Of summer workouts, sweating and heat stroke

Sweating to lose weight is a dangerous fallacy says Kerry Senchyna, owner of West Coast Kinesiology

West Coast Kinesiology.

Of summer workouts, sweating and heat stroke

During the dog days of summer, temperatures can soar and heat can be a problem, especially when exercising. What happens when we sweat, and what are the dangers with heat build-up in the body?

Sweating due to hot temperatures and exercise is controlled by the hypothalamus in the brain, where heat-sensitive nerves are located. The function of the hypothalamus is also affected by inputs from temperature receptors in the skin. High skin temperatures reduce the thermostat setting for sweating and amplify the sweat response by dilating the blood vessels near the surface of the skin. You can tell this is happening because the veins on your feet and hands appear to get bigger and all of your skin becomes reddened, flushed with blood.

When these capillaries open up, they allow heat to get nearer the surface of the skin and water can then evaporate from the skin. This evaporation has a dramatic cooling effect on the skin and blood. When the humidity is low, that means that heated water will easily evaporate off the skin and releases heat into the air.

But there is a big problem when the humidity is high combined with exercise on a hot day. The sweat response is triggered, but water vapour isn’t easily created because there’s so much water vapour in the air already. Water forms at the skin, but doesn’t cool you because there’s little evaporation – the water just rolls off your body. This can cause a build-up of heat, and this can be dangerous, even life-threatening.

When you exercise hard, even at moderate temperatures, you can lose two litres of water per hour – and up to four litres in extreme conditions. Imagine the amount of heat it would take on a stove-top to boil away a pot containing just two litres of water in one hour. That’s an incredible amount of heat, and it’s a great deal of water to lose. Each litre weighs about two pounds, so that’s about four pounds of body weight lost, per hour.

If you do not drink that water back, you are asking for severe problems.

The blood has less water and becomes more sludgy. That means it can’t move as easily through your heart and blood vessels, doesn’t pick up oxygen as easily from the lungs, nor deliver it to the muscles well.

The same goes for all the waste products your muscles produce like lactic acid and carbon dioxide. Less oxygen to the muscles and brain means your performance in your sport starts to suffer – you become uncoordinated, clumsy, light-headed, even disoriented.

Even slight dehydration can negatively affect performance. A 200-pound man only needs to sweat away about two to four pounds of water before his performance is hampered. If you keep pushing yourself to exercise in this dehydrated state, the skin will start to reduce blood flow, so the muscles can continue to get blood, but this further reduces sweating and cooling, and dangerously increases core temperature.

This is why people who reach a state of heat-stroke (body temperature reaching 40.6  C or 105.1 F) are hot to the touch with dry skin.

Believe it or not, the practice of exercising while wearing a garbage bag in order to (mistakenly) lose weight still goes on. Every so often, you hear a news report about some teenager pushed to play hard in the sweltering heat wearing full football equipment, who dies due to heat stroke.

You are not going to lose weight by artificially sweating it away – you’ll gain all the weight back when you drink after your exercise session.

Another common belief is that sweating rids the body of toxins – it doesn’t. Sweat is just water, some salts and urea, all common compounds that the body deals with every day. The whole idea of detoxifying is problematic since the liver and kidneys do all the detoxifying we need, thank you very much.

To avoid poor performance due to dehydration, heat exhaustion or heat stroke, just remember, it’s best to drink a glass of water about 30 minutes before exercise, drink five to 0 ounces of cool fluid (water or sports drink, but not juice) every 15 to 20 minutes during exercise, then juice or sports drink to rehydrate after exercise. The amount depends on the temperature, humidity, duration and intensity of exercise, but basically drink a litre of fluid for every two pounds lost during exercise.

If in doubt, drink more.


Kerry Senchyna holds a bachelor of science degree in kinesiology and is owner of

West Coast Kinesiology.