Hello summer heat!
The body has many mechanisms to help adapt to a range of environmental conditions. Heat acclimatization plays a major role in the body’s physical responses and overall ability to perform the heat.
What is heat acclimatization?
Heat acclimatization (or acclimation) involves biological adaptations that reduce physiological strain (ex. heart rate and body temperature), improve comfort, improve exercise capacity and reduce the risks of serious heat illness during exposure to heat stress. Heat acclimatization can occur in a hot and dry environment as well as a hot and humid environment. Acclimatization varies person to person and also depends on the volume of exercise, the intensity of exercise, hydration status and fitness level.
Trained athletes and those with less body surface area will generally (but not always) have more physical advantages to tolerating the heat compared to the untrained or those with more body surface area.
Humidity
On humid days, when the air is already saturated with water, sweat evaporates more slowly. This explains why it feels so much hotter (and harder) to train in high humidity. When relative humidity reaches a high enough level, the body's natural cooling system simply can't work. Sweat evaporates very slowly, if at all, and the body heats up. No matter how much you drink or try to cool yourself, if your body is overheated and you can no longer cool yourself, your body will no longer be able to safely train in the heat.
Physiological strain
There is a great cardiovascular strain when exercising in the heat. Cardiac output (volume of blood pumped by the heart per minute, a function of heart rate and stroke volume) becomes inadequate to fuel the dual demand for skin and muscle blood flow. Even though heart rate increases during thermal stress, the amount of blood that the heart can pump per beat is reduced because of overall low blood volume. This is called cardiac drift. The heart is trying to compensate for the reduced blood volume by beating faster. This makes it extremely difficult to cool the body and support the working muscles. Ultimately, the skin will win and your muscles will not be able to properly contract.
When training in the heat, the body will struggle to maintain core temperature, keep up with muscle contractions and maintain optimal gastric emptying. This is why so many athletes experience GI distress when training in the heat. And when you get dehydration, this makes training in the heat even worse.
Importance of sport nutrition
Restoring fluids and meeting hydration needs during workouts is a must as this is the only way to maintain muscle contractions and reduce the risk of heat stress. However, don't expect your gut to tolerate taking in sport nutrition on a hot summer day if you haven't been consistent with your fueling/hydration over the past few months (in cooler temps). It can take up to 6-8 weeks to train the gut to tolerate digesting and absorbing nutrition while exercising. If fluid replenishment is not adequate (ex. rationing your fluids, not planning your refill stops appropriately, not drinking on a schedule, etc.), blood volume will decrease which also boosts heart rate to compensate for the reduced blood volume.
Dehydration reduces blood volume and as the body compensates by retaining more sodium in the blood, there is increased pressure in major blood vessels as blood becomes more concentrated, thicker and more difficult to circulate. Dehydration also increases glycogen utilization (depleting carbohydrate stores quicker). The breakdown of glycogen during training leads to an increase in intracellular acids (ex. lactic acid) which decreases pH thus causing skeletal muscle fatigue. When athletes experience heat stress and dehydration, they typically complain that a normal easy or stustainable given effort feels much more difficult. Ultimately, performance declines and as exercise continues, health can be negatively compromised.
How to adapt
When preparing for heat acclimatization, it's important to first have a solid base level of fitness. In other words, avoid starting a training plan (or new exercise routine) when the weather turns warm or hot. For the competitive athlete or those preparing for a summer or fall endurance event, this stresses the importance of maintaining a good level of fitness all year long.
Because heat acclimatization is a complex series of physiological adaptations that occur when exposed to the heat, you need to train in the heat in order to acclimate to the heat. While you can attempt to acclimate passively with the use of a sauna protocol, the best strategy is to acclimate actively by completing a few of your workout sessions in the heat, over the course of about 3 weeks. While it may feel uncomfortable and difficult at first, you are teaching the body to better tolerate exercising in the heat. Overtime, your physiology will change to reduce the cardio, thermal and metabolic strain on the body due to the heat. Prolonged exercise in the heat forces the cardiovascular system to provide blood flow to the working skeletal muscles to meet metabolic demands and to provide blood flow to the skin to dissipate the heat released by the working muscles. Additionally, although you will continue to sweat to cool your body, you will lose less sodium in your sweat.
Most athletes will find that performance is not significantly compromised when training in the heat for short distances and that blood can support the muscle demand and skin demand. However, when exercise is prolonged or intensified, the body may fail to maintain thermal homeostasis and will move into positive heat storage. Because of this, you must take training in the heat seriously. You must adjust your effort/intensity and volume. And you must check your ego at the door and be willing to acclimate slowly and patiently.