The science of the obese brain

15 January 2019 | Story Carla Bernardo. Photo Wikimedia Commons. Read time 8 min.
In the United States, childhood obesity has more than doubled since the 1990s, with young adults being diagnosed with obesity-related diseases every year.
In the United States, childhood obesity has more than doubled since the 1990s, with young adults being diagnosed with obesity-related diseases every year.

Neuroscientist Dr David Hume led participants at the University of Cape Town’s (UCT) 2019 Summer School through a study of obesity, helping them understand the roles played by birth, bacteria, the brain, the food industry and eating behaviours.

The first lecture of Hume’s two-day short course, titled “The Obese Brain: The Neuroscience Behind Weight Gain”, covered “Brain-body Connections” and took place on Thursday, 10 January. The second, “Optimising Obese Interventions”, followed the next day.

In his Thursday lecture, UCT academic Hume looked at the three systems that are implicated in weight gain and the obesity phenomenon. He also explored the endocrine system’s relationship with insulin and ketones, the nervous system’s response to input from the gut, and eating behaviours.

The impact, he said, begins from the start, in utero. Doctors are trained to encourage vaginal birth rather than a caesarean delivery for a number of reasons. But the reason pertinent to his lecture is that unless the mother is immunocompromised, or there are problems with the uterus, the baby is sterile before it passes through the vaginal canal. On passing through, the baby is colonised with millions of different kinds of healthy bacteria – all crucial for normal bodily functions.


“Our guts are like a garden [where] we decide what we cultivate and what will thrive.”

A microbiome then starts to develop on and inside the child, a process which takes two years to complete. This microbiome contains bacteria, viruses, fungi and various other organisms. Mother’s milk also has an important role to play as it contains special sugars that support and maintain the healthy bacteria.

Hume explained the three different types of bacteria in the human body, the first of which are harmless and occupy space in order to keep out harmful intruders. The second are those that can cause harm, but over which our bodies have a certain degree of control. An example are the microbes that cause bad breath or tooth decay.

The third and most important group for Hume’s lecture are the bacteria that support important bodily functions such as tissue regeneration. These include the bacteria in the gut that aid digestion, assisting the existing enzymes which cannot do the job alone by degrading the food we eat to the correct level, at which point the body takes over.

That isn’t where the relationship between bacteria and health ends, however.


“There might actually be crosstalk between the gut and the brain,” said Hume.

Two examples of this are found in the production of serotonin and the link between gut bacteria and immune cells.

Serotonin is the hormone that promotes an overall a state of well-being, deficiencies of which have been implicated in mental disorders such as depression. Over 90 percent of serotonin is produced in the gut.

The second instance of crosstalk is that gut bacteria link up with important immune cells in our gastrointestinal tracts. They cause a series of changes that sends a signal to the brain to act early to protect itself against any harm.

“The amazing thing about the microbiome in the gut is that it changes depending on what we eat,” Hume explained.

Different types of bacteria feed off different things. Some bacteria like fibre and leafy greens, some like sugars and starches, and others like greasy foods.

“In this way, our guts are like a garden [where] we decide what we cultivate and what will thrive.”

So, if we eat processed foods, the bacteria that love unhealthy food increase and take up more space. In addition, those bacteria start to stimulate the brain to crave more processed foods.

“This really is the perfect storm because not only are you craving more food, you’re craving more processed foods, and that really is a recipe for inevitable weight gain.”

This bad microbiome, plus obesity, inevitably leads to myriad health problems such as cancers, heart disease and diabetes.


“What has happened is the people in the food industry have become masters at manipulating the food that we consume.”

Cheap processed food

So why then are we eating processed foods to the point of ill health?

A lot of the food we eat today is packed with sugar and processed elements. And in countries like South Africa, processed foods are cheaper and easily accessible, motivating people to buy them.

“The problem is so profound that childhood obesity is now a huge concern,” said Hume.

In the United States, childhood obesity has more than doubled since the 1990s. Young adults are being diagnosed with obesity-related diseases every year. Both children and young adults are suffering from diseases previously seen only in predisposed populations or in people who have led unhealthy lifestyles for decades.

But how does this relate to the brain?

“Something has changed in our diets which potentiates obesity at a frightening rate,” said Hume.

“What has happened is the people in the food industry have become masters at manipulating the food that we consume.

“Food has been altered to the point that it packs an extra pleasure punch which has now been scientifically termed as the ‘bliss point’ in food.”

It has been engineered to be just the right smell, texture and taste.

Hume explained that when we eat it regularly, certain pathways in the brain are stimulated. These are called reward pathways or hedonic hotspots which, if stimulated frequently, prompt a craving for processed food. So people eat it, and then they crave even more.

 “It happens because we are biologically programmed to seek energy-dense foods and to consume it when we can, because previously, we didn't always have food available,” he said.

“It’s really frightening to consider that food which has the potential to alter our guts, our organs and our brains is being tampered with to the extent that itʼs altering our actual brain tissue… It’s a really frightening thought.”

And 75% of the food industry’s profits come from these processed foods.

The good news

There are however ways to maintain a healthy lifestyle eating foods that are relatively affordable and easily accessible, and Hume said these include a low-carbohydrate, high-fat diet, along with the correct combination of foods.

The quality of carbohydrates and fats is however important. For the latter, Hume discussed the importance of omegas 3 and 6, stressing that while both are good, they must be balanced. While omega 6 is often found in processed foods because of its association with palatability, Hume cautioned against being duped into thinking added nutrients make processed food any healthier.

For those who are not genetically predisposed to high cholesterol or who don’t have an error of metabolism, a quality low-carbohydrate, high-fat diet provides advantages such as promoting satiety, a healthy calorie intake and a maximum of four teaspoons of sugar compared to the average South African’s 40 teaspoons.

However, warned Hume, “it is important not to be an extremist in any kind of situation”.

“I doubt any good can come from omitting everything and anything from your diet in huge amounts.”

His final suggestion is a return “to basics”. He encouraged participants to consider the foods associated with disease and those that promote health and longevity.

“The good news is that a bad gut is reversible,” Hume said.

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