Diet affects the brain's insulin sensitivity, promoting obesity

While a poor diet and insufficient exercise are frequently cited as causing obesity, the mechanisms in the body that lead to this chronic illness are more complex. Now, a study by the University Hospital of Tübingen, the German Center for Diabetes Research (DZD) and Helmholtz Munich has offered intriguing new insights into the origins of type 2 diabetes and obesity, as well as the brain’s function as a critical control centre.

In a healthy state, insulin has an appetite-suppressing effect in the brain. However, in people with obesity in particular, insulin no longer regulates eating behaviour properly, resulting in insulin resistance. There have been numerous signs indicating insulin causes neurodegenerative and metabolic disorders — especially in the brain — with unhealthy body fat distribution and chronic weight gain linked to the brain’s sensitivity to insulin. The researchers have now shown in the journal Nature Metabolism that even a brief consumption of highly processed, unhealthy foods causes a significant alteration in the brains of healthy individuals, which may be the initial cause of obesity and type 2 diabetes.

The study saw 29 male volunteers of average weight split into two groups. For five days in a row, the first group had to supplement their regular diet with 1500 kcal from highly processed, high-calorie snacks. The extra calories were not consumed by the control group. Both groups underwent two separate examinations following an initial evaluation. One examination was conducted immediately following the five-day period, and another was conducted seven days after the first group had resumed their regular diet.

The researchers used magnetic resonance imaging (MRI) to look at the liver’s fat content and the brain’s insulin sensitivity. Not only did the fat content of the liver of the first group increase significantly after five days of increased calorie intake, but the significantly lower insulin sensitivity in the brain compared to the control group also persisted one week after returning to a normal diet. This effect had previously only been observed in obese people.

“Interestingly, in our healthy study participants, the brain shows a similar decrease in sensitivity to insulin after a short-term high calorie intake as in people with obesity,” said study leader Prof Dr Stephanie Kullmann, from the Division of Diabetology, Endocrinology and Nephrology at the University Hospital of Tübingen.

Prof Dr Andreas Birkenfeld, the study’s final author, serves as Medical Director of the Division of Diabetology, Endocrinology and Nephrology, Director of the Institute for Diabetes Research and Metabolic Diseases (IDM) at Helmholtz Munich, and a DZD board member. He urges more research on how the brain contributes to the development of obesity and other metabolic illnesses in light of the current findings.

“We assume that the brain’s insulin response adapts to short-term changes in diet before any weight gain occurs and thus promotes the development of obesity and other secondary diseases,” he concluded.

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