Most people who have struggled with obesity will have felt encouraged by an initial loss of weight when starting a diet before hitting a disheartening plateau or even regaining weight as the body seemingly compensates. A new study suggests the blame could be attributed to our brains, which direct our bodies to stop burning fat and adjust energy expenditure in response to a reduction in caloric intake.

Yo-yo dieting is a term likely familiar to anyone who has tried to lose weight and found themselves unable to maintain their weight loss long term or avoid putting the weight back on. In an attempt to uncover the underlying causes for such dispiriting cycles, a team at Cambridge University set out to understand the underlying mechanism responsible for how the body regulates its energy expenditure when confronted with a less calories being consumed.

"We know that the brain must regulate this caloric thermostat, but how it adjusts calorie burning to the amount of food we've eaten has been something of a mystery," says Dr Clémence Blouet, the leader of the study.

The scientists looked at a group of neurons located in the hypothalamus known for regulating appetite, called "agouti-related neuropeptide" (AGRP) neurons. When activated, AGRP neurons stimulate hunger, making us eat, and conversely, when these neurons are inhibited, severe anorexia results.

Using genetically modified mice they developed a process to switch the AGRP neurons on and off, while also measuring the animal's temperature, a known proxy for energy expenditure. They found that when the AGRP neurons were activated in the absence of food, they signal the body to limit how many calories we burn. And once food becomes available again, the AGRP neurons deactivate and energy expenditure rises again.

"Our findings suggest that a group of neurons in the brain coordinate appetite and energy expenditure, and can turn a switch on and off to burn or spare calories depending on what's available in the environment," Dr Blouet explains. "If food is available, they make us eat, and if food is scarce, they turn our body into saving mode and stop us from burning fat."

While this mechanism can be evolutionarily understood as helping us conserve energy when suffering through long periods of famine, today most people in the developed world would only encounter such triggers upon limiting caloric intakes when dieting to try and lose weight.

The study offers a compelling insight into the neurological mechanism that explains what many dieters would already anecdotally understand. But the real development could come in helping scientists work on new therapies to combat obesity. If a way could be found to interrupt the energy conservation messages from AGRP neurons, then weight loss results from simple lower-calorie diets could be sustained.

On a more pragmatic and immediate level, the scientists say combining exercise with a lower-calorie diet is currently the best approach for the moderately overweight, as increasing one's energy output should mildly mitigate the changes in these energy expenditure signals from our brains.

The study was published in the journal eLife.