By Michael F. Bergeron, Ph.D.
Your body produces heat during tennis -- lots of it! And as intensity and duration of play increase, you face an growing challenge to eliminate the accumulating heat, especially in hot weather. The best way for a tennis player to get rid of internal body heat during play is by sweating. But if it’s hot and humid, even sweating doesn’t eliminate heat effectively.
In warm to hot conditions, most adult tennis players will lose between 1.0 and 2.5 liters of water during each hour of competitive singles, although sweat rates of 3.5 liters per hour have been observed during play in very hot (above 95° F) conditions. Sweat rate increases as:
1. the environment gets hotter and more humid,
2. as intensity of play increases, and
3. as a player becomes more aerobically fit and acclimatizes to the heat.
And although women generally sweat less than men, this is not always the case.
Sweat is mostly water, but it contains a fair amount of sodium (Na ) and chloride (Cl-). In contrast, there is usually very little potassium (K ) in sweat. Furthermore, contrary to what many tennis players and coaches have heard, clinical evidence clearly supports a relationship between heat-related muscle cramps and a high sodium loss, not potassium. Players will generally lose 3-10 times as much sodium as potassium during play. And without adequate sodium replacement, the cumulative effect of such losses can lead to a progressive sodium deficit after several days of playing or training in the heat. The effects of inadequate sodium replacement may include incomplete rehydration between matches (from a decreased retention of fluid) as well as muscle cramps or increased heat exhaustion during subsequent play. Again, regarding electrolytes, players should be more concerned about replacing sodium losses after a match, not potassium losses. And notably, bananas will not resolve or prevent heat-related muscle cramps; although appropriate salt and fluid intake often will.
Even with relatively short matches, it’s not uncommon for tennis players to finish play with water deficits equal to or greater than 2% of their respective pre-match body weights. So, why don’t these players drink more during play? One reason that tennis players don’t always offset sweat losses with fluid intake is that, during a match, thirst is usually not a rapid enough indicator of body water losses. Consequently, players don’t always feel the need to drink as much as they should. Therefore, tennis players should follow a predetermined hydration plan, whether they are thirsty or not. But, is a 2-3% body weight deficit a big deal? After all most players drink enough to avoid serious heat disorders. Well, very little research has specifically examined varying levels of a body water deficit on tennis performance. However, other research studies have shown significant decreases in muscular strength, muscular endurance, physical work capacity, and even mental performance, with only marginal to moderate body water losses. Further, current research shows that fluid ingestion reduces internal body temperature, as well as muscle glycogen use, during prolonged exercise; these factors can clearly contribute to improved performance.
So what should you do? First of all, recommendations for managing fluid and electrolyte losses should be individual specific. However, several general recommendations can be made;
Prior to playing in a hot environment, make sure that you are acclimatized to the heat. This can be accomplished in 7-10 days, so long as you exercise for 1-2 hours each day in the same heat. Your sweat rate will increase, while your rate of sodium loss will decrease. Both of these are positive effects. If you are prone to heat-related muscle cramps during hot weather, you might consider increasing your salt intake (via foods and liquids containing salt). Ensure that you are well hydrated and that your carbohydrate intake is adequate. Water is usually fine for a pre-match beverage, but only if sufficient carbohydrates and electrolytes are provided by your food intake. Avoid drinks that contain caffeine or alcohol; they will accelerate fluid loss.
During play, you should drink enough, during each changeover, to feel comfortably full, whether you are thirsty or not. The specific amount will depend on your sweating rate and the average number of changeovers per hour; but for many players, 8 ounces (about 8 swallows) during each changeover is appropriate. If you begin a match well-hydrated and with carbohydrate stores fully replenished, and you know that the match is going to be short (around 1 hour or less), then water consumption during play is probably fine. Otherwise, a carbohydrate (CHO)-electrolyte beverage is recommended. Why? Most researchers have shown enhanced fluid absorption with a carbohydrate-electrolyte drink versus plain water. In addition, consumption of a carbohydrate-electrolyte drink will help you maintain your blood glucose level; this may delay fatigue. And, the sodium in the drink will replace a portion of the sodium lost in sweat. Sodium may increase drink palatability (which may cause you to drink more) and can play an important role in restoring body water content.
After play, your primary nutritional concern should be to immediately begin the process of replacing lost fluid, electrolytes and carbohydrates. If you are going to play another match within 30-60 minutes, then rehydration should be with a carbohydrate-electrolyte drink. A small amount of easily digestible solid food can also be consumed at the same time (i.e., immediately following the previous match). If your next match is not for several hours or more, then water and an appropriate meal should provide enough of the required nutrients for subsequent play. If sweat losses from the previous match were excessive and/or you are prone to heat-related muscle cramps, then adding more salt to your fluids and food may be appropriate. If you choose to use salt tablets, make sure you crush and dissolve them in plenty of water (usually about 2 tablets per liter of water).
Dr. Michael Bergeron is a member of the USTA Sport Science Committee.