Waiting two hours before you swim saves you from nothing

They told you at the edge of the pool, sandwich still in your hand: two hours before you go in. In some houses it was ninety minutes, in others three hours, in others until the colour came back to your face. Nobody explained where the number came from, and the number did not come from any hospital.

The earliest trace is in a 1908 scouting handbook, and the two hours you inherited were set by the Australian swimmer Annette Kellerman in her book How to Swim, of 1918.

When somebody finally went looking for the cases, they were not there. In 2011 the scientific advisory council of the American Red Cross reviewed the available evidence on swimming after eating, found nothing linking having eaten to drowning, and filed the recommendation as a myth. The Spanish Society of Emergency Medicine agrees. The "digestive cut-off", as people tell it, does not exist.

The popular theory has an apparent logic, and that is what has kept it alive for a hundred years. When you eat, some of the blood is redistributed towards the digestive system; if the muscles are left without supply, the reasoning goes, you cramp and you sink. The problem is the scale. The redistribution is real and it is modest, and it does not leave your arms without oxygen. The cramps swimmers actually get come from fatigue, from cold or from hours without drinking, never from the steak. If eating prevented swimming, no long-distance swimmer could touch food during a three-hour crossing, and they all eat during the crossing.

Immersion syndrome, also called hydrocution, does exist. It is a reflex. When the skin goes abruptly from a lot of heat to a fair amount of cold, the nervous system reacts all at once. The surface vessels close, blood pressure rises, the heart may bolt and in severe cases it stops. An involuntary gasp also appears, and if that gasp catches you with your face in the water, you swallow. The rest is an ordinary drowning, with one peculiarity worth knowing: it starts within the first half minute, before you have got used to the water.

What decides is the difference in temperature and the abruptness with which you cross it. Prevention advice talks about gaps of more than five degrees between skin and water. The risk rises when the water is clearly below the temperature of your skin, when you have spent a long stretch in the sun, and when you go in with a dive. A heavy meal and alcohol make the picture worse, but as companions. The clock on the meal makes no difference.

So the useful rule is not "wait two hours". It is this one, and it holds as well at eight in the morning as at five in the afternoon:

Go in by parts and in this order: feet, wrists, nape, chest. At the nape and at the wrists the blood runs very close to the skin, so wetting them first warns the body what is coming.

Do not dive or jump in head first after a stretch lying in the sun, even if the water feels mild to your foot. What the foot registers tells you nothing about the blow the torso will take.

If you have been in the sun a long time, take a shower first or sit five minutes in the shade. What that brings down is the degrees of difference, which is the thing that can floor you.

With children, it makes no difference when they ate. What counts is that an adult is watching the water and not the phone.

If you have been drinking, your problem is no longer the temperature.

And the warning sign, so that you know it: if on going in you feel sudden dizziness, ringing in your ears, tingling in your arms or a cold that climbs all at once, get out. Do not wait to see whether it passes. Get out of the water, sit down and wait for it to go.

So eat the sandwich whenever you like. What you have to do is the other thing: wet the back of your neck and your wrists, go in feet first, and take half a minute to put your chest under.