This is an episode from the predecessor of the regular column ‘5 questions’: QED. Each instalment focused on a single question.
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According to the founder of the Scout movement, Baden Powell, anyone who swims too soon after eating risks drowning. Is he right?
It is a rule of thumb that many children find deeply irritating on beaches and at the edges of swimming pools: wait an hour after eating before going in the water. Swimming too soon is supposed to be dangerous. But is there actually anything to it?
‘Swimming after eating is not dangerous’, says exercise physiologist Eric van Breda (University of Antwerp / Doping Authority). ‘It can be uncomfortable, though.’ Eating a large or fatty meal poses a challenge for the stomach. ‘The stomach is a large muscle that tries to knead your food. If there’s a big lump to deal with, that becomes difficult’, Van Breda explains. ‘Fatty food requires bile for digestion, which takes place after the stomach. Whereas other food is broken down into ever smaller pieces by gastric juices and the like, fatty food remains a larger mass. That can result in the feeling of a stone sitting in your stomach.’ Not pleasant – and no less so when you’re swimming.
Gastric emptying
This may actually be worse during swimming, Van Breda notes. ‘In a horizontal position, as is often the case when swimming, gastric emptying works less efficiently. Some food can therefore remain behind – that stone in your stomach. When you’re upright, gravity helps more effectively to move food out of the stomach.’
Although a lump of half-digested food may be unpleasant, it poses no real danger. ‘In the past, it was thought that after eating, more blood flows to the digestive system, leaving less for the muscles, meaning you would be unable to move – or swim – properly. But there is simply no evidence for that. On the contrary, if you swim intensively – whether or not you have eaten beforehand – more blood goes to your muscles. That takes priority’, says Van Breda. The same phenomenon can have an unpleasant consequence for marathon runners, for example: ‘You sometimes see them getting diarrhoea. It runs out of them, because digestion has temporarily been more or less shut down.’
Eating a large or fatty meal poses a challenge for the stomach
Not pretty, but not life-threatening either. So where does this persistent myth come from? Van Breda points to Scout movement founder Baden Powell. In his handbook (Scouting for Boys, 1908) the rule can indeed be found: ‘Never bathe in deep water very soon after a meal, it is very likely to cause cramp, which doubles you up, and so you get drowned.’ That claim appears to have been plucked from thin air. Although stomach cramps after eating are possible, they do not in principle seriously affect a person’s ability to keep their head above water. Even with muscle cramp, getting to the side is generally not a problem for a competent swimmer, and according to Van Breda there is in any case no evidence that eating is linked to the onset of muscle cramp.
Blood sugar
The only thing that could potentially be dangerous when swimming after eating is so-called reactive hypoglycaemia. ‘In response to the sugars that enter the bloodstream from food, the body produces insulin, causing blood sugar levels to fall again. If you exercise at that moment, your muscles will also be demanding sugar. Blood sugar levels can then drop too far. In extreme cases, you could faint’, Van Breda explains. ‘If you were in the water at that point, that could be dangerous. That said, people who feel such a dip will probably not go swimming.’
Awkward questions from pupils? Send your questions to redactie@bionieuws.nl.