to certain food factors as being strikingly important in adding years to one's life.

Most famous of all longevity diets is that developed by Dr. Henry C. Sherman of Columbia University. One of the half dozen leaders in the field of nutrition, Dr. Sherman's name is intimately linked to several of the vitamins through its use in identifying units of vitamin dosage in early stages of investigation.

Riboflavin, or Vitamin G, is essential to what Dr. Sherman calls "preservation of the characteristics of youth" that is, vigor, clear skin, enthusiasm, lack of wrinkles, and the like. By merely changing the proportions of natural foods in the diet, Dr. Sherman believes it quite possible to increase the average life span by 10 per cent. The Biblical threescore and ten thus becomes, through benefit of science, not 70 years but 77 - an extra 7 years of terrestrial activity available to those who study their nutrition lessons.

The families of white rats at Columbia University that made this heartening promise possible lived 10% longer than others who apparently had normal diets. What was added to their feedings? Principally, additional quantities of milk solids. The essential elements that made for longer life are, as Dr. Sherman identifies them, Vitamins A and G and the mineral calcium.

Nor are the extra years mere feeble and disabled ones added to old age. "They are best conceived as inserted at the apex of the prime of life," says Dr. Sherman. It is one's productive years, not the declining ones of senility, that are extended by riboflavin and Vitamin A and calcium in the diet.

Since gray hair is usually associated with aging processes of the body, the marvelous B complex vitamins may reasonably be assumed to play an important part in longevity. Rats that turn white on diets deficient in the anti-gray hair vitamins also exhibit early signs of senility. These extraordinary vitamins are still so new that their functions have hardly been catalogued. However, it is probable indeed, it is almost certain that liberal intake of the B-rich foods will play a vital role in increasing life expectancy.

Even in this fascinating business of living longer, proteins rear their gluey heads. There is every evidence that liberal intake of good protein in quantities considerably above the minimum usually prescribed makes for a long life and a merry one. On this basis a minimum intake of some 400 protein calories a day a large part of them preferably from such superior protein foods as milk, eggs, and meat is a splendid idea for anyone who wishes to postpone liquidation of his life insurance policies.

"Does man gain in vigor, happiness, or longevity by eating an amount of protein which is materially in excess of the minimum?" asks Dr. James S. McLester, famous nutrition authority of the University of Alabama whose classic textbook is a diet Bible of physicians. He answers his own question: "Animal experiment and clinical experience, as well as studies in racial development, warrant, I believe, an affirmative answer."

Proteins, calcium, Vitamins A and G and the B complex have in part, the answers of science to date as to how to eat for a longer life. Always, of course, these vital elements should be taken in foods which provide just enough calories to keep one at ideal weight or slightly below it, after middle age.