Supplements and their side-effects

• Vitamin C: Many people take many times more than the recommended daily dose without problems. But very high doses - more than 1g (1,000mg) -can cause diarrhoea.

• Beta-carotene is converted to vitamin A in the body. Long-term overdose can lead to bone-thinning. Vitamin A overdose is very dangerous for pregnant women, because of the effects on the unborn baby. For this reason pregnant women are advised not to take vitamin supplements containing vitamin A (or fish oil supplements, which are rich in the vitamin), and not to eat liver.

• Calcium: Very high doses (more than 1,500mg a day) can lead to stomach upsets.

• Iron: Too much can cause constipation. The official guidance is that 17mg per day should be safe for most people, but some people actually need more, for example if they are being treated for anaemia.

• Zinc: Too much can cause stomach aches and nausea.