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Is it bad to do aerobic exercise in the heat ? It seems to me that the whole point of aerobic exercise is to load your heart. Heat does that - it requires your heart to pump extra blood to your skin to regulate body temperature. Of course, it doesn't matter if that blood is oxygenated or not, unlike the blood that fuels your muscles; but that's immaterial to the work your heart does pumping the blood.

One often hears warnings about exercising in the heat. But is that simply because it increases the load on the heart beyond what the activity normally requires (in more temperate weather), to potentially dangerous levels it current fitness is not up to the task ? Or is it somehow intrinsically harmful ?

Now, of course, if one is training for activity in hot weather (e.g. hiking in the Grand Canyon) then there is some additional value in exercising in the heat, in order to increase heat acclimatization. But that's a separate issue.

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The article aptly named Exercise in the heat: challenges and opportunities (Journal of Sports Sciences, 2004, 22, 917–927) has the following to say in the abstract (bolded mine):

Exercise in the heat poses a formidable challenge to the body’s ability to control its internal environment due to the high rates of metabolic heat production and heat gain by physical transfer from the environment. In an attempt to restrict the rise in core temperature, an increased rate of sweat secretion onto the skin is invoked. This may limit the rise in core temperature, and can prolong the time before a limiting temperature is attained, but it does so at the cost of a loss of body water and electrolytes. The effects of the diminished blood volume are offset to some extent by cardiovascular adaptations, including an increased heart rate and an increased peripheral resistance, but these are insufficient to maintain functional capacity when blood volume is reduced. Prior dehydration will impair performance in both prolonged exercise and short-term high-intensity exercise. Athletes living and training in the heat may experience chronic hypohydration due to inadequate replacement of fluid losses. The negative consequences of exercise in the heat are attenuated to some extent by a period of adaptation, and by the ingestion of water or other appropriate fluids. Optimum fluid replacement strategies will depend on the exercise task, the environmental conditions and the individual physiological characteristics of the athlete. Manipulation of pre-exercise body temperature can also influence exercise performance and may be a strategy that can be used by athletes competing in stressful environments.

Long story short: Yes, there is a stress on the cardiovascular system but it is negligible as long as the body is adequately rehydrated so that the viscosity of the blood is not getting worse and the lost minerals are refilled. Prior heat adaptation by prolonged exposition and cooling down before and after exercise help as well.

Feel free to read the full text for further information.

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Exercising in the heat may present certain challenges and risks to the body, but that doesn't necessarily mean it's inherently harmful.

When you exercise, your body generates heat, and one of the main mechanisms for dissipating heat is through sweating and increased blood flow to the skin. When you exercise in hot conditions like high heat or high humidity, your body has to work harder to regulate your core temperature because the external environment has exacerbated heat stress.

There are many factors to consider when doing aerobic exercise in the heat: cardiovascular stress, dehydration, heat-related illnesses. That means staying well hydrated, dressing in breathable and lightweight clothes, and getting out and exercising at the right time.

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Heat [loads your heart] - it requires your heart to pump extra blood to your skin to regulate body temperature.

No it does not. The small increase in heart rate that comes with an increase in body temperature is not a significant stress and cannot be compared to cardiovascular exercise.

One often hears warnings about exercising in the heat. But is that simply because it increases the load on the heart beyond what the activity normally requires (in more temperate weather), to potentially dangerous levels it current fitness is not up to the task ? Or is it somehow intrinsically harmful ?

It is nothing to do with fitness - exercise generates heat which the body must remove, and it is more difficult to remove heat in a hot environment. If the body cannot offload heat that it needs to, then hyperthermia and eventually heatstroke will occur, and those are intrinsically harmful.

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  • Not so sure about this. "Your heart beats faster and pumps harder, and may circulate two to four times as much blood each minute as it does on a cool day" health.clevelandclinic.org/…. Commented Jul 15, 2023 at 18:05

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