Behaviour changer? Daniel Day/Getty

A low-carb diet might do more than affect your health – it could make you a more tolerant person. People who ate fewer carbohydrates for breakfast made more forgiving decisions in a money-sharing game they played a few hours later.

“Extreme [low-carb] diets might be influencing people’s behaviour,” says Soyoung Park of the University of Lübeck in Germany. This could be because less starchy meals tend to have more protein, which boosts levels of dopamine in the brain, involved in decision making.

Standard advice is that we should base our meals around starchy carbohydrates, such as bread, potatoes and pasta. Low-carbers tend to have a higher protein intake because they replace these foods with protein-rich meat, dairy and nuts.


Dietary protein affects the levels of an amino acid that is a precursor to dopamine in our blood. Since increasing the amino acid increases dopamine, and dopamine affects decision-making, Park wondered if a low-carb diet might change people’s behaviour. To find out, her team asked people to participate in the “ultimatum game”, in which you are split into pairs and your partner is given some money and they decide how much to share with you. If you accept the offer, both of you get the cash, but if you reject it, no one gets anything.

Urge to punish

Although in theory people should always accept – because even a small sum is better than nothing – in practice, people often reject low offers. We seem to have an urge to punish those who split the money unfairly, even if we suffer a small loss, says Park. It may reflect urges to deter antisocial behaviour. “It’s trying to punish cheaters and is supposed to foster a good society,” she says.

First, Park’s team asked 87 people what they had had for breakfast that morning and then got them to play the game. Those who had eaten a low-carb meal were more likely to accept unfair offers – 76 per cent did so compared with 47 per cent of the high-carb group.

Then they asked 24 people to come in for breakfast before playing several rounds of the game on two different days. The volunteers ate either a high-carb meal including bread, jam and fruit juice or a low-carb one including ham, cheese and milk, then switched meals on the second day. The team found people were more forgiving after a low-carb meal, accepting about 40 per cent of unfair offers compared with 31 per cent after the high-carb breakfast.

Since low-carb meals can affect our bodies in many ways, such as causing less of a blood sugar spike, the team took blood samples from the volunteers to work out what caused the effect. When they measured levels of the precursor to dopamine, a compound called tyrosine, they found that the low-carb meal raised people’s tyrosine more, and that high tyrosine correlated with forgiving behaviour. There was no such link seen with a range of other blood measurements, including glucose.

Reward signaller

Dopamine might have this effect because it is involved in signalling that we have experienced a reward. Perhaps people with higher baseline dopamine levels from their breakfast found a lower sum of money offered by their partner more satisfying and were therefore more likely to find their low offer acceptable, speculates Park.

On the other hand, people could accept lower offers for other reasons. They may feel less aggressive, says Park – or even more rational, since accepting low offers is economically the right thing to do. But irrespective of why, people’s breakfast did seem to be changing their behaviour.

Bahador Bahrami of University College London says that diet does seem to affect people’s decision making in this particular setting – but we don’t yet know how much it changes other kinds of behaviour. “This is a very specific probe of human cost-benefit analysis. We need the same to be shown in a number of other social decisions,” he says.

A previous study found that judges were less likely to approve prisoners for parole just before their meal breaks. It was thought this was because the judges felt hungry – but perhaps it was because they had low dopamine levels, says Park.

Journal reference: PNAS, DOI: 10.1073/pnas.1620245114

Further reading: Carb your enthusiasm: Are bread, pasta and spuds making you fat?