103 HOW TO LIVE LONGER AND FEEL BETTER
Chapter 10
Biochemical Individuality
The genetic mutation that deleted the capacity to manufacture vitamin C in the primate line presents one vivid example of the countless genetic variations from which natural selection produced the diversity of biological organisms we know in the world today. Such biochemical insight permits us to see evolution, as it were, from the inside. It gives a quantitative measure of the wealth of differences among individuals within a single species upon which natural selection acts in choosing the "fittest." It shows each of us human beings to possess a biochemical individuality that is scarcely expressed in (but only partly accounts for) the differences we observe in one another.
Let us consider some genetic characteristic, such as the weight of the liver relative to the total weight of the human being or the concentration of a certain enzyme in the red cells of the blood. It is found that, when a sample of a hundred human beings is studied, this characteristic varies over a wide range. The variation often is approximately that given by the standard, bell-shaped probability function. It is customary to say that the "normal" range of values of the characteristic is that range within which 95 percent of the values lie and that the remaining 5 percent of the values, representing the extremes, are abnormal. If we assume that five hundred characteristics are independently inherited, then we can calculate that there is only a small chance, 3 percent, that one person in the whole population of the world would be normal with respect to each of these five hundred characteristics.
It is estimated, however, that a human being has a complement of one hundred thousand genes, each of which serves some function, such as controlling the synthesis of an enzyme. The number of characteristics that can be variable, because of a difference in the nature of a particular gene, is presumably somewhere near one hundred thousand rather than only five hundred: and accordingly we reach the conclusion that no single human being on earth is normal (within the range that includes 95 percent of all human beings) with respect to all characteristics. This calculation is, of course, oversimplified. It helps emphasize, however, that human beings differ from one another and that each human being must be treated as an individual, biologically as well as morally.
The species Homo sapiens is more heterogeneous, with respect to genetic character, than most other animal species. Nevertheless, heterogeneity has been found also for laboratory animals such as guinea pigs. It was recognized long ago that guinea pigs fed the same scurvy-producing diet, containing less than 5 milligrams (mg) of ascorbic acid per day per kilogram of body weight, differed in the severity of the scurvy that they developed and in the rapidity with which they developed it. A striking experiment was carried out in 1967 by Williams and Deason. These investigators obtained some male weanling guinea pigs from an animal dealer. After a week of observation during which the guinea pigs were on a good diet, including fresh vegetables, they were placed on a diet free of ascorbic acid or with known amounts added. They were divided into eight groups, each of ten to fifteen guinea pies, with one of the groups receiving no ascorbic acid and the other groups receiving varying amounts through a pipette into the mouth. About 8O percent of the animals receiving no ascorbic acid or only 0.5 mg per kilogram per day developed signs of scurvy, whereas only about 25 percent of those receiving between 1 mg and 4 mg per kilogram per day, and none of those receiving 8 mg per day or more, developed these signs. These results agree with the customary statement that about 5 mg per kilogram per day of ascorbic acid is required to prevent scurvy in guinea pigs.
It was observed, however, on the one hand, that two animals receiving only 1 mg per kilogram per day remained healthy and gained weight over the entire period of the experiment (eight weeks). One of them showed a total gain in weight larger than that for any animal receiving two, four, eight, or sixteen times as much ascorbic acid.
On the other hand, seven of the guinea pigs receiving 8, 16, or 32 mg per kilogram per day were unhealthy and showed very small growth during the first ten days on the diet. They were then provided with a larger amount of the vitamin, five of them with 64 mg per kilogram per day and two of them with 128 mg per kilogram per day. These animals showed a remarkable response: whereas they had grown only 12 grams (g), on the average, in a period of ten days on the smaller amounts of ascorbic acid, their growth during the ten-day period after beginning to receive the larger amounts was, on the average, 72 g. The indicated conclusion is that these animals, seven of the thirty that were given between 8 mg and 32 mg per kilogram per day, required more vitamin C for good health than the others. Williams and Deason (1967) reached the conclusion that there is at least a twentyfold range in the vitamin-C needs of individual guinea pigs in a population of a hundred. They pointed out that the population of human beings is presumably not more uniform than that of the guinea pigs used in their experiments and that accordingly the individual variation in the vitamin-C needs of humans is probably just as great.
