Vitamin C (ascorbic acid) can be controversial. There are debates about how much is safe to supplement and a widespread lack of knowledge about how it is stored and used in the body.
Vitamin C in the body.
Vitamin C is used in the body in a number of ways:
- Formation of collagen, a connective tissue protein that is present throughout our body. Vitamin C contains 4 hydroxyl groups (HO), which can be added to collagen, changing its conformation and making it stronger. Collagen is involved with the function of blood vessels, tendons, ligaments and bones as well as contributing to a firm skin tone. Without it we would wrinkle up and our blood vessels would start leaking.
- Formation of norepinephrine a neurotransmitter that affects our mood and our ability to get going. Norepinephrine levels are increased by some anti-depressant drugs.
- Synthesizing carnitine a molecule that transports fat into our mitochondria that improves our ability to burn fat. Lack of carnitine could contribute to fatigue, one of the early symptoms of the vitamin C deficiency disease, scurvy.
- As an anti-oxidant that prevents our cells from being damaged by harmful free radicals. Its ability to regenerate other antioxidants such as vitamin E, makes it particularly useful in this regard.
From the above list you will probably conclude that it would be beneficial to have sufficient vitamin C in your body at all times.
Vitamin C storage.
Vitamin C is a water soluble vitamin, and I've heard people say that it is easily excreted from the body in our urine. This is not true. In fact vitamin C is retained in our body by most of our organs and also in our blood plasma. Levels in blood plasma are kept to about 1.4mg/dL. If they go below this level the kidneys then reabsorb any vitamin C that passes through them. If it goes much above this then kidney reabsorption decreases and more vitamin C is excreted. Our organs try to store vitamin C at levels well above that in blood plasma. For instance the adrenals, pituitary, thymus and retina all have more than 100 times the concentration of vitamin C than blood plasma.
What we see then is that our kidneys are tightly controlling the amount of vitamin C that is present in the body based on our vitamin C status. Our vital organs need a fair amount of the stuff to be present and if it is not then our plasma levels drop and we start conserving more vitamin C. However we do need to have enough of it. The question is how much is enough.
The recommended daily amount of Vitamin C.
The RDA of vitamin C used to be 60mg and has now gone up to 90mg. RDAs are normally set at levels that are sufficient to prevent overt deficiency symptoms in 95% of the population. When you think about it, that means it must be far below what people should be having. Why is that? Well consider overt deficiency, which in the case of vitamin C is called scurvy. This overt deficiency disease occurs over a period of months. The RDA takes no account of long term disease processes such as heart disease which are also thought to be affected by vitamin C status. Also, if you are just above the overt deficiency threshold you are in danger of falling below it if you get injured or ill. Any physical trauma tends to lower vitamin C levels quite markedly1. Smoking counts as physical trauma for these purposes also, and it has been shown that despite the fact that most smokers also consume less vitamin C, they also have lower levels due to their smoking alone2. It is also worth noting that frequent aspirin use and the birth control pill both decrease vitamin C levels.
It is also commonly thought that you can overdose on vitamin C, with symptoms such as kidney stones being frequently mentioned. The evidence for this is poor with many studies showing no increased likelihood for kidney stones even with regular daily intakes of greater than 1500mg. Even greater amounts of vitamin C are regularly used intravenously by some practitioners of complementary medicine, for instance 28,000mg every four days. The practice is fairly widespread for the treatment of cancer, fatigue and chronic infection. Investigations into the practice of intravenous megadosing of vitamin C seem to indicate that it is surprisingly safe, with very few people experiencing any negative side effects3. Both my experience and research indicate that high levels of supplemental vitamin C are safe for all but people with kidney insufficiency. People with the common male hereditary disorder, G6PD deficiency, may also be adversly affected, but serious complications for them are highly unlikely. Having said the above, quite a few people will experience stomach distress such as nausea and diarrhoea if they take large quantities of oral vitamin C. The best advice is to try it and limit it to below the level at which any stomach distress occurs.
Why do we need vitamin C?
Vitamin C is synthesized by most animals. However humans, chimps, monkeys, bats, guinea pigs and capybara are among a few that don't. It is interesting to note that the animals that do synthesize vitamin C produce one heck of a lot of it. For instance, goats weighing the same as a slim human adult, have been reported as producing 13g of vitamin C per day. That is 140 times more than the current RDA. Equally other primates, who cannot produce their own vitamin C reportedly consume at least 10 times as much vitamin C in their diet as the current RDA. While some of the functions of vitamin C may be carried out by uric acid in humans it seems very likely given the evolutionary evidence from other animals that we should also be consuming a lot more than the current RDA.
Vitamin C is also associated with reduced risk of some major illnesses. Risk of heart disease is likely reduced by 25% or more in those who supplement more than 400mg daily4. Risk of cancer is also likely reduced as vitamin C can help to starve a rapidly growing tumour of its oxygen supply5. Gout is one other condition where there is good evidence that supplemental vitamin C can help in prevention or reduce symptoms4.
So how much is recommended?
Given the evidence above I am convinced that vitamin C should be consumed at amounts significantly greater than the current RDA. My recommendation for most people is to consume a diet containing plenty of vitamin C. Depending on how much you eat somewhere between 300-600mg per day from fruits and vegetables would be a reasonable aim. For those with conditions that may benefit from supplemental vitamin C amounts greater than 1g (1000mg) may well be worth trying.
Does vitamin C prevent colds?
Vitamin C has a mixed record in trials to demonstrate its ability to prevent colds or reduce their duration. However a lot of the trials have tested amounts as little as 200mg/day, which may be too small to be effective anyway. Trials with those under duress such as people doing hard exercise or who have been ill do seem to help prevent colds and reduce their duration and severity. For the ordinary couch potato it seems that prevention is not so effective, but that the severity and duration of colds can be reduced by regular supplementation. My own preference is to take a mixture of vitamin C and zinc at the first sign of a cold, see here for more on this.
Where can I get my C?
Vitamin C is available primarily from plant sources. The list below shows the amount of vitamin C available in 100g of certain plants high in vitamin C:
- Peppers 150-250mg
- Parsley 130mg
- Kiwi fruit 90mg
- Greens 30-90mg (Broccoli 90mg, spinach 30mg)
- Oranges 50mg
- Banana 9mg, Apples 6mg - so oranges have far more than banana and apples!
Vitamin C is also available from animals although not in as great an amount as many plants:
- Liver 12-36mg
- Oysters 30mg
- Milk 2-4mg
- Meat <0.1mg so no good for vitamin C