An international group of scientists have solved the mystery of a genetic flaw which greatly increases the risk of obesity in one in six people.
A version of an obesity gene, called FTO, had been linked to a bigger belly, but the reason why was uncertain.
A study, published in The Journal of Clinical Investigation, showed FTO gene made fatty foods more tempting and altered levels of the hunger hormone, ghrelin.
Obesity experts said drugs targeting ghrelin might reduce weight gain.
There is a strong family link with obesity, and a person’s genetic code is thought to play a major role in the risk of them becoming overweight.
People have two copies of the FTO gene – one from each parent – and each copy comes in a high and a low-risk form. Those with two-high risk copies of the FTO gene are thought to be 70% more likely to become obese than those with low-risk genes.
But no-one knew why.
FTO gene made fatty foods more tempting and altered levels of the hunger hormone, ghrelin
A team, led by researchers at University College London, tested two groups of men. All were a normal weight, but one group had the high-risk FTO genes and the other was low risk.
The first tests looked at levels of the hormone ghrelin either side of a meal in 10 men from each group.
Levels of the hormone, which makes people hungry, did not fall as far in the high-risk patients after the meal. Their ghrelin levels also began to climb more quickly.
In separate tests, a series of brain scans after a meal showed further differences between the two groups. Men with the high-risk genes found pictures of high-fat foods more appealing than the low-risk men.
Dr. Rachel Batterham, the head of the centre for obesity research at University College London, said: “Their brain is set up to be particularly interested in anything to do with high-calorie food.”
She said they were “biologically programmed to eat more”.
Dr. Rachel Batterham said understanding how FTO affected the odds of becoming overweight would help patients.
She said exercise such as cycling was an excellent way to lower ghrelin levels and there was a significant amount of research from pharmaceutical companies working on the hormone.
Dr. Rachel Batterham added: “Also protein meals do lower ghrelin more, so anything that suppresses ghrelin is more likely to be effective in FTO patients.”
The FTO mutations were probably life-saving at one point in human history when piling on the pounds in the summer would help people survive the winter.
Being underweight or severely obese did cut life expectancy.
The researchers at the US National Centre for Health Statistics looked at 97 studies involving nearly 2.9 million people to compare death rates with Body Mass Index (BMI) – a way of measuring obesity using a person’s weight and height.
A healthy BMI is considered to be above 18.5 and below 25. However, overweight people (with a BMI between 25 and 30) were 6% less likely to die early than those considered to have a healthy weight, the study reports.
Mildly obese people (BMI between 30 and 35) were no more likely to die prematurely than people with a healthy BMI.
A new study suggesting being overweight can lead to a longer life has caused controversy among obesity experts
The study said being “overweight was associated with significantly lower all-cause mortality”.
Possible explanations included overweight people getting medical treatment, such as to control blood pressure, more quickly or the extra weight helping people survive being severely ill in hospital.
However, the researchers point out they looked only at deaths and not years spent free of ill-health.
On Tuesday, the Royal College of Physicians called for the UK to rethink the way it tackles obesity.
Prof. John Wass, vice-president of the college, said: “Have you ever seen a 100-year-old human being who is overweight? The answer is you probably haven’t.”
He said the largest people will have died years before and pointed to health problems and higher levels of Type 2 diabetes.
“Huge pieces of evidence go against this, countless other studies point in the other direction.”
Other experts criticized the research methods.
“Some portion of those thin people are actually sick, and sick people tend to die sooner,” according to Donald Berry, from the University of Texas.
Dr. Walter Willett, from the Harvard School of Public Health said: “This is an even greater pile of rubbish than a study conducted by the same group in 2005.”
Tam Fry, from the National Obesity Forum in the UK, said: “It’s a horrific message to put out at this particular time.
“We shouldn’t take it for granted that we can cancel the gym, that we can eat ourselves to death with black forest gateaux.”