Stop the diet: Is it better to be bigger?

If you're tubby and tired of being lectured about it, here's good news: the latest research suggests overweight people may be less likely to die early

In the restless world of medicine, where one piece of definitive research is followed within weeks by another that's wholly contradictory, this is the study many of us may have hoped for.

After the World Cancer Research Fund concluded that we should try to stay as slim as possible (within healthy limits) to prevent cancer, a study by the Centers for Disease Control and Prevention (CDC), an American federal agency, says that being chubby may not be so bad.

Actually, the report says more; it says that overweight (though not obese) people – calculated by body mass index (BMI) – have a higher risk of dying from kidney disease and diabetes than people of normal weight, but a lower risk of dying from a range of conditions including emphysema, pneumonia and lung disease. And it found that overweight people are no more likely to die from heart disease and cancer than those of normal weight.

As a whole, the research says that the overweight have a lower mortality rate than people who are underweight, obese or normal. In other words, a little fat appears to have a protective effect. And, the study says, obese people have higher death rates than normal people, but this is driven by a higher incidence of heart disease; they have little higher risk of dying from cancer.

There are nuances in that finding. Obese people were more likely to die from a range of cancers that have been associated with extra weight, such as colon, breast, kidney and pancreas. But overweight people weren't.

Still, for those with a BMI of 30 or more, the message is the same: slim down or risk an early death. For those carrying an extra stone or two, it's less clear. Lead researcher Dr Katherine Flegal says she wouldn't make recommendations to the public "that differ in any way from current public health recommendations to eat right, get some exercise and not smoke". But she admits that "the relation of weight to mortality is a fairly complex issue".

For the public, the issue of weight and health now seems very complex. Official advice is to aim within a "normal" BMI range of 18.5-25. Now, Flegal's research suggests that "normal" may not be optimum in terms of life expectancy.

The discrepancy is partly down to focus. "Our data are based on mortality, not on incidence of disease," Flegal says. "We don't address the probability that someone gets cancer or heart disease, but rather the probability that someone dies from them." One explanation may be that overweight people get ill as much as people of a normal weight, but die of illness less often.

Put simply, during serious illness, overweight people may have additional nutritional resources to draw on. But, according to other CDC researchers, that may not be the only explanation for the discrepancy. In fact, one of the researchers, Dr David F Williamson, questions whether there is any discrepancy at all. "If one carefully examines the graphs in Chapter Six in the World Cancer Research Fund report, it is very difficult to see evidence of a marked increase in either cancer incidence or mortality in the 'overweight' BMI range," he says.

So two studies fail to show a marked link between moderate excess weight and cancer, and the CDC report shows no increased risk of dying from heart disease. But other experts point out that the CDC study only looked at mortality rates, andthat good health is about far more than staying alive.

So health groups are not changing their message: "People who are overweight or obese are more likely to suffer from coronary heart disease, type 2 diabetes, gallstones, high blood pressure and some types of cancer," says Dr Joanne Lunn of the British Nutrition Foundation.

And some question how the CDC report has been interpreted. "Much has been made of the absent or lower risks in the overweight range, but it would be a mistake to interpret this as meaning that being overweight is protective," says Dr Shiriki Kumanyika, a professor of epidemiology and part of the expert panel for the World Cancer Research report. Kumanyika says the processes that lead to cancer in some obese people may have started when they were overweight but only showed up much later.

But there does seem some acknowledgement that, for the mildly overweight, living healthily may be more important than frantically slimming. Lunn says an overweight person should eat well and be active. Preventing further weight gain is, however, important. Judy O'Sullivan, a cardiac nurse at the British Heart Foundation, agrees that, for heart disease, one of the big risks is the ease with which "overweight" can slide into "obese", particularly as we get older."

Dr Mitchell Gail, another CDC report author, is less equivocal. "If you are in the pink and feeling well and getting a good amount of exercise," he told The New York Times, "and if your doctor is happy with your lab values and other test results, then I am not sure there is any urgency to change your weight."