Article content

If you were ever told to wait 30 minutes after eating before swimming, that was well-meaning but misguided advice. The fear is that you will get a cramp and drown. And while not entirely far-fetched, the biological reality is far more nuanced.

The basic premise of the argument goes something like this: Eating a large meal will leave a large bolus of food in your stomach that your body has to digest. You will need to redirect extra blood flow to your stomach to aid in digestion, which means less blood is available to send to your muscles. Now deprived of oxygen, your muscles will cramp up, you won’t be able to swim, and you will risk drowning.

Article content

Superficially, the physiology is not wrong. The sympathetic nervous system regulates the fighter-flight response that increases blood flow to the muscles and away from the intestine during exercise — whereas the parasympathetic nervous system is nicknamed “rest and digest” because it does the opposite and increases digestion. The rationale here is that eating a large meal would upregulate the parasympathetic nervous system and downregulate the sympathetic.

This is probably true to a degree. A large meal is going to make exercise more difficult. After all, most people don’t eat a steak dinner and then immediately go run a 5K.

The science somewhat bears this out. One study tested 40 people who suffered from exercise-related transient abdominal pain (ETAP), the scientific name for a stitch in your stomach, which is actually distinct from a muscle cramp. This study had them exercise after drinking water, sports drinks or fruit juices. The fruit juice induced more symptoms of bloating and ETAP, which suggests the high sugar load of the fruit juice was the responsible factor.

Article content

But there are some problems with this argument. First is the timing. It takes more than 30 minutes for you to digest your food. It takes a few seconds for food to pass from your mouth, down your esophagus, into your stomach. But it spends roughly a couple of hours in your stomach before passing into your small intestine where the real digestion happens. After several hours, what’s left passes into your large intestine so most of the water in the food can be reabsorbed. Roughly one day after you ate it, the food is finally expelled. Waiting 30 or even 60 minutes is likely trivial when you consider that it takes you roughly 24 hours to pass a meal through your entire gastrointestinal system.

One survey of 848 participants in a community run in Australia found that consuming a large meal one to two hours before running increased the risk of ETAP, so the commonly suggested 30-minute window is probably too short to do anything meaningful.

But an important distinction has to be made between athletes going for a run and someone splashing around in the sea during a beach vacation. The purported danger of eating and then swimming assumes that you are going to be swimming alone in deep water with no one to help you when you get such a debilitating muscle cramp that you would be unable to even float on your back. This is not how most people enjoy the seaside. If you’re planning to lounge in waist-deep water at the beach while floating on a pool pillow, even if you did get a severe cramp you could simply stand up and walk back to your deck chair with ease.

Alcohol, not food, increases the risk of drowning. Alcohol is involved 70 per cent of the time and is the one thing you should avoid if you plan to swim or go boating in deep water. Otherwise, enrol your kids in swimming lessons, use life-jackets on boats and put a fence around your backyard pool. There are many ways to prevent drowning. But waiting 30 minutes after a meal isn’t one of them.

Christopher Labos is a Montreal physician, co-host of the Body of Evidence podcast and author of Does Coffee Cause Cancer?

Share this article in your social network