Could changing your mealtimes improve your health?
Six years ago, during the London Olympics, a documentary I’d made, Eat, Fast, Live Longer went out on BBC Two. It was up against the 200m finals, featuring Usain Bolt, so I wasn’t optimistic that it would be a ratings success. It actually did better than I was expecting and in time, unleashed huge interest in intermittent fasting, IF. The idea behind IF is that you can lose weight and get other health benefits by intermittently cutting your calories.
At the time I was investigating the benefits of cutting calories a couple of days a week, as part of the 5:2 diet. But there are other forms of IF, including Time Restricted Feeding (TRF). TRF normally involves eating within an 8-hour window and fasting for 16 hours a day. But what is the evidence that this is healthier than normal patterns of eating?
The 16:8 approach is based largely on the work done by Prof Satchin Panda from the Salk Institute. In a 2014 study, he and other researchers subjected hundreds of mice to different lengths of daily fasts, ranging from 12 to 15 hours. They found that the benefits were proportional to the length of the fast, but as long as the mice went at least 12 hours without eating they remained healthier and slimmer than those who ate the same number of calories spread out. What was surprising was the protective effects were maintained, even when the mice were allowed unlimited access to food during weekends.
All this is great if you are a mouse, but how well does TRF work in us? There have been surprisingly few human studies, though the ones that have been done are generally positive. There was, for example, a small study carried out by the University of Alabama where 11 overweight people were asked to spend four days eating between 8am and 2pm and four days eating between 8am and 8pm. They ate the same number of calories both times and completed testing under supervision. The researchers found that eating only between 8am and 2pm reduced daily hunger swings and increased fat burning during the night. It also improved metabolic flexibility, which is the body’s ability to switch between burning carbs and fats. Crucially, it didn’t lead to weight loss.
On Trust Me, I’m A Doctor, we decided to test TRF. With the help of Dr Jonathan Johnston at the University of Surrey, we recruited 16 volunteers and randomly assigned them to one of two groups, the blues or the reds.
The blues, the control group, were asked to carry on as normal. The reds were asked to stick to their normal diet but move their breakfast 90 minutes later, and dinner 90 minutes earlier. This meant that for three extra hours each day they went without food. Everyone kept a food and sleep diary to ensure that they were eating the same amount as normal. After 10 weeks we found that the group who had eaten breakfast later and dinner earlier had, on average, lost a bit more body fat and seen significant falls in blood sugar levels and cholesterol levels, compared to the control group.
This was a small study and it didn’t run for long. But there do seem to be some benefits of extending your daily ‘fast’ – and it is a good idea to avoid the midnight cheeseburger.