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If you eat three meals a day, does it make a difference if they are taken within, say, an 8-hour window or a 14-hour one?

Lewis O鈥橲haughnessy, Nottingham, UK

Human metabolism is incredibly complex and no one has developed the perfect way to cheat the system and lose weight.

Time-restricted fasting has become a popular way to lose weight or retain a healthy weight, but, on the whole, studies have shown little impact to body weight when the only variable is the time over which we eat. Where there have been reports of weight loss resulting from time-restricted fasting, these impacts are likely to result from one of two causes.

First, restricting the time in which you can eat makes it harder to eat too much without getting full. This is especially true if you eat less-calorie-dense foods like vegetables, which can physically fill your stomach. If you ate exactly the same food over an 8 or 14-hour window, there would be negligible impact on your weight.

Second, as with any diet, time-restricted fasting makes us more aware of what we are eating. This means many people who follow it tend to have diets that are better balanced or eat less overall. Eating main meals late at night, however, can have detrimental effects on sleep and overall health, as a result of fluctuations in hormone levels.

Mark Mattson,, Johns Hopkins University, Baltimore, US

Evidence suggests that 16-18 hour daily fasting periods provide clear health benefits in terms of reducing risk factors for diabetes and cardiovascular disease.

A defining feature of the eating patterns in intermittent fasting is that they must include periods of time with no calorie intake. These must be of sufficient duration to deplete the glucose stores in the liver and then release fatty acids from fat cells which are then converted into chemicals called ketones. The liver stores about 400 to 500 calories of glucose, which will last for about 10 or 12 hours. For the metabolic switch from glucose to ketones to be fully engaged, a person must fast for at least 14 hours.

鈥淔asting for 16 to 18 hours a day has clear health benefits in terms of reducing the risk of cardiovascular disease and diabetes鈥�

Exercise can accelerate the onset of the metabolic switch during fasting. For example, if a person goes on a 1-hour run in the morning before eating breakfast, they will deplete their liver glucose stores and switch to 鈥渇at burning mode鈥� during their run. Recent research suggests that intermittent fasting can enhance many of the health benefits of exercise.

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