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© flashpop/GettyIs it worth squeezing all your food into just 6 hours?
Not eating carbs after 6pm is a common diet tip, but here's a new idea. A small study of overweight men suggests that not eating anything at all after 3pm reduces appetite, cuts blood pressure, and may prevent diabetes.

Time-restricted eating has been found to stabilise blood sugar levels and reduce diabetes risk in mice, but rigorous studies in people have been lacking. To address this, Courtney Peterson at the University of Alabama at Birmingham and her colleagues tested a diet in 8 overweight men who were all on the threshold of developing type 2 diabetes.

For five weeks, the volunteers ate identical breakfasts, lunches and dinners under supervision. Half were assigned to eat all three meals within a 6-hour period ending no later than 3pm, while the other four ate theirs within a more normal 12-hour time frame. After five weeks, the groups swapped for a further five weeks.

The team found that limiting food to a 6-hour window led to a big increase in sensitivity to the hormone insulin - a sign of improved sugar control.

The time-restricted diet also reduced overall appetite and cut blood pressure by an average of 10 mmHg - about the same amount usually achieved by taking blood pressure medication.

Seize the day

These effects were not due to weight loss, since all participants were fed enough calories to maintain their weight. Instead, eating earlier in the day may have aligned better with their natural circadian rhythms.

Growing evidence suggests that humans are better at processing food early in the day. That's when our blood sugar control is at its best and when our close primate relatives usually choose to eat. In contrast, shift workers who eat during the night are known to be more prone to diabetes and obesity.

"We've evolved to be active during the day, so it makes sense for our metabolism to rev up at the beginning of the day and rev down at night to be as efficient as possible," says Peterson.

Feeling full

Eating for 6 hours and fasting for 18 hours per day may be easier than eating for 5 days and fasting for 2 days per week - as advocated by the popular 5:2 diet. All participants in the latest study, except for one, said they easily adapted to fasting in the evening. Their only complaint was that eating all their daily meals within 6 hours "made them feel stuffed" says Peterson.

Her team is now investigating whether giving people less food and extending the eating window from 6 hours to 8 hours per day makes the diet easier to follow and also leads to weight loss. This "8-hour diet" has been shown to promote weight loss in mice and has been popularised by US nutrition guru David Zinczenko, with celebrity followers reportedly including Hugh Jackman, Nicole Kidman and Miranda Kerr.

Journal reference: Cell Metabolism, DOI: 10.1016/j.cmet.2018.04.010