R E Hughes. Cambridge World History of Food. Editor: Kenneth F Kiple & Kriemhild Conee Ornelas. Volume 1. Cambridge, UK: Cambridge University Press, 2000.
In delineating the history of a vitamin, we can often recognize four chronological phases. First, there is the description of a disease of unknown etiology, and second, there is the description of an empirical cure for the disease. Following this step, and often closely associated with it, is the identification of the curative factor—which perforce then becomes known as a vitamin. In the fourth phase the mode of action of the vitamin in preventing the deficiency disease is characterized.
The history of vitamin C (ascorbic acid) conforms to this general pattern. The characterization of the deficiency disease (scurvy) and the empirical discovery of a cure for it are, properly speaking, a part of the history of scurvy and have been dealt with elsewhere in this work. But this chapter is concerned with the subsequent history of the antiscorbutic factor, which conveniently presents itself in three chronological stages: (1) the somewhat ill-defined and open-ended period—from the beginning of the nineteenth century to the 1920s—when the vitamin had the existence of an “unrevealed presence” and was known to exist only because of its preventive influence on the disease scurvy (just as, during the same period, the perturber of Uranus was known to exist long before the “discovery” of the planet Pluto); (2) the 1920s and the 1930s, when vitamin C was named, isolated, and its molecular structure revealed (in that order); and (3) the modern post-1940 period, with its emphasis on the characterization of the biochemical role of vitamin C in preventing scurvy and, more recently, the debatable “extra-antiscorbutic” roles sometimes attributed to it.
The Antiscorbutic Factor
The early history of vitamin C cannot readily be separated from that of the demise of scurvy. By the beginning of the nineteenth century it was generally accepted that it was possible to prevent and cure scurvy by the use of citrus fruits: James Lind’s contribution in establishing this belief was of paramount significance. But it was in essence a pharmacological concept rather than a nutritional one; the belief that the citrus fruits were replacing a missing dietary component would have been alien to medical thought at the beginning of the nineteenth century; even Lind himself did not regard fruit and vegetables as obligatory dietary principles in the prevention of scurvy. In other words, not until the end of the nineteenth century was there any general acceptance that scurvy was a deficiency disease resulting from a lack of a specific dietary principle and that the disease could be prevented or cured by appropriate dietary manipulation. Moreover, even this acceptance was complicated by the advent of the germ theory of disease which, some have argued, caused reversion to an infection theory to explain scurvy’s etiology.
One of the earliest thinkers to discuss these new ideas was George Budd (1808-82), Professor of Medicine at King’s College, London—although as K. C. Carter has indicated, Budd should perhaps be regarded as a developer rather than as an innovator of the “deficiency disease theory” (Hughes 1973; Carter 1977). In 1842, Budd published in the London Medical Gazette a series of articles entitled “Disorders Resulting from Defective Nutriment.” He described “three different forms of disease which are already traced to defective nutriment” and argued that such conditions resulted from the absence of dietary factor(s) other than carbohydrate, fat, and protein, and that the absence of each of these specific factors would be associated with a specific disease—an idea that lay in abeyance for some 40 years until experimentally proved by N. Lunin. There can be little doubt that the three diseases described by Budd were avitaminoses A, C, and D.
L. J. Harris, himself a significant figure in the later history of vitamin C, aptly described Budd as “the prophet Budd” and referred to an article in which Budd expressed the belief that scurvy was due to the “lack of an essential element which it is hardly too sanguine to state will be discovered by organic chemistry or the experiments of physiologists in a not too distant future” (Budd 1840; Harris 1937: 8).
Little happened, however, to fulfill Budd’s prophesy until the beginning of the twentieth century. In 1907, A. Holst and T. Fröhlich of Norway reported experiments in which they had demonstrated that scurvy could be induced in guinea pigs and cured by dietary manipulation (Holst and Fröhlich 1907;Wilson 1975). They used guinea pigs to assess the antiscorbutic value of different foodstuffs and to show the thermola-bile nature of the antiscorbutic factor. At the same time there were parallel, but independent, developments in the general theory of vitamin deficiency diseases. F. G. Hopkins, developing earlier work by Lunin, C. A. Pekelharing, W. Stepp, and others, in 1912 published his classic paper in which he demonstrated the presence of growth factors in milk and showed their essential dietary nature (Hopkins 1912); in the same year, Casimir Funk introduced his “vitamin hypothesis,” in which he attributed scurvy to the absence of an “anti-scurvy vitamine” (Harris 1937: 1-21).
The use of the guinea pig assay technique for the assessment of the antiscorbutic factor was extended, and in 1917, H. Chick and M. Hume published an important paper in which they reported the factor’s distribution in a number of foodstuffs (Chick and Hume 1917). The following year A. Harden and S. S. Zilva published their fractionation studies on lemon juice, in which they demonstrated that the antiscorbutic potency was not attributable (as had been suggested by earlier workers) to the citric acid content (Harden and Zilva 1918). The year after that, J. C. Drummond designated the factor “Water soluble C” (Drummond 1919).
Identification of Vitamin C
Work now began in earnest to identify and isolate the antiscorbutic factor. The Medical Research Council’s 1932 publication Vitamins: A Survey of Present Knowledge may be referred to for a detailed account of the large number of papers published during the 1920s on what was by then known as “vitamin C.” Foremost in these early efforts was Zilva, working at the Lister Institute, London. The essential feature of Zilva’s procedure was the precipitation of the factor with basic lead acetate after removal of the bulk of the other organic acids with calcium carbonate. He applied this technique to a variety of sources, such as lemon juice and swede (rutabaga) tissues, and he succeeded in increasing the concentration of the antiscorbutic factor some 200 to 300 times (Hughes 1983).
Other workers were similarly occupied, notably N. Bezssonoff in France and C. G. King in the United States. King has stated that during this period “many investigators had abandoned or failed to publish their work for various reasons.” He referred, specifically, to Karl Link of Wisconsin, who had prepared several grams of crude calcium ascorbate during the 1920s but carried his work no further because of lack of financial support (King 1953).
The purest of these early “concentrates” still contained much impurity, and it was not until 1932 that W. A. Waugh and King published their paper “The Isolation and Identification of Vitamin C,” which included a photograph of vitamin C crystals (Waugh and King 1932).
These attempts to isolate and characterize vitamin C were paralleled by two separate, but nevertheless highly relevant, developments in related areas. J. Till-mans and P. Hirsch, German government chemists, extensively studied the capacity of lemon juice preparations to reduce the redox dye 2,6-dichlorophenolindophenol, and they claimed that the reducing power of their preparations was always in proportion to their antiscorbutic potency; the indophenol dye technique later became a standard method for the assay of vitamin C. Zilva, however, disagreed with the German findings and appears, at that point, to have been diverted from his main endeavor by an attempt to disprove them (Zilva 1932).
The other significant development at this time was Albert Szent-Györgyi’s isolation of hexuronic acid. Szent-Györgyi, a Hungarian biochemist working on plant respiration systems at Groningen in Holland, became interested in a reducing compound present in his preparations. Hopkins invited him to Cambridge to extend his studies, and in 1927, Szent-Györgyi isolated his “Groningen reducing agent” in a crystalline, from oranges, lemons, cabbages, and adrenal glands (Szent-Györgyi 1928).
He proposed to name his crystalline sample “ignose”—thus indicating its apparent relationship to sugars while at the same time underlining his ignorance of its true nature. But Harden, the editor of the Biochemical Journal at the time, according to SzentGyörgyi, “did not like jokes and reprimanded me.” A second suggestion “godnose” was judged to be equally unacceptable. Szent-Györgyi finally agreed to accept Harden’s somewhat more prosaic suggestion “hexuronic acid”—”since it had 6 Cs and was acidic” (Szent-Györgyi 1963: 1-14).
Hexuronic acid was a strongly reducing compound. So, too, according to Tillmans and Hirsch, was the antiscorbutic substance (vitamin C). The suggestion that hexuronic acid and vitamin C were actually one and the same substance appeared in print in 1932 in papers by both J. L. Svirbely and Szent-Györgyi and by Waugh and King, but there can be little doubt that the idea had been mooted some years previously. Who first made the suggestion is, however, unclear, and even the main participants in the drama later appeared uncertain and confused. King (1953) claimed that it was E. C. Kendall in 1929, but according to Hopkins (reported by King) it was Harris in 1928 (King 1953)—and he had, in any case, already attributed the idea to Tillmans and Hirsch (Harris 1937: 95). But E. L. Hirst (a member of the team later involved in chemical studies on the structure of vita-min C) named Waugh and King (Hirst 1953: 413).
Hopkins had already, in 1928, sent a sample of SzentGyörgyi’s hexuronic acid to Zilva for comments on its vitamin C potency.According to King, Hopkins was disturbed because Zilva (who,naturally perhaps,was reluctant to admit that his “antiscorbutic preparations” were in reality identical with hexuronic acid) had replied that the sample was not vitamin C, but did so without reporting the evidence of his tests (King 1953).
By 1932, however, evidence in favor of the identity of hexuronic acid as vitamin C was substantial.Waugh and King had shown that their “crystalline vitamin C” cured scurvy in guinea pigs (Waugh and King 1932), and earlier the same year, Svirbely and Szent-Györgyi (now working in his native Hungary) had described the antiscorbutic potency of a sample of “hexuronic acid” isolated from adrenal glands (Svirbely and SzentGyörgyi 1932).
In 1933, in a single-sentence letter in Nature, Szent-Györgyi and W. N. Haworth drew attention to the chemical inaptness of the term “hexuronic acid” and suggested the term “ascorbic acid,” thus formally acknowledging the antiscorbutic nature of the compound. Harris and his colleagues at Cambridge demonstrated the positive correlation between the hexuronic acid content and the antiscorbutic potency in a wide range of foodstuffs and published a highly convincing “eight-point” proof of the identity of the two substances.
Their three most important points were as follows: (1) Hexuronic acid paralleled antiscorbutic potency; (2) destruction of hexuronic acid by heat or by aeration was accompanied by a corresponding fall in the antiscorbutic activity; and (3) hexuronic acid disappeared from the organs of scorbutic guinea pigs (Birch, Harris, and Ray 1933).There could now be little doubt that hexuronic acid (Tillmans and Hirsch’s reducing compound) and vitamin C were one and the same substance.
The situation was not without its human aspects, and even today the question of priority in the discovery of vitamin C still elicits discussion. “The identification of vitamin C is one of the strangest episodes in the history of vitamins,” wrote T. H. Jukes in commenting on the appearance in 1987 of a book by R. W. Moss that placed, in Jukes’s opinion, too great an emphasis on Szent-Györgyi’s contribution (Moss 1987; Jukes 1988: 1290). Moss had implied that King had rushed off his claim for the identity of vitamin C and hexuronic acid after it became clear to him that Szent-Györgyi intended making the same point in a note to Nature—a situation curiously reminiscent of the suggestion that Charles Darwin behaved similarly on learning in 1858 that Alfred Russel Wallace was about to publish his theory of evolution.
The emphasis now shifted to the elucidation of the structure of vitamin C. Haworth, a Birmingham (U.K.) chemist, had received from Szent-Györgyi a sample of his “hexuronic acid,” and in 1933, in a series of impressive papers, the Birmingham chemist, using both degradative and synthetic procedures, described the structure of the molecule (Hughes 1983). The molecule was synthesized simultaneously, but independently, by T. Reichstein in Switzerland and by Haworth and his colleagues in Birmingham, both groups using essentially the same method.
The synthesis—which, as it later emerged, was quite different from the biosynthetic pathway—was based on the production of xylosone from xylose and its conversion with cyanide to an imino intermediate that, on hydrolysis, gave ascorbic acid. The Swiss group published their results just ahead of the Birmingham workers (Ault et al. 1933, Reichstein, Grussner, and Oppenheimer 1933). The picture was completed the following year when the Birmingham workers joined forces with Zilva to demonstrate that synthetic ascorbic acid produced at Birmingham had exactly the same antiscorbutic potency as a highly purified “natural” sample from the Lister Institute (Haworth, Hirst, and Zilva 1934).
The annual report of the Chemical Society for 1933, with perhaps unnecessary caution, stated that “although it seems extremely probable that ascorbic acid is vitamin C … it cannot be said that this is a certainty.” Other were less circumspect. A. L. Bacharach and E. L. Smith, addressing the Society of Public Analysts and Other Chemists in November 1933, said that “Vitamin C can now be identified with a sugar acid known as ascorbic acid. … Contrary to expectation, it is the first vitamin not merely to have assigned to it a definite molecular formula, but actually to be synthesised by purely chemical means• (Hughes 1983). Budd was correct in his 1840 prophecy that both physiologists and chemists could contribute to the identification of the “antiscorbutic factor.” But his “not too distant future” proved to be a period of 93 years!
Biosynthesis and Metabolism of Vitamin C
By the end of the 1930s, serious research had commenced on the biological role of vitamin C. In particular, biochemical reductionists sought to explain the nature of the relationship between the clinical manifestations of scurvy and the biochemical involvements of vitamin C. It was recognized that vitamin C was a powerful biological reductant, and there were early attempts to explain its nutritional significance in terms of its involvement in oxidation-reduction systems—a major theme in prewar biochemistry. But the first clear advance in the biochemistry of vitamin C came from studies of its biosynthesis, and by the early 1950s, the pathway for its formation from simple sugars had been worked out. L. W. Mapson and a colleague at the Low Temperature Research Station at Cambridge (U.K.) fed different possible precursor molecules to cress seedlings and measured the formation of vitamin C. And in the United States, King and co-workers used labeled glucose to chart the biosynthetic pathway in rats (Mapson 1967: 369-80).
The biosynthetic pathway proved to be a comparatively simple one. D-glucuronate (formed from glucose) is converted to L-gluconate and then to Lgulono-gamma-lactone, which in turn is further reduced (via L-xylo-hexulonolactone) to L-ascorbic acid (2-oxo-L-gulono-gamm-lactone). The final enzymatic step is catalyzed by L-gulonolactone oxidase (EC 18.104.22.168.)—in the liver in evolutionarily “advanced” species such as the cow, goat, rat, rabbit, and sheep and in the kidney in other species such as the frog, snake, toad, and tortoise—and it is this enzyme that is lacking in those species unable to synthesize vitamin C.
To date, this biochemical “lesion” has been detected in a small, and disparate, number of species—higher primates (including, of course, humans), guinea pigs, certain bats, birds, insects, and fish (Chat-terjee 1973; Sato and Uderfriend 1978). Whether all these species are necessarily scurvy-prone is not quite so clear. A survey of 34 species of New World microchiropteran bats showed that L-gulonolactone oxidase was apparently absent from the livers of all of them (and from the kidneys of at least some of them), but nevertheless, the tissue levels of ascorbic acid (even in species that were fish-eaters or insect-eaters) were similar to those in species that could biosynthesize the vitamin (Birney, Jenness, and Ayaz 1976).
This finding would suggest that the vitamin was being synthesized in organs other than the liver and kidney; or that the metabolic requirement for it was remarkably low; or that there were extremely efficient mechanism(s) for its protection against degradative changes. The whole question of the evolutionary significance of vitamin C—in plants as well as in animals—remains a largely uncharted area.
The rate of endogenous biosynthesis of vitamin C in those species capable of producing the vitamin shows considerable interspecies variation, ranging from 40 milligrams (mg) per kilogram (kg) body weight daily for the dog to 275 for the mouse (Levine and Morita 1985). These values are well in excess of the amounts of the vitamin required to prevent the appearance of scurvy in species unable to synthesize it—a finding that has frequently been used to buttress the claim that vitamin C has a number of “extra-antiscorbutic” roles requiring daily intakes well in excess of the recommended daily amounts.
The total body pool of ascorbic acid in a 70 kg man has been estimated at about 1.5 grams (g) (but according to Emil Ginter it could be three times as great as this [Ginter 1980]), which is attainable in most people by the sustained daily intake of 60 to 100 mg. A daily intake of 10 mg vitamin C results in a body pool of about 350 mg. Scorbutic signs do not appear until the pool falls to below 300 mg (Kallner 1981;”Experimental Scurvy” 1986).
Plasma (and less conveniently, leucocyte) concentrations of ascorbic acid are often taken as an index of the body status of the vitamin. The normal concentration range in the plasma of healthy persons on an adequate plane of nutrition is 30 to 90 micromoles per liter (m mol/L) (0.5-1.6 mg/100 ml). The Nutrition Canada Interpretive Guidelines are often referred to in this respect; these guidelines suggest that values between 11 and 23 m mol/L are indicative of marginal deficiency and that values below 11 m mol/L point to frank severe deficiency—but differences of sex, race, metabolism, smoking habits, and, particularly, of age (factors known to influence plasma ascorbic acid concentrations) reduce the validity of such a generalization (Basu and Schorah 1981; Hughes 1981b).
During a period of vitamin C depletion there is a comparatively rapid loss of vitamin C (a reduction of about 3 percent in the body pool daily) resulting from the continued catabolism of the vitamin and the excretion of its breakdown products in the urine. In humans, the main pathway identified involves the conversion of the ascorbic acid to dehydroascorbic acid, diketogulonic acid, and oxalic acid (in that order), with the two latter compounds accounting for the bulk of the urinary excretion of breakdown products. Smaller amounts of other metabolites, such as ascorbic acid-2-sulphate also occur, and in the guinea pig there is substantial conversion of part of the ascorbic acid to respiratory CO2. It has sometimes been argued that the excess formation of these catabolites (particularly oxalic acid) should signify caution in the intake of amounts of vitamin C substantially in excess of the amount required to prevent scurvy.
Biochemical Role of Vitamin C
It was noted in the early experiments of Holst and Fröhlich, and confirmed by many subsequent workers, that defective formation of connective tissue was a primary pathological feature of experimental scurvy, and at one time it was believed that this lesion could account for most of the known pathological sequelae of the disease—the petechial hemorrhages, the breakdown of gum tissue, and the impairment of wound repair tissue. Attempts to characterize the biochemical modus operandi of vitamin C in preventing scurvy, therefore, centered initially on the metabolism of collagen—the essential glycoprotein component responsible for imparting strength to connective tissue.
By the 1970s, there was suggestive evidence that the biochemical lesion was located in the hydroxylation of the proline and lysine components of the collagen polypeptide and that vitamin C had an essential role in the process (Barnes and Kodicek 1972). The hydroxylases involved in collagen biosynthesis (prolyl 4-hydroxylase, prolyl 3-hydroxylase, and lysyl hydroxylase) require ferrous iron as a cofactor, and it appears that vitamin C, a powerful biological reductant, has an almost obligatory role in maintaining the ferrous iron in the reduced form. Thus emerged a simplistic and reductionist explanation for the role of vitamin C in preventing the emergence of the main clinical features of scurvy.
Yet although there can be little doubt that vitamin C plays a critical role in the biosynthesis of collagen, recent studies have suggested that the simple “defective hydroxylation” theory is, perhaps, not the complete story. Studies have indicated that the activity of prolyl hydroxylase and the formation of collagens by fibroblast cultures is not influenced by ascorbic acid; furthermore, ascorbic acid deficiency does not always result in severe underhydroxylation of collagen in scorbutic guinea pigs (Englard and Seifter 1986).
There is increasing evidence that vitamin C may also influence the formation of connective tissue by modifying the nature and formation of the extracellular matrix molecules (Vitamin C Regulation 1990). B. Peterkofsky (1991) has recently suggested that the role of vitamin C in collagen biosynthesis is a dual one—a direct influence on collagen synthesis and an indirect one (mediated perhaps via appetite) on proteoglycan formation.
The complement component Clq, which has a central role in disease resistance, contains a collagen-like segment that is rich in hydroxyproline, and it has been suggested that this segment could offer a link with the putative anti-infective powers widely suggested for vitamin C (Pauling 1976; see also the section on megatherapy). Studies over the last 15 years, however, have failed to demonstrate that the complement system, unlike connective tissue collagen, reflects vitamin C availability (Thomas and Holt 1978; Johnston 1991). Indeed, the belief that vitamin C had anti-infection powers probably stemmed from reports by Harris in 1937 of lowered vitamin C in persons suffering from certain diseases, particularly tuberculosis.
During the 1960s and the 1970s, however, some 25 epidemiological studies were completed in different parts of the world to assess the validity of claims that vitamin C had anti-infection powers, particularly with respect to the common cold. The general conclusion drawn from the results of these studies was that the evidence for a protective/curative role for vitamin C in the common cold was far from convincing (Hughes 1981b: 22-6; Carpenter 1986: 213-16).
There is accumulating evidence, however, that vita-min C may have additional involvements in a range of enzymatic changes unrelated to the formation of collagen. There are three systems of considerable physiological significance in which vitamin C plays an important, and possibly obligatory, role: (1) as the immediate donor for dopamine B-hydroxylase, a key reaction in the conversion of tyrosine to norepinephrine (Englard and Seifter 1986; Fleming and Kent 1991); (2) in the peptidylglycine alpha-amidating monooxygenase system, whereby peptidyl carboxyl-terminal residues are amidated, a process that requires molecular oxygen, copper, and ascorbate and is important in the biosynthesis of a number of neuroendocrine peptides (Englard and Seifter 1986; Eipper and Mains 1991); (3) in the hydroxylation reactions in the biosynthesis of carnitine from lysine and methionine (Englard and Seifter 1986; Rebouche 1991).
The exact physiological significance of these and other reactions vis-à-vis the clinical manifestations of scurvy is unclear. The first two, having obvious involvements in the endocrine and nervous systems, could well be causally related to various functional derangements of scurvy; and as carnitine has an important role in the transport of fatty acids into the mitochondria, where they may be oxidized to provide energy, it has been suggested that the carnitine involvement could account for the lassitude and fatigue that have been invariably noted as an early feature of scurvy (Hughes 1981a).
Should such involvements require an availability of ascorbic acid greater than that required to prevent the emergence of “classical” scurvy—and there is some evidence that this is so, at least in the case of carnitine biosynthesis—then a revision of the currently accepted Recommended Dietary Allowance/Reference Value would be called for (Hughes 1981a). The current recommended daily intake of vitamin C (60 mg in the United States and recently raised from 30 to 40 in the United Kingdom) is, after all, the amount estimated to prevent the emergence of the classic (“collagen”) features of scurvy—and in the United Kingdom, it is based, essentially, on a single experiment completed almost half a century ago on a non-representative population sample.
Source of Vitamin C
Vitamin C is a heat-labile, water-soluble, and readily oxidizable molecule, and its distribution among foodstuffs and the losses resulting from processing and food preparation have been well documented. Studying the losses induced in the vitamin C content of various foodstuffs by simple culinary procedures must be one of the commonest and oft-repeated projects in basic college and university courses, and the amount of unpublished data resulting from these studies must be immense.
The mean daily intake of vitamin C in the United Kingdom (based on noncooked purchases) is about 60 mg daily with potatoes, citrus fruits, and cabbage accounting for 20, 12, and 6 percent, respectively, of the intake.The losses resulting from cooking are substantial, and these are further increased if the cooked food is allowed to stand around before being eaten. Nevertheless, because of the comparatively widespread distribution of the vitamin in plant foodstuffs, and the role of technology in increasing the availability of uncooked plant and vegetable material during the whole year, very few persons today appear to suffer from clinically defined hypovitaminosis C; consequently, frank scurvy is an almost unknown condition.
A recent survey of vitamin C intakes in European countries revealed an interesting, and almost providential, reciprocity between the consumption of two important sources of vitamin C. Of 27 countries studied, Iceland, Switzerland, and France had the lowest annual consumption of cabbage (less than 5 kg per capita) but a high consumption of citrus fruit (over 20 kg); Romania, Poland, and the former Soviet Union, in contrast, had the lowest consumption of citrus fruit (less than 4 kg) but the highest consumption of cabbage (more than 30 kg) (Kohlmeier and Dortschy 1991). Only where a person, for ideological, economic, or supposed “health” reasons subsists on a diet devoid of fruit and vegetables (such as one based on nuts, grain, and/or cooked meat/fish) is scurvy likely to emerge.
The foliage of many flowering plants has an unexpectedly high concentration of vitamin C, with concentrations of up to 1 percent wet weight being attained in some members of the Primulaceae family. The mean concentration for 213 species examined (162 mg per 100 g) was some three times that of those culinary vegetables usually regarded as good sources of the vitamin; and the mean value for the leaves of 41 woody shrubs and trees examined was 293 mg per 100 g—significantly higher than black currants, which are usually cited as the dietary source par excellence of vitamin C (Jones and Hughes 1983, 1984).
The historically important “antiscorbutic herbs” are among the poorest sources of vitamin C (Hughes 1990). William Perry, who stowed boxes of mustard greens and cress on board his ship in an attempt to fend off scurvy during his Arctic expedition of 1818 (Lloyd and Coulter 1963: 108), would have done better to adorn his stateroom with primrose plants, a single leaf of which, chewed in the mouth daily, would have sufficed to offer complete protection. The exact reason, if any, for these high (and often disparate) concentrations of ascorbic acid in angiosperms is not known; nor is the role of ascorbic acid in plant biochemistry understood. It has been suggested that there is a positive correlation between the concentration of ascorbic acid in plants and corresponding concentrations of phenolic compounds, but the extent to which this reflects a biochemical relationship is a matter of conjecture (E. C. Bate-Smith, personal communication).
Vitamin C Megatherapy
The practice of ingesting daily doses of vitamin C grossly in excess of the amount believed to protect against scurvy and even in excess of the amount known to produce tissue saturation is one of the more controversial aspects of current nutritional thought. The arguments for vitamin C “megatherapy” were initially outlined in the United States by Irwin Stone and later elaborated by Linus Pauling, winner of two Nobel Prizes (Hughes 1981b: 47-53). Stone disputed the adequacy of current recommended daily intakes, basing his case primarily on the rate of biosynthesis of the vitamin by animals producing their own supply and on G. H. Bourne’s estimate that the natural diet of the gorilla provides it with a daily intake of some 4.5 g ascorbic acid (Bourne 1949). His arguments for daily intakes of grams rather than milligrams were enthusiastically embraced and extended by Pauling.
Closely interwoven with the megatherapy theory is the claim that vitamin C has a number of extra-anti-scorbutic functions (protection against infection and, particularly, the common cold, detoxication, cerebral function, lipid metabolism, longevity, and so forth) that might require significantly raised amounts of the vitamin (Hughes 1981b: 14-34). For example, E. Ginter has for many years carefully presented the thesis that vitamin C plays a part in lipid metabolism, particularly by enhancing the conversion of cholesterol to bile salts, and that it would, therefore, have a hypocholesterogenic function (Ginter and Bobek 1981).
To date, however, there is little evidence that these putative relationships are reflected by a specific and increased demand for vitamin C. And as indicated earlier, some of these supposed secondary roles have now been subsumed in enzymatic terms by the advances of reductionist biochemistry. Secondary (or extra-antiscorbutic) roles for vitamin C could, conceivably, require intakes greater than those necessary for the prevention of classical scurvy, but such increased requirements would, in biochemical terms, scarcely justify the massive intakes recommended by the megatherapists.
Apart from the lack of satisfactory evidence, there are other arguments against vitamin C megatherapy (Jukes 1974; Hughes 1981b: 47-53).Adverse reactions elicited by massive doses of vitamin C and the possibly toxic influence of its breakdown products could well disadvantage the body. Moreover, the ingestion of large amounts of ascorbic acid is a self-defeating exercise as the absorption of large doses is a relatively inefficient process, with less than one-half of a 1 g megadose being absorbed from the gastrointestinal tract and only one-fourth of a 5 g dose (Davies et al. 1984;”Experimental Scurvy” 1986).And, in any case, it is generally accepted that tissue saturation in humans may be satisfactorily attained by a daily intake of 100 to 150 mg or even less. The faith of the megatherapists would have appeared to blind them to the normal canons of scientific assessment.
In the mid-1970s, Pauling espoused perhaps the most controversial of all his vitamin C beliefs. In collaboration with a Scottish surgeon, Ewan Cameron, he began to write extensively on the supposed antitumor activity of vitamin C; more specifically, Cameron and Pauling published the results of a clinical trial in which it was claimed that a megadose (10 g daily) of vitamin C quadrupled the survival time of terminally ill cancer patients (Cameron and Pauling 1976). The methodology of this trial was widely criticized, and a carefully controlled attempt to repeat it at the Mayo Clinic in the United States failed to confirm the Cameron-Pauling claims. For the next 15 years, and in the face of growing reluctance on the part of the scientific press to publish his papers, Pauling continued to present his arguments for the efficacy of vitamin C in the treatment of cancer. An account of this drawn-out battle between Pauling and the American scientific establishment has recently appeared (Richards 1991).
In more general and theoretical terms, it has been suggested that the antioxidant and free-radical scavenger roles of vitamin C support its possible function in the prevention (as contrasted with the cure) of cancer. G. Block has assessed some 90 studies of cancer and vitamin C/fruit intake relationships and has concluded that there is evidence that in the majority of cancers vitamin C may have a significant prophylactic role (Block 1991). In this respect, the possible relationship between vitamin C and nitrosamine-induced cancers has attracted some attention.
It has been speculated that endogenously produced N-nitroso compounds may be important initiators of human cancers. Significant in this respect is the formation of N-nitrosamines and related compounds. Nitrosamines may be formed when nitrate, a suitable “nitrosable” amine, and bacteria coexist—as in the gastrointestinal tract. Nitrate (the main dietary sources of which are fish and root vegetables) is converted by bacterial action to nitrite, which then reacts with amines to produce carcinogenic nitrosamines. Some foods, particularly cured meat products, contain nitrosamines formed during processing.
There is evidence that vitamin C may prevent the formation of carcinogenic nitrosamines from nitrate and may even reduce the carcinogenicity of pre-formed nitrosamines (Hughes 1981b: 27-9; MAFF 1987). It has been suggested, for example, that a reduction in nitrosamine formation, attributable to citrus fruit vitamin C, may be a contributory factor in determining the comparatively low incidence of large bowel cancer in the “citrus belt” of the United States (Lyko and Hartmann 1980). Sodium nitrite is used in the large-scale preparation of cured meats, bacon, and sausages (primarily to prevent the activity of the highly toxic Clostridium botulinum), and these products, consequently, contain a range of preformed nitroso compounds. There may, therefore, be good scientific reasons for regarding orange juice as a useful dietary accompaniment to a fried breakfast!
Vitamin C and Industry
Many tens of thousands of tons of vitamin C are produced synthetically each year from glucose; the initial stages in conversion involve reduction to sorbitol followed by bacterial oxidation to sorbose. Much of this vitamin C finds its way into health-food stores for sale to megatherapy enthusiasts as a putative dietary adjuvant.A substantial proportion is used industrially as a “technological aid” some is used in meat-curing processes to promote pigment conversion (in this application, it also has an adventitious and unintended role in reducing the formation of volatile Nnitrosamines).
Vitamin C is also widely employed as a permitted antioxidant (sometimes as ascorbyl palmitate) to prevent the formation of rancidity in stored fat products and the phenolic browning of commodities such as dehydrated potatoes. It is used as a flour improver in the Chorleywood Bread Process, where its oxidation product (dehydroascorbic acid) modifies the availability of glutathione in dough development, thereby shortening the period of fermentation.
The use of vitamin C in these technological processes finds general approval on the grounds that one is, after all, adding a beneficial vitamin rather than some untried additive of unknown toxicity. It should be pointed out, though, that little of this additive vita-min C is recoverable from the marketed product, which will, however, contain substantial amounts of vitamin C breakdown products—many of them unidentified and almost all of them of unknown toxicity. Bread is vitamin C-free despite the substantial amounts that may have been added during the Chorleywood process. It has been estimated that the average consumer may ingest up to 200 mg a week of vitamin C breakdown products from additive sources (Thomas and Hughes 1985).
Why vitamin C should have attracted so much attention in nutritional circles—orthodox and otherwise—is difficult to understand. Its almost limitless appeal to health enthusiasts and pseudonutritionists is matched only by the time and attention devoted to it by academic nutritionists. The annual global publication of some 2,000 papers bearing on vitamin C implies an annual research expenditure of some £40,000,000 (about 60 to 70 million U.S. dollars)—a not inconsiderable sum for studying a molecule whose nutritional significance is, at the most, marginal. Vitamin C deficiency is today a rare occurrence, and the evidence for extra-antiscorbutic requirements in excess of the mean daily intake is slender. Perhaps the biochemical versatility of the vitamin C molecule makes it attractive to biochemists who feel that it deserves a much more significant role than that of a somewhat prosaic involvement in the biosynthesis of collagen.
There are some questions that remain unanswered. For example, the apparent negative correlation between blood and tissue concentrations of vitamin C and age is puzzling. Many very elderly subjects—particularly if institutionalized—have virtually no ascorbic acid in their blood, a situation that would almost certainly be associated with the emergence of clinical scurvy in a younger age group.Yet these octogenarians and nonagenarians seem to be in no way disadvantaged by the apparent absence of the vitamin. Is there, then, a negative correlation between aging and dependency upon vitamin C? Such a relationship, if true, would be a remarkably fortuitous one as a substantial proportion of the institutionalized elderly have intakes of vitamin C well below the recommended daily amount. It is a somewhat sobering thought that in these days of scientifically attuned dietetics the mean intake of vitamin C by the institutionalized elderly in the United Kingdom is no greater than it was in hospitals a century and a half ago (Jones, Hughes, and Davies 1988).
In the more rarefied atmosphere of academic biochemistry, however, it is possible to point to real advances in our knowledge of vitamin C over the past 40 years. Today, modern high-performance liquid chromatographic techniques are replacing the classical indophenol dye method for the determination of vitamin C, with an increase in sensitivity and specificity. Our knowledge of possible biochemical involvements of the vitamin has advanced substantially. Sadly, however, one cannot point with equal certainty to any corresponding expansion of our knowledge of the nutritional significance of vitamin C beyond its role in the prevention of classical scurvy.