Weight gain through increasing skeletal muscle mass (hypertrophy) is often advantageous in many sporting contexts and activities. To increase weight, an athlete must achieve a positive energy balance with muscle hypertrophy only occurring when muscle protein synthesis exceeds the rate of protein breakdown for a prolonged period of time [10]. The two principal determinants of skeletal muscle protein synthesis in adults are physical activity and nutrient availability [11].

Utilising protein ingestion with physical activity, particularly resistance exercise, promotes an optimal anabolic environment in the skeletal muscle compared to either stimulus alone [12]. The addition of protein ingestion following a bout of resistance exercise has repeatedly been shown to augment the stimulation of muscle protein synthesis, which over a period of resistance training with increased protein consumption can lead to muscular hypertrophy. The anabolic effects of nutrition are driven by the transfer and incorporation of amino acids captured from dietary protein sources into skeletal muscle proteins. The amino acid leucine has been highlighted to be particularly important in stimulating protein synthesis and appears to have a controlling influence over the activation of protein synthesis [13]. As such, rapidly digested leucine-rich proteins such as whey, in conjunction with resistance exercise, are advised for individuals wishing to increase muscle mass. In terms of protein quantity, 20–25 g of high-quality protein with at least 8–10 g of essential amino acids [14] has been shown to maximally potentiate exercise-induced rates of muscle protein synthesis in healthy young adults [15]. In total, athletes are recommended to consume ~1.3–1.8 g.kg⁻¹.d⁻¹, consumed as four meals while attempting to gain weight through increasing muscle mass [16]. It should be noted that these recommendations are dependent on training status and more protein should be consumed during periods of high-frequency/high-intensity training.

Weight loss is not an uncommon goal for athletes and is often motivated by factors relating to performance issues. This usually involves weight loss either to enhance performance or for aesthetic reasons. Excess fat is often detrimental to performance of physical activities requiring the transfer of body mass either vertically (such as in jumping) or horizontally (such as in running). This is because it adds mass to the body without providing any additional capacity to produce force. Excess fat can also be detrimental to performance through increasing the metabolic cost of physical activity that requires movement of the total body mass.

Weight loss can occur when a negative energy balance is created. Thus, weight loss can be achieved by restricting energy intake, increasing the volume/intensity of training or, most often, a combination of both these strategies. It is important for athletes and coaches to recognise that with extreme energy restrictions, losses of both muscle and fat mass may adversely influence an athlete’s performance [17]. Therefore, in most cases, it is important for an athlete to preserve their fat-free mass during periods of weight loss. There is a growing body of evidence suggesting that higher protein intakes during energy restriction can enhance the retention of fat-free mass [18, 19]. A reduction in dietary fat and carbohydrate may allow athletes to achieve higher protein intakes without the excessive restriction of a particular macronutrient. Current recommendations advise athletes aiming to achieve weight loss without losing fat-free mass to combine a moderate energy deficit (~500 kcal.d⁻¹) with the consumption of between ~1.8 and 2.0 g.kg⁻¹.d⁻¹ of protein in conjunction with performing resistance exercise [14].

It is not uncommon that extreme sports athletes may be frequent travellers due to the nature of their sport and competition; thus they may face frequent trips that may involve long travel times that can cause fatigue. Having access to nutritious balanced meals and adequate fluid can be challenging; however, ample pre-planning meal, snack and fluid arrangements can enhance an athlete’s dietary preparation when travelling [20]). Table 2.2 provides a practical summary of key dietary strategies that can help support teams cope with challenging travel demands.

Exercise is associated with high rates of metabolic heat production, eliciting high rates of sweat secretion in order to attenuate the rise in body temperature that would otherwise occur. If exercise is prolonged, this leads to progressive hypohydration and a loss of electrolytes, particularly in hot environments where sweat rates may exceed 2 l/h [22].

Significant hypohydration can occur during many types of exercise activity and poses a challenge to both an individual’s performance and health. Hypohydration can have a negative impact on exercise outcomes through impairing thermoregulation and performance of prolonged aerobic exercise [23], cognitive function [24] and gastric emptying and comfort [25]. These performance impairments can be detected when fluid losses are as low as 1.8 % of body mass [22]. A body mass loss of more than 4 % during exercise may lead to heat exhaustion and heat illness [22]. Even in winter sports environments, where sweat rates are expected to be lower, fluid loss can be significant [26]. Nordic skiers competing in 15–30-km races can typically lose 2–3 % of body mass, and collegiate cross-country skiers lost 1.8 % of body mass after 90 min of ski training [27]. Thus, strategies to minimise the degree of hypohydration should be undertaken before, during (if possible) and in recovery from exercise activities.

Sweat rates and electrolyte losses can vary widely amongst different individuals and between different activities under the same environmental conditions. Therefore, it is difficult to accurately prescribe fluid and electrolyte intakes without knowledge of individual sweat rates under the specific environmental conditions. Hydration status can be monitored by employing simple urine and body mass measurements. Changes in body mass can reflect sweat losses during exercise and can be used to calculate individual fluid replacement needs for specific exercise activities and environmental conditions. Urine osmolality also provides a clear indication of hydration status and can be measured quickly and simply using a portable osmometer. A urine osmolality of 100–300 mOsmol/kg indicates that an individual is well hydrated. Values of over 900 mOsmol/kg indicate that an individual is relatively dehydrated. Portable urine osmometers can be useful to assess hydration status in the field as they are small, reliable and convenient and thus can be a good tool to monitor hydration status objectively.

Accurately measuring electrolyte losses is a more challenging, as the composition of sweat is difficult to measure and the methods such as sweat patch testing have poor reliability. Electrolyte losses vary greatly between individuals but also vary with changing sweat rates over time [28]. The major electrolytes lost in sweat are sodium and chloride. These are the major ions of the extracellular space; therefore, the replacement of these ions, especially sodium, should be a priority. The perception of thirst as the signal to drink is unreliable. This is because a considerable degree of dehydration, sufficient enough to impair athletic performance, can occur before the desire to drink is evident [29]. The sensation of thirst results increases the secretion of the antidiuretic hormone from the posterior pituitary gland, which acts on the kidneys to reduce urine excretion. However, thirst is quickly alleviated through drinking fluid before a significant amount of fluid is absorbed in the gut [22]. Thus, the use of thirst alone should not be used as indicator of fluid balance.

Electrolytes play a key role in promoting postexercise rehydration. This was first highlighted by Costill and Sparks [30], who showed that the ingestion of a glucose-electrolyte solution after a relatively severe degree of hypohydration (4 % of the pre-exercise body mass) resulted in a greater restoration of plasma volume than water alone. A higher urine output was also observed in the water trial. The ingestion of drinks containing sodium following exercise promotes rapid fluid absorption in the small intestine, allows the plasma sodium concentration to remain elevated during rehydration and helps maintain thirst while delaying the stimulation of urine production. Drinks containing multiple transportable carbohydrates such as glucose and fructose can also aid hydration through enhancing gastric emptying rates, improving the delivery of fluid consumption compared to a single carbohydrate drink (this is covered in more detail in the carbohydrate supplements section) [31]. The addition of carbohydrate can also make the drink more palatable, aiding the rehydration process.

Gonzales-Alonso et al. [32] confirmed that a dilute carbohydrate-electrolyte solution (60 g/l carbohydrate, 20 mmol.l⁻¹ Na+, 3 mmol.l⁻¹ K+) was more effective in promoting postexercise rehydration than either plain water or a low-electrolyte diet cola. The difference between the drinks was primarily the volume of urine produced. As previously mentioned, individual sodium content of sweat varies widely, and no single formulation will meet requirements for all individuals in all situations. The upper end of the normal range for sodium concentration (80 mmol.l⁻¹), however, is similar to the sodium concentration of many commercially produced oral rehydration solutions intended for use in the treatment of diarrhoea-induced dehydration. In contrast, the sodium content of most sports drinks is in the range of 10–30 mmol.l⁻¹. Most commonly consumed soft drinks contain virtually no sodium, and these drinks are, therefore, unsuitable for rehydration. The problem with high sodium concentrations is that this may exert a negative effect on taste, resulting in reduced consumption. Therefore, it is important that a balance between electrolyte content and palatability is achieved.

The rise in core body temperature observed when exercise is performed in hot environmental conditions is associated with reduced motor output during self-paced exercise [41, 42], as well as the termination of exercise during time to exhaustion protocols [43, 44]. The central nervous system is thought to reduce motor output following elevations in core temperature and terminate exercise once critically high internal temperatures are attained, in an attempt to limit the development of catastrophic heat illness [45].

The subjective perception of effort is an important consideration. If the exercise feels hard, the duration will often be cut short and adherence is likely to be poor. It is well recognised that the subjective rating of perceived exertion is higher when exercise is performed in warm environments than in cool environments [46] and is also increased by even moderate levels of hypohydration.

Total body water can have a critical influence on thermoregulation and exercise performance in the heat. Total body water usually remains relatively constant [47]; however, physical exercise and heat exposure will increase water flux to support thermoregulation [48]. In a hot environment, sweat evaporation is the primary avenue for dissipating body heat absorbed from the environment or produced by the exercising muscle. Therefore, the most notable effect of exercise in a hot climate is increased fluid loss.