Study shows brain's response to appetite hormone

“Scientists have demonstrated for the first time how a hormone may hold the key to explaining why people carry on eating, even if they have already eaten enough to fill them up”, The Independent reported.

A practical use for this research was also highlighted by the BBC, which said: “The researchers hope a greater understanding of how appetite is controlled could help tackle the obesity crisis - 23% of the adult UK population is classified as obese.”

This was a small experimental study in eight healthy male volunteers whose weight was normal. This study does help scientists to understand what areas of the brain are normally affected by the hormone peptide YY (PYY), which plays a role in appetite regulation. However, further studies will be needed to see if brain activity in response to PYY differs in obese people and in people with eating disorders such as anorexia.

In itself, this study does not suggest any new treatments for obesity, as a trial of a nasal spray containing PYY for obesity is already ongoing. We should wait for the results of this trial before we draw any conclusions about the effectiveness of PYY.

Where did the story come from?

Drs Rachel Batterham, Steven Williams and colleagues from University College London, and King’s College London carried out this research. The study was funded by the Medical Research Council, Rosetrees Trust, and the Travers’ Legacy. The study was published in the peer-reviewed scientific journal Nature.

What kind of scientific study was this?

This was an experimental study looking at brain activity in eight healthy adult male volunteers (average age about 30 years old) and how it was affected by the hormone peptide YY (PYY) or placebo. PYY is known to affect hunger and is naturally produced by the body after a meal to suppress appetite. The volunteers were of normal weight and had remained at roughly the same weight over the past three months.

Researchers instructed volunteers to eat similarly sized meals between 7pm and 8pm the day before the experiment, and not to eat anything after this. The following morning, volunteers were placed into a magnetic resonance image (MRI) scanner so that researchers could look at their brain activity during the experiment.

Researchers monitored brain activity for 10 minutes, before gradually injecting half of the volunteers with the hormone PYY and the other half with placebo (a salt solution) over a period of 90 minutes. Giving a dose of PYY mimics what happens in the body after a meal is eaten.

Every 10 minutes participants were asked to rate how they were experiencing 10 feelings (four of which were food-related and six were non-food related) on a scale of nought to 100. The food-related feelings related to how hungry they were, how sick they felt, how much food they thought they could eat, and how pleasant it would be to eat. The researchers looked at activity in different parts of the brain as these questions were asked and blood samples were also taken every 10 minutes during scanning. Thirty minutes after the injections were completed, volunteers answered the questions again, and a blood sample taken.

They were then offered a large buffet lunch, and how much they ate and drank was measured. After the meal they answered the questions about feelings again, and were asked to rate how pleasant the meal was.

Seven days after this experiment, it was repeated again. This time the volunteers who had received PYY in the first experiment received placebo, and volunteers who had received placebo in the first experiment were given PYY.

What were the results of the study?

The researchers found that PYY affected the activity of areas of the brain known to be involved in regulating the amount animals, including humans, eat (the hypothalamus and brainstem). They also found that PYY affected activity in different – higher function - regions of the brain (corticolimbic and higher cortical regions), which are known to be involved in experiencing pleasurable reward sensations.

They found that when volunteers were given PYY, the level of activity in these higher function areas of the brain was associated with how many calories they ate at the buffet meal, whereas when they had been given placebo, it was the activity of the hypothalamus that predicted their calorie consumption.

What interpretations did the researchers draw from these results?

The researchers concluded that their study provides the first evidence about which brain areas respond to the signals that regulate food intake in humans, and that their findings may lead to a better understanding of how obesity arises and how it might be treated.

They suggest that finding treatments that can override the need to eat to get pleasurable feelings will be very important in the fight against obesity, and that looking at how the identified brain regions are affected by potential treatments, may help to predict which of them will be effective.

What does the NHS Knowledge Service make of this study?

This was a complex experimental study looking at how PYY affected brain activity. Its results will help scientists understand which areas of the brain may be involved in controlling appetite.

However, this study is in a very small number of people, all of whom were of a healthy weight. The effects of PYY on the brains of people who are obese or who are anorexic may be different, and researchers will need to investigate this further.

Sir Muir Gray adds...

Until the off switch is fully understood and controllable, you should go and walk 2000 steps every time you feel hungry.