Obesity is defined as having a body mass index (BMI) of 30 or more. Being overweight or obese increases the risk of health problems such as high blood pressure and type 2 diabetes. Schedules and individual sleep, eating and exercise rhythms can also impact health by either complementing or contradicting the body’s natural circadian rhythm. According to research, the time of day that energy is burned could be a key factor in why some people struggle with obesity.

Lean People Use More Energy During the Day

In fact, a recent study from Oregon Health & Science University published in the journal Obesity found that people at a healthy weight expend more energy during the day, when most people are active and eating, while people with obesity expend more energy at night, when most people sleep, burn more energy. Researchers also found that people with obesity have higher levels of the hormone insulin during the day – a sign that the body is working harder to use glucose, an energy-rich sugar. It was surprising for researchers to learn how much the timing at which the body burns energy differs among people with obesity, but it is not clear what causes this. Burning less energy during the day could lead to obesity or be the result of obesity.

Every 24 hours, people experience numerous changes that are triggered by the human body’s internal clock. These changes usually occur at certain times of the day to best meet the body’s needs at any given hour. The experts focused their research on how ciradian rhythms and sleep affect the human body. While previous research has suggested that circadian rhythm misalignment affects energy metabolism and glucose regulation, these studies largely involved participants who had a healthy weight. To investigate this further, researchers organized a study involving people of different heights. A total of 30 people volunteered to take part in the study, in which subjects stayed in a specially designed circadian research laboratory for six days. The study followed a strict circadian research protocol that included a schedule designed so that participants were awake and asleep at different times each day.

After each sleep period, the volunteers were awakened to eat and took part in various tests for the remainder of each day. In one test, they exercised with a mask connected to a device called an indirect calorimeter, which measures exhaled carbon dioxide and helps estimate energy expenditure. Blood samples were also taken to measure glucose levels in response to an identical meal each day. Next, the research team plans to study eating habits and hunger in people with obesity as well as those at a healthy weight. This new study also follows up on a 2013 study that found circadian clocks naturally increase cravings for food at night.

How Obesity Affects the Brain

Obesity leads to altered energy metabolism and reduced insulin sensitivity of cells. The so-called “anti-obesity drugs” are increasingly being used to treat obesity and are attracting great interest, particularly in the USA. Researchers at the Max Planck Institute for Metabolism Research in Cologne have now shown in people with obesity that reduced insulin sensitivity impairs the learning of sensory associations. In order to control our behavior, the brain must be able to form associations. This involves, for example, linking a neutral external stimulus with a consequence that follows the stimulus (e.g. the stove burns red – you can burn your hand). In this way, the brain learns what effects our interaction with the first stimulus has.

Associative learning is the basis for the formation of neural connections and gives stimuli their motivational power. It is essentially controlled by a brain region called the dopaminergic midbrain. This region has many receptors for the body’s own signaling molecules, such as insulin, and can thus adapt our behavior to the physiological needs of our body. But what happens when the body’s insulin sensitivity is reduced due to obesity? Does this change our brain activity, our ability to learn associations and therefore our behavior?

Anti-Obesity Drug Can Improve Learning Ability

Researchers at the Max Planck Institute for Metabolic Research measured how well association learning works in participants with normal body weight (high insulin sensitivity, 30 volunteers) and in participants with obesity (reduced insulin sensitivity, 24 volunteers), and whether this learning process is influenced by the Anti-obesity drug liraglutide is influenced.

Low insulin sensitivity reduces the brain’s ability to associate sensory stimuli. In the evening, the researchers injected the participants with either the drug liraglutide or a placebo in the evening. Liraglutide is a so-called GLP-1 agonist that activates the GLP-1 receptor in the body, stimulates insulin production and causes a feeling of satiety. It is often used to treat obesity and type 2 diabetes and is administered once daily. The next morning, the subjects were given a learning task that the researchers could use to measure how well associative learning worked. They found that the ability to associate sensory stimuli was less pronounced in overweight participants than in those of normal weight, and that brain activity was reduced in the areas that encode this learning behavior. After just one dose of liraglutide, participants with obesity no longer showed these impairments, and no difference in brain activity was found between normal-weight and obese participants. The drug returned brain activity to the state of normal-weight subjects.

These findings are fundamental because they show that basic behaviors such as associative learning depend not only on external environmental conditions but also on the body’s metabolic state. While it is encouraging that available medications have a positive effect on brain activity in obesity, it is alarming that changes in brain performance also occur in young people with obesity without other medical conditions. According to the researchers, the prevention of obesity should therefore be the focus.