Guinness isn't that good for you, and other dietary myths exposed

As an NHS dietician I regularly hear diet myths used to justify eating habits.

Sometimes these notions have a basis in fact, but more often they're based on outmoded beliefs that don't bear any relation to the way our bodies actually work.

Food myths come from a variety of sources, but a common theme is that people believe them to be true - and then alter their diet accordingly.

Here are the most common myths and a scientific explanation of why they are flawed...

MYTH: Eating late at night makes you put on weight

It is a commonly held view that a late meal eaten before going to bed leaves calories unused and promotes weight gain. Research shows that eating late at night does not pile on the weight - as long as your daily calorie needs match your body's requirements.

A calorie is a calorie, it doesn't matter when you eat it - but the total number of calories eaten daily does matter. If you eat late at night there's a temptation to fit in an early-evening snack to control hunger, so boosting daily energy intake overall. But an identical meal eaten at 5pm or at 10pm has exactly the same effect, calorie-wise, in the body.

MYTH: You should not exercise immediately after eating

After a meal some ten to 15 per cent of our usual hourly blood flow is directed to the gut to aid digestion. When we exercise, muscles require more blood, too, to supply oxygen and nutrients. Both exercise and eating make competing demands on our circulation - the basis of this myth.

It's fine to exercise immediately after eating, so long as it is not so intense that the muscles take so much oxygen that the stomach struggles - leading to cramps.

So, no need to delay that after-dinner brisk walk or swim, although probably best not to start heavy exercise immediately after Sunday lunch. It will slow digestion and the food will slosh around in your stomach for longer.

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