Posted on May 26, 2017, 6 a.m.
Recent study may shed light on why weight-loss diets can be an inefficient way to lose weight.
A team of researchers has discovered a mechanism in the brain that puts a limit on weight loss when mice were fed a low-calorie diet. This may explain why in humans, dieting can have a minimal effect as our brains prevent the body from burning calories (a carryover from our early ancestors when conserving energy was a necessity if food was scarce). According to Dr. Clémence Blouet from the University of Cambridge, weight loss diets are an inefficient way to lose weight because our bodies are like a caloric thermostat that the brain uses to regulate how many calories are to be burned after eating. As we eat less, our bodies seem to burn fewer calories, but just how the brain makes these adjustments to calorie burning has remained a mystery until now. The study was published in the journal eLife.
AGRP Neurons Play Key Role in Regulating Appetite
Researchers experimented with mice since they are biologically similar to humans, and they made a useful model for testing purposes. What researchers were interested in was a group of neurons which lay in the hypothalamus region of the brain. Called AGRP neurons, they are known for playing a significant role in regulating appetite. When these neurons are activated they induce hunger, but when they are inhibited they suppress hunger, so much so that it can lead to anorexia.
The mice were implanted with temperature probes then isolated in special chambers where measurements like energy expenditure (depending on food availability) were taken. Using a genetic trick to manipulate the neurons in mice, the researchers were able to turn AGRP neurons on or off. The experiments showed theses neurons are key contributors that controlled the caloric thermostat in the test mice (regulated calories burned). The study suggests the same should be true for humans as we have the same AGRP neurons that act to conserve energy when we limit the amount of food we eat. As soon as we start eating normally again, the neurons are interrupted and energy expenditure goes up to normal levels. This explains why dieting can be so difficult.
Mechanism Controls how many Calories to Burn
Researchers also found a mechanism that AGRP neurons use to control their own activity. This mechanism detects how much stored energy we have in our bodies and then determines how many calories to burn. According to lead author Dr. Blouet, these neurons help the brain coordinate energy expenditure and appetite by turning on (make us eat if food is available), or off (stop eating if food is scarce). This mechanism may be the result of evolution that helped us survive during famines when food was scarce.
Nowadays, our bodies only encounter this energy saving mode when we deliberately try and lose weight. The study helps explain why dieting on its own over the long run has little effect. According to co-author of the study Dr. Luke Burke, the research could lead to future new and improved therapies to assist people struggling with overeating and obesity. For now, the best solution for people (moderately overweight) wanting to lose weight is to combine modest calorie reduction along with exercise.
Luke K Burke, Tamana Darwish, Althea R Cavanaugh, Sam Virtue, Emma Roth, Joanna Morro, Shun-Mei Liu, Jing Xia, Jeffrey W Dalley, Keith Burling, Streamson Chua, Toni Vidal-Puig, Gary J Schwartz, Clémence Blouet. mTORC1 in AGRP neurons integrates exteroceptive and interoceptive food-related cues in the modulation of adaptive energy expenditure in mice. eLife, 2017; 6 DOI: 10.7554/eLife.22848