International Business Department           Liu Bojia           March 04, 2025

  Obesity is one of the most common health problems in modern times. However, as the process of weight gain is often gradual, many people will only suddenly realise that they have gained weight when there is a noticeable change in their body shape. This also gives some people a lucky break: since it takes a certain amount of time for weight gain to happen, it's fine to have a short-term binge and then return to a healthy diet.

  But a recent paper in Nature Metabolism may put that idea to rest. Scientists from the University of Tübingen and the German Diabetes Research Centre found that volunteers only needed to consume a high-calorie diet for five consecutive days to build up a significant amount of fat in their livers. Even more far-reaching is the fact that an individual's brain response to insulin is disrupted and adapts to short-term dietary changes. This means that the brain maintains a negative insulin response for as long as there is a period of inactivity, setting the stage for the development of obesity in the future.

  According to the paper, the researchers recruited 29 healthy male volunteers with a body mass index between 19 and 25 to participate in the trial. One group was kept on a normal diet, while the other was required to consume high-calorie foods for five days, which equated to an additional 1,500 kilocalories on top of their normal diet. The types of food include a variety of ultra-processed snacks such as crisps and chocolate. The researchers will follow up and collect data from all individuals after 5 days. The volunteers in the experimental group would then return to their regular diet and be followed up again a week later.

  The researchers found that the experimental group volunteers surprisingly did not have significant changes in their body weight and body fat distribution despite the dramatic increase in calorie intake over the 5 days. However, analyses showed a significant increase in liver fat content in the experimental group of volunteers, while the control group remained largely unchanged. Accumulation of liver fat is an early sign of fatty liver, which in turn is closely related to insulin resistance, type 2 diabetes and other metabolic diseases. In other words, although the weight has not changed, the body has already begun to ‘sound the alarm’.

  On the other hand, the researchers also delivered insulin to the volunteers via an intranasal spray, which was able to enter the brain via the nasoencephalic pathway and influence brain activity. The researchers then observed the brain's response with the help of functional magnetic resonance imaging (fMRI). It was found that after just five days of indulgent eating, regions of the volunteers' brains associated with food reward and appetite regulation, such as the right insula cortex and the right midbrain/brain bridge, were significantly more responsive to insulin. This means that the brain's ‘craving’ for high-calorie foods became stronger.

  One week after they resumed their normal diet, the hippocampus and the fusiform gyrus were significantly less responsive to insulin. These brain regions were able to actively perform memory and learning functions in response to insulin, but the arrival of a high-calorie diet altered this trend, potentially setting the stage for cognitive decline.

  In addition to changes in insulin response, the study also found that high-calorie diets affect the brain's ‘decision-making system’. Specifically, volunteers in the experimental group were less sensitive to rewards and more sensitive to punishments after five days. This means that they may be less likely to derive satisfaction from healthy foods and more likely to be tempted by high-calorie foods. This change persists even 1 week after returning to a normal diet, but the effect is reduced.

  These results suggest that even a brief dietary indulgence can have a profound effect on the metabolic health of the brain and body. If you think ‘it's okay to eat more every now and then,’ it will be harder to resist high-calorie foods in the future. The best way to avoid obesity is to stay away from high-calorie foods and eliminate their negative effects at the source.