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 difficulties, and despite reams of learned papers, these questions have remained unanswered among scientists as well as the public.
Now a group of British 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.
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.
‘At three months, the ones who are going to be fat have exactly the same resting energy expenditure as the normal babies. Yet their total energy expenditure was reduced. So we have shown that the reason for this was that they were using up less energy in activity – they were more placid,’ says Dr. Alan Lucas, a Dunn scientist.
This finding, that the eventual chubbiness of the ‘fat’ babies was not due to their eating any more than usual, has sparked worldwide interest. The study was published in the New England Journal of Medicine, along with a report by American researchers of a similar study among a group of Pima Indians in Arizona, chosen because they are known to be extremely likely to gain weight – by their early 20s, as much as 80%-85% of the Pima population is obese.
By keeping track of the Indians over a period of two years, the US scientists found that those who later gained the most weight were burning fewer calories, rather than eating more. It was only about 80 calories fewer a day than was normal for their body sizes. But Dr. Eric Ravussin, the study director, calculated that that was enough for a weight gain of 9lb a year.
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.
It was that tendency for obesity to run in families that helped the Cambridge team to set up their research. In all, 18 babies took part, all of them born at the Rosie Maternity Hospital in Cambridge. Six came from thin mothers, and these, as expected, all developed as regular sized babies. The other 12 were the babies of fat mothers, chosen in the expectation that a fair proportion of these would grow into fat babies.
In the event, half of them did exactly that. A baby was judged as being fat if its weight relative to its length put it among the top 10% for babies nationally. At nine months, that meant weighing an average of about 24-251b (11 kilos), compared with about 20lb (9 kilos) for the ‘thin’ babies.
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.
In Cambridge, however, Lucas does not claim that the study findings have proved obesity to be the result of some biochemical abnormality in the way fat people burn energy. The discovery that the two groups of babies metabolized their food at the same rate while asleep points way from any simple biochemical explanation.
‘There could be an inherent problem, a biological difference that makes the babies who become fat less active,’ he said. ‘Alternatively, they may be less active for social reasons, such as that the fatter mothers may treat their babies differently – they may be less active themselves. That is unlikely to account for all the differences in the babies, however, because in such young babies we would expect much of their activity to be spontaneous.
‘Our next studies will explore this question of whether he differences are social or biological. At this stage, we are excited simply to have been able to put these babies into the category of reduced energy expenditure rather than overeating.’
The main aim behind the research is to find out eventually whether early diet, and energy expenditure patterns, are important to the quality of later life.
Britain has been declared a nation of fatties by authoritative medical bodies such as the Royal College of Physicians, which in a recent report estimated that up to 30% of the population at different ages were overweight – and warned that they could all be at risk of developing heart disease as a result. In the US, panel, which concluded that any degree of overweight, even 5lb or 10lb, may be hazardous to health.
But not all experts agree that people whose plumpness earns them the medical description of ‘overweight’ should actually be regarded as abnormal. It may be that a much wider variation around the average should be regarded as normal, they argue.
‘I’m not at all convinced that moderate obesity is a health hazard or even a biological abnormality,’ says Professor John Durnin of Glasgow University’s Institute of Physiology.
In Cambridge, Lucas thinks there probably is a relationship between fatness in early life and later tendencies towards certain kinds of health problems. ‘If that is so,’ he says, ‘understanding its causes will be of immense importance.’ But there may also be a genetic propensity for some people to weigh more, he says.
While the mothers of plump babies can take comfort from the study that they are not over-feeding their infants, the findings provide no grounds for the truly obese to argue that their size has nothing to do with their eating habits.
The reason they may 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.