Dr Rajendra Pachauri, who chairs the UN Intergovernmental Panel on Climate Change, wants us to eat less meat.

Pachauri is an economist (and a vegetarian) who believes that reducing meat consumption could also cut greenhouse gas emissions. The idea seems to make sense, since about one-fifth of global emissions are produced by the meat industry. I am already a semi-vegetarian: I avoid meat and only occasionally eat fish. This may be good for the environment, but is it any good for my health?

The question has always been hard to answer scientifically, for a number of reasons. Studies on the relationship between diet and health are often conducted retrospectively, which can cause problems. When, for example, a victim of a heart attack is asked about his or her dietary history, the response will be affected by “recall bias”. This means that present habits and events may influence perception of the past. In addition, it can be hard to ensure that individuals’ vegetarian diets are similar enough to act as a uniform comparison to carnivorous ones.

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