Losing weight through dieting can be difficult. Some diets will work and others won't despite our best efforts. So why does dieting not work? A new study finds a mechanism that may explain how our body limits weight loss, working against us when we are trying to lose weight.

New research published in the journal eLife has discovered a mechanism in mice, which share adequate similarities with the human body, that may be responsible for those disappointing moments in a dieter's life when nothing seems to work. 

Researchers from the Metabolic Research Laboratories at the University of Cambridge in the United Kingdom examined a group of neurons in the brain's hypothalamus and their role in regulating appetite. (The hypothalamus is responsible for producing the hormones that regulate a multitude of bodily functions from body temperature, hunger and sleep). 

The hypothalamus contains a group of neurons called "agouti-related neuropeptides" (AGRP), which play a key role in regulating appetite. When AGRP neurons are activated we want to eat, but when these neurons are deactivated, they can make us stop eating almost completely. 

The researchers used genetics to switch these neurons "on" and "off" in mice. The mice were examined in special "metabolic chambers" that can measure energy expenditure. 

The experiments revealed that "artificially activating the neurons in mice that don't have access to food increases the animals' activity levels but reduces the rate at which they burn calories."

This helps the mice to maintain the same weight. However, when the small rodents were allowed to eat - or even just smell or see the food - their energy expenditure levels went back to normal. 

"Finally, exposing mice to a high-fat diet for several days inhibits their AGRP neurons, and causes the animals to burn calories at a faster rate," report the authors. 

In other words, AGRP neurons regulate our appetite depending on the amount of food that is available. 

From an evolutionary perspective, such a mechanism may have evolved in order to help animals cope with famine. In the case of dieting, the brain cannot tell that the person is intentionally trying to lose weight. 

The study's lead investigator explains, "Weight loss strategies are often inefficient because the body works like a thermostat and couples the amount of calories we burn to the amount of calories we eat. When we eat less, our body compensates and burns fewer calories, which makes losing weight harder. 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. 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."