A Japanese research team has discovered a mechanism potentially explaining why people gain weight easily in middle age.

Nagoya University professor Kazuhiro Nakamura and his team found that in rats, the primary cilia of neurons in the hypothalamus, a region of the brain controlling metabolism and food intake, become shorter with age.

A similar mechanism is believed to exist in humans as well. It is hoped that this discovery could lead to a fundamental treatment for obesity.

The results of the research were published in the U.S. journal Cell Metabolism on Thursday.

While a decline in overall metabolism due to aging is often cited as a cause of middle-age weight gain, the specific mechanism behind it has been unclear until now.

Nakamura's team focused on melanocortin 4 receptors, or MC4Rs, found in neurons in the hypothalamus, which detect excess nutrients and stimulate metabolism.

A study of rats given an antibody developed to make MC4Rs visible found that the receptors were concentrated in primary cilia that extended from some hypothalamus neurons like antennas.

Comparisons of rats of various ages showed that the primary cilia gradually shortened with age. When the team shortened the primary cilia in rats through genetic modification, the rats displayed lower metabolism and increased food intake, resulting in weight gain.

The team also found that the primary cilia shrank faster in rats on a high-fat diet, while they grew longer or shortened at a slower pace in rats on restricted diets.

"Overeating leads to the anti-obesity mechanism wearing out," Nakamura said. "By maintaining a moderate diet and not consuming too many calories, we may be able to keep the anti-obesity effect even as we age."