(The information in this article was taken or adapted from the High Performance Coaching Program Study Guide.)
Carbohydrates are sugars. Before they are absorbed into the blood, carbohydrates that are consumed are broken down by digestion into single sugar units, such as glucose or fructose. Glucose is the body’s main source of “energy” and is used to fuel the working cells. Fructose (the very sweet sugar of fruit and also found in soft drinks and some sport drinks) can also be used as fuel, but first must be converted to glucose in the liver before it can be used for energy.
Breads, cereals, rice, pasta, fruits, and vegetables are all good primary sources of carbohydrate that should be regularly included in a tennis player’s diet. Other foods such as sport drinks and sport bars can help too.
It is often recommended that 55% to 70% of an athlete’s daily dietary calories should be in the form of carbohydrates. This goal is not always appropriate or practical, particularly if a player needs to consume many calories to offset those that are burned off on the court. A better guideline for a competitive tennis player is to ingest at least 3 grams of carbohydrate per pound of body weight each day. This is equivalent to 465 grams (or 1860 calories from carbohydrates) for a 155-lb person, which would represent roughly 62% of a 3000-calorie intake for a given day. Calorie and carbohydrate intake should be considerably greater when playing and/or training a lot; up to 5 grams of carbohydrate per pound of body weight may need to be consumed each day.
Carbohydrates are commonly classified by how fast they are broken down by the digestive system, and how fast the sugar enters the blood stream so it can be used by working muscle. This classification is often based on what is called a food’s “glycemic index.” Carbohydrates that break down slowly and raise blood sugar (glucose) slowly are described as having a low glycemic index. Conversely, foods that raise the blood sugar level a lot and quickly are described as having a high glycemic index. The glycemic effect can be very important for tennis players. For example, if a player needs a rapid energy boost on the court, certain foods such as plain bagels, ready-to-eat cereals (e.g., corn flakes, Cheerios, etc.), white bread, crackers, pretzels, honey, certain candies, and some sport drinks (those with carbohydrate primarily from glucose, sucrose, or a glucose polymer) will raise the blood sugar quickly. Foods like these can provide a rapid and more readily utilizable energy source. On the other hand, apples, yogurt, and fructose-predominant sport drinks, for example, will provide energy at a slower rate, because the carbohydrate will not be absorbed as quickly and because it must be first converted to glucose in the liver. Even a banana has only a moderate glycemic index – it tastes sweet, but it does not give you energy very fast.
In fact, a player who has a high consumption of fructose during play (e.g., fruit or a fructose-based sport drink) may get a feeling of gastrointestinal distress because the absorption of fluid and carbohydrate occurs more slowly.
Tennis players need fat for a number of important biological functions as well as for energy during play. Current recommendations for dietary fat intake suggest that fat should make up between20 - 30% of the total daily calories consumed. Additionally, saturated fats such as butter, coconut oil or lard, should make up less than 10% of the total calories consumed during a day. However, even though fat is important to the tennis player, consumption of fat during or just prior to play is not necessary or appropriate.
Importantly, using fat for energy still requires a continual and simultaneous breakdown of carbohydrate. Therefore, all players, regardless of the intensity of play, will eventually feel the effects of depleting carbohydrate stores if the match is long and carbohydrate is not consumed during play. Therefore, carbohydrate for energy during play should still be the emphasis. Further recommendations for in-competition nutrition are provided in future sections.
Some players’ daily fat intake regularly exceeds the daily-recommended amount, usually because of convenience or a player’s preference for eating high-fat meals. For players involved in extensive playing or training, eating a high-fat diet is often a practical means to help maintain body weight without having to consume an excessive amount of food to match a very high calorie expenditure. If the daily carbohydrate requirement (e.g., 3-5 grams per lb. of body weight) is still met, and the player is not putting on too much body fat, then from a performance point of view, a periodic high-fat diet may be okay (it’s fairly common among many other sports). However, from a long-term health perspective, excessive fat intake is likely to adversely affect the diet-related risk factors for coronary heart disease to some degree, even in a fit population.
The recommended daily protein intake (for adults) is about 0.3-0.4 grams of protein per pound of body weight (or about 10% to 15% of the daily total calories consumed). However, during and immediately after play or training, there is an increase in protein breakdown followed by an increase in protein building during recovery. Thus, most tennis players should try to get closer to 0.55-0.75 grams per pound of body weight each day (85-116 grams of protein for a 155 pound tennis player). Regular strength training may further increase the daily protein needs to 0.7-0.9 grams per pound of body weight (109-140 grams of protein for a 155 pound player). Fortunately, such an increase in dietary protein is likely already met by the typically higher daily caloric intake that active players usually have. So unless an athlete is inappropriately restricting calories, protein supplements are generally not needed. An exception might be when traveling and typical protein sources (meat, fish, dairy products) are not available or convenient.
Water and Electrolytes
In warm to hot conditions, most adult tennis players will lose between 1 and 2.5 liters of sweat during each hour of competitive singles play or on-court training. Notably, sweat rates of up to 3.5 liters per hour have been observed with some players in extreme conditions. Clearly, it would not be difficult for some tennis players to lose as much 10 or more liters of fluid in a long match if water was not replaced on a regular basis.
Sweat is mostly water, but it also contains a number of other elements found in the blood, including a variety of minerals in varying concentrations. These minerals are collectively called electrolytes and they help to maintain fluid balance in the body and are necessary for proper muscle contraction and nerve impulse transmission. The most common electrolytes found in sweat are sodium (Na ) and chloride (Cl-), which make up normal table salt. Sodium (especially) and Chloride levels as well as the rates that these electrolytes are lost through sweating vary tremendously in players. In a given liter of sweat, the amount of sodium could range from 100 to 2300 milligrams (mg). In contrast, potassium (K ) and magnesium (Mg2 ) losses in sweat, for example, are typically much lower. In fact, players will generally lose 3-10 times as much sodium as potassium during play.
With high sweating rates and sweat that contains only a moderate concentration of sodium, a player could readily lose up to 5000 mg of sodium per hour of play. Such a player would have a severe challenge in maintaining sodium concentrations and fluid balance in the body. This player would be at high risk for heat-related problems on court, such as extreme fatigue and/or muscle cramps, unless fluid and mineral intake was carefully managed during practice or play.
Probably the most common heat related injury encountered by tennis players is heat cramps. Heat-related muscle cramps often occur during or following prolonged playing (one or several matches) when there have been previous extensive and repeated fluid and sodium losses. With a significant body water and sodium deficit, nerve endings connecting to the muscles may become hyper-excitable and overly sensitive, resulting in seemingly spontaneous muscle contractions (i.e., cramps).
Lack of conditioning and fatigue can cause a ‘normal’ muscle cramp. This type of cramp is usually localized and passive stretching, massage, or icing can often resolve it. Such is not the case with heat cramps. Heat cramps can eventually spread over many areas of the body, including the stomach, arms, and even fingers and facial muscles. Drinking plenty of water may help to delay muscle cramps, but to completely restore the proper fluid and electrolyte balance throughout the body (and eliminate the heat cramps) the salt that was lost through sweating must be replenished as well. Therefore, extra salt intake is appropriate when playing or training in hot conditions or any time that sweating is expected to be extensive.
Vitamins and Minerals
Vitamins are organic substances and minerals are inorganic substances (like iron, calcium, or zinc) that are essential for the human body to function properly. In the majority of cases, vitamins and minerals cannot be produced by the body and must be consumed in the foods we eat. There are recommended daily amounts of vitamins and minerals that should be consumed each day. If these recommended allowances are not met, the tennis player should consider supplementing his or her diet with a multivitamin (The topic of supplementation is discussed in more detail in a later section of this competency).