Vitamin C

Vitamin C

itamin C (chemical names: ascorbic acid and ascorbate) is a six-carbon lactone which

is synthesised from glucose by many animals. Vitamin C is synthesised in the liver in

some mammals and in the kidney in birds and reptiles. However, several species –

including humans, non-human primates, guinea pigs, Indian fruit bats, and Nepalese redvented

bulbuls – are unable to synthesise vitamin C. When there is insufficient vitamin C in

the diet, humans suffer from the potentially lethal deficiency disease scurvy (1). Humans and

primates lack the terminal enzyme in the biosynthetic pathway of ascorbic acid, lgulonolactone

oxidase, because the gene encoding for the enzyme has undergone substantial

mutation so that no protein is produced (2).

Role in human metabolic processes

Background biochemistry

Vitamin C is an electron donor (reducing agent or antioxidant), and probably all of its

biochemical and molecular functions can be accounted for by this function. The potentially

protective role of vitamin C as an antioxidant is discussed in the antioxidants chapter of this

report.

Enzymatic functions

Vitamin C acts as an electron donor for 11 enzymes (3, 4). Three of those enzymes are found

in fungi but not in humans or other mammals (5, 6). They are involved in reutilisation

pathways for pyrimidines and the deoxyribose moiety of deoxynucleosides. Of the 8

remaining human enzymes, three participate in collagen hydroxylation (7-9) and two in

carnitine biosynthesis (10, 11); of the three enzymes which participate in collagen

hydroxylation, one is necessary for biosynthesis of the catecholamine norepinephrine (12, 13),

one is necessary for amidation of peptide hormones (14, 15), and one is involved in tyrosine

metabolism (4, 16).

Ascorbate interacts with enzymes having either monooxygenase or dioxygenase

activity. The monooxygenases dopamine β-monooxygenase and peptidyl-glycine α-

monooxygenase incorporate a single oxygen atom into a substrate, either a dopamine or a

glycine-terminating peptide. The remaining enzymes are dioxygenases which incorporate two

oxygen atoms in two different ways. The enzyme 4-hydroxyphenylpyruvate dioxygenase

incorporates two oxygen atoms into one product. The other dioxygenase incorporates one

oxygen atom into succinate and one into the enzyme-specific substrate.

Miscellaneous functions

The concentrations of vitamin C in gastric juice were several fold higher (median, 249 μmol/l;

range, 43–909 μmol/l) than those found in the plasma of the same normal subjects (39 μmol/l,

14–101 μmol/l) (17). Gastric juice vitamin C may prevent the formation of N-nitroso

compounds, which are potentially mutagenic (18). High intakes of vitamin C correlate with

reduced gastric cancer risk (19), but a cause-and-effect relationship has not been established.

Vitamin C

Vitamin C protects low-density lipoproteins ex vivo against oxidation and may function

similarly in the blood .

A common feature of vitamin C deficiency is anaemia. The antioxidant properties of

vitamin C may stabilise folate in food and in plasma, and increased excretion of oxidized

folate derivatives in human scurvy was reported (21). Vitamin C promotes absorption of

soluble non-haem iron possibly by chelation or simply by maintaining the iron in the reduced

(ferrous, Fe2+) form (22, 23). The effect can be achieved with the amounts of vitamin C

obtained in foods. However, the amount of dietary vitamin C required to increase iron

absorption ranges from 25 mg upwards and depends largely on the amount of inhibitors, such

as phytates and polyphenols, present in the meal (24).

Overview of significant scientific information

From the 15th century, scurvy was dreaded by seamen and explorers forced to subsist for

months on diets of dried beef and biscuits. Scurvy was described by the Crusaders, during the

sieges of numerous European cities, and as a result of the famine in 19th century Ireland.

Three important manifestations of scurvy – gingival changes, pain in the extremities, and

haemorrhagic manifestations – preceded oedema, ulcerations, and ultimately death. Skeletal

and vascular lesions in scurvy probably arise from a failure of osteoid formation. In infantile

scurvy the changes are mainly at the sites of most active bone growth; characteristic signs are

a pseudoparalysis of the limbs caused by extreme pain on movement and caused by

haemorrhages under the periosteum, as well as swelling and haemorrhages in areas of the

gums surrounding erupting teeth (25). In adults one of the early, principle adverse effects of

the collagen-related pathology may be impaired wound healing (26). Vitamin C deficiency

can be detected from early signs of clinical deficiency, such as the follicular hyperkeratosis,

petechial haemorrhages, swollen or bleeding gums, and joint pain, or from the very low

concentrations of ascorbate in plasma, blood, or leukocytes. The Sheffield studies (26, 27) and

later studies in Iowa (28, 29) were the first major attempts made to quantify vitamin C

requirements. The studies indicated that the amount of vitamin C required to prevent or cure

early signs of deficiency was between 6.5 and 10 mg/day. This range represents the lowest

physiologic requirement. The Iowa studies (28, 29) and Kallner et al (30) established that at

tissue saturation, whole body vitamin C content is approximately 20 mg/kg, or 1500 mg, and

that during depletion vitamin C is lost at 3 percent of whole body content per day.

Clinical signs of scurvy appear in men at intakes lower than 10 mg/day (27) or when

the whole body content falls below 300 mg (28). Such intakes are associated with plasma

ascorbate concentrations below 11 μmol/l or leukocyte levels less than 2 nmol/108 cells.

However, the plasma concentrations fall to around 11 μmol/l when dietary vitamin C is

between 10 and 20 mg/day. At intakes greater than 25–35 mg/day, plasma concentrations start

to rise steeply, indicating a greater availability of vitamin C for metabolic needs. In general,

plasma ascorbate closely reflects the dietary intake and ranges between 20 and 80 μmol/l.

Note that during infection or physical trauma, an increase in the number of circulating

leukocytes occurs and these take up vitamin C from the plasma (31, 32). Therefore, both

plasma and leukocyte levels may not be very precise indicators of body content or status at

such times. However, leukocyte ascorbate remains a better indicator of vitamin C status than

plasma ascorbate most of the time and only in the period immediately after the onset of an

infection are both values unreliable.

Intestinal absorption of vitamin C is by an active, sodium-dependent, energyrequiring,

carrier-mediated transport mechanism (33) and as intakes increase, the tissues

progressively become more saturated. The physiologically efficient, renal-tubular

reabsorption mechanism retains vitamin C in the tissues up to a whole body content of

ascorbate of about 20 mg/kg body weight (30). However, under steady state conditions, as

intakes rise from around 100 mg/day there is an increase in urinary output in so that at 1000

mg/day almost all absorbed vitamin C is excreted (34, 35).

Definition of population at risk

The populations at risk of vitamin C deficiency are those for whom the fruit and vegetable

supply is minimal. Epidemics of scurvy are associated with famine and war, when people are

forced to become refugees and food supply is small and irregular. Persons in whom the total

body vitamin C content is saturated can subsist without vitamin C for approximately 2 months

before the appearance of clinical signs, and as little as 6.5–10 mg/day vitamin C will prevent

the appearance of scurvy. In general, vitamin C status will reflect the regularity of fruit and

vegetable consumption but also socio-economic conditions, because intake is determined not

just by availability, but by cultural preferences and cost.

In Europe and the United States an adequate intake of vitamin C is indicated by the

results of various national surveys (36-38). In the United Kingdom and Germany, the mean

dietary intakes of vitamin C in adult men and women were 87 and 76 (37) and 75 and 72

mg/day (36), respectively. In addition, a recent survey of elderly men and women in the

United Kingdom reported vitamin C intakes of 72 (SD 61) and 68 (SD 60) mg/day,

respectively (39). In the United States, in the National Health and Nutrition Examination

Survey (38), the median consumption of vitamin C from foods during the years 1988–91 was

73 and 84 mg/day in men and women, respectively. In all these studies there was a wide

variation in vitamin C intake and 25–30 percent of the US population consumed less than 2.5

servings of fruit and vegetables daily. Likewise a survey of Latin American children in the

United States suggested that less than 15 percent consumed the recommended intake of fruits

and vegetables (40). It is not possible to relate servings of fruits and vegetables to an exact

amount of vitamin C, but the World Health Organization (WHO) dietary goal of 400 g (41)

aimed to provide sufficient vitamin C to meet the 1970 Food and Agriculture Organization of

the United Nations (FAO)/WHO guidelines – that is, approximately 20–30 mg/day – and

lower the risk of chronic disease. The WHO goal has been roughly translated into the

recommendation of five portions per day (42).

Reports from India show that the available supply of vitamin C is 43 mg/capita/day,

and in the different states of India it ranges from 27 to 66 mg/day. In one study, low-income

children consumed as little as 8.2 mg/day of vitamin C in contrast to a well-to-do group of

children where the intake was 35.4 mg/day (43). Other studies done in developing countries

found plasma vitamin C concentrations lower than those reported for developed countries, for

example, 20–27 μmol/l for apparently healthy adolescent boys and girls in China and 3–54

μmol/l (median 14 μmol/l) for similarly aged Gambian nurses (44, 45), although values

obtained in a group of adults from a rural district in Northern Thailand were quite acceptable

(17–118 μmol/l, median 44 μmol/l) (46). However, it is difficult to assess the extent to which

sub-clinical infections are lowering the plasma vitamin C concentrations seen in such

countries.

Data describing a positive association between vitamin C consumption and health

status are frequently reported, but intervention studies do not support the observations. Low

plasma concentrations are reported in patients with diabetes (47) and infections (48) and in

smokers (49), but the relative contribution of diet and stress to these situations is uncertain

(see Chapter 17). Epidemiologic studies indicate that diets with a high vitamin C content

have been associated with lower cancer risk, especially for cancers of the oral cavity,

oesophagus, stomach, colon, and lung (39, 50-52). However, there appears to be no effect of

consumption of vitamin C supplements on the development of colorectal adenoma and

stomach cancer (52-54), and data on the effect of vitamin C supplementation on coronary

heart disease and cataract development are conflicting (55-74). Currently there is no

consistent evidence from population studies that heart disease, cancers, or cataract

development are specifically associated with vitamin C status. This of course does not

preclude the possibility that other components in vitamin C – rich fruits and vegetables

provide health benefits, but it is not yet possible to separate such an effect from other factors

such as lifestyle patterns of people who have a high vitamin C intake.

Dietary sources of vitamin C and limitations to vitamin C

Ascorbate is found in many fruits and vegetables (75). Citrus fruits and juices are particularly

rich sources of vitamin C but other fruits including cantaloupe, honeydew melon, cherries,

kiwi fruits, mangoes, papaya, strawberries, tangelo, watermelon, and tomatoes also contain

variable amounts of vitamin C. Vegetables such as cabbage, broccoli, Brussels sprouts, bean

sprouts, cauliflower, kale, mustard greens, red and green peppers, peas, tomatoes, and

potatoes may be more important sources of vitamin C than fruits. This is particularly true

because the vegetable supply often extends for longer periods during the year than does the

fruit supply.

In many developing countries, limitations in the supply of vitamin C are often

determined by seasonal factors (i.e., the availability of water, time, and labour for the

management of household gardens and the short harvesting season of many fruits). For

example, mean monthly ascorbate intakes ranged from 0 to 115 mg/day in one Gambian

community in which peak intakes coincided with the seasonal duration of the mango crop and

to a lesser extent with orange and grapefruit harvests. These fluctuations in dietary ascorbate

intake were closely reflected by corresponding variations in plasma ascorbate (11.4–68.4

μmol/L) and human milk ascorbate (143–342 μmol/L) (76).

Vitamin C is also very labile, and the loss of vitamin C on boiling milk provides one

dramatic example of a cause of infantile scurvy. The vitamin C content of food is strongly

influenced by season, transport to market, shelf life, time of storage, cooking practices, and

chlorination of water. Cutting or bruising of produce releases ascorbate oxidase. Blanching

techniques inactivate the oxidase enzyme and help to preserve ascorbate as also will low pH,

as in the preparation of sauerkraut (pickled cabbage). In contrast, heating and exposure to

copper or iron or to mildly alkaline conditions destroys the vitamin, and too much water can

leach it from the tissues during cooking.

The use of citrus fruits by the British navy in the 18th century gave rise to the term

‘limey’, a colloquial term for British sailors. However, it is important to realise that the

amount of vitamin C in a food is usually not the major determinant of a food’s importance for

supply, but rather regularity of intake. For example, in countries where the potato is an

important staple food and refrigeration facilities are limited, seasonal variations in plasma

ascorbate are due to the considerable deterioration in the potato’s vitamin C content during

storage; the content can decrease from 30 to 8 mg/100 g over 8–9 months (77). Such data can

indicate the important contribution the potato can make to human vitamin C requirements

even though the potato vitamin C concentration is low.

An extensive study has been made of losses of vitamin C during the packaging,

storage, and cooking of blended foods (maize and soya-based relief foods). Data from a US

Agency for Internation Development programme show that vitamin C losses from packaging

and storage in polythene bags of such relief foods are much less significant than the 52–82

percent losses attributable to conventional cooking procedures )78).

Information used to derive dietary requirement of vitamin C

Calculating the dietary intake from the physiologic requirements

Adults

At saturation the whole body content of ascorbate in adult males is approximately 20 mg/kg,

or 1500 mg. Clinical signs of scurvy appear when the whole body content falls below 300–

400 mg, and the last signs disappear when the body content reaches about 1000 mg (28, 30).

In these experiments, ascorbate in the whole body was catabolised at an approximate rate of 3

percent/day (2.9 percent/day, SD 0.6) (29).

There is a sigmoidal relationship between intake and plasma concentrations of vitamin

C (79). Below 30 mg/day, plasma concentrations are around 11 μmol/l. Above this intake,

plasma concentrations increase steeply to 60 μmol/l and plateau at around 80 μmol/l, which

represents the renal threshold. Under near steady state conditions, plateau concentrations of

vitamin C are achieved by intakes in excess of 200 mg/day (Figure 8) (34). At low doses

dietary vitamin C is almost completely absorbed, but over the range of usual dietary intakes

(30–180 mg/day), absorption may decrease to 75 percent because of competing factors in the

food (35, 80).