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Eating meals later on weekends than during the week may cause weight gain by messing with the body’s metabolic rhythms.

We already know that shift workers and people with disrupted sleep patterns are prone to weight gain. This is probably because they are more likely to eat meals at night when our bodies aren’t used to processing food, which seems to lead to the storage of extra fat.

María Fernanda Zerón-Rugerio and Maria Izquierdo-Pulido at the University of Barcelona in Spain and their colleagues wondered if smaller disruptions to normal eating schedules – like eating meals later on weekends – might have similar effects. “It’s common to sleep in on weekends, so we end up having breakfast later and then lunch and dinner tend to be a bit delayed too,” says Zerón-Rugerio. “We call this eating jet lag.”

The team surveyed more than 1100 university students in Spain and Mexico to find out what time they normally ate breakfast, lunch and dinner on weekdays and weekends.

Almost two-thirds had an hour or more of eating jet lag on weekends, meaning the midpoint between their first and last meal was at least an hour later on weekends than on weekdays. Breakfast was the most delayed meal, tending to become brunch.

The greater the students’ eating jet lag, the more likely they were to be overweight. Those who reported more than 3.5 hours of eating jet lag on weekends had body mass indexes (BMIs) that were 1.3 units higher on average than those with no eating jet lag. This wasn’t related to the quality of their diet, how long they slept or how much they exercised.

Bye bye, brunch

This suggests that eating at the same time every day may help people lose stubborn excess weight, says Zerón-Rugerio. Dropping 1.3 BMI units is equivalent to someone who is 170 centimetres tall and weighs 90 kilograms losing 4 kilograms.

The reason why eating jet lag is associated with weight gain is probably because our internal biological clocks – known as circadian systems – prepare our metabolisms to process food at specific times of the day, says Izquierdo-Pulido. “Say you usually have breakfast at 7 am but then on weekends you have it at 9 am. Your biological clock doesn’t know it’s the weekend so it’s going to prepare your body to eat at 7 am, and then it gets confused when you actually eat at 9 am,” she says.

Jonathan Cedernaes at Uppsala University in Sweden agrees. “Molecular 24-hour clocks in all our cells determine what time of day the body should be prepared and optimally able to take up or utilise specific nutrients,” he says. “If we keep to a certain meal schedule – and also maintain a regular sleep-wake schedule – then this allows our metabolism to consistently allocate a time of day when we will be able to burn off excess nutrients.”

Zerón-Rugerio and Izquierdo-Pulido now plan to investigate the long-term impact of eating jet lag by comparing weight change in students with consistent and variable eating schedules. They are also interested in testing whether eating meals at the same time every day helps people lose weight.

Journal reference: Nutrients

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