Anyone who’s ever struggled with their weight knows all the tricks for taming appetite: don’t eat mindlessly in front of the telly, don’t bolt your food and resist the triggers for non-hungry eating like boredom and stress.

But what if you’d acquired a faulty gene that gave you such a powerful appetite that none of these strategies worked?

That’s the problem for three to six per cent of people who become very obese – often the ones who the rest us think need ‘more willpower’.

They’re unlucky enough to inherit a defective MC4R gene – and it stops the brain from getting the message that they’ve had enough to eat.

”For people with this gene there’s a genuine reason why they overeat – they have an uncontrollable appetite because their brain keeps telling them to eat,” says Dr Daniel Chen from the Diabetes and Obesity Research Program at Sydney’s Garvan Institute of Medical Research.

”It’s important to raise the profile of MC4R because people with a defect in this gene can pass it on to their children. But if we can identify a two year old with the gene, for instance, then they may be candidates for more targeted therapy.”

Science has found around 60 different gene variations that can influence our weight. Although most have a small effect on their own, some people inherit a faulty gene that can cause significant obesity. 

Fortunately these ‘broken’ genes aren’t widespread, but a defective MC4R is the most common of them – you only need to inherit a single copy from one parent to be stuck with a massive appetite. Its effects often become obvious in early childhood, with even toddlers becoming very overweight.

Another clue that someone in the family may have this gene – along with a big appetite – is blood pressure that’s unexpectedly healthy for someone who’s overweight, adds Chen who’s looking for overweight volunteers to have a blood test to see if they have the gene.

This faulty gene isn’t new but it’s likely to be having more of an effect now than in the past because of more high kilojoule food and less physical activity, says Dr Jerry Greenfield, Head of Endocrinology at St Vincent’s Hospital and lead investigator in the obesity genetics project at the Garvan.

”We’re not saying that lifestyle factors, such as increased food intake, aren’t important. But research shows that the drive to eat has a strong genetic influence, a fact that is often forgotten. People with a defect in MC4R don’t get the message in the brain to stop eating.”

 Given that that Harvard obesity researcher Dr David Ludwig suggested this month that severely  overweight children may be better off away from their parents’ custody, this raises an interesting issue –  in some cases ‘blaming’ the parents may be unhelpful as the problem may be in the genes, Greenfield says.

It was work by his former colleagues at the UK’s University of Cambridge that resulted in children with a rare genetic problem – leptin deficiency – being removed from the child protection register.

It’s easy to say that weight loss is simple – it’s about eating fewer kilojoules than you burn up in physical activity. But for some people there are genetic influences that make it harder to stick to a healthy weight.

Weight loss surgery may help some people with a faulty MC4R gene but no studies have been done to find out, says Greenfield – but the gene may explain why some people who have weight loss surgery don’t do so well.

”At the moment there’s a blanket approach to weight loss and we need more targeted treatments – we need to find a way to reverse the defect in the MC4R gene and other inherited causes of obesity,” he says.

Have you had problems with your appetite – and if so have you been able to overcome them?

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