People who take a daily vitamin usually assume it is doing something useful for their body. A British doctor says the trial evidence behind most of them is close to empty.

Dr Xand van Tulleken examined the science behind supplements for his new book, Make Me Well. He says he found nothing credible he could recommend to the general population.

He is not against wellness, and says he wanted proof that these products worked. His search began because the claims had started to confuse him.

What the Vitamin D Trials Found

Van Tulleken began with vitamin D, expecting a clear success. When he trained in the late 1990s, it was linked to lower rates of heart disease, cancer, dementia and multiple sclerosis.

Those links came from observational studies, which watch people rather than test a treatment. Correlation is not causation, and the later trials told a different story.

In the major trials, supplements appeared to do nothing except prevent deficiency, which causes conditions such as rickets. These are now very uncommon in the UK, he says.

The NHS still advises that everyone over the age of one should consider a 10mcg supplement daily in autumn and winter, when sunlight drops. Van Tulleken calls that advice 'excellent'.

The aim of that advice is to stop anyone becoming seriously ill from deficiency, he explains. It was never designed to make a healthy body work any better.

Negative findings often go unpublished because a study that shows nothing can feel like a failure. That leaves patients with a rosier picture than the science supports.

Why High Doses Can Harm

More is not better. Randomised trials cited by the American Medical Association found that 2,000 international units of vitamin D a day did nothing for fractures, falls, cancer or heart disease.

Other randomised trials suggested that doses of 4,000 units a day may contribute to bone loss, while some high-dose regimens may raise the risk of falls. Strengths of 5,000 to 10,000 units sit openly on shop shelves.

Vitamin E carries a sharper warning. A clinical trial of more than 29,000 men found the supplement raised prostate cancer risk rather than lowering it.

'Natural' does not mean harmless either. The herbs comfrey and kava can seriously damage the liver, and vitamin K reduces the effect of some blood thinners.

Supplements in the United States are regulated as food, not medicine, so they need no proof of effect. The Food and Drug Administration estimates more than 100,000 products are on sale.

When Supplements Do Help

Some supplements do work, but for narrow medical reasons. Folic acid in pregnancy helps prevent neural tube defects, a spinal problem, and iron treats iron deficiency.

Vitamin B12 comes mostly from meat, fish and dairy, so vegans may need it. Those whose gut has been altered by surgery may need supplements for life.

The National Institutes of Health reports that more than half of Americans take supplements regularly or occasionally. Its advice is that a varied diet supplies what most people need.

Patients should still tell their doctor what they take. Some products cause side effects, clash with prescribed medicines or create problems before surgery.

Sport is one of the few areas with positive evidence, though the gains are small. Sports nutrition guidance backs only a short list of products.

Even then, van Tulleken was told to fix sleep, diet and training first. Only after a full season should an athlete consider supplements.

For everyone else, his advice is blunt. He suggests walking away from the supplement aisle and eating whole foods, because the body seems to take up nutrients better.

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Frequently Asked Questions

  • What did Dr Xand van Tulleken find about vitamin supplements?
    Dr Xand van Tulleken found that most vitamin supplements do not have credible evidence supporting their effectiveness for the general population.
  • What is the NHS advice regarding vitamin D?
    The NHS advises that everyone over the age of one should consider a 10mcg vitamin D supplement daily in autumn and winter to prevent deficiency.
  • Can high doses of vitamin D be harmful?
    Yes, high doses of vitamin D above 4,000 units a day can cause bone loss and increase the risk of falls.
  • Are there any supplements that are beneficial?
    Yes, some supplements like folic acid in pregnancy and iron for iron deficiency are beneficial for specific medical reasons.
  • What should patients do before taking supplements?
    Patients should inform their doctor about any supplements they take, as some can cause side effects or interact with prescribed medicines.