Ever noticed that chilli is such a popular ingredient in hot countries?
Drinking warm chai tea in India, slurping hot phở in Vietnam or getting that kick of spicy salsa in Mexico will surely make you sweat. While you might rather opt for a chilled drink come summer, eating hot or spicy foods can actually cool you down on a warm day – if the conditions are right.
Keeping the balance
Our bodies are well-equipped to balance the heat. It's produced by metabolic processes (eating) with mechanisms which keep us cool if we are in a hot environment, or it will retain that heat if we feel relatively cold.
Sweating, or evaporative cooling, is the main physiological way that we release excess heat from our bodies. Blood flow to the skin, feet and hands first increases to transfer heat to our periphery; then heat is carried away from our skin when sweat turns into water vapour. Humans have millions of sweat glands in our skin: in a patch the size of a 5 cent piece, you can have anywhere between 440 and 1000 sweat glands.
The human body is so adept at controlling its temperature with this mechanism, not to mention a toolbox of others, that we can keep our core temperature stable within one degree or less (36.5-37°C) when in a comfortable resting state. With exercise and environmental fluctuations, we regulate our body temperature. “2-3°C either side of this thermoneutral state”, says Nigel Taylor, Associate Professor of Thermal Physiology at the University of Wollongong – thermoregulation is strict business.
Make you sweat
Spikes in temperature, whether internally or in the external environment, make us sweat rapidly. Receptors, or nerve endings, sensitive to temperature changes are called thermoreceptors. They notify the hypothalamus - the thermostat in our brains - if the mercury is rising (or falling) and our body swiftly responds.
When you consume hot drinks, the sweat response is triggered from thermoreceptors in and around the stomach that notice the additional heat; an increase of your deep-body temperature between 0.2-0.5°C is all that is required.
In spicy foods it is capsaicin, the active ingredient in chilli, that stimulates thermoreceptors. However, unlike hot drinks in the stomach, this time it's in the mouth, and we perspire. This response is called gustatory sweating since it relates to taste. Sweating is concentrated around the forehead and neck and it can be triggered by other compounds like caffeine, too.
Critically, whether sweating is an effective cooling mechanism or just an unpleasant inconvenience depends on airflow and how much water is already in the air. The sweat beading on our skin will evaporate faster if it is a “dry” day with low ambient humidity and if evaporation is not obstructed by clothing. If the air is humid, the sweat has nowhere to go.
Has it been tested?
Dr Ollie Jay, who heads up of the Thermal Ergonomics Laboratory at the University of Sydney, has for many years been studying with how humans regulate their body temperature under different conditions. His research has shown that if you consume a hot drink the overall amount of heat stored in the body is less than if you have a cold one. The cooling effect of sweating outweighs the added heat.
“As long as the sweat can evaporate, you can theoretically be better off but the effect is not that large,” Jay says. “The main thing is that our bodies shut sweating down when drinking a cold drink – so you are actually no better off with a 1.5°C drink than you are with a 37°C drink.”
However, if you can’t swallow a steaming hot drink this summer, Jay suggests that avoiding ice-cold drinks on a hot day or when exercising intensely and instead, drinking fluid at a palatable temperature will likely cool you down.