PT on the Net Research

Overheating While Cycling


Question:

I have a 50-year-old client who has a problem when training outdoors. He is a very fit cyclist, but he experiences problems with overheating during exercise. He gets hot and then gets really faint and disorientated. He cools down when putting water on himself, but this is a problem during races. He always hydrates properly, but when he gets hot like this, he can’t urinate or sweat.

Answer:

The answer to this question is multidimensional. Three factors must be discussed to provide an optimal nutritional plan: dehydration, heat exhaustion and the effect of age on thermoregulation.

Dehydration

When the environmental temperature and humidity rise, so does an athlete's core temperature. While dehydration may sometimes be unavoidable due to the inability for athletes to balance fluid intake with fluid loss, it is of utmost importance for athletes to make a concerted effort to stay hydrated on a daily basis before exercising as well as instilling sound hydration strategies during exercise. General recommendations from the National Athletic Trainer's Association position paper (2000) indicate that an athlete should begin exercise well hydrated, and the easiest way to determine daily hydration status is by the color of the urine. Urine that is pale yellow to clear in color most times throughout the day will indicate a well-hydrated state. Drink 17 to 20 ounces of fluid two to three hours before and seven to 10 ounces of fluid 10 to 20 minutes before exercise. If the athlete enters the exercise session in a well-hydrated state, it is much easier to prevent dehydration. During exercise, it is important to drink only enough to match sweat loss to avoid any gastrointestinal issues. Seven to ten ounces of fluid every 10 to 20 minutes generally can be followed for most athletes, unless they are very heavy sweaters and the environmental temperature is hot. The lack of urination during a longer exercise session is highly correlated with dehydration, so it is important to enter exercise well hydrated and implement the before and during hydration guidelines to prevent a serious loss of body weight (more than two percent) when performance suffers and health could be compromised. After exercise, athletes should drink 20 to 24 ounces of fluid for every pound of body weight lost. It is important to note that the most effective fluids to consume before, during and after exercise are those that are cool, flavorful and contain water, carbohydrates and sodium.

While pouring water on the skin in an effort to cool the body feels refreshing, it does not rehydrate the body and can in fact deter from the body's efficiency of evaporating sweat from the skin.

Heat Exhaustion

Heat exhaustion, usually experienced in hot and humid conditions, results from strenuous exercise combined with environmental heat stress. It is characterized by the body's inability to sustain adequate cardiac output and is usually recognized in athlete's who have difficulty continuing exercise in the heat and by mild hyperthermia (usually defined as a temperature from 97-104 degrees F). Common symptoms include physical fatigue and dizziness, dehydration, syncope, lightheadedness, elevated body core temperature, decreased urine output, profuse sweating, headache, nausea, vomiting, diarrhea, stomach or intestinal cramps and persistent muscle cramps. It is important to realize that dehydration can lead to this type of heat illness and can therefore be prevented with proper planning.

Aging

It is also important to note that the biology of aging affects heat tolerance and exercise. The main factor that is affected with age relative to exercise is the skin blood flow. Research has shown that older adults have a significantly lower (25 to 40 percent) skin blood flow, and this is most likely due to structural changes within the cutaneous vessels. This decreases vasodilation, which does not allow the body to cool itself as efficiently by moving the blood to the skin's surface for evaporation to take place. Of particular interest is the possibility of a reduced thirst response and increased excretion of water by the kidneys in older athletes. This can lead to a state of hyperosmolar hypohydration, which can lead to reduced heat tolerance. Research has shown that when older athletes are compared to younger athletes during heat exposure in a dehydrated state, the older athletes have a reduced blood volume and increased plasma osmolality, and the older athletes rate themselves as less thirsty. This inability to maintain plasma volume during exercise in hotter environments can lead to a more severe level of dehydration.

Based on the biology of aging and the fact that older athletes do not feel as thirsty, it could be speculated that older athletes are not drinking as much as their bodies require, even though they report to be hydrated. Some medications also affect hydration status and should be discussed in detail with a physician.

Conclusion

In summary, older athletes are more thirst challenged due to certain physiological responses associated with the biology of aging. Education is crucial for any athlete exercising in the heat, and heat illnesses are preventable in most cases. By remaining hydrated on a daily basis, following the before and during exercise hydration recommendations and trying to exercise during less hot times of the day, athletes will be able to be more successful in preventing dehydration and any associated heat illnesses such as heat exhaustion.

References:

  1. Binkley, H. et al. (2002). National Athletic Trainers' Association Position Statement: Exertional Heat Illnesses. Journal of Athletic Training, 37(3), 329-343.
  2. Casa, D. Preventing exertional heat illness: a consensus statement. Gatorade Sports Science Institute (www.gssiweb.com)
  3. Casa, D. et al. (2000). National Athletic Trainers' Association Position Statement: Fluid Replacement for Athletes. Journal of Athletic Training, 35(2), 212-224.
  4. Kenney, L. (1993). The older athlete: exercise in hot environments. Gatorade Sports Science Exchange (www.gssiweb.com), volume 6, number 3.
  5. Kenney, W.L. & Havenith, G. (1991). Aging, skin blood flow and heat tolerance. Proc. International Conference on Human-Environment System, Tokyo, Japan, pp. 87-90.
  6. Kenney, W.L. & Hodgson, J.L. (1987) Heat tolerance, thermoregulation and aging. Sports Med. 4:446-456.