How does breakfast affect our mood?

Are we what we eat?

Laura likes to eat a sweet muesli in the morning. Your partner Reto prefers to eat hearty, preferably an omelette with ham. Later, the two want to go shopping and then go to lunch. Over the course of the day, they will make a variety of decisions, such as how tolerant they are of the woman huddling at the grocery store checkout or how much to tip the waiter after lunch. Would it be possible that Laura and Reto behave differently here - and that only because of the different diets that they ate early in the morning? We deal with these and similar questions in the field of nutrition and decision-making research.

It is undisputed that our diet has a major impact on our physical health. It is known that a diet with lots of fresh fruit and vegetables lowers the risk of cardiovascular diseases, while fast food and industrially highly preprocessed foods, so-called convenience food, carry an increased risk of diabetes. Researchers from psychology and neuroscience are now increasingly interested in the impact our food has on the brain and our behavior. For example, high daily sugar consumption causes the brain to age faster and make us more forgetful.

In addition, it has been shown several times that the brain volume of people who eat unhealthy food for a long time is significantly smaller than that of people who eat a balanced diet. These are long-term effects that take effect over the years. But what influence does nutrition have on immediate and everyday social and economic decisions? Does a certain meal have a direct impact on our brain performance?

A full belly makes you more benevolent
An Israeli study led by neuroscientist Shai Danziger provided an initial indication that diet can play a role in social decisions: judges ruled more benevolently and more favorably after eating breaks. However, the researchers failed to provide us with any information about the composition of the judicial meals. However, recent research shows that foods rich in simple carbohydrates (colloquially known as sugar) make you tired and severely impair memory.

Food provides important nutrients. It can be divided into three energy suppliers, namely carbohydrates, fats and proteins, which are collectively referred to as macronutrients. A protein-rich diet is particularly popular with athletes and figure-conscious people who want to build muscle or reduce their weight through this diet.

But what influence could a high protein diet have on behavior? In an evolutionary animal study by Chang Han from the South Korean Kyung Hee University, it has already been shown that a changed composition of the macronutrients, i.e. the ratio of proteins to carbohydrates, can influence the social behavior of animals. Cricket males raised on a high-protein diet were later more willing to mate than those who grew up on a high-carbohydrate diet. But can such social effects also be transferred to people?

Research into decision-making behavior
Neuroscientist Soyoung Park from the German Institute for Nutritional Research was one of the first to conduct an experimental study to investigate whether the composition of a meal influences people's decision-making behavior. The so-called ultimatum game was used for this, a scientifically established method to investigate socio-economic decision-making behavior. It collects information on how people behave in situations that can be perceived as fair or unfair.

In the game, the test subject was assigned to a fictional partner behind whom a computer program was located. The program also receives a fictitious amount of money and can then «decide» how much of it it would like to share with the test person. If, for example, she receives ten francs, the program suggests predetermined offers (fair, medium-fair, unfair), for example five, three or two francs. Viewed objectively, the offer of five francs appears fair, as the total amount is divided equally. The third offer, however, is rather unfair. Here the test person receives only 20 percent of the available money. She then has to decide whether to accept the offer or not. If she accepts, she and the partner receive the amount offered; if it refuses, both receive nothing. The test person can thus punish the behavior of the fictitious partner.

The behavior of the test subject, i.e. which amounts he accepts or rejects, can be used to determine the social decision-making behavior: From a purely economic point of view, the test person should accept any amount of money, since even two francs is better than nothing. However, many studies confirm that low offers are actually perceived as unfair and are therefore often rejected. It is believed that this reflects the test subjects' social drive for justice.

Food shifts the tolerance threshold
Since the results of the ultimatum game suggest how people react to fair or unfair offers, i.e. whether they are more willing to punish their partner by rejecting unfair offers or not, the research group around Soyoung Park chose this Method for two consecutive studies that were carried out at the University of Lübeck. In the first study, a group of students were asked at lunchtime to play the ultimatum game. Before that, however, the researchers asked the students what they had had breakfast that morning. The subsequent analysis of the data showed that 76 percent of the test persons who had eaten a protein-rich breakfast were willing to accept unfair offers.

In contrast, only 47 percent of those who had a high-carb breakfast that day were willing to do so. So while a protein-rich breakfast was associated with greater social tolerance for unjust behavior, a carbohydrate-rich breakfast showed a greater urge for social justice. Does this mean that people who eat more carbohydrates act more justly or reject injustice more strongly? And if so, which metabolic processes lead to such behavior? These questions could not be answered by the study. Although a clear connection was identified, it initially did not allow any conclusions to be drawn about a possible causality. It would also have been possible that more tolerant people generally tend to have a high-protein diet.


In order to check whether breakfast really changed the behavior of the students, the researchers carried out a second, controlled laboratory study. In turn, students from the University of Lübeck were invited to come to the laboratory on two different days. There they were served a breakfast on both days, in which the ratio of carbohydrates and proteins was changed so that the test subjects either ate a breakfast rich in protein or a breakfast rich in carbohydrates. After breakfast, blood was also drawn from the test subjects in order to determine the amino acids in the blood and thus the metabolic processes to which a possible change in behavior can be attributed. They were then asked to play the ultimatum game.


The Lübeck research group was able to confirm the results of the first study by means of this targeted experimental change in the composition of the meal. After consuming the high-carbohydrate breakfast, the test subjects were more sensitive to unfair behavior by the fictitious partner than after consuming the high-protein breakfast: They refused unfair offers significantly more often. The same person made different decisions based on what they had for breakfast. How is that possible?

The role of the neurotransmitter
The nutrients in our food affect our metabolism in the short and long term. We take in carbohydrates, fats and proteins in different proportions with every meal. Animal and vegetable foods provide proteins that consist of amino acids. These acids are precursors to neurotransmitters: messenger substances that transmit messages in the brain and the rest of the body.

A meal with a higher content of protein in relation to carbohydrates causes the tyrosine level in the blood to rise. The amino acid tyrosine is a precursor to the neurotransmitter dopamine. In contrast, a meal that contains proportionally more carbohydrates than proteins affects the tryptophan level in the blood: this also rises. The amino acid tryptophan in turn is a precursor for the brain neurotransmitter serotonin. Dopamine and serotonin are neurotransmitters that may play key roles in the psychological exploration of decision-making. They have different roles in various decision-making mechanisms, for example in learning, processing rewards, but also in creating and maintaining moods.

Due to the different proportions of carbohydrates and proteins in the students' breakfast, the balance of tyrosine and tryptophan in the blood could be manipulated in a targeted manner. This enabled the researchers to show that the carbohydrate-rich breakfast increased the tryptophan level in the blood, while the protein-rich breakfast increased the tyrosine level in the blood. Another analysis also showed that high tyrosine levels predicted the more likely acceptance of unfair offers in the ultimatum game. By evaluating the personality data of the students, it could be ruled out that the changes in the social decision were not caused by differences in mood or personality, but were actually caused by the different breakfasts.

Based on these results, the researchers suspected that the protein-rich breakfast, which changed the test subjects' tyrosine levels, indirectly influenced the dopamine concentration in the brain. Previous studies had shown that dopamine plays a role in social decisions and reward expectations. The Lübeck study also showed that a change in the meal via tyrosine, the precursor to dopamine, can predict social decision-making behavior.

It is still unclear to which other areas of social decision-making behavior these results can be transferred. The duration and strength of the effect is currently being researched. It would be exciting to find out whether a diet tailored to the person could positively change their decision-making behavior in the long term. If this works, it would amount to a kind of neurodoping.