Vitamin C, or ascorbic acid, is a water soluble vitamin whose chemical structure is closely related to glucose. Most animals can, in fact, synthesise vitamin C from glucose and galactose sugars, but humans lack the enzyme (l-gulonolactone oxidase) needed for this conversion.
The only other animals who are unable to synthesise their own supplies of vitamin C are:
- Primates
- Guinea pigs
- The Indian fruit bat
- The Red-vented bulbul (an Asian songbird)
- Rainbow trout
- Coho salmon
- A single strain of Japanese laboratory rat.
Why our ancestors lost or never acquired the ability to synthesize vitamin C is one of the greatest mysteries of human biochemistry. It is thought to have resulted from a genetic accident occurring around 60 million years ago which, according to some scientists, mean we all suffer from a genetic disease, named hypoascorbaemia. This genetic defect may increase our risk of a number of common illnesses such as viral infections, raised cholesterol levels, coronary heart disease and cancer as well as reducing our ability to cope with stress. The goat, for example, which weighs around the same as an adult man, produces between 2g and 13 g of vitamin C per day depending on its levels of stress and illness.
Because our primitive ancestors ate a vegetarian diet full of vitamin C-rich plants such as purslane, their vitamin C intake was higher than in the modern diet at an estimated 392 mg a day, allowing them to survive despite this genetic defect.
Why You Need Vitamin C
Vitamin C is an important antioxidant, especially the eye lens. It regenerates other antioxidants, such as vitamin E, and is needed for at least 300 metabolic reactions to proceed normally. It is essential for conversion of the amino acid, proline, to hydroxyproline during collagen synthesis and is also involved in the metabolism of stress hormones and the regulation of immune reactions.
The presence of vitamin C in food increase absorption of iron in the gut by converting ferric iron (Fe3+) to ferrous iron (Fe2+).
Within the EU, the European Food Safety Authority has authorised health claims that vitamin C contributes to:
- The normal function of the immune system during and after intense physical exercise
- Normal collagen formation for the normal function of blood vessels, bones, cartilage, gums, skin and teeth
- Normal energy-yielding metabolism
- Normal functioning of the nervous system
- Normal psychological function
- Normal function of the immune system
- The protection of cells from oxidative stress
- The reduction of tiredness and fatigue
- The regeneration of the reduced form of vitamin E
- Increased iron absorption.
Food Sources
Vitamin C is mainly found in fruit and vegetables, especially:
- blackcurrants
- berries
- guava
- kiwi fruit
- citrus fruit
- mango
- capsicum peppers
- green leafy vegetables.
Animal sources include kidney, liver and whale skin, which is an important source within the Inuit diet.
Vitamin C is unstable, however, and up to two thirds is lost during food processing, cooking and storage. Fruit juices rapidly lose their vitamin C content when exposed to air, even if chilled.
Vitamin C Deficience
Lack of vitamin C causes scurvy, a deficiency disease associated with poor collagen production, poor wound healing leading to bleeding skin and gums and loss of teeth. If untreated, scurvy is fatal. A minimum daily intake of 10mg vitamin C is needed to prevent this – although 20 mg per day is needed for proper wound healing.
Symptoms that may be due to vitamin C insufficiency
- frequent colds and other infections
- dry, rough, scaly skin
- broken thread veins around hair follicles
- dry, fissured lips
- muscle and joint pain
- fatigue
- weakness.
Symptoms that may be due to major vitamin C deficiency
- poor wound healing
- easy bruising
- anaemia
- misshapen, tangled, brittle, ‘corkscrew’ hair
- hair loss
- inflamed, swollen, bleeding gums,
- loose teeth
- bleeding skin, eyes and nose
Vitamin C and General Health
Diabetes complications: Vitamin C has beneficial effects on glucose control, yet people with diabetes have circulating levels of vitamin C that are around 30% lower than in people without diabetes, suggesting that requirements are increased to help offset the metabolic abnormalities associated with this condition.
When blood glucose levels are raised, some glucose is converted to sorbitol inside cells which contributes to diabetic complications affecting the eyes (retinopathy, cataracts) and nervous system (peripheral neuropathy). Vitamin C reduces sorbitol formation by blocking one of the enzymes needed to convert glucose to sorbitol (aldose reductase). Taking 1g vitamin C per day for just 2 weeks can reduce the amount of sorbitol within red blood cells by over 12%. Taking 2g vitamin C daily reduces the build-up of sorbitol in red blood cells by 44.5% in people with diabetes.
Vitamin C also improves blood vessel dilation and blood flow in people with diabetes, which is often impaired due to free radical damage.
Cardiovascular disease: Vitamin C protects cholesterol in the blood stream from oxidation to help protect against heart attack and stroke. Vitamin C also improves the liver’s ability to convert cholesterol into bile acids.
A study in Norfolk, UK, involving over 20,000 adults found that those with the highest blood levels of vitamin C from fruit and vegetables were 22% less likely to have high blood pressure than those with low levels, and were half as likely to die from any cause, including cardiovascular disease, during the follow up period of four years. However, a large analysis of data from 15 trials, involving over 188,000 people found that, overall, antioxidant vitamin supplements appeared to have no significant effect on risk of heart attack or stroke.
Cataracts: Vitamin C is an important antioxidant in the eye, and the level of vitamin C in the lens is 60 times that found in the circulation. A number of studies suggested that long-term consumption of vitamin C supplements might reduce the development of age-related cataracts. However, a recent large analysis of data from 9 trials, involving over 117,000 people aged 35 plus, concluded there was no evidence that antioxidant supplements (betacarotene, vitamin C or vitamin E) could prevent or slow the progression of age-related cataracts.
Common colds: Vitamin C is one of the most popular supplements to prevent or treat the common cold. Vitamin C suppresses the activation of viral genes so the virus cannot survive when high vitamin C levels are present. Studies involving school children and students suggest that vitamin C can reduce the risk of catching a cold by as much as 30%. Heavy physical exercise increases the likelihood of respiratory infections, and vitamin C supplements can protect military troops and endurance athletes in training by halving the risk of developing cold symptoms at doses of 600mg to 1g vitamin C per day. A large analysis of data from 29 trials involving 11,300 people, also showed a consistent reduction in the duration of a cold by 8% for adults and 14% for children. The severity of cold symptoms was also reduced.
Gallstones: Vitamin C affects the breakdown of cholesterol to bile acids and, in women, may reduce the risk of symptomatic gallstones by 13%. A similar relationship was not found for men, however.
Gout: Vitamin C can lower uric acid levels. Data from 13 trials, involving 556 people with recurrent gout showed that taking 500mg vitamin C per day significantly reduces serum uric acid levels. A study involving almost 47,000 males measured vitamin C levels every 4 years for 20 years. Compared with men who had vitamin C intakes of less than 250mg per day, those with intakes of 500mg – 999mg per day were 17% less likely to develop gout; those with intakes of 1000 – 1499 mg per day were 34% less likely to develop gout, and those with intakes of 1500mg per day or more were 45% less likely to experience gout.
Osteoporosis: Vitamin C is essential for the synthesis of collagen, a structural protein that makes up 30% of bone volume. Vitamin C stimulates bone-building cells (osteoblasts), enhances vitamin D activity and boosts calcium deposition in bone. In postmenopausal women, those with osteoporosis have significantly lower dietary vitamin C intakes than those without osteoporosis.
Osteoarthritis: Vitamin C may reduce the risk of cartilage loss and disease progression in people with osteoarthritis. The risk of having osteoarthritis, for example, is 1.9 times reduced in people who obtain recommended intakes of vitamin C, or higher, than those with lower intakes.
Sperm health: Vitamin C is found in semen at concentrations eight times higher than in the circulation. It is vital for sperm health in stopping sperm from clumping together and in protecting the genetic material (DNA) of sperm against oxidative damage. Taking 1g vitamin C twice a day for 2 months has been shown to increase average sperm count from 14.3 x106 per ml to 32.8 x 106 per ml; the number of sperm with normal morphology from 43% to 66.7% and the number of sperm with normal motility from 31.2% to 60.1%.
Skin damage: Ultraviolet light damages skin so that long-term sun exposure causes photo-ageing with skin becoming increasingly thickened, yellow, scaly, mottled and wrinkled with a coarse, leathery texture. Vitamin C protects skin during UV exposure to reduce sunburn and is now added to many cosmetic creams designed to slow the visible signs of skin ageing.
How Much Vitamin C Do You Need?
The EU RDA for vitamin C is 80mg and the US DV is 60mg.
Requirements increase during pregnancy and lactation.
It has also been suggested that smokers should consume an additional 35 mg vitamin C per day than non-smokers to help offset the increased oxidative damage caused by exposure to cigarette smoke.
Typical Intakes
The average Western diet supplies 64 mg vitamin C per day.
Supplements
The efficiency of vitamin C absorption depends on the amount consumed. At intakes of up to 200 mg as a single dose, absorption of vitamin C is almost complete through an active transport process. At single doses of over 500 mg, it is also absorbed through a process of diffusion which is less efficient so that only around half of a 1.5g dose is absorbed, and only 16% of a massive 12g dose.
Safety
Some vitamin C is metabolised to oxalic acid, but claims that large doses could trigger oxalate kidney stones have proved unfounded. However, recurrent stone formers who have a defect in ascorbic acid or oxalate metabolism, and people with renal failure are usually advised to restrict daily vitamin C intakes – for example, to no more than 100 mg.
Large doses of vitamin C may trigger indigestion or diarrhoea. This is largely due to the acidity of vitamin C rather than a specific sign of toxicity. Some people are more sensitive to the acidity of vitamin C than others. Changing to a supplement form know as ester-C (which is non-acidic) can overcome this effect.
Individuals with the iron-storage disease, haemochromatosis, should not take vitamin C supplements except under medical advice.
Suddenly stopping high-dose vitamin C supplements can cause temporary symptoms of deficiency (rebound scurvy) when enzymes activated by high levels of vitamin C are suddenly deprived of the extra vitamin C they need to work properly.
The European Food Safety Authority has determined a tolerable upper intake for long-term use from supplements as 1gram.
By Dr Sarah Brewer, originally published on www.drsarahbrewer.com. Twitter: @DrSarahB
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