People who are overweight may be able to partly blame their muscles for storing too much fat, a study suggests.
Exercise encourages the body to burn rather than store fat
The muscle of severely obese people appears to be programmed to amass more fat compared to that of lean people, US researchers at Duke University found.
This programming, which is down to a gene, might also explain why some obese people find it hard to keep weight down despite cutting calories, they said.
Targeting the gene with drugs may help, they told Cell Metabolism journal.
Until then, plenty of exercise seems to be the most expedient way to weight loss among individuals with the gene, although a healthy diet is also important, they say.
Dr Deborah Muoio and colleagues studied the muscle cells of severely obese and lean humans in the lab.
They found that the fat-building enzyme stearoyl-CoA desaturase-1 (SCD-1) was three times more abundant in the muscle from the obese people.
SCD-1 slows down fat burning and promotes storage of fat droplets in the muscles.
When the researchers used genetic techniques to alter the cells of the lean individuals so they also had higher levels of the enzyme the cells began to store more fat.
The study authors believe people either inherit the SCD-1 genetic predisposition to obesity or develop it at some point in their life, possibly triggered by a poor diet.
Dr Muoio said exercise would probably help fight this type of obesity because regular physical activity encourages changes in the body to burn rather than lay down fat.
She added: "Obesity is a very complex disease and this metabolic pathway does not fully explain obesity but it is a likely contributor.
"The cells of obese people remember their metabolic programme, which could help explain, in part, why losing weight and maintaining weight loss is so difficult. The good news is it's possible to change your energy balance through exercise."
Dr Toni Steer, a nutritionist with the Medical Research Council, said: "We know that genes only explain about 30% of obesity. The other 70% is down to environment and other lifestyle factors.
"This work contributes to the reasons why people might be at risk of gaining weight but it is by no means the one and only answer.
"Also, a number of genes may be involved.
"The best way to lose weight is to eat healthily and to exercise. Even if there was a drug available, adopting healthy eating habits and being more physically active would still be important."