As a fat person who’s struggled all his life to stay on the acceptable side of obese (and largely – haha! – failed), I was slap bang in the target audience for the first of a new series of Horizon, which set out to redress the balance of research into weight gain and weight loss. 99% of all studies look at people who are already fat and try to work out how they got that way. This time, Horizon took 10 slim people and investigated what would happen to them if they were forced to spend four weeks eating twice as many calories as they normally do. The result was one of the most fascinating insights into body size ever broadcast.

Show me a fat person who claims not to be envious of those slim people who can eat what they like and not put on an ounce, and I’ll show you a liar. Yet what this programme ultimately showed, both through experiment and discussion of additional research, is that they can’t help it any more than the fat person can.

Basically your body is programmed, through a combination of genetics, prenatal conditions, and childhood experiences, to have a natural body weight. That weight is different for everybody, and your body will do its damnedest to achieve and stick to that weight, pretty much no matter what you eat. For some people, that means a lifetime of slimness and acceptance by our celebrity-obsessed culture. For others it means a lifetime of chubbiness, and people looking down their noses at you wondering why you can’t control yourself.

It’s not a licence to stuff your face. Indeed, the images of people forcing platefuls of burger and chips into their gobs were almost as revolting as the scientist with the bucketful of cow fat that he alternately prodded and held up for our collective scrutiny. But the message is, if you eat what naturally feels right for you, then you’ll be either thin or fat, depending on that “natural” body weight.

Was your mum a little older than average when you were born? Then on average you’ll be fatter. Were you breastfed? Then on average you’ll be slimmer. Did you overeat as a child? Then you may very well have made more fat cells than you need, and you’ll never get rid of them. Have you come into contact with adenovirus? Then you’re three times more likely to be up to 20% fatter than average, as this little monster infects your fat cells and makes them replicate. There’s a fortune to be made from anyone who can come up with a cure for that one!

Fat wasn’t always bad. In our caveman days, the chubbier people were the ones who survived the famines, so there was considerable evolutionary pressure in favour of fatties. Sadly in our modern environment, where we’re never without food for more than a few minutes, there’s little reward for those who can go months without eating. Piling on the pounds with zero chance of ever having to fast is not a good idea.

Hunger is controlled by hormones, which in turn are controlled by genes. How these differ from person to person was demonstrated through a clever experiment with 4-5-year-old children. Having been fed until every one of them reported being “full” they were then sat at tables with a variety of colouring materials alongside plates filled with chocolatey, cakey snacks. Many of the kids pushed away their plates of snacks and couldn’t be persuaded to eat more, even by the encouragement of their peers, some of whom happily troughed their way through their entire plate without stopping for a second thought. It’s likely that those kids for whom feeling full is no barrier to further snacking have the “high risk” version of the FDO gene, and will go on to a lifetime of being overweight. The kids with a natural appetite control have the “low risk” version of FDO and will be able to control their eating without too much conscious effort.

Like the ten volunteers, who were uniformly and sickeningly lath-like in appearance. Two of them – Ben and Ayisha – had such powerful appetite controls that they found it impossible to eat the target number of calories during the four-week experiment. Physically impossible. Either their throats closed up or they felt sick, or they did actually vomit. Others had different in-built mechanisms to maintain their body weight at its natural slim level. After four weeks of constant stuffing, one guy’s basal metabolic rate had increased by 30% to burn off the extra calories, much of which he had converted to extra muscle rather than extra fat.

Two girls had unconsciously controlled their weight gain through increased fidgeting. Even the “winner” – gaining 9.5% of his starting body weight (6.5kg) – had returned to his original weight within two weeks of the end of the experiment. Conversely, in a dimly lit laboratory somewhere in the depths of hell, some poor volunteers had dieted down from their starting weight by 10% and were being kept on a calorie level intended to maintain that lower weight. They reported constant feelings of hunger, which the scientists interpreted as the body trying to restore their weight to its natural level.

So for all you frustrated dieters out there, the message is simple. You are the weight you’re meant to be. Eat if you’re hungry. Don’t eat if you’re not hungry, and live with the consequences, whatever your waistline. Vive la difference!