Overweight people’s brains have different 'appetite control centre’

Researchers have shown that the hypothalamus, a crucial part of the brain that regulates appetite, differs in the brains of obese and overweight individuals from those of people who are of a healthy weight.

The findings provide additional support for the connection between brain structure and eating habits and body weight.

According to recent estimates, there are over 1.9 billion overweight or obese people in the world. Nearly two-thirds of adults in the UK are overweight or obese, according to the Office for Health Improvement & Disparities. This raises a person's risk of contracting a variety of illnesses, such as type 2 diabetes, heart disease, stroke, cancer, and worsened mental health.

Our genetic make-up, the way our hormones are regulated, and the environment in which we live are just a few of the many variables that affect how much and what kinds of food we eat. It is not entirely clear what happens in our brains to signal when we are hungry or full, but studies have shown that the hypothalamus, a tiny area of the brain the size of an almond, plays a crucial role.

Stephanie Brown from the Department of Psychiatry and Lucy Cavendish College, University of Cambridge, said: “Although we know the hypothalamus is important for determining how much we eat, we actually have very little direct information about this brain region in living humans. That’s because it is very small and hard to make out on traditional MRI brain scans.”

Animal studies provide the bulk of the evidence for the hypothalamus' function in controlling appetite. These demonstrate the existence of intricately interconnected pathways in the hypothalamus, where various cell populations work in concert to signal when we are hungry or full.

To circumvent this, Brown and colleagues used a machine learning-based algorithm to analyse MRI brain scans from 1,351 young adults with a range of BMI scores. They were looking for variations in the hypothalamus when comparing people who are underweight, healthy weight, overweight, and obese.

In a study that was just published in Neuroimage: Clinical, the team discovered that young adults who were overweight or obese had significantly larger hypothalamus overall volumes. In fact, the team discovered a strong correlation between the hypothalamus' volume and body mass index (BMI).

These volume differences were most noticeable in the hypothalamic subregions that regulate appetite by releasing hormones that balance feelings of hunger and fullness.

One possibility is that the change is related to inflammation, even though the exact significance of the finding is unclear, including whether the structural changes are a cause or an effect of the changes in body weight.

Previous animal studies have demonstrated that a high-fat diet can result in hypothalamic inflammation, which then leads to insulin resistance and obesity. Three days of a diet high in fat are sufficient for mice to experience this inflammation.

Other studies have shown that this inflammation can raise the threshold at which animals are full – in other words, they have to eat more food than usual to feel full.

Brown, the study’s first author, added: “If what we see in mice is the case in people, then eating a high-fat diet could trigger inflammation of our appetite control centre. Over time, this would change our ability to tell when we’ve eaten enough and to how our body processes blood sugar, leading us to put on weight.”

Inflammation may explain why the hypothalamus is larger in these individuals, the team say. One suggestion is that the body reacts to inflammation by increasing the size of the brain’s specialist immune cells, known as glia.

Professor Paul Fletcher, the study’s senior author, from the Department of Psychiatry and Clare College, Cambridge, said: “The last two decades have given us important insights about appetite control and how it may be altered in obesity. Metabolic researchers at Cambridge have played a leading role in this.

“Our hope is that by taking this new approach to analysing brain scans in large datasets, we can further extend this work into humans, ultimately relating these subtle structural brain findings to changes in appetite and eating and generating a more comprehensive understanding of obesity.”