Millions of people are confused by health advice. It appears so contradictory that the simplest thing is to disbelieve it all.

Nowhere is this truer than in advice over diet. This week the World Cancer Research Fund stigmatised bacon – along with other processed meats – by advising those who want to avoid cancer to cut it out of their diet. What’s their beef? They have to be kidding, surely?

At issue here is the whole question of how we know what we know – what philosophers call epistemology. So this is a page about epistemology, a lovely word that seldom creeps into even a newspaper as upmarket as The Times.

Where do all these claims about diet and health come from?

They come from studies launched by scientists to try to unravel the causes of disease. We know that many diseases are caused by germs, but thanks to vaccines and antibiotics most of these infectious diseases are now under control. We are left with the diseases caused by age, diet and lifestyle: principally heart disease and cancer, which between them are the cause of more than half of all deaths in developed countries.

Hang on. You’ve just said that heart disease and cancer are caused by age, diet or lifestyle, without any evidence. How do we know that?

Both are commoner in older people than younger ones. And both are commoner in some communities than in others, while some lifestyle links – between smoking and both cancer and heart disease, for example – have been well proven. So it is certainly a valid hypothesis that there are features of modern life and diet that contribute to disease and it is worth trying to find out what they are.

What’s the best way to do that?

The best way would be the way that new medicines are tested, in a double-blind placebo-controlled trial. One group would be fed on the food under suspicion, the other given a matching but harmless placebo, and they would be followed until they developed cancer, or died. Neither group would know which they were getting, nor would those responsible for running the trial, to avoid accidental bias. This is the gold standard, but it’s entirely impracticable in most cases for dietary studies in free-living human beings. Life’s too short, especially if you are in the group randomised to bacon.

And the next best?

There are many alternative ways of studying dietary effects, generally known as observational studies. They fall into two broad groups: cohort studies, and case-control studies. They have their strengths, and weaknesses.

An example, please?

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