[Part 3 of 4 in a series on Vitamin C and the Common Cold]
In the chapter “Vitamin C and Evolution” from his book Vitamin C and the Common Cold, Pauling wondered about the reasons why the rest of the animal world can synthesize vitamin C, while human beings, along with a very small group of mammals, cannot. His answer was gene mutation, using the instance of thiamine as evidence.
All animals need thiamine as an essential vitamin; in its absence they develop a disease similar to beriberi. Pauling theorized that over 500 million years ago, when the common ancestor to present-day birds and mammals lived, there existed an environment imbued with an abundance of green plants containing thiamine. By way of gene mutation, one of the animals living during that era must have lost the mechanism which allowed it to synthesize thiamine. This was advantageous to the animal – which was probably plant-eating – because it could obtain the thiamine it needed from the plants it ingested while simultaneously conserving the energy that it would have used to manufacture the vitamin.
Pauling pointed out that possessing this extra energy would have caused the animal to flourish and to have more offspring than others of its kind. The advantageous mutation would be passed on to certain of the progeny, who would in turn pass it on to their own offspring, and so on. Eventually the mutation would spread, and a few million years later all mammals and birds would possess the mutation.
Pauling believed that in the same way that all animals lost the biochemical machinery to produce thiamine, so too did human beings, primates, guinea pigs and a particular Indian fruit-eating bat lose the ability to synthesize vitamin C. A mutation that results in the inability to synthesize a substance is simple and occurs often; it only requires a single gene to be damaged or deleted. The reverse process is more complex and takes much longer. The mutation that removed the ability to synthesize vitamin C probably took place about 25 million years ago, in the ancestor of modern primates and humans.
In his book, Pauling next asked the question, why didn’t all mammals and birds lose the ability to synthesize vitamin C the way that they lost the ability to synthesize thiamine? Pauling theorized that the change likely occurred in the guinea pig and the Indian fruit-eating bat independently of the common precursor of the primates, due to an abundance of vitamin C in their diets. The fact that the majority of animals possess the ability to synthesize vitamin C indicates that there is not sufficient vitamin C in their dietary environment for them to obtain the vitamin solely from their nutrition intake. To Pauling, this also suggested the existence of a deficiency of ascorbic acid in the human diet.
Dr. Irwin Stone, a biochemist in Staten Island, New York, was the person responsible for sparking Linus Pauling’s interest in vitamin C. Dr. Stone, a leader in the ascorbic acid field at the time, sent a letter to Pauling in 1966 informing him of a high-level ascorbic acid regimen that he had been developing over the past three decades, which Pauling and his wife began to follow. Stone believed that humans need between 3 and 5 grams of vitamin C per day, reinforcing this claim by citing the British researcher G. H. Bourne’s evidence that gorillas ingest about 4.5 g of ascorbic acid per day.
Gorillas, like humans, do not synthesize vitamin C, and so need to obtain it from their diet. In 1949 Bourne pointed out that before the development of agriculture, humans lived mainly off of raw, green plants with little meat; a diet similar to that of the modern gorilla. Bourne concluded that
it may be possible, therefore, that when we are arguing whether 7 or 30 mg of vitamin C a day is an adequate intake we may be very wide of the mark. Perhaps we should be arguing whether 1 g or 2 g a day is the correct amount.
Irwin Stone also took into consideration the amount of ascorbic acid that other animals, such as rats, manufacture. The rat synthesizes vitamin C at a rate of between 26 mg and 58 mg per day per kilogram of body weight. If the same rate of manufacture were applied to humans, a person weighing 70 kg (154 lbs) would need to ingest between 1.8 g and 4.1 g of ascorbic acid per day.
From there, Pauling verified the amounts of various vitamins contained in 110 different raw fruits and vegetables corresponding to a diet of 2,500 kilocalories per day, and found that “for most vitamins this amount is about three times the daily allowance recommended by the Food and Nutrition Board.” For ascorbic acid, the difference was much more drastic: the average amount of ascorbic acid in a day’s ration of the 110 raw foods was 2.3 g, which was about 42 times the recommended amount. Pauling argued that
If the need for ascorbic acid were really as small as the daily allowance recommended by the Food and Nutrition Board the mutation would surely have occurred 500 million years ago, and dogs, cows, pigs, horses, and other animals would be obtaining ascorbic acid from their food, instead of manufacturing it in their own liver cells.
Pauling found that the average ascorbic acid content for the fourteen most vitamin C-rich plant foods is 9.4 g per 2,500 kilocalories, leading him to the conclusion that the optimum daily vitamin C intake for an adult human being is between 2.3 g and 9 g – quantities in line with what he saw as existing in the natural diet of the human lineage and numbers far beyond the recommendations issued government nutritional authorities, then or now.