Fat people eat too much don’t they? Or is it that their bodies don’t burn energy as fast as those of slim people? Are they less able to convert it into heat? Did nature mean them to be bigger?

In other words, are you overweight because you eat too much, or because you have a slower body metabolism that makes you fat?

The study of obesity has been riddled with myths and difficulties, and despite reams of learned papers, these questions have remained unanswered among scientists as well as the public.

Now a group of babies has helped researchers to provide the first really clear-cut answers. Doctors at the Medical Research Council’s Dunn Nutrition Unit in Cambridge, using new techniques for measuring human energy intake and expenditure in an ultra-precise way, have shown that early in infancy babies who are later destined to become fat eat about the same as babies who grow to an average size.

This lets the mothers of many fat babies off one guilt inducing hook: that of being accused of over-feeding their offspring.

Fat Babies Expend Less Energy

The study, which followed the babies during their first year of life, showed that those babies which put on more weight expended less energy than those whose weight stayed normal, even though they were all eating the same.

But the study also discovered that at three months, while the babies were still the same size and their fat or thin propensities had not yet been expressed, both groups burned exactly the same amounts of energy while they were asleep.

This suggests that it was not different “resting” metabolisms that were the reason for the differences in weight that emerged but different levels of energy expended when the babies were active: the ones who would become fat were less lively when awake, and therefore burning off fewer calories, than the “thin” group.

Is Obesity Inherited?

The finding that the eventual chubbiness of the “fat” babies was not due to their eating any more than usual has sparked worldwide interest.

The researchers found similar lower rates of energy expenditure among family members, in agreement with previous studies showing that a tendency to become obese is inherited.

Controversy continues, however, over whether the study results mean nature or nurture is responsible for obesity. In America, the findings by Lucas, a consultant pediatrician, and his coworker, Dr. Susan Roberts, have been seized upon as vindication for the theory that the tendency to obesity is innate – that is, that some people are born to be fat.

The reason why some people seem to put on weight with unfair ease is that once gluttony in adult life exceeds a certain level, the body changes in ways that allow it to absorb more readily a higher level of food consumption.

The average adult has about 25 billion fat cells distributed in clusters around the body. When calorie intake exceeds expenditure, the cells can swell to almost three times their normal size. But beyond a certain point, they begin to multiply, up to a maximum of about five times normal. And the process has never been known to reverse.

Rather like a former alcoholic who cannot risk a drink, former fatties really do have to be much more careful of what they eat.