With people being urged to save electricity again this summer due to the crisis at the Fukushima No. 1 nuclear power plant, they have to be careful of possible heatstroke - a condition caused when the body loses its ability to control its temperature due to heat.
The most important measure to prevent heatstroke is to replace water lost in sweat.
When people sweat, not only water but also minerals including sodium, the main component of salt, are lost from their bodies. Therefore if people drink only water, the concentration of minerals in their bodies drops, which could cause nausea and muscle cramps.
Sports drinks effectively supply water, salt and sugar. However drinking them frequently is expensive.
Hideki Taniguchi, a professor at Kanagawa University of Human Services, a Kanagawa prefectural university specializing in health and welfare, recommends a homemade rehydration water. "It's possible to make an excellent drink for rehydration that contains components similar to commercially available sports drinks," he said.
Taniguchi says to mix three grams of salt (1/2 teaspoon) and 40 grams of sugar (4-1/2 tablespoons) into one liter of water. Stir the mixture well until the grains dissolve. You can use tap water, but boil the water once to remove chlorine and cool it to room temperature.
Next, drop a twist of lemon or grapefruit into the mixture. The juice flavors the water and adds potassium, which helps muscle function. Cool it in a refrigerator before drinking. Cool water lowers body temperature and is said to stimulate the small intestine, which absorbs water.
It is not healthy to drink the rehydration water every day in lieu of normal water. This is because excessive intake of salt and sugar could lead to lifestyle-related diseases, including high blood pressure. It is best to drink the rehydration water only when you have a poor appetite due to summer heat or when you sweat a lot after jogging or hiking.
"If you drink a large volume of [rehydration] water at once, it could easily be lost through urine. I recommend drinking the water in several sittings, for example, one cup (about 200 milliliters)," Taniguchi said.
Especially when exercising in the summer, people should pay careful attention to water supply as muscle exertion creates a lot of heat and the risk of heatstroke increases. In the case of intense exercise, even if for only a short time or when the temperature is not very high, a person should not forget the importance of drinking water.
Even though people lose water through sweat while exercising, they don't feel thirsty right away. As a result, people tend to suffer a water deficiency.
However, excessive hydration is also unhealthy. When people drink too much water in a marathon or other endurance activities, they could develop water intoxication caused by a decrease in sodium concentration in the blood. The imbalance is caused by excessive water intake, which could lead to problems such as pulmonary edema in serious cases. There have been deaths from this in marathons.
So how much water should a person drink during exercise?
According to the Sports Medicine and Science Research Committee of the Japan Sports Association in Shibuya Ward, Tokyo, it is desirable to rehydrate to ensure water loss does not exceed 2 per cent of body weight. For example, a 60-kilogram person should drink enough so that water loss does not reach 1.2 liters.
The committee recommends that people weigh themselves before and after exercise. If the weight loss is less than 2 per cent of body weight, the person is sufficiently hydrated and should be fine.
The association sets a target volume of water for different activities. In the case of soccer, players should drink 250 milliliters to 500 milliliters before the game and 500 milliliters to 1,000 milliliters during the game. It is recommended to drink several cups of water over time, rather than drinking it all at once. As salt is also lost in sweat, 0.1 per cent to 0.2 per cent salt should be included in the water.
Shizuo Ito of the association's Sports Science Laboratory said: "These figures are just guidelines. The important thing is to prepare an environment in which people can rehydrate during exercise when they want to do so."
Elderly need to take extra care
Elderly need to take extra care
Seiichi Nakai, a professor of exercise hygiene at Kyoto Women's University, said, "The elderly are prone to heatstroke, so they must pay careful attention to hydration."
There are several reasons they develop heatstroke symptoms.
The first is that elderly people have a low sensitivity for heat. When it's hot, young people naturally feel thirsty and actively replenish water. But as aging desensitizes the elderly to heat, they tend not to rehydrate themselves.
Second, elderly people's bodies have less water. Water occupies about 60 per cent of a young person's body weight, but the amount in the elderly is only about 55 per cent. The third reason is that the body's heat regulating mechanism in the elderly is poor and body heat is not easily dispersed.
According to the Fire and Disaster Management Agency, the number of people taken to the hospital by ambulance due to heatstroke from July through September last year was about 39,000 nationwide. About 17,000 of them, or more than 40 per cent, were elderly.
Nakai recommends elderly people take in 2.2 liters to 2.5 liters of water a day because that is about the amount they lose through urine and sweat.
For example, for a person who takes in 2.5 liters of water, drinking water makes up about 1.2 liters--water drunk after waking up, tea with meals, coffee, etc. One liter comes from water contained in food. The remaining 0.3 liter is produced by metabolism, which oxidizes energy-containing substances in food.
"The target figures are the minimum required volume. If elderly people sweat a lot in the heat, they should take in salt through sports drinks [in addition to the target amount]," Nakai said.