Evolution and the Need for Ascorbic Acid

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Linus Pauling, 1970

Linus Pauling’s belief in the value of vitamin C emerged from many sources, but key among them was the fact that humans, for most of their history, have been unable to produce their own ascorbic acid. This stands in stark contrast to nearly every other animal, virtually all of whom are able to synthesize their own ascorbic acid internally. Pauling viewed this human characteristic as having emerged from an evolutionary adaptation that, in his view, had sentenced modern humans to lives of sub-optimal health.

In December 1970, Pauling detailed this point of view in an article titled “Evolution and the Need for Ascorbic Acid,” which was published in the Proceedings of the National Academy of Sciences. In it, Pauling began by stating that the minimum daily requirements then espoused for vitamin C – 35 mg for an infant and 60 mg for an adult – were only enough to stave off scurvy and remained grossly insufficient to supporting ideal human functioning. In so doing, Pauling framed the onset of scurvy as not just the first symptom of low ascorbic acid levels, but rather the last symptom before death.

Pauling then pointed out that, along with the guinea pig, the Indian fruit-eating bat, and an early ancestor of the Passeriformes bird, humans are among a tiny minority of the world’s animals who are incapable of synthesizing their own ascorbic acid. The question is, why?


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Table 1 (excerpted) from Pauling’s 1970s PNAS article.

Pauling took an evolutionary view as he searched for an answer. In his article, he began by defining the eobiontic period – a two to three billion year period after the “hot thin soup” era – as a phase characterized by profound biochemical evolution. It was during this time period, about 25 million years ago, where Pauling believed that humans lost the ability to self-produce ascorbic acid.

To demonstrate how this might have happened, Pauling detailed a similar circumstance with thiamine, which is also an essential nutrient for mammals. At some point during the eobiontic period, certain species also began to lose their ability to synthesize thiamine and many researchers, including Pauling, believed that this was because “the supply of food available to an earlier ancestor provided an adequate supply of these vitamins, enough to make it advantageous to discard the mechanism for synthesizing them.” According to the theory, those species that did not discard this mechanism were disadvantaged because maintaining synthetic production became a burden. “[I]t cluttered up the cells,” Pauling wrote, “added to the body weight, and used energy that could be better used for other purposes.”

Pauling believed that the abundant availability of foods rich in vitamin C also led humans to evolve away from synthesizing ascorbic acid. Pauling listed 110 of these foods in a table within his article. They included sweet red peppers, sweet green peppers, hot red chili peppers, parsley, black currants, and broccoli spears among many others.


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Irwin Stone. (Image by Oscar Falconi)

Pauling also examined the research of three colleagues to add support for his theory: British researcher G.H. Bourne, American biochemist Irwin Stone, and American physician Edmé Régnier. Pauling looked to these three in particular to try and calibrate the level of ascorbic acid intake that would result in ideal human functioning.

In 1949, G.H. Bourne conducted a study focusing on the diets of gorillas and found that they consumed nearly 4.5 g of ascorbic acid per day through green foods. The variety of foods consumed by gorillas was also deemed by Bourne to be similar to that likely consumed by humans prior to the development of agriculture. By comparing the diets of the two, as well as their proportional body weights, Bourne determined that contemporary humans should strive to consume closer to 1 or 2 grams of ascorbic acid per day, rather than the the 7 to 30 mg recommended at the time.

Later, in the mid-1960s, Irwin Stone performed a set of experiments with a similar aim. After discovering that the daily rate of vitamin C synthesis for rats ranged from 26 mg kg-1 to 58 mg kg-1, Stone determined that the best intake of ascorbic acid for optimum human health was between 1.8 g to 4.1 g per day – the levels that individuals of varying sizes would produce if the rat synthesis rate were scaled accordingly.

Only a couple years after Stone released his hypothesis, Edmé Régnier produced his own theory that settled on a regiment of 5 g of ascorbic acid per day. Further, after several trials in which Régnier administered varying amounts of ascorbic acid to study participants, Régnier concluded that 45 out of 50 colds had been prevented by doses of 600 mg of ascorbic acid. Not long after, Pauling would write a book that did much to popularize the use of vitamin C in the treatment and prevention of the common cold.


After considering the research of the previous three scientists as well as conducting trials of his own, Pauling theorized that optimal human intake of ascorbic acid likely ranged from 2.3 g to 9.5 g. Pauling’s minimum recommendation was 2.3 g because that was the average amount of ascorbic acid provided by the 110 natural foods listed in his table. Likewise, Pauling deduced that the amount required to achieve optimal health would not exceed 9.5 g, because that was the high-end total available through a smaller selection of foods described in the same table.

Pauling also recognized the importance of biochemical individuality, age, size, and gender, and considered all of these factors in publishing his 2.3 g to 9.5 g range. He likewise took comfort in knowing that his conclusions were similar to those of Stone and Bourne, and this corpus of research convinced Pauling, for the remainder of his life, that vitamin C was an essential key to achieving optimal health.

Vitamin C Deficiency in Humans: An Issue of Evolution?

Linus Pauling and Irwin Stone, 1977.

[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.