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When I have a cold or otherwise feeling ill, I like to take vitamin C supplements. I've noticed that they almost invariably have amounts way above the recommended daily allowance.

For example, one that I'm looking at now contains 1000mg, where the RDA is 90mg. That is around 11x more than is needed. I've noticed similar things on other vitamin supplements.

What is the purpose in providing such large doses? This answer for example, suggests that 1000mg of vitamin C can actually be harmful. Why not just provide the RDA?

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I don't know why your taking vitamin C supplements when you are ill. Vitamin supplements are usually prescribed when the vitamins in the said person is known to be deficient. They are not prescribed for illness and shouldn't be taken as such as over-dosage causes hyper-vitaminosis or vitamin toxicity. Having said that, I think you meant to say that the RDA is 90mg(as 90g is way too high). I'm aware that this value is different depending on the country as well as the type of person. The reason why vitamin supplements are available in high doses is because these are meant to be used for deficiency syndromes(sometimes they're used as placebos as well). They are meant to replenish the lost stores of vitamins in the body. Also note that 1000mg of oral vitamin C does not mean that your body will absorb 100% of it. Infact in 1000mg of oral vitamin C, only about 50% is absorbed by the body. This is what is referred to as "bio-availability." This is because a series of metabolic activities take place in the body, and about 50% is absorbed and 50% is excreted [please note that bioavailability of different drugs will be different]. There is also no point in making vitamin supplements to contain the exact RDA as required as (a) You would have consumed the RDA equivalent if you are eating a balanced diet (b) Hence there is no need to create a supplement as such (c) For 90mg of vitamin C to be absorbed by the body, the dosage of the supplement should around 125mg(here at lower doses bioavailability of vitamin C is around 70-80%). This does not help people will deficiency syndromes

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  • Thanks for pointing out the error. Going with the "vitamin supplements are only prescribed for deficient persons" theory doesn't explain the high availability of off-shelf multi-vitamin tablets (a shotgun approach, rather than filling in for a specific deficiency). Also, I'm aware of the fact that absorption is lower than what you ingest. But presumably that applies equally to RDA? If we consider 1000mg = 500mg absorbed, then an RDA of 90mg = approx 70mg absorbed. – JBentley May 31 '17 at 8:03
  • Also note that my question about why these supplements contain above the needed amount. Your answer focuses mainly on who should take them, but not on why the dose should be so high for those people. In fact you've even said a supplement should be around 125mg, so we're still missing an explanation as to why they're typically more like 1000 - 2000mg. – JBentley May 31 '17 at 8:06
  • RDA 90mg means you should at least have consumed 125mg(approx) to get that much. If you see labels of food items that show the vitamin contents(example: certain cereals boxes) you'll see that the value is written as a percentage as it represents the approximate percentage of the RDA you're body is most likely going to get(this also differs from person to person) – Chikku May 31 '17 at 9:34
  • Also I think I was not clear on point (c). I merely meant to say that if you want 90mg of Vitamin C to be absorbed you will need to consume approx 125mg. Normal people do this everyday and therefore there is no need for such a supplement. Also for any vitamin deficiency to show it takes a really long time. Its not like starvation that shows in few days after you stop eating . It takes years before signs and symptoms of vitamin deficiency start to show and it also takes months to restore these lost vitamins. Hence a high dose of 1000mg is recommended. – Chikku May 31 '17 at 9:40

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