Dieting is extremely difficult because our brains are not hardwired to do so – hunger-sensitive cells make resisting snacks almost impossible, researchers have discovered.
The answer as to why this is lies in our evolutionary history, scientists from the Howard Hughes Medical Institute say. The team found neurons responsible for the uncomfortable feelings that hunger cause.
Early on in our history, these feelings would have served an evolutionary purpose. Perusing food at the time was a dangerous venture and to take the risk, we would have needed some encouragement.
"We suspect that what these neurons are doing is imposing a cost on not dealing with your physiological needs," research leader Scott Sternson said. In an environment where food is available all the time, ignoring these signals can be very tough.
Publishing their finding in the journal Nature, the authors said the brain cells responsible are known as AGRP neurons. While they do not directly drive the animal to eat, these neurons teach them to respond to sensory cues signalling the presence of food.
"We suspect that these neurons are a very old motivational system to force an animal to satisfy its physiological needs. Part of the motivation for seeking food is to shut these neurons off," Sternson said.
While it has long been known hunger affects almost all cells in the body, why hunger is unpleasant was not clear. Sternson said an earlier theory had suggested that neurons were to blame, but all of those tested had the opposite effect, with eating increasing positive feelings about food ie hunger makes food tastier.
AGRP neurons are located in the part of the brain called the hypothalamus and scientists know they become active when the body lacks energy – prompting it to eat.
The researchers looked to find out why through a series of experiments on mice and two flavoured gels that contained no nutrients. They switched them on while they sampled one of the flavours. In subsequent tests, the mice avoided that flavour gel because it was associated with false hunger signals.
In a reverse experiment, they switched the neurons off while hungry mice ate one of the flavours – which led to them developing a preference for the flavour choice that led to the silencing of the AGRP neurons.
This suggests they had been motivated to turn off the unpleasant signal. Findings also showed mice sought out places where the AGRP neurons had been switched off and avoided those where they were switched on.
In another experiment, scientists looked inside the mice brains while they searched for food. Findings showed that as soon as they saw the food, the signals stopped – a response that would not have happened if the neurons made food taste better.
Researchers now plan to investigate how to interfere with the AGRP neurons, which could have implications for dieting success.