One in three women now takes supplements, but certain vitamins and minerals can be harmful in high doses says Carina Norris
One in three women now takes supplements, but certain vitamins and minerals can be harmful in high doses. Registered nutritionist Carina Norris sheds some light on the matter
Vitamins and minerals are a doubleedged sword – getting enough protects us from deficiency symptoms, as well as reducing our risk of serious chronic diseases such as heart disease, strokes and cancer. But anything that’s powerful enough to do you good can harm you when taken in excess.
The Committee on Medical Aspects of Food and Nutrition Policy (COMA) and the Department of Health have devised recommendations for how much we need of the main micronutrients – these are called Dietary Reference Values or DRVs. But new research is constantly emerging suggesting that increased dosages – higher than you could get from food – could protect us from disease. So, should we consider the DRVs a bare minimum and think about taking more in order to gain extra benefits?
Many of us do, as one in three women (and one in four men) take supplements. But are they safe? This debate was thrown into sharp relief by a 2008 review of the scientific literature by the Cochrane Collaboration, on the subject of antioxidants (beta-carotene, vitamin A, vitamin C, vitamin E and selenium) for reducing cancer risk. The big surprise was the lack of evidence that supplements reduced the risk of dying. Taking beta-carotene, vitamin A and vitamin E actually increased mortality rates. And while there was no evidence that vitamin C and selenium did any harm, they appeared not to help, either.
There are two levels of safety recommendations, set by the Expert Group on Vitamins and Minerals (EVM). ‘Safe Upper Levels’ are the amount that a healthy person could take every day on a life-long basis. But for some nutrients there isn’t enough scientific data available to calculate a Safe Upper Level – and for these, the EVM issues ‘guidance’ on how much we could expect to take safely, based on the limited data available.
The uptake of some nutrients depends on the levels already in our bodies, so it’s harder to overdose. With the water soluble vitamins (B and C), most of the excess can be flushed out in our urine if we take too much. But the fat soluble vitamins (A, D, E and K), plus many minerals, can be stored in the body, accumulating over time to potentially harmful levels.
With the watersoluble vitamins most of the excess can be flushed out in our urine if we take too much, but the fat-soluble vitamins can be stored in the body, accumulating over time
These can accumulate in the body. But they pose no danger if you stick to the safe limits.
Problems occur when retinol (the ‘pre-formed’ form of vitamin A) is taken over a long time, building up stocks too great for the liver to destroy or store. Research suggests that longterm dosages over 1,500ug daily can reduce bone density, increasing your osteoporosis risk.
All red meat is a good source, but liver or liver products such as pâté are super-rich in vitamin A. Many multivitamins contain vitamin A and fish liver oil is also very high in retinol. So, if you eat a vitamin A-rich diet, be careful with supplements.
Too much vitamin A is linked with birth defects, so pregnant women should avoid eating liver or liver products and avoid taking supplements containing vitamin A.
The body converts this phytochemical into usable vitamin A, but only as and when we need it, so it’s a safer way of getting your vitamin A. There’s no evidence that beta-carotene from food could be harmful, but some research showed an increased incidence of lung cancer among smokers who were taking beta-carotene supplements (20-30mg, alone and in combination with other nutrients). There’s no data suggesting that beta-carotene is harmful to nonsmokers, but to be safe, the Food Standards Agency says don’t take more than 7mg daily, and smokers are advised not to take it at all.
There’s no evidence that doses up to 20ug daily cause side effects, and long-term supplementation up to 25ug appears to be fine for most people. However, one study suggested that 25-50ug may increase kidney stone risk in susceptible people.
Although this is accepted as a very ‘safe’ vitamin, high doses can interfere with the body’s use of other fatsoluble vitamins, as well as causing headaches, nausea and muscle weakness.
Vitamin B1 (thiamin)
Dosages in the 1,000s of mg can sometimes cause symptoms such as headaches, nausea and irritability.
Vitamin B2 (riboflavin)
Riboflavin has a low absorption rate in the gut, so many times the minimum recommendation can be taken.
Vitamin B2 (niacin)
Overdoing the DRV of niacin (which is generally taken as nicotinic acid or nicotinamide) by just three times can cause flushed and itchy skin, nausea and vomiting. The EVM guidance says stick to less than 17mg nicotinic acid supplements or 500mg or less of nicotinamide.
Too much vitamin B6 can cause a loss of feeling in the arms and legs, known as peripheral neuropathy. This can be caused by one-off doses of 200mg, or lower doses (10 – 200mg daily) over the long term.
Folic acid (folate)
Pregnant women and those planning a pregnancy should take a 400ug supplement to reduce the risk of neural tube defects in their baby. The EVM guidance states a maximum of 1,000ug daily.
Elderly people are less able to absorb this nutrient, and some nutritionists recommend they take more than the minimum recommendation (pregnant and breastfeeding women also have higher requirements).
Vitamin C is involved in immunity and wound healing. Its antioxidant properties reduce our risk of cardiovascular disease and cancer and many nutritionists believe the recommendation for this vitamin is too low. Also, because it’s water-soluble, it’s important to keep our levels topped up.
We’re advised to get at least 40mg daily (slightly more for pregnant and breast-feeding women), but supplements commonly provide up to 1,000mg. The main side effect from high doses is diarrhoea. Vitamin C is very safe – in two supplementation trials up to 1,000 mg/day was taken for up to five years without side effects.
Intake should be below 6g salt or 2.5mg sodium daily because of its blood pressureraising effect, which can increase our risk of heart disease and stroke.
The body controls calcium metabolism so tightly that it’s virtually impossible to take too much. Taking more than the EVM guidance could lead to diarrhoea and stomach pain.
Dosages from 50 to 220mg daily can cause constipation, nausea and vomiting. Up to 17mg is safe for most people, but some need more. For example, iron deficiency anaemia is common in pregnancy, and your doctor may prescribe a higher dose.
Too much causes stomach cramps and nausea, especially if taken on an empty stomach.
There’s little data on magnesium supplements. Taking too much for a short time can cause diarrhoea, and we don’t know the effects of high intakes long-term.
The EVM’s guidance is that up to 10mg of the trivalent form of chromium would be safe. Less is known about the chromium picolinate form.
|FOOD COMBO||WHY IT WORKS|
|Baked beans on toast with a glass of orange juice||The vitamin C in the orange enhances the uptake of the iron in the beans|
|Veggie chilli made with tinned tomatoes||The vitamin C in the tomatoes enhances the uptake of iron from the beans|
|A salad of sliced tomato with a walnut oil dressing||The oil enhances the uptake of the phytonutrient lycopene from the tomatoes|
|Baked egg custard||The vitamin D in the egg yolk enhances the uptake of the calcium in the milk|
|Steamed broccoli drizzled with olive oil||The fat in the oil helps the absorption of the vitamin E in the broccoli|
|Wholemeal toast with olive spread||The fat in the spread enhances the absorption of the vitamin E in the whole wheat grains|
|Tofu and spinach stir-fry with sesame seeds||The tofu is a rich calcium source, while the spinach and seeds are also good for magnesium, enhancing calcium uptake and use in bone-building|
|Cheese omelette||The cheese is good for calcium, while eggs provide vitamin K, which helps with calcium uptake|
Some points to bear in mind
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