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Journal of Evolution and Health
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1. Introduction
Vitamin C is one of the most widely taken nutritional supplements [1,2]. Health professionals
as well as laymen attribute a number of health benefits to vitamin C such as boosting the
immune system or preventing the common cold and cancer. Whilst the role of vitamin C in
physiologic processes is well-established, there is little or no high-grade evidence supporting
that taking vitamin C as a supplement is indeed beneficial. We will summarize existing
research data with a special emphasis on aspects that are seldomly mentioned in the context of
vitamin C. As such, we invoke the glucose-ascorbate antagonism (GAA) theory, which
proposes that availability of vitamin C for cells is determined by glucose concentration [3].
We review evidence from cellular physiology as well as dietary studies carried out in Inuit
people to support the GAA theory and the view that the source of vitamin C as well as
composition of diet is crucial for optimal vitamin C supply. In this review we propose a
concept which may be suitable for synthesizing data coming from diverse sources and/or that
are apparently conflicting. The frames of the hypothesis are formed by a major, albeit often
neglected principal: evolution. We put forward that apparent controversies regarding vitamin
C can be readily resolved by an evolutionary approach.
2. Biosynthesis and biological significance of vitamin C
In most mammals vitamin C is produced from glucose in the liver. Species that are unable to
synthesize this nutrient rely on dietary uptake from its food sources [4]. Vitamin C, in synergy
with vitamin E, is known to have a role in reducing lipid peroxidation [5]. Vitamin C also acts
as a co-factor in at least eight enzymatic reactions including those involved in the synthesis of
collagen and carnitine [6].
Vitamin C exists in two redox states: ascorbic acid (AA), and its oxidized form,
dehydroascorbic acid (DHAA). Most actions of vitamin C can be attributed to AA which acts
as a reducing agent donating electrons to various reactions [7]. The oxidised form of vitamin
C is then recycled back to AA. Both AA and DHAA are absorbed in the small intestine [7].
Absorption is almost complete at doses <200 mg but the degree of absorption decreases as
intake increases [8]. Uptake of AA from the intestine relies on an active transport mechanism
of sodium dependent vitamin C transporters (SVCT) while uptake of DHAA relies on
facilitative diffusion by facilitative glucose transporters (GLUT) [9]. Within the enterocytes,
absorbed DHAA is converted to AA thereby producing low intracellular DHAA
concentration which by a gradient facilitates further DHAA uptake. DHAA may be taken up
from the blood by several cell types that reduce it to AA. DHAA can be filtered from the
plasma by renal corpuscules and then reabsorbed in the renal tubules for subsequent reduction
[10]. Some organs accumulate vitamin C (AA) 10-50-fold higher than the blood level of it.
These include tissues of high metabolic activity such as adrenal glands, thymus, eye lens,
retina, brain, pancreas, kidney, lymph nodes and lymphocytes [11].
AA cannot penetrate the blood brain barrier thus vitamin C is taken up in its oxidised
form DHAA which is then reduced back to AA for retaining it within brain cells [12]. A
similar transport mechanism exists for vitamin C entering the mitochondria [9]. AA is
accumulated within the mitochondria where it has a role to scavange free radicals abundantly
produced by mitochondrial function [13]. Several pathological conditions have been shown to
be associated with impaired redox cycling of vitamin C. For example, conditions associated
with inflammation such as diabetes, trauma, surgery, sepsis and wound healing are also
known to be characterized by decreased AA concentrations along with elevated DHAA
concentrations in both plasma and leucocytes [10]. This is believed to be due to increased
level of oxidants such as hydroxyl radicals, peroxyl radicals and superoxide anion requiring
more AA than could be regenerated from DHAA [10].
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Clemens and Tóth: Vitamin C from the evolutionary perspective
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3. Shortage and overdose of vitamin C
Shortage of vitamin C causes scurvy, presenting with symptoms including malaise, gum
disease, poor wound healing, shortness of breeth and bone pain. Scurvy occurred frequenty
among salesmen in mediaeval times. In current western societies it may occur in subjects with
alcoholism where it typically presents along with deficiencies of other vitamins [14]. Oral
megadose (>1000 mg) of vitamin C may cause diarrhea, nausea, abdominal bloating and
heatburn [4]. There is evidence that vitamin C supplementation increases the risk of forming
kidney stones [4]. Megadoses of vitamin C may lead to the development of vitamin B12
deficiency [4], a phenomenon explained by redox reactions of AA converting vitamin B12
into a biologically inactive analogue [6]. As an additional side effect, in glucose-6-phosphate
dehydrogenase deficiency AA has been shown to have a hemolytic effect [6]. Intrevenous
megadose of vitamin C may result in diarrhea, nausea, headache, decreased appetite and
fatigue [16].
4. Evolutionary perspective
Vitamin C is an essential nutrient for humans who are unable to synthesize and thus have to
obtain it from its dietary sources. Loss of the ability to synthesize vitamin C is not unique
among mammals: e.g. guinea pigs, monkeys and apes also lack this ability. Non-synthesizing
species, including humans, lack the L-gulonolactone oxidase (GULO) enzyme which is
required in the last enzymatic step of synthesis of vitamin C from glucose [4]. In the genome
of humans and that of the anthropoid primates a non-functional gene is present instead of the
GULO gene. Loss of the ability to synthesize vitamin C in primates is believed to have
occurred about 60 million years ago at the time of the split of the two primate suborders:
Strepsirrhini and Haplorhini [17]. Strepsirrhine primates including lemurs, lorises, and
galagos are able to produce vitamin C while haplorhine primates such as tarsiers, monkeys
and apes obtain it from dietary sources.
In the medical literature losing the ability to synthesize vitamin C in the course of
evolution is usually interpreted as an imperfection leaving our health vulnerable. However
with an evolutionary attitude such an ”imperfection” is inconceivable given that species that
are currently living represent the highest level of adaptation to their environment. Non-human
species lacking the GULO gene are not regarded imperfect either. It would be hard to believe
that the Homo genus would have been so successful in terms of survival and spatial spread
with a deteriorating genetic mutation. We assume that in the ancestral environment, where our
hominoid predecessors lived for 2.6 million years, loss of internal synthesis of vitamin C was
not disadvantegous. As one possible explanation, Ames et al. [18] proposed that during the
primate evolution uric acid might have taken over the antioxidant function of vitamin C, a
hypothesis based on the striking paralellism between the inability to break down uric acid and
the loss of the ability of vitamin C synthesis in primates [19].
5. Dietary sources of vitamin C
Recommended Dietary Allowances (RDA) of vitamin C, as suggested by the Institute of
Medicine, is 90 mg for males and 75 for females. According to the NHANES survey between
2003 and 2004, 7% was vitamin C deficient (serum concentration <11.4 μmol/L) in an US
population where 37% of men and 47% of women was taking vitamin C supplements [20].
The estimated average requirement (EAR) is set at 75 mg/d on the basis that it will
lead to 80% saturation of neutrophil AA concentrations without substantial urinary excretion,
thereby maximizing antioxidant effects. The RDA is derived from the EAR by assuming a
coefficient of variation (CV) of 10% in nutritional needs and adding twice the CV to the EAR
to yield an RDA that presumably covers the needs of 97-98% of the population [21]. Failure
or inconclusiveness of intervention studies with vitamin C may be due to the fact that vitamin
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C in these studies was given to people who had already been meeting or nearly meeting their
requirement as defined by the RDA.
Current dietary recommendations concentrate on plant sources of vitamin C. The
WHO, for example, recommend a minimum of 400g of fruit and vegetable per day also in an
attempt to ensure other micronutrients [22]. The belief that plant sources are ideal for vitamin
C supply roots back to James Lind who in 1747 discovered that scurvy, a devasting disease of
English sailors, can be prevented and reversed by fresh fruits. In fact, several lines of
evidence indicate that people may remain free of scurvy on diets lacking fruits and vegetables.
In hunter-gatherer societies organ meats and especially liver, marrow and brain are
highly favoured. These latter organs, as also described above, accumulate high level of
vitamin C [23,24] and are rich in other vitamins too [25]. At odds with dietary
recommendations in the western world, the majority of hunter gatherer societies are subsisting
on meat based diets yet remain free of scurvy [26,27,28]. In the study of Cordain et al. [26]
168 of the 229 hunter gatherer societies subsisted on diets containing at least 55% hunted
and/or fished food. In fact the ratio of animal food may be even higher given that hunther
gatherers also eat insects, invertebrates and other small animals that are collected by gathering
[26]. If both plant and animal foods are available, hunther gatherers clearly prefer the latter
[26,27].
Traditionally living arctic people represent extreme examples of animal food relience.
Among the Inuit scurvy was not observed until the 20th century [23]. Studies show that the
traditional diets of northern populations, including the Inuit diet, provide enough amount of
vitamin C. Fediuk [23], for example, reported an average intake of 38 mg of vitamin C/day
from meat and organs. However, on special occasions such as after successful whale hunts
intake may reach 340 mg/day [23]. Animal parts with highest vitamin C content include
”muktuk”, epidermis of the beluga whale (containing 36 mg/100 mg of vitamin C) and the
liver (containing 24 mg/100 mg of vitamin C) as well as the brain (containing 15 mg/100 mg
of vitamin C) of land and sea mammals [23,24]. In the 1970th, in the study of Geraci and
Smith [29] daily intake of vitamin C in a hunting Inuit community was in the range 10-120
mg, much higher than found earlier in a national survey of Canada assessing Inuit populations
in larger settlements with transitional cultures [29]. Westernalization of native societies
brought substantial changes in life style along with a decline in the access to traditional foods.
In parallel, an increasing portion of the Inuit people became at risk for scurvy [23].
Several examples show that not only native people may subsist on meat-fat based
diets. It is known that arctic travellers of European descent living on canned Western type
food were frequently affected by scurvy but this was not the case in those who had access to
traditional foods [25]. The arctic explorer Vilhjalmur Steffanson, for example, lived on the
Inuit diet for nine years and remained completely healthy. In 1930 an experiment was set up
in which Steffanson along with a fellow explorer lived on an exclusive meat diet for one year
[30]. No sign of vitamin deficiency, including scurvy, was noticed despite of the absence of
vitamin supplementation. Voegtlin, first proponent of the human evolutionary diet, put
forward that the diet humans are evolutionary adapted to is based on animal fat, meat and
offal, and is of full nutritional value. Along with this, he argued against the usefulness of
supplementing vitamin C [25]. We would like to emphasize that those hunter gatherers
subsisting on diets containing larger amounts of fruits and vegetables, may also have access to
enough vitamin C. This may be due to the combined sources of animal and plant vitamin C
along with a carbohydrate intake being still much lower than on an average Western type diet.
Anyhow, the Kitavan islanders for example, who are known to consume much carbohydrate
as compared with other indigenous people, exhibit low blood glucose levels (on average 3.5
mmol/l in the young population) [31].
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The authors of the present paper are rehabilitating patients with chronic diseases by
using a diet we refer to as the paleolithic ketogenic diet [32-36]. This is an animal fat-meat
based diet which is close to the diet originally proposed by Voegtlin [25]. The paleolithic
ketogenic diet excludes foods that were not available for preagricultural humans such as
cereal grains, milk and dairy, vegetable oils, nightshades, legumes, refined sugars and foods
with additives. It also excludes foods that may be included in the popular versions of the
paleolithic diet such as oilseeds, coconut, coconut oil, artificial sweeteners, vitamin as well as
other supplements. The paleolithic ketogenic diet also differs from the popular paleolithic diet
in that it restricts vegetables and fruits to an amount of <30% (by weight) and thus ensures
ketosis. The suggested fat:protein macronutrient ration is 2:1 (in grams). We encourage red
and fat meats and the regular intake of organ meats. Like other proponents of the paleolithic
diet [25,26] we advise patients against taking vitamin C supplements. Our clinical experience,
also shown by the example of our published cases [32-36], indicate that neither scurvy nor
other nutritional deficiency emerge in the absence of vitamin C supplementation while
adhering to the paleolithic ketogenic diet. As an important distinction, this is not the case with
the classical form of the ketogenic diet where scurvy may occur [37] likely due to limited
intake of both animal and plant sources of vitamin C.
Another potential benefit of animal derived vitamin C pertains to cooking. It is widely
known that vitamin C content of foods degrade at higher temperatures. However, cooking-
related loss of vitamin C seems to be smaller in animal-derived foods as compared with plant-
derived ones as assessed both by the analysis of the Inuit diet [29,23] and by comparison of
food items in publicly available food databases. For example, according to the USDA's
National Nutrient Database for Standard Reference in the raw spinach there is 28.1 mg/100 g
of vitamin C while cooked spinach contains only 9.8 mg/100 g of it. At the same time, raw
and cooked forms of pork liver contain comparable amounts of vitamin C: while the former
contains 25.3 mg/100 g the latter contains 23.6 mg/100 g of vitamin C.
6. Vitamin C and disease
In the following we review data from those clinical studies representing the highest grade
evidence available and have a hard clinical endpoint. In such an attempt we have searched
pubmed and google scholar databases for studies that include terms ”vitamin C” or ”ascorbic
acid” in combination with ”mortality”, "cancer”, ”cardiovascular disease”, ”stroke”,
”hypertension” and ”common cold”. We also searched for ”intravenous vitamin C therapy”.
We then reviewed search results for each of the above condition to select those studies that (1)
include the most subjects/patients, (2) have a sound methodology and (3) and assess a hard
endpoint/clinically meaningful variable. Parameters which were regarded as hard clinical
endpoint include: bivariate variables (such as death or the occurrence of specific events
including stroke or cardiovascular events, and having a diagnosis of a specific disease),
survival time, incidence of common cold, and blood pressure itself in hypertension. We
disregarded studies that rely on soft clinical endpoints of disease e.g. cholesterol level in
cardiovascular disease, the role of which in cardiovascular disease is debated. Prospective
cohort and correlational studies are presented separately from intervention studies. For
prospective and correlational studies we only included studies that assessed blood level of
vitamin C and excluded those that rely on estimated dietary intake of vitamin C. Reasons for
this include probable inadequacies in estimation of the vitamin C content of foods and the
low/variable correlation between dietary intake and the blood level of vitamin C [38]. The
low measured correlation between dietary and blood vitamin C in these studies is due to fact
that blood vitamin C not only depend on intake but on other factors including differences in
relative partitioning between plasma and intracellular concentration as well as differences in
excretion and oxidation rates [6]. From our point of view it is of upmost importance that
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DOI: 10.15310/2334-3591.1030
dietary studies assessing vitamin C intake, either epidemiological or interventional studies,
limited the estimation of vitamin C to the plant sources while neglecting the animal sources of
it. The reason for selecting the conditions of common cold, cardiovascular disease and cancer
in the present manuscript lies in the fact that large studies of vitamin C (either prospective or
randomized controlled studies) are available for these conditions only. Out of the three
conditions, cancer will be in the focus, given the predominance of studies assessing cancer.
6.1. Prospective cohort and correlational studies: blood level of vitamin C and disease
For an overview of prospective cohort and correlational studies of blood level of vitamin C
and disease including mortality, cancer, cardiovascular disease, stroke and hypertension see
Table 1.
Table 1.
Major prospective cohort and correlational studies of vitamin C and disease
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Clemens and Tóth: Vitamin C from the evolutionary perspective
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* number of patients in the gastric cancer group
Note that all studies indicate an inverse association between blood concentration of AA and
mortality/morbidity.
6.1.1. Mortality
In the EPIC-Norfolk prospective population study, plasma concentration of AA was inversely
related to mortality from all-causes as well as from cardiovascular disease and ischaemic heart
disease. Mortality risk in the top quintile was half the risk in the lowest quintile [39]. A study
based on an NHANES II. database found an inverse relation between dying from any cause
and low AA in men but there was no association in women [40]. In one study with elderly,
AA concentration was shown to be inversely associated with subsequent mortality in both
males and females [41] while in another study AA concentration was inversely associated
with mortality from stroke but not from coronary heart disease [42]. Male cardiovascular
mortality in the Basel Prospective Study was also inversely associated with plasma level of
vitamin C [43]. In hemodialysis patients, low level of vitamin C was shown to be a risk factor
for cardiovascular morbidity and mortality which was explained by vitamin C ameliorating
vascular dysfunction generally seen in patients with renal failure [44].
5.1.2. Cancer
In the EPIC-Norfolk study, vitamin C level was inversely related to cancer mortality in males
but not in females [40]. In an NHANES II. study, cancer risk was inversely related to AA in
males [45]. Unexpectedly, in females high level of AA was found to be associated with
increased cancer risk [45]. A case control study showed increased risk for gastric cancer for
those with low plasma level of vitamin C [46] but no association was found for dietary
vitamin C intake [46]. In hospice patients low plasma concentrations were associated with
shorter survival [47].
6.1.3. Cardiovascular disease
In an NHANES II. study, serum AA was inversely related with the prevelance of coronary
heart disease and stroke [48]. In several studies, including the Epic-Norfolk prospective
population study, low plasma concentration of vitamin C was associated with increased stroke
risk [49]. In cross-sectional studies, blood pressure in middle aged and elderly [50] as well as
in young adults [51] was found to be inversely associated with blood AA.
6.2. Intervention studies of vitamin C
6.2.1. Oral supplementation of vitamin C
For an overview of the oral vitamin C intervention studies related to mortality, common cold,
cardiovascular disease and high blood pressure see Table 2. Given the high number of
intervention studies with vitamin C available in the literature, in the present review we
concentrate on RCTs and metaanalyses.
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Table 2.
Major metaanalyses and RCTs of oral vitamin C supplementation related to mortality,
common cold, cardiovascular disease and hypertension. No study report a clinically
meaningful benefit.
CVD: cardiovascular disease; MD: missing data; SBP: systolic blood pressure
6.2.1.1. Mortality
A recent Cochrane review examining mortality as an endpoint found that supplementation of
antioxidants including vitamin C have no effect on overall mortality [52].
6.2.1.2. Common cold
The latest Cochrane review [53] concluded that vitamin C supplementation has no effect on
the incidence of common cold. However, a modest reduction of sympoms was consistently
found in the reviewed studies [53].
6.2.1.3. Cardiovascular disease
The Women's Antioxidant Cardiovascular Study, a randomized controlled study (RCT), found
no effect of vitamin C supplementation in the secondary prevention of cardiovascular disease
[54]. Another RCT, the Physicians' Health Study II., did neither find vitamin C to be
beneficial in the prevention of cardiovascular events [55]. In regards to high blood pressure, a
metaanalysis of twenty-nine trials of blood pressure indicated a statistically significant yet
clinically not meaningful decrease of 3.8 mmHg in systolic blood pressure [56].
6.2.1.4. Cancer
The idea that vitamin C may be beneficial in cancer treatment stems from Linus Pauling who
argued that high doses of vitamin C (10 g/day) given intravenously may be useful in the
treatment of cancer [57]. This assumption was based on a study of 100 patients with advanced
cancer who as compared to historical controls survived three to four times longer [57]. The
study was later criticized because of the absence of an appropriate control group. The study
was repeated with improved methodology and the authors still reported survival benefit in the
vitamin C group [58,59]. However, subsequent prospective controlled studies that were
carried out at the Mayo clinic were unable to repeat results. These studies, however, used oral
administration of vitamin C rather than intravenous administration [60,61]. Unlike the
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Clemens and Tóth: Vitamin C from the evolutionary perspective
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original study [57], Creagan et al. [60] included patients who previously received
chemotherapy while the study by Moertel et al. [61] included cancer patients with no prior
chemotherapy.
Prospective RCTs, like the Physicians’ Health Study II., did not find vitamin C
supplementation to be effective in the prevention of total cancers, prostate and in other solid
tumors [62]. No effect of vitamin C supplementation on cancer incidence and mortality was
seen in another randomized trial, the Women's Antioxidant Cardiovascular Study [63]. A
metaanalysis by Coulter et al. [64] also concluded that vitamin C provide no prevention of
cancer.
For an overview of the oral vitamin C intervention studies related to cancer see Table
3.
Table 3.
Cancer-related intervention studies of oral vitamin C.
a patients received IVC therapy for 10 days followed by oral vitamin C
b besides oral vitamin C some patients received IVC therapy too
c number of patients in the vitamin C group
* Number of patients treated with vitamin C
RCT: randomised controlled trial; NA: not applicable
6.2.2. High dose intravenous vitamin C (IVC) therapy in cancer
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Vitamin C is known to poorly absorb when administered orally at higher doses: immune cells
saturate at 100 mg daily, and renal excretion of AA increases above this dose [8]. Therefore, it
was claimed that blood concentrations of vitamin C in the Mayo Clinic studies [60,61] might
not have reached a concentration producing the cytotoxic effect to tumor cells seen in in vitro
studies [65]. With intravenous administration, the tightly regulated absorption and transport of
vitamin C is bypassed and therefore result in a much (30- to 70-fold) higher than
physiological blood level of vitamin C [66]. Cytotoxicity of AA to cancer cells was based on
in vitro studies which suggested that AA in concentrations higher than physiological in the
extracellular space act as a pro-oxidant and through the formation hydrogen peroxide kill
cancer cells [67]. However, in a recent study the cytotoxic effect was abolished at
physiological concentrations of iron which prevented the accumulation of hydrogen peroxide
[68]. It was suggested that previous studies disregarding in vivo concentration of iron had
significantly overestimated the anticancer effect of AA [68].
Currently, no more than a single RCT is available that assessed the effect of IVC
therapy in cancer patients [69]. This indicated a reduction of chemotherapy induced side
effects in ovarian cancer patients but no significant effect in terms of survival was found [69].
In an uncontrolled study of metastatic pancreatic cancer patients, Monti et al. [70] reported
minor decrease in the size of the tumor without evidence of prolonged survival. In another
study, of the 16 patients with solid tumors no one experienced an objective tumor response
[71]. In a study by Mikirova et al. [72], decreased level of inflammatory markers was seen
following IVC therapy but no positive effect was reported on disease progression and
survival. Clearly, no antitumor effect was evident in phase I. clinical trials studies including
studies by Welsh et al. [73] and Hoffer et al. [74]. Two other clinical group studies by
Riordan [75] and Yeom et al. [76] did neither provide evidence for an anticancer effect. A
more recent phase I-II. clinical study by Hoffer et al. [77] reported transient (lasting for 3-13
months) stable disease in 3 out of the 16 patients in the study. Actually all the existing group
studies have failed to provide evidence for a benefit in hard clinical endpoints such as
survival. At the same time, it is important to mention that except for the seminal study of
Cameron and Pauling [57] all the above IVC studies included patients with prior or
concurrent chemotherapy. Thus the possibility suggested by Creagan et al. [60] that cancer
patients with no chemotherapy may benefit more from IVC therapy cannot be entirely
excluded.
Currently only case reports are available describing survival benefit following IVC
therapy [78,17]. Overall, from the available group studies it only seems that high dose IVC
therapy is relatively safe, reduces chemotherapy induced side effects but provide no benefit in
terms of hard clinical endpoints including survival in cancer patients also receiving
chemotherapy.
For an overview of the IVC studies related to cancer see Table 4.
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Clemens and Tóth: Vitamin C from the evolutionary perspective
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Table 4.
IVC therapy studies in cancer. Except for a small RCT (Ma et al. 2014) all studies contain
only a single group. Each study included patients with prior or concurrent chemotherapy. No
study reported an apparent benefit in terms of survival.
ICV: intravenous vitamin C; RCT: randomized controlled trial
6.3. Summary of the prospective cohort and intervention studies of vitamin C
Prospective cohort studies unequivocally found an inverse relation with the blood level of
vitamin C for mortality, cancer and cardiovascular disease for each of the specific disease
populations. Two prospective studies [40,45] when assessing males and females separately,
reported the inverse association in males only. We assume that the absence of an inverse
association in females may be due to differential habits of males and females in taking
supplements. Although in the two studies it was not possible to retrieve data on supplement
use, females are known to be more prone to take vitamin C supplements [20], and thus a
higher serum level of AA in females may reflect the effect of supplementation which may not
be preventive. Overall results from prospective studies are in sharp contrast with the
intervention studies where none of the studies reported a clinically meaningful benefit as
regards mortality, incidence of common cold, cardiovascular events, and prevention or
treatment of cancer.
7. The glucose-ascorbate antogonism theory
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The glucose-ascorbate antagonism (GAA) theory was first proposed by John Ely as early as
the 1970s [3]. The theory postulates that given the structural similarity between glucose and
vitamin C, the two molecules compete for same transport system to enter cells [10]. It has
been shown that cellular uptake of both AA and DHAA may be competitively antagonized by
elevated glucose levels. Specifically, AA uptake by the small intestine enterocytes was shown
to be inhibited by elevated glucose concentration [9]. DHAA transport into cells was shown
to be impaired by high blood glucose concentration in most cell types including adipocytes,
erythrocytes, granulosa cells, neutrophils, osteoblasts and smooth muscle cells [10].
A potential limitation of the GAA theory pertains to the lack of in vivo studies in
animals and in humans. Diabetes, however, can be regarded as a natural model to study
interaction between blood glucose and vitamin C in vivo in humans. Studies with diabetic
patients are in line with the GAA therory. Two studies in type 2 diabetes [79,80] indicated
that in spite of similar dietary intake of vitamin C, patients have decreased levels of plasma
AA as compared to normal controls. Furthermore, serum level of AA inversely correlated
with glucose levels [80] and in another study with glycated hemoglobin of diabetic patients
[81]. Diabetes is also known to be associated with impaired renal reabsorption of AA [82]
which likely contributes to the low level of AA in patients.
Uptake of glucose and DHAA also share the feature of insulin-dependency on the
GLUT4 glucose transporter primarily found in muscle and adipose tissue [83]. In type 1
diabetes, deficiency of insulin has been shown to impair DHAA uptake of lymphoblasts [84].
Such a decreased DHAA uptake, through impaired AA accumulation, may lead to
compromized immune system function in type 1 diabetes patients [84]. The effect of glucose
seems to be immediate on intracellular AA given that intravenous glucose administration
results in a prompt decrease in the AA concentration of leukocytes [85]. Also consistently
with the GAA theory, blood level of vitamin C is inversely related with obesity [86] a
correlate of increased carbohydrate intake.
Controlling hyperglycemia has also been suggested as an adjunct to cancer therapy
[87]. Hyperglycemia in cancer patients is known to be associated with reduced intracellular
AA concentration. Such a decrease results in impaired actions of AA, including a decreased
activity of the hexose monophosphate shunt, a patway important in optimal immune cell
functioning [87]. In cardiovascular disease, the association of high blood glucose and low AA
concentration is also consistent with the GAA mechanism, and may plausibly explain
functional impairments such as lipid peroxidation and endothelial dysfunction which are
known to contribute to the generation of atherosclerosis [87].
As an additional parallelism, the study by Johnstone et al. [88] compared the effect of
two high protein diets: one with low (in fact ketogenic) and another with medium
carbohydrate content. To the surprise of the investigators, higher concentration of blood
vitamin C was found on the low carbohydrate diet. Yet, the difference was explained by other
factors.
AA is known to have a role in scavenging reactive oxygen species within the
mitochondria. Reactive oxygen species within the mitochondria have been suggested to have
a role in the development of degenerative disorders including cancer [89]. Given the fact that
carbohydrate based Western type nutrition is associated with increased production of reactive
oxygen species in the mitochondria [89], it is plausible to speculate that the need for AA
would be decreased when on a low carbohydrate diet.
In human brush border membrane vesicles evaluated ex vivo, AA uptake is
competitively antagonized by glucose [9] coming abundantly from carbohydrate based diets
may explain why according to a metaanalysis dietary intake and plasma level of AA were
only moderately correlated [38]. Importantly, estimation of AA intake in this study relied on
plant sources only. Negligation of animal sources of vitamin C is typical in the case of the
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Clemens and Tóth: Vitamin C from the evolutionary perspective
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other dietary studies too. This fact together with the loose association between the intake
estimate and blood level indicate that plants are not ideal sources of vitamin C. Additionally,
flavonoids abundantly found in fruits and vegetables were also shown to inhibit vitamin C
uptake by enterocytes [90]. We are not aware of studies assessing the relation between animal
sources of vitamin C and blood level of it. The striking dissociation between epidemiological
and intervention studies, as revealed in this review for mortality, cardiovascular disease and
cancer, may be regarded as an indication that vitamin C taken as a supplement may not be an
optimal way to obtain this nutrient.
Chronic diseases such as obesity, diabetes, neurodegenerative diseases, cancer and
cardiovascular diseases being associated by the cluster of carbohydrate overconsumption [91],
mitochondrial dysfunction [92], increased production of reactive oxygen species [92] and
decreased blood level of vitamin C also point to the GAA as a common mechanism beyond
these pathologic conditions. It can be speculated that increased dietary intake of carbohydrates
and resulting low level of blood AA are important factors contributing to the development of
chronic degenerative disorders.
As a further parallelism, the hypothesis proposed in the present paper also fits well
with the Warburg theory postulating that cancer is originating from a metabolic dysfunction
including insufficient mitochondrial oxidative phosphorylation and compensatorily enhanced
glycolysis [93]. Cancer cells are highly dependent on glucose and are unable to use fat or
ketones for energy [94], a metabolic failure ketogenic diets are aimed to exploit. We suggest
that an animal based, low carbohydrate high fat diet (instead of the currently used ketogenic
diets) may provide vitamin C not only in sufficient amounts but also in a bioavailable form,
and thus may be more appropriate for the treatment of cancer as compared with the versions
of the classical ketogenic diets that are currently used in clinical trials e.g. [95].
8. Conclusion
From the prospective cohort and correlational studies it is clear that higher blood level of AA
is associated with lower mortality and morbidity in several chronic conditions. At the same
time, high-grade evidence from clinical intervention studies indicate that vitamin C taken as a
supplement provide little or no benefit in the prevention or treatment of chronic diseases.
Indeed, apart from intervention studies of common cold where minor benefit was reported, no
single RCT is available that found a clinically meaningful benefit in hard clinical endpoints of
chronic diseases including cancer and cardiovascular diseases. We put forward that the
discrepancy between correlational and interventional studies, as regards the role of vitamin C,
is only apparent and may be resolved by introducing the GAA theory. As detailed above,
metabolism of vitamin C, including absorption and its uptake by several cell types, is
inhibited by increasing glucose concentration. Western type diets resulting in carbohydrate
overconsumption and high blood level of glucose may inhibit utilization of vitamin C also
when taken as a supplement. Current dietary guidelines, instead of animal sources,
concentrate on plant sources of vitamin C, thereby also increasing carbohydrate load and
intake of polyphenols which both inhibit vitamin C utilization. By contrast, studies of
contemporary hunter-gatherer societies as well as documentations of the arctic people from
the 18th and 19th century indicate no signs of scurvy despite subsisting on diets predominated
by foods of animal origin and using no vitamin supplements. Our own clinical experience
with the paleolithic ketogenic diet also shows improving health parameters and long-term
sustainability of meat-fat based diet in the absence of vitamin C supplementation. It may be
anticipated that supplementing vitamin C while on a Western type diet may not reproduce the
biochemical and physiological complexity of the evolutionary adapted way to utilize vitamin
C. Even though the amount of dietary vitamin C consumed on an animal meat-fat based diet
may be lower as compared to dietary intake from some fruits and vegetables, the former may
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DOI: 10.15310/2334-3591.1030
ensure a higher bioavailability of vitamin C. We believe that loss of vitamin C synthesis was
not deleterious in the ancestral environment humans are evolutionary adapted to. Rather, a
mismatch between our current diet and ancestral physiology may explain why deficient levels
of vitamin C are associated with disease. Instead of supplementing vitamin C, changing our
nutrition as a whole and adopting a meat-fat based diet, even if it may sound a radical
solution, may be a better choice to support vitamin C homeostasis. Bearing in mind that
”nothing in biology makes sense except in the light of evolution” [96] this concept is in
accordance with the major organizing principle of the living world: evolution.
Conflict of interest
The authors declare that there are no conflicts of interest.
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