I grew up thinking it was dangerous to eat more than three eggs a week. That’s not true. I thought salt would give me high blood pressure. That’s not true either. I believed margarine was healthier than butter. Guess what? Not true. Why did I believe all those things? Because I was told, over and over again, that this is what health experts said.

Some health experts did make those claims. The problem is they jumped the gun. And they were helped by activists (who’re often unscrupulous) and journalists (who often don’t understand science). Just because a study suggests some food is bad for you doesn’t mean it’s true.

There could be all sorts of other factors involved. That’s why experiments have to be repeated several times, by different teams, and checked carefully before researchers can be sure the results mean anything important. Unfortunately that’s not always what happens.

It’s easy for someone to push their own agenda by selectively quoting results – picking the studies that back their opinion, and ignoring the ones that don’t. This doesn’t happen much in mainstream science – nobody’s challenging the theory of gravity, and outside the USA and the Islamic world evolution is completely uncontroversial – but it’s rife in health and medical science. A lot of the researchers in this field are also activists and they use the science to advance their own goals.

If you look at many controversies in health you’ll find the same names popping up over and over again. If there’s a story about how sugar is toxic and addictive (it’s not) you’ll probably hear from Robert Lustig in the USA, or Aseem Malhotra in the UK. When electronic cigarettes hit the headlines the go-to experts are Stanton Glantz and Martin McKee.

Most other hot-button issues have similar figures that pop up in the media every time the subject is mentioned. It’s natural to assume they’re the leading researchers, but are they? Lustig is an endocrinologist, but his views on sugar are far from mainstream. Malhotra is a cardiologist, so not an expert on nutrition.

McKee is an epidemiologist, while Glantz doesn’t even have a medical degree – he studied aeronautical engineering.

The people who give interviews about the latest health problem aren’t usually doing it because they’re the leading experts on the subject. Often it’s simply because they have the snappiest soundbites and the best network of media contacts. This gives them immense power to push their own beliefs, which are usually sincere but also often completely wrong. And this has the power to cause immense damage to public health.

Too much salt is bad for you, but how much is too much? There’s growing evidence that the federal government’s recommended limit of 2,300mg a day isn’t just unnecessarily low; it could be dangerously low.

A recent study from Denmark found that risk of premature death goes down if you eat twice that much, and a British team linked low levels of chloride – a main component of salt – to a 20% higher risk of dying early.

Then there’s saturated fats. Too much of any kind of fat is bad for you, but in the 1970s and 1980s saturated fat – mostly found in meat and dairy – was particularly demonized. The result was people (and the food industry) switched en masse to trans-unsaturated fats, by for example replacing butter with margarine. The problem is that saturated fats aren’t uniquely dangerous, but trans fats are. The best estimate is that they now cause 30,000 heart disease deaths in the USA every year; deaths brought about by bad health advice.

Many medical historians now believe that while we live longer than the Victorians did they stayed healthy longer than we do, despite having a diet that, by modern standards, was appalling. But 19th century doctors focused on a balanced diet, not on trying to eliminate one particular nutrient as a magic bullet solution. Their 21st century counterparts seem obsessed with the search for that magic bullet, but too often they miss the target with lethal results.

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