“Quitters day” falls on January 9 this year and marks the point at which people are most likely to give up their New Year’s resolutions. As usual, weight loss is one of the most common goals. But there are compelling reasons to suggest that ditching the diet is not a failure at all. In fact, it may be a sensible and healthy choice.

The science behind weight and health is not a straightforward story of cause and effect. It is widely assumed that carrying extra body fat automatically makes someone unhealthy, but this does not stand up to scrutiny.

Certainly the health problems associated with higher body weight are a serious challenge in today’s world, but the evidence that a few extra pounds are always harmful is surprisingly weak.

The amount of body fat a person can carry before health problems develop varies widely between people. Biological sex, ethnicity and genetics all play a role. However, the most common way we measure body size, the body mass index or BMI, does not reflect this complexity. BMI is simply a ratio of weight to height and does not measure body fat, fitness, diet, physical activity or metabolic health.

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As a result, it is entirely possible to have a high BMI and be metabolically fit and healthy, meaning that indicators such as blood sugar, cholesterol and blood pressure fall within healthy ranges. Conversely, someone with a BMI categorised as “normal” may still experience poor health.

There is also strong evidence that being underweight can carry serious health risks, particularly in later life. In short, body size alone tells us very little about........

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