Bad luck with the cut-off date: the month of birth influences how healthy schoolchildren eat

Anna Sandner

16.2.2026

Translation: machine translated

How the month of birth determines health: The youngest children in a class eat fewer vegetables, drink more soft drinks and are more often overweight.

If you were always one of the youngest at school, you know the feeling of constantly having to catch up. A new study by the University of Trier shows that this «relative age effect» not only plays a role in grades or sports, but also on the plate. The result: the youngest children in a class eat more unhealthily and have a measurably higher risk of being overweight.

Dr Sven Hartmann and his international team scrutinised a huge amount of data. They analysed the data from around 600,000 schoolchildren aged between ten and 17 from 30 European countries. The data was provided by the WHO long-term study «Health Behaviour in School-Aged Children» from 2001 to 2018.

A small age difference with a significant effect

Specifically, the researchers found that the youngest children in a year group were around two percentage points more likely to be overweight than their older classmates. Sounds like little? But it's not.

You can visualise it like this: If you take a group of 100 older schoolchildren, statistically around 14 of them will be overweight. If you take an equally large group of the youngest schoolchildren in the same year, the figure is 16, which means that two more young people are affected in every group of 100 simply because of their month of birth. What seems like a small fluctuation has a significant effect on the population as a whole.

More soft drinks, fewer vegetables

However, it's not just about weight, but also about the patterns of daily eating. Younger people eat fewer vegetables, drink more soft drinks and skip meals more often. This effect is particularly noticeable in boys. Paradoxically, they also diet more often. Important for categorisation: the researchers statistically factored out other factors such as family wealth or household size. In addition, only countries in which all children start school on the same date were analysed - federal systems with different deadlines, such as in Germany or Switzerland, were excluded in order to keep the data clean.

Already known for some time: Younger people have a harder time

The phenomenon is not new in science. People who have their birthday shortly before the deadline for starting school are often up to twelve months younger than the person sitting next to them. A UK study involving over one million children showed back in 2019 that the age difference within a class is not without consequences. According to the study, the youngest children have an increased risk of being diagnosed with ADHD. They are also more susceptible to depression. It was not until 2025 that a Norwegian register study, based on national health data from over 1.1 million adolescents, reconfirmed these relative age effects.

The Trier study now provides the missing piece of the nutritional behaviour puzzle and also found lower grades and less sport among the «late-borns».

Why do younger people eat less healthily?

Why the date of birth determines whether an apple or a chocolate bar ends up in the stomach has not yet been conclusively clarified. However, study author Hartmann has a hunch: «The relatively younger pupils may have more mental problems, which in turn can reinforce unhealthy eating behaviour». There may also be social pressure. Those who are younger often orientate themselves towards their elders and also adopt their bad habits. However, these mechanisms remain hypotheses. Although the study shows the statistical correlations, it provides no proof that stress or lack of sleep are the direct triggers.

An interesting observation: younger people skip breakfast more often. The researchers interpret this as a possible time problem that has a direct impact on the scales. Younger children often need more sleep. When school starts early, time is short and breakfast falls victim to time pressure, which in turn favours cravings and unhealthy snacks during the day.

School meals mitigate the effect

In addition to the bad news, the study also has solutions up its sleeve. The country comparison showed that the negative effects are significantly smaller where school meals are provided across the board. Hartmann therefore advises people to pay attention: «It is important to sensitise parents and teachers to the particular challenges faced by relatively young pupils». After all, it remains to be seen whether eating behaviour will automatically return to normal in adulthood due to a lack of long-term data.

The time of day could also be changed. Starting school later would even help twice over. Young people would get the sleep they need and have time for a proper breakfast in the morning.

Anna Sandner

Senior Editor

Anna.Sandner@galaxus.de

Science editor and biologist. I love animals and am fascinated by plants, their abilities and everything you can do with them. That's why my favourite place is always the outdoors - somewhere in nature, preferably in my wild garden.

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