“FIBRE diets don’t prevent cancer,” many headlines proclaimed this week. They were prompted by the publication, in the Journal of the Amercian Medical Association, of a study of pooled data involving 750,000 people, which showed little or no association between diets high in fibre and reduced risk of colorectal cancer.

For those forced to endure bowlfuls of the cardboard box shavings popularly known as muesli, it was an excuse to wave goodbye to regimens where pulses and flatulence reigned supreme. But for those weaned on the idea that fibre was good, it was another puzzling health reversal.

How come scientists seem to get it so wrong? Depressingly perhaps, it’s because they are right only until someone else proves them wrong.

So how does this process normally work? A researcher puts up a hypothesis, which may be based on an observation — with fibre, it was that people living in countries with high-fibre diets have lower rates of colon cancer — and then thinks of ways to prove or disprove it. But there are many ways in which results can mislead.

Size is one. For instance, in the mid-1980s a number of small trials suggested that low-dose aspirin might reduce pre-eclampsia, a potentially fatal disease of pregnancy, by as much as 70 per cent. There was great excitement and talk of giving aspirin to all pregnant women. But when the Medical Research Council instituted an international trial called Clasp, involving 9,000 women, the benefit was shown to be minimal except for those at the highest risk. This process of discovery took well over a decade.

Another factor, is whether what you are seeing is a true effect. Early trials seemed to suggest that hormone replacement therapy (HRT) prevented heart disease. But the women in these studies chose to take HRT. They also looked after themselves, exercised and ate healthily. This was protecting them from heart disease, not HRT.

There’s a similar argument with fibre. High-fibre diets are likely to be a sign that people are eating more fruit and veg and living a healthier lifestyle. So how can you tease out whether it’s the fibre alone that’s preventing colon cancer? In most studies, you can’t.

Confused? Luckily there is one area of certainty. In a study of more than 100,000 nurses in the US, there was an 85 per cent reduction in mortality (and this for women in their mid-forties) between the most healthy subjects and those who smoked, drank, ate a terrible diet and took no exercise. Yep, live healthily and you’ll live longer.

With this evidence behind us, we know that Christmas indulgence comes with a health warning. But be aware, this advice may change.

WELL, WELL...

“The best I can give you is a full tin of baked beans on brown toast, lightly buttered”
Melvyn Bragg, broadcaster, on his cooking skills

“The NHS has two options, either radical reform to improve productivity or local rationing, rising waiting lists and falling staff morale”
Professor Nick Bosanquet, health economist, Imperial College

“Just the pulse in my fingertips could have destroyed the whole thing, so I had to put myself into a meditative state before I began”
Willard Wigan, who created a 0.005mm tall sculpture of Santa

“If a woman thinks to herself, ‘If I buy this dress my friends are going to love it and my status will increase’, then she essentially believes it is helping her survival”
Felix Economakis, NHS psychologist

“The obesity epidemic is totally out of control and Scotland is worse than England. This is more than just a warning signal; it’s a red light” Professor Philip James, head of the Obesity Task Force

“I’d like to live a long life and then get three weeks’ notice”
Steve Coogan, comedian

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