When young adults start university, they often gain weight. In the United States, they have a name for this phenomenon: the "freshman 15", referring to the 15lbs typically accrued during students’ first year of living away from home. In part, this weight gain can be explained by the substitution of home-cooked meals for ready meals and fast food, combined with reduction in physical activity.
Increasingly, however, scientists are fingering an additional suspect: circadian disruption, brought about by a culture of late-night eating, drinking, and inconsistent sleep patterns.
For decades, we’ve been told that weight gain, together with associated diseases such as type 2 diabetes and heart disease, are a simple matter of the quantity and type of food we consume, balanced with the number of calories we expend through exercise. But mounting evidence suggests that timing is also important: it’s not just what you eat, but when you eat that matters.
The idea that our response to food varies at different times of day dates back a long way. Ancient Chinese medics believed that energy flowed around the body in parallel with the sun’s movements, and that our meals should be timed accordingly: 7-9am was the time of the stomach, when the biggest meal of the day should be consumed; 9-11am centred on the pancreas and spleen; 11am-1pm was the time of the heart, and so on. Dinner, they believed, should be a light affair, consumed between 5pm and 7pm, which was when kidney function predominated.
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Although the explanation is different, modern science suggests that there is plenty of truth in that ancient wisdom.