What Does Sweat Loss Actually Do During Exercise?

Metabolism specialist Dr Piero Fontana explains in an interview how to measure your sweat loss and from what point you should compensate for salt loss.

Dr Piero Fontana, how much does a person sweat at rest, and how much during physical activity?

During moderate physical activity, sweat loss is around 0.5 to 2 litres per hour. The higher the intensity of the activity, the more heat the body produces. This in turn triggers increased sweat production to cool the body down. Intense exertion in a warm environment can therefore lead to sweat losses of up to three litres per hour.

What's the simplest way to work out your personal 'sweat volume'?

By weighing yourself before and after physical exertion. However, you also need to factor in the amount of fluid taken in during the activity, as well as any urine passed over that period. Ideally, you should weigh yourself naked, to avoid measurement errors caused by sweat-soaked clothing.

At what level of fluid loss does performance start to suffer?

Physical performance can already be negatively affected by a body weight reduction of 1 to 2 per cent, which for an 80 kg man corresponds to roughly 1 to 1½ kilos. However, just like sweat production itself, the relationship between sweat loss and reduced performance is highly individual and difficult to quantify.

Which factors influence sweating, and above all, how do they influence it?

Besides physical activity, it's mainly environmental factors such as temperature and humidity that influence sweat production. Sweating draws heat away from the body because the sweat evaporates on the skin. Hot, humid climates, however, prevent the sweat from evaporating, so it just drips off instead, which has no cooling effect. As a result, the body increases sweat production and loses more fluid. Cooler, drier climates, for example at altitude, help the body release heat, so sweat production is lower there. Clothing also plays a role. Clothing generally reduces air circulation close to the surface of the body. This in turn hinders heat loss through sweating. Good clothing should offer protection from the weather while also being well ventilated, so the sweat can evaporate properly. Depending on the material, clothing absorbs moisture and sweat to differing degrees. The more it holds, the heavier the clothing becomes.

Which substances do you lose through sweat?

Sweat mainly contains electrolytes such as sodium, chloride and potassium. But various minerals, such as calcium, copper, magnesium, iron and zinc, are also dissolved in it as ions.

How much salt is lost, and how much does it vary? Are there people who sweat out considerably more salt than others?

Just as with the volume of sweat lost, individual factors also play an important role in electrolyte loss. Acclimatising to a hot climate lowers both electrolyte and mineral levels, so that the greater sweat losses don't also lead to greater electrolyte and mineral losses. Similar regulatory mechanisms are expected to develop through regular physical activity.

Which substances should you replenish during physical activity?

The most important thing is to take in fluid regularly during physical activity. Ideally, intake should match your previously determined individual sweat loss, but during intense exertion it shouldn't exceed one litre per hour, to avoid placing too much strain on the digestive system. For physical activity lasting two hours or more, it's recommended to drink beverages with a salt content of 1.2 to 1.8 grams of sodium chloride per litre. The remaining mineral losses don't need to be covered straight away through fluid intake — that can happen later through normal food intake.

Source: Fit For Life, 7-10 / Interview: Andreas Gonseth