For occupational, military and leisure-related reasons, interest in the nutritional requirements of activities has a long history (Porter, 1999). Hippocrates (c 460-370BC) advised athletes and propounded health-related aspects of diet. Galen (c AD129-216) was appointed by Roman Emperor Marcus Aurelius to tend to the medical and surgical needs of gladiators. This extended to advising gladiators' trainers and related staff and included those who designed and prepared the combatants' diets. Sometime between AD384-389), Vegetius wrote his treatise Epitoma rei militaris (Epitome of military science) (Whipp et al., 1998) that included consideration of the nutritional requirements of marching Roman legionnaires. Some two thousand years later, bodies such as the English Institute of Sport employ nutritionists and dieticians to advise and guide athletes both to support their training and preparation for competition.
This advice is also an important part of modern-day tennis. Tennis is a pan-global sport that is played year-round in both hemispheres. The economic impact of tennis is substantial both in terms of prize money that is available and associated sponsorships of tournaments and coverage by various media. It is big business. This places notable demands on the physical and psychological preparation of players and included in these demands are nutritional and fluid requirements both of training and match play.
Conducting nutrition research in tennis presents several challenges because of the numerous variables involved but mainly because of the lack of valid, standardised and reliable performance protocols that are sensitive enough to detect meaningful changes in performance before and after an intervention. Nonetheless, there are sufficient studies that have been conducted in tennis and other racket sports with similar demands to provide nutritional recommendations for tennis competitors. The purpose of this article is to provide evidence-based nutritional recommendations for tennis players.
Anthropometric and physiological characteristics of tennis players
Table 1 presents typical anthropometric and physiological characteristics of modern tennis players. Notably tennis players do not excel in any particular characteristic but are well adapted in all areas. This is likely a result of the varied nature of tennis match play and training demands (Reid et al., 2008).
Energy expenditures in tennis
Tennis is broadly considered an intermittent sport, that comprises brief periods (4-10 s) of activity interspersed with short active recovery durations (10-20) and longer passive recovery bouts (60-90 s) (Fernandez-Fernandez et al., 2006). However, matches can last for three or more hours although only about 15% of total time is actual match-play. The physiological demands of tennis match-play are complex and depend on highly variable interactions between technical, tactical, physical and environmental constraints. Typically, the type of court surface, style of play (serve and volley, baseline player), duration of rally, phase of play (service or return game) and ambient temperature and humidity combine to influence the energy demands and make tailoring match-specific nutritional strategies a major challenge. Moreover, factors such as playing surface influences bounce and ball speed which in turn affects rally duration and consequently the energy expenditure.
As well as court surface, playing style also influences rally duration. Players who play from the baseline are likely to have longer rallies than those who prefer to serve and volley (Smekal et al., 2001). Therefore, players who use baseline tactics on clay courts are more likely to be involved in longer rallies than players who opt for serve-and-volley tactics on grass courts and therefore, expend more energy. Longer rallies on clay courts lead to larger percentage playing times than on hard courts (approx. 25% vs. 21% respectively) (Christmass et al., 1998; Fernandez et al., 2007; 2008; Girard et al., 2006; Martin et al., 2011; Mendez-Villanueva et al., 2007; Smekal et al., 2001) and potentially, longer match durations and greater energy expenditures.
Environmental constraints such as ambient temperature and humidity influence both tennis match play and energy demands through autonomic and/or behavioural mechanisms (Morante and Brotherhood, 2008). The duration of rallies has been found to be positively correlated with rectal temperature and thermal perception as well as skin temperature (Morante and Brotherhood, 2008). However, the intermittent activity profile of tennis means that in most environments (
Energy expenditures of 30.9 [+ or -] 5.5 and 45.3 [+ or -] 7.3 kJ x [min.sup.-1] have been reported in women and men players respectively regardless of court surface as indicated in Table 2. These data are based upon assumptions of mean body mass, maximum oxygen uptake (table 1) a fractional utilization of 55% [VO.sub.2max], 1 L of [O.sub.2] equivalent to 21 kJ (5 kcal) of energy and a mean RQ of 0.9. Assuming that all fluid lost by sweating must be replaced, fluid requirements in a range of ambient temperatures are estimated from sweat rate using the regression equation of Morante et al. (2007) assuming a mean body mass from Table 1.
Future research should identify energy expenditures of both men and women players using more accurate methods such as doubly labelled water, during different training phases and tournaments in elite-standard ([less than or equal to] 100 rank) tennis players.
General macronutrient and energy intake recommendations for tennis
The nutritional challenges facing elite-standard tennis players are unique. Year-round competition with a heavy travel programme and unpredictable time spent in competitive match-play make for a complicated nutrition strategy. It might be important to integrate training, and training nutrition, into a competition programme if tournaments dominate the calendar. This requires a carefully planned strategy, allowing for periods of physical training, skill acquisition, competitive performance and sufficient time for recovery and adaptation. Independent of the time of year, adequate energy must be consumed to support the volume, intensity and duration of activity.
Estimated energy expenditure during tennis play for between 1-5+ h for men can range from 2.72 [+ or -] 0.44 MJ to 13.58 [+ or -] 2.19 MJ (649 kcal [+ or -] 105 to [less than or equal to] 3244 kcal [+ or -] 524) (Table 2). One study that investigated seasonal changes in the diets of four women tennis players (age 19.3 y [+ or -] 15.0) reported no differences in energy intake across the seasons, but did report variation in macronutrient intakes (Nutter, 1991). Notably, the diets were lower in carbohydrate during the in-season (49 % and 55 % for in- and post-season respectively) and lower in fat (33 % and 28 % for in- and post-season, respectively). The findings of this study should be interpreted with caution because they are unlikely to be representative of professional tennis players who do not have such clearly defined seasons. Elite-standard tennis players are required to maintain their optimal body mass and composition all year round and thus have to adjust their energy intakes during short periods of rest or travel.
Dietary carbohydrate intake recommendations
Carbohydrate intake during tennis match play will be considered in greater detail in the supplements section, therefore this section will address general carbohydrate recommendations for tennis players. It has been understood for many years that a high carbohydrate diet leads to increased muscle glycogen stores (Bergstrom, 1967), which contributes to optimal performance particularly in endurance-type activities (Hargreaves, 2004). It is also known that a low carbohydrate diet (
Elite-standard tennis players should ensure that they are adequately fuelled and hydrated before each match; however this could be challenging without knowing how long a match will last. Furthermore, a delayed start in a tournament could disrupt rehearsed pre-match meal routines and cause players either to be undernourished and hungry at the onset of a match, or to play on a stomach of undigested food. Either way, this could be detrimental to performance and could create gastrointestinal problems. Other issues could arise when tennis players compete in more than one match per day e.g. if they enter both singles and doubles match-play in tournaments. This situation can leave players with an inadequate or sometimes an unknown duration of recovery which makes post-match refuelling difficult. Recovery may therefore be with carbohydrates only due to their ease of digestion and protein could be sacrificed, thus compromising optimal recovery. In addition, an unexpected result or an early exit from a tournament could mean athletes have overeaten during their preparations. Tennis players should seek guidance from a suitably qualified sports nutrition specialist to address these issues to maximise performance.
Dietary protein intake recommendations
There are limited data on the dietary intakes and requirements for protein in racket sports, with most published guidelines aimed specifically either at solely strength- or endurance-trained athletes. While tennis includes aspects both of strength and endurance performance, it is not directly comparable to either. It is thus more appropriate to estimate the protein requirements of tennis players based either on the volume and intensity of training or competition. One study investigated the dietary intakes of four women collegiate-standard tennis players and reported daily protein intakes of 1.3 g x [kg.sup.-1] x [d.sup.-1] and 1.2 g x [kg.sup.- 1] x [d.sup.-1] in season and post-season, respectively (Nutter, 1991). In addition, Gropper et al. (2003) investigated dietary intake of seven women tennis players aged 19 yrs who were training for 4 h/d, 6 d/wk. This study...