by Karen Newby, BSc Nutrition
Your body is very clever and absorbs what vitamin C it needs. Anything that isn’t used is then safely excreted, although the body can hold onto vitamin C for a short period in the gut.
There’s much debate about how effectively the body absorbs vitamin C. Some research shows that up to 90% of the vitamin is absorbed at intakes of up to 180mg and reduces to 50% for dosages over 500mg (1). However these studies don’t take into account the body’s individual daily need states, which might be far greater and lead to greater absorption levels. This may be especially true at times of illness or stress when demands for this nutrient are likely to be higher (2).
If vitamin C or any other nutrient is excreted in the urine, it doesn’t necessarily mean it didn’t have a positive effect on the body as it passed through. It actually shows it’s been absorbed! Everything excreted through the urine has been absorbed by the body and then filtered from the blood via the kidneys and out again through the urine. Surely this then poses a massive advantage to the body – with circulating plasma vitamin C the body is potentially better primed to deal with viruses and bacteria than if the circulated vitamin C was low. Even though any excess is excreted, it surely bodes well for the body to have additional levels of this antioxidant should it be required.
So although a pee test might show a high level of vitamin C, perhaps this is still an advantage – it’s been in the blood should we have needed it. So expensive pee, maybe (although vitamin C is relatively inexpensive) but definitely not a waste.
- 127 Mora JR, Iwata M, von Andrian UH. Vitamin effects on the immune system: vitamins A and D take centre stage. Nature reviews Immunology.2008;8(9):685-698.