Life in the (intermittent) fast lane: the health benefits of time restricted diets

Have breakfast a little later, and dinner a little earlier, and reap the health benefits.

One of the biggest health trends in 2019 was ‘intermittent fasting’. There is every sign that interest will continue in 2020, and I like to think I am partly responsible.Seven years ago I wrote a book called The Fast Diet, all about the benefits of intermittent fasting. The book featured a new approach to dieting called the 5:2, which involves cutting your calories to around 600 calories for two days a week, and eating healthily for five days.

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In the book, I also wrote about Time Restricted Eating (TRE). The idea here is that you simply extend your normal overnight fast. If you stop eating at 20:00 and don’t start eating again until 10:00, that is a 14-hour fast. That way you get some of the benefits of intermittent fasting without worrying about calorie counting.

The original TRE studies were in rats, and it took a while for human studies to get going. In fact, the BBC series Trust Me, I’m A Doctor was one of the first to do a human TRE study with Dr Jonathan Johnston of the University of Surrey.

For this study, which was later published in 2018 in the Journal Of Nutritional Science, we recruited 16 healthy volunteers and measured their body fat, blood sugar and cholesterol levels. They were then randomly assigned to TRE or a control group.

The TRE people were asked to stick to their normal diet but to move their breakfast 90 minutes later, and their dinner time 90 minutes earlier. This meant that for three extra hours each day they were without food (fasting). Everyone kept a food and sleep diary to ensure that they were eating the same amount as normal.

Life in the (intermittent) fast lane: the health benefits of time restricted diets © Ovadia Benishu
© Ovadia Benishu

Ten weeks later, we repeated the tests. We found that the group who had eaten breakfast later and dinner earlier had, on average, each lost around 1.6kg of body fat. They had also seen bigger falls in blood sugar and cholesterol than the control group.

A more recent study, published in the journal Cell Metabolism on 5 December 2019, did something similar. This time, researchers took 19 overweight volunteers with metabolic syndrome (raised blood sugar, blood pressure and cholesterol) and asked them to eat within a 10-hour window.

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Although the recruits were asked to eat as much as normal, many of them said they were eating less, due to the shorter eating window. As with the Trust Me study, the participants found that extending their overnight fast led to a significant reduction in body fat, blood pressure and total cholesterol. Blood sugar and insulin levels also improved.

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Metabolic syndrome is a risk factor for developing type 2 diabetes. Therefore if we can prevent people with prediabetes from developing diabetes by adapting their fasting window, then it could save the NHS billions of pounds a year. I look forward to seeing more intermittent fasting studies in 2020.