Extreme fasts, such as replacing meals with liquids, shouldn’t be done at home

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Being biologically younger than your actual age may lower your risk of heart disease, cancer and dementia. A new discovery suggests it may also aid with weight loss, possibly by making the body more efficient at switching from using food for energy to using stored fat.

In February, a review of existing research concluded that intermittent fasting – such as the 5:2 diet, where you eat as normal on five days a week and restrict calories on the other two – is probably no more effective for weight loss than doing nothing at all. But fasting can also involve eating no, or very little, food for a single, continuous stretch of time, which is sometimes done for religious reasons.

Focusing on this type of fasting, Steve Horvath at the University of California, Los Angeles, and his colleagues recruited 32 volunteers with no known health conditions, who consumed just 200 to 250 calories a day in the form of fruit juice, vegetable broths and honey for 12 days. “This is not something to try at home,” says Horvath. “It was a medically supervised inpatient programme at a specialist clinic, with clinical monitoring throughout.”

These calories were consumed in three windows: 7am to 9am, 11.30am to 1pm and 6.30pm to 8pm. This put the participants – who were aged between 25 and 72 – in a fasting metabolic state, says team member Robin Mesnage at King’s College London, which occurs when the body shifts from burning energy from food to energy stored in the body.

The team used 12 different epigenetic clocks to estimate and track the volunteers’ biological ages over the course of the study. These mostly work by looking at patterns of chemical tags on DNA, which change in predictable ways with age, but can also be altered by our lifestyle, such as how much we exercise.

The clocks varied in their responses to the fast. Some ticked upwards to an older age, suggesting that the extreme fast put negative stress on the body. But one clock, which was built to detect changes in body mass index, went down.

Incongruous results are a known issue between different epigenetic clocks. But one finding was fairly consistent: the participants who were generally deemed to have a young biological age, relative to their chronological age, at the start of the study lost more weight than those who were biologically older. 

The researchers weren’t expecting this result, according to Hovarth. “Honestly, we don’t know the mechanism,” he says. But he thinks the most likely explanation is that biologically younger people have better metabolic adaptability, which means they can swiftly switch from using food for energy to stored fat when calories are scarce. 

This is plausible, because biologically younger people tend to have more efficient mitochondria, which leads to faster switching to fat burning, says Paulina Correa-Burrows at the University of Chile.

Biologically younger people may also have better pathways for sensing nutrients in cells, which could also enable more efficient switching between fuel sources to burn fat. But Correa-Burrows cautions that the group of volunteers was small, so the results are inconclusive.

Horvath now wants to see investigations into whether biologically younger people also lose more weight under less restrictive diets. “A 12-day medically supervised fast is not a long-term diet and it isn’t meant to be one,” he says.

In the meantime, a healthy lifestyle can help lower our biological age. “The habits with the strongest evidence for healthy epigenetic aging are the familiar ones: do not smoke, [do] physical exercise, keep a healthy body weight and maintain good relationships,” says Hovarth.

Do not change your diet without first speaking to a doctor.