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Nutrition and Performance in Sport

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Christophe Hausswirth

Chapter 3. Nutrition and lifestyle

Topic 3. The importance of dairy products in the athlete’s daily nutrition

Julien Louis, Yann Le Meur et Christophe Hausswirth

Résumé

Achieving a high level in sport requires physiological adaptation and development, which depend on the training stimulus and a number of other factors. Among these factors, nutrition plays a major role. Today, it is well established that modifying food intake can positively influence the body’s adaptive reaction to training and recovery processes. However, in practice, athletes often struggle to find their way through the vast array of products and advice offered by companies and colleagues alike. Much research suggests that milk and dairy products are a convenient, cheap and particularly efficient option for athletes, at many levels. The results of scientific research highlight the various benefits that consuming skimmed cow’s milk presents for athletes, including for those with lactose intolerance (who can drink delactosed milk). The abundance of scientific evidence currently available allows us to assert that ingesting milk daily contributes to good bone health, to post-exercise muscle regeneration (even to increasing lean mass, when this is an objective) and to reducing fat mass. Some recent studies also highlighted that, to favour recovery after endurance exercises, milk is as efficient as commercialized energy drinks as a recovery drink. Due to its electrolyte content, milk is particularly effective in optimizing post-exercise rehydration. Finally, some preliminary data suggest that milk also has a positive impact on sleep.
The objective of this chapter is to explore in what contexts and which forms dairy products can help meet the specific needs of high-level athletes and those of occasional athletes who want to continue to make progress.

Texte intégral

1. Bone health

1The expression “bone health” refers to bone resistance and the bone’s capacity to avoid fractures, for example in the case of a fall, strike, torsion or any other mechanical stress to which bones can be subjected. Bone mineral density is associated with bone health; this density is used as an indicator of the bone’s resistance to mechanical stress, when seeking to detect bone diseases, and to follow up treatment for these diseases. When bone mineral density increases, the bone’s resistance increases. Regularly practicing certain sports can have a direct effect on bone resistance and can contribute to reducing the risk of fracture by up to 20 to 35%. In effect, healthy bone tissue presents a capacity to adapt to the mechanical constraints associated with muscle contractions: it is more solid, and therefore less subject to fractures. The importance of the role played by mechanical solicitations was shown by the results of a study of ten professional tennis players, where the bone mineral content was found to be, on average, 15% higher in the radius of the racquet arm (Haapasalo et al. 1998). However, some disciplines, such as running, swimming, diving or synchronized swimming, are considered to be high-risk sports in terms of bone health, especially for female athletes (Manore, 2002). Indeed, in activities where a lean shape is the norm, there is often a restriction of calorie and calcium intake, especially when athletes exclude dairy products from their diet or reduce their consumption of these products. Several studies have shown that, in female athletes, daily calcium intake varies between 500 and 1623 mg in female athletes, but that the majority of female athletes consumed less than 1000 mg per day (Beals and Manore, 1998; Kopp-Woodroffe et al. 1999). The recommended daily calcium intake is 1200 mg for young women aged 9 to 18, and 1000 mg for adult women aged 19 to 50. An inadequate calcium intake affects bone structure regeneration and increases the risks of reduced bone density and the subsequent prevalence of stress fractures (Manore, 1999; Nattiv, 2000).

2Optimal calcium absorption and bone health are not only linked to calcium intake (Clarkson and Haymes, 1995); vitamin D status is also important, yet it is often low in athletes living in zones where the climates offers limited sunshine. A serum vitamin D level of at least 80 nM is necessary for good bone health (Heaney and Weaver, 2003; Grant and Holick, 2005). It is thus recommended that athletes have regular check-ups, especially those competing in high-risk sports, and to make sure they balance any deficit by adapting their diet, consuming oily fish (salmon, tuna, trout) two or three times a week, as well as favouring the consumption of vitamin D-enriched milk, particularly at the end of autumn and throughout winter.

3Milk, the best source of calcium. The increase in bone density induced by physical activity is influenced by the level of calcium intake. The relationship between bone density and physical activity is stronger when calcium intake is between 800 and 1000 mg per day. A lower intake would reduce the effect of activity on bone density, while intake above this threshold does not increase the effect of activity.

4It is acknowledged that the best sources of calcium are foods from the “milk and dairy products” group. Other foods also provide this element in satisfactory amounts, notably some green vegetables, as well as calcium-enriched foods. However, many vegetables, even though they are known to have a high calcium content, are not good substitutes for dairy products. Indeed, for bones and the rest of the body to benefit from calcium, it must be properly absorbed from food. Most vegetables contain significant amounts of oxalates and phytate, which can impair calcium absorption. For example, cooked spinach contains 129 mg of calcium per 125 ml (half cup). This level is much higher than the calcium content of most other foods (apart from milk – which contains 315 mg per 125 ml – and other dairy products), but it is estimated that only 5% of the calcium contained in spinach is absorbed, i.e. 6 mg/125 ml in absolute terms. This is not much compared to milk, from which 32% of the calcium is absorbed (i.e. 101 mg/125 ml in absolute terms). Thus, in terms of calcium absorption, 15 portions of spinach would be equivalent to one portion of milk. Having said that, green vegetables contain many other vitamins and minerals and should therefore be included in a balanced diet. It is also important to understand that the Dietary Reference Intake was determined based on the calcium content in food, but does not take into account that only 30 to 40% of this calcium will be absorbed, on average. This is why, among food sources, dairy products meet the daily calcium requirements most efficiently. For this reason, dairy products should provide two-thirds of the daily calcium requirements. A diet low in dairy products cannot provide more than 500 mg of calcium per day, which is insufficient. Calcium deficiency jeopardizes bone health, as calcium required for other functions is resorbed from the bones to ensure normal blood calcium levels. This demineralization can result in reduced bone mass, osteoporosis and a higher risk of fractures, especially when it is associated with a low-calcium diet and low vitamin D levels.

2. Muscle recovery and gain in muscle mass

5Strength sports and exercises where muscle building is a major objective are associated with high-intensity mechanical constraints that can generate an increase in the membrane permeability of muscle cells. The muscle microlesions caused by exercise trigger decreased muscle performance (Byrne and Eston, 2002a, 2002b; Twist et al. 2008), increased circulating muscle enzyme concentrations (Sorichter et al. 2001) and delayed onset muscle soreness (DOMS) (Semark et al. 1999; MacIntyre et al. 2001). Various studies have shown that the nutritional strategy adopted during the postexercise period plays a major role in muscle recovery (Koopman et al. 2007). These studies showed that the amplitude of muscle synthesis mechanisms depended on food intake, including proteins. Indeed, ingesting proteins, preferably when the body is at rest, stimulates muscle anabolism by directly activating protein synthesis. For muscle regeneration to be effective, the nitrogen balance, which is the difference between the amount of muscle proteins synthesized and the amount of muscle proteins degraded, must be positive. Some research has shown that even though bodybuilding in itself generates an increase in protein synthesis, it alone cannot compensate for the amount of proteins degraded, and a negative nitrogen balance appears after intense training (Phillips, 2009). These observations reinforce the need to plan and adapt macronutrient intake to balance post-exercise protein levels, thus accelerating muscle recovery while also avoiding injuries.

6Many studies have investigated the effect on post-exercise protein metabolism of ingesting amino acids (Tipton et al. 1999), proteins (Tipton et al. 2004), carbohydrates (Roy et al. 1997; Roy et al. 2000; Borsheim et al. 2004), and a combination of carbohydrates and proteins (Rasmussen et al. 2000; Roy et al. 2000; Tipton et al. 2001). These studies showed that combining the intake of carbohydrates and proteins was the best nutritional strategy when seeking to achieve a positive nitrogen balance immediately after training. It is interesting to note that these components are the main macronutrients contained in milk. Milk contains a large amount of branched-chain amino acids (valine, leucine and isoleucine), which are strongly implicated in muscle protein synthesis pathways (Lunn et al. 2011). This explains why several researchers tested the effect on muscle recovery of consuming milk after bodybuilding sessions. The results reveal that drinking milk after exercise can limit the modifications of the muscle microlesion indicators triggered by physical exercise (Cockburn et al. 2008, 2010, 2012), either by reducing protein degradation or by increasing protein synthesis (Elliot et al. 2006; Wilkinson et al. 2007).

7Milk and recovery from DOMS. Elliot et al. (2006) studied the influence of several drinks containing milk on the protein reaction following a bodybuilding session. These authors compared one hour after a bodybuilding session, the effect of the ingestion of skimmed milk (237 g), whole milk (237 g) and of a portion of skimmed milk representing the same calorie intake as the 237 g of whole milk (393 g). The results showed a positive nitrogen balance for the three situations tested, thus indicating that consuming milk rapidly after a bodybuilding session is advantageous. A study performed by Wilkinson et al. (2007) established the relationship between these results and the increase in post-exercise protein synthesis. Additionally, research has shown that protein synthesis was increased more after consuming 500 ml of skimmed milk than after drinking a portion of soy milk representing an equivalent energy and macronutrient intake (745 kJ, 18.2 g of proteins, 1.5 g of lipids and 23 g of carbohydrates). According to the authors of this study, this difference could be due to the fact that skimmed milk takes longer to digest than soy milk. Skimmed milk could therefore provide a more sustained plasma amino acid concentration than soy milk, which contributes to prolonging the duration of post-exercise protein synthesis. Moreover, soy proteins appear to be mainly used for splanchnic protein synthesis and are more readily converted into urea than animal proteins (Fouillet et al. 2002). Based on these results, consuming cow’s milk (preferably skimmed), immediately after strength-based training sessions, can help promote muscle recovery. Some recent studies further support this affirmation, showing that drinking cow’s milk could reduce DOMS induced by a bodybuilding session and accelerate the recovery of strength production capacity in trained athletes (Cockburn et al. 2008, 2010, 2012). Cockburn et al. (2012) recently showed that 500 ml of skimmed milk was the optimal volume to achieve this result. Milk proteins could also be beneficial for endurance athletes who regularly participate in long-distance events. In this type of event, muscle proteins are exposed to both metabolic and structural degradation which can be repaired through appropriate nutritional intake. In this context, milk could be beneficial during the acute recovery period following exercise (Moore et al. 2014).

8Milk and increase in muscle mass. To stimulate protein synthesis, essential amino acids must be available, this requirement reinforces the benefits of consuming milk during the recovery period after each training session. The soluble proteins (lactoserum proteins) present in cow’s milk are a better source of protein than casein in terms of stimulating muscle growth. Their improved effect can be explained by their high leucine content (Cribbe et al. 2006; Hulmi et al. 2009). The biological value of soluble milk proteins exceeds that of protein contained in egg by ~15% and in meat by ~35% (Smithers, 2008). In addition, liquid sources of proteins (such as milk) also make amino acids more rapidly available (~25 min) than solid food sources (~100 min) (Burke et al. 2012).

9Within this context, consuming milk as an integral part of a bodybuilding programme seeking to increase muscle mass was investigated in several experimental studies. The first one was carried out by Rankin et al. (2004), who compared the influence on body composition of regularly ingesting low-fat chocolate milk (providing 5 kcal, 0.21 g of proteins and 0.92 g of carbohydrates per kilogram of body weight) and consuming a carbohydrate-containing drink with an equivalent calorie value.

10Both drinks were consumed immediately after bodybuilding sessions, over a ten-week period with three weekly sessions. The results found an increase in the level of maximal strength as well as an improved body composition in both of the tested groups, with no statistical difference between the two. However, interestingly, in subjects in the milk group, a non-significant trend towards an increase in lean mass was noted. Increases of 1.6 ± 0.4 kg were noted in the “Milk” group, whereas this gain was only of 0.8 ± 0.5 kg in the “Carbohydrate drink” group. These results, which suggest that milk may have a positive effect on the evolution of body composition during a bodybuilding programme, were supported by subsequent research. Hartman et al. (2007) compared the influence of three recovery drinks consumed by inexperienced bodybuilders during a 12-week training period (with five training sessions per week). The three drinks tested were a) skimmed milk (two portions of 500 ml), b) soy milk (two portions of 500 ml) and c) an energy drink containing maltodextrins. Each of the drinks had a similar calorie content. The results obtained reinforced those presented by Elliot et al. (2006), with a significant increase in the muscle mass in the bodybuilders who consumed milk regularly. In contrast, a recent study by Mitchell et al. (2014) showed no effect of consuming 500 ml of chocolate milk after each training session during a 12-week strength-training programme. This result was reported both in young and older people, and could be mainly explained by the fact that the amount of milk ingested after each session was probably insufficient (500 ml, corresponding to 14 g of proteins). Based on other results, it is very likely that doubling the dose would result in an ergogenic effect. Consuming at least 20 g of milk proteins rapidly after exercise appears to maximize muscle protein synthesis (Moore et al. 2009; Areta et al. 2013). Beyond the amount of milk ingested, the timing of its ingestion is also crucial when seeking to stimulate protein synthesis, with ingestion immediately after exercise being optimal. For example, West et al. (2011) showed that consuming one dose of 25 g of milk proteins immediately after a resistance training session was more effective than consuming ten doses of 2.5 g over the three hours following exercise. Recent data suggest that a leucine threshold must be reached to stimulate maximal protein synthesis, and that this threshold increases with age (Breen and Philipps, 2011).

11Josse et al. (2010) showed that these results also applied to female athletes. These authors compared the effect of consuming 2 x 500 ml of milk to the effect of an isocaloric carbohydrate-containing drink; in both cases, the drink was consumed immediately after each training session, and then one hour later, over a 12-week programme with five training sessions per week (Fig. 1). Their results showed an average greater increase in muscle mass gain of 72% (+ 1.9 ± 0.2 kg) for the “Milk” group compared to the “Carbohydrate” group (+ 1.1 ± 0.2 kg).

Figure 1: Evolution of lean mass before and after 12 weeks of bodybuilding training for groups consuming milk (n= 10) or an isoenergetic carbohydrate-rich drink (n= 10) after the sessions.

Figure 1: Evolution of lean mass before and after 12 weeks of bodybuilding training for groups consuming milk (n= 10) or an isoenergetic carbohydrate-rich drink (n= 10) after the sessions.

*: significantly different from the group consuming a carbohydrate-containing drink compared to the reference value (P<0.05).
The values presented are average ± standard error. Adapted from Josse et al. (2010).

3. Reducing mass

12In many disciplines, reaching a high level of performance requires body fat mass to be limited. Control of body composition can also reflect aesthetic demands, attempts to reduce the energetic cost of locomotion, or be used to modify an athlete’s weight category. Two main, possibly complementary, strategies can be contemplated to reduce fat mass (Donnelly et al. 2009). The first one consists in increasing the amount of training, in terms of volume and/or intensity, without modifying eating habits. However, for high-level athletes, this strategy is often difficult to implement, given the high volume of training they already undertake. The second strategy consists in reducing the number of calories provided by food, and often seems easier to adopt. This strategy also has other advantages; for example, when making changes, it can be a good opportunity to improve the quality of the food eaten at the same time. This approach will enhance the effects of calorie restriction. Several studies show that consuming cow’s milk could help reduce fat mass while also favouring more efficient use of the macronutrients provided by food. Indeed, studies carried out by Hartman et al. (2007), Rankin et al. (2004) and Josse et al. (2010) [see above, “2. Muscle recovery and gain in muscle mass”] all showed a more significant reduction of fat mass in subjects who consumed skimmed milk after a bodybuilding session compared to those who ingested a carbohydrate-rich drink (Fig. 2). A similar result was also described in studies performed with overweight populations (Zemel, 2009). The mechanism explaining how drinking cow’s milk affects fat mass reduction are being elucidated (Christensen et al. 2009; Zemel, 2009). The three main hypotheses emerging from current scientific data are that the increase in calcium intake due to increased milk consumption could: increase faecal fat excretion (Christensen et al. 2009); improve appetite control (Zemel et al. 2000); and/or enhance the mobilization of fat for energy expenditure through a calcium-vitamin D mediated mechanism (Zemel, 2003, 2009).

13During a restrictive diet, it is important to maintain an adequate protein intake to preserve muscle mass (Donnelly et al. 2009). If protein intake is not maintained, muscle atrophy can occur, which could generate a decrease in strength and speed qualities (Filaire et al. 2001; Degoutte et al. 2006). Considering this, it seems wise to schedule a snack rich in branched-chain amino acids (protein) after intense training sessions to limit post-exercise protein catabolism and to promote a rapid return to a positive nitrogen balance. Here again, skimmed cow’s milk appears to be an effective drink to meet the athlete’s specific needs.

Figure 2: Evolution of fat mass before and after 12 weeks of bodybuilding training for two groups of female athletes who consumed either milk (n= 10), or a carbohydrate-rich drink (n= 10) after training sessions.

Figure 2: Evolution of fat mass before and after 12 weeks of bodybuilding training for two groups of female athletes who consumed either milk (n= 10), or a carbohydrate-rich drink (n= 10) after training sessions.

*: significantly different from the group consuming a carbohydrate drink compared to the reference value (P<0.05). The values presented are average ± standard error. Adapted from Josse et al. (2010).

4. Energy recovery

14The relationship between glycogen storage and fatigue in intense aerobic activities is well established (Coggan and Coyle, 1987). Indeed, the progressive depletion of intramuscular glycogen stores and the associated decrease in blood sugar levels has been linked to fatigue during endurance events (Coyle, 2004). In this type of event, one of the major roles of the recovery period between two training sessions or two competitive events is to ensure rapid replenishment of muscle and liver glycogen stores.

15Many studies have attempted to identify the most effective strategies contributing to recovery. For recovery to be as efficient as possible, the athlete should consume 2 g carbohydrate per kilogram of body weight. This carbohydrate portion must be consumed as soon as possible after completing training as glycogen resynthesis is maximal during the two hours directly following exercise, due to activation of the glycogen synthase enzyme. Glycogen synthase ensures that glucose molecules are stored as glycogen (Ivy et al. 1988). In addition, other studies have shown that ingesting proteins (Zawadzki et al. 1992) or protein hydrolysates (van Loon et al. 2000) could increase glycogen resynthesis levels when associated with the ingestion of carbohydrates. Given that the carbohydrate concentration of milk is similar to that of many commercialized recovery drinks, and that it is also rich in proteins (i.e. casein) and essential amino acids (leucine, glutamine, serine), some authors tested its effect on recovery after endurance exercise (Shirreffs et al. 2007; Watson et al. 2008).

16Few studies have directly examined the effect of milk consumption on the replenishment of glycogen stores after this type of exercise. A recent study by Lunn et al. (2011) investigating muscle glycogen concentrations after a 45-minute race performed at 65% of VO 2 max did not report any significant benefit from consuming 500 ml of low-fat chocolate milk compared to a carbohydrate-rich isocaloric drink. However, it is worth noting that this event only induced a mild decrease in glycogen stores in participants, which probably limited the influence of the recovery drink on this postexercise parameter. Kammer et al. (2009) compared the respective effects on recovery of eating milk and cereal (77 g carbohydrates, 19 g protein, 3 g lipids) and of consuming a carbohydrate-based recovery drink (78 g carbohydrates) in trained cyclists after two hours’pedalling at 60-65% of VO 2 max. Once again, no difference was found in terms of replenishment of muscle glycogen between the two conditions tested (Fig. 3). This equivalent efficiency of milk compared with carbohydrate recovery drinks was confirmed by other studies which examined the level of performance as a function of the drink ingested after exercise. In one of these studies (Karp et al. 2006), a group of endurance athletes was studied to compare the efficiency of different recovery drinks – of which chocolate milk was one. Athletes took part in a cycling event performed at 70% of VO2 max until exhaustion, two hours after an intermittent work session. The carbohydrate recovery drink given to subjects was adapted to ensure an equivalent calorie intake to that of the milk drink. No statistically significant difference was found between the two drinks. A similar result was reported by Pritchett et al. (2009).

Figure 3: Muscle glycogen concentration measured in the vastus lateralis in trained cyclists, immediately and one hour after having pedalled for two hours at 60 to 65% of VO2 max. A significant increase in glycogen levels was observed after consuming a bowl of cereal with skimmed milk or a carbohydrate-rich drink providing an equivalent number of calories.

Figure 3: Muscle glycogen concentration measured in the vastus lateralis in trained cyclists, immediately and one hour after having pedalled for two hours at 60 to 65% of VO2 max. A significant increase in glycogen levels was observed after consuming a bowl of cereal with skimmed milk or a carbohydrate-rich drink providing an equivalent number of calories.

*: significantly different from “Immediately” (P<0.05). Adapted from Kammer et al. 2009.

5. Rehydration

17When an exercise is performed in a hot atmosphere, the water losses linked to sweating often exceed the amount of water ingested, resulting in frequent dehydration (Broad et al. 1996; Burke and Hawley, 1997; Maughan et al. 2004; Shirreffs et al. 2005). A negative water balance during one session can potentially jeopardize performance levels during a subsequent training session. Several studies show that dehydration has a negative impact on some physiological functions (Cheuvront et al. 2003; Coyle, 2004), but that post-exercise rehydration could help to minimize its effects. While it is quite easy to balance water losses when the athlete does not train more than once a day, the rehydration strategy must be carefully considered when several daily training sessions are scheduled.

18Athletes often find it difficult to balance their water losses during exercise and it is important to be well hydrated at the beginning of a subsequent session. As a result, post-exercise rehydration strategies have been comprehensively studied (Shirreffs et al. 2004). Several experiments showed that when electrolytes are included in recovery drinks the rehydration process is significantly increased; with sodium directly influencing the amount of liquid actually assimilated (Maughan and Leiper, 1995; Shirreffs et Maughan, 1998). Similarly, other studies showed a positive effect on postexercise rehydration of adding potassium to recovery drinks (Nielsen et al. 1986; Yawata, 1990; Maughan et al. 1994). Most research into rehydration strategies focused on experimental drinks prepared in laboratories or commercialized energy drinks, and yet milk should be considered a potential “candidate” given its high electrolyte concentration and the fact that it contains a similar level of carbohydrates to most commercialized energy drinks.

19Three recent studies (Shirreffs et al. 2007; Watson et al. 2008; Desbrow et al. 2014) examined to what extent milk could benefit post-exercise rehydration. Shirreffs et al. (2007) first compared the effects of ingesting skimmed milk (plain or sodium-enriched), carbohydrate-rich sports drinks or water in athletes having lost 1.8 ± 0.2% of their body mass after an endurance exercise performed in a climatic chamber. The subjects consumed 150% of the mass they had lost in one hour. Their urine was collected before exercise and during the four hours following exercise. The results showed that after drinking milk, post-exercise urine production had not increased after one or two hours compared to the situation at rest. This result contrasts with urine production in the “Water” and “Energy drink” conditions (Fig. 4). Thus, the amount of urine produced post-exercise was on average two-fold lower when subjects had consumed milk compared to water or a carbohydrate-rich drink. The results went on to show that the subjects who ingested water or an energy drink returned to a negative water balance within one hour, while those who consumed milk (sodium-enriched or not) maintained a positive hydration balance throughout the four hours following exercise (Fig. 5).

Figure 4: Urine production after the ingestion of four different drinks following an exercise inducing a loss of 1.8% of body mass.

Figure 4: Urine production after the ingestion of four different drinks following an exercise inducing a loss of 1.8% of body mass.

a, b, c, d: significantly different compared to water, energy drink, milk and sodium-enriched milk, respectively. Adapted from Shirreffs et al. (2007).

Figure 5: Evolution of water balance after the ingestion of four different drinks following an exercise having generated a 1.8% drop in body mass.

Figure 5: Evolution of water balance after the ingestion of four different drinks following an exercise having generated a 1.8% drop in body mass.

a, b, c, d: significantly different compared to water, energy drink, milk and sodium-enriched milk, respectively. Adapted from Shirreffs et al. (2007).

20This result was confirmed by a second study by Watson et al. (2008), based on a relatively similar protocol, but involving a time to exhaustion cycling event performed after three hours’ recovery in a hot and humid atmosphere (35 °C and 63% relative humidity). Despite improved rehydration, no effect of the recovery drink on performance was shown.

21More recently, Desbrow et al. (2014) confirmed their previous results by comparing the effects of four different drinks (cow’s milk, soy milk, a milk-carbohydrate liquid supplement, a carbohydrate-rich sports drink) on rehydration in cyclists having lost 2% of their body mass after intermittent exercise. The participants consumed 150% of the body mass lost during the hour following exercise. Body mass was better restored by drinking cow’s milk, soy milk or the milk-carbohydrate liquid supplement than by drinking the sports drink. Urine excretion was significantly greater with the sports drinks compared to the other drinks. Based on this study, drinks rich in proteins, carbohydrates and sodium enhance fluid retention compared to drinks rich in carbohydrates alone. The combination of different macronutrients and the higher calorie content could slow the rate of gastric emptying, which is likely to improve the absorption capacity and fluid retention in the small intestine (Kwiatek et al. 2009), thus enhancing rehydration.

22Even though the number of studies on the topic is still limited, all these results confirm that milk presents advantages over commercial sports drinks in terms of balancing exercise-related water and electrolyte losses.

6. Sleep

23To keep up with training and to adapt to the workload it represents, athletes must optimize their recovery. If this is not done, an overloaded state, characterized by a temporary decrease in performance, can appear. While the quantitative and qualitative needs of the athlete in terms of sleep have not been studied much up to now, there are many examples showing that sleep is the main recovery factor for this population due to the major role it plays in replenishing cognitive and physiological performances (Halson et al. 2008).

24Some studies have shown that a lack of sleep has a negative effect on sporting performance due to psychophysiological mechanisms. At the physiological level, a lack of sleep triggers diminished immune resistance and reduced aerobic performance (Reilly and Edwards, 2007). At the psychological level, sleep deficits affect mood and perturb cognitive performances (Reilly and Deykin, 1983). While the precise relationship between overload, overtraining and sleep remain to be established, some studies have indicated a negative relationship between sleep quality and training load (Jurimae et al. 2002, 2004).

25Sleep is controlled by the production of a neurohormone in the central nervous system known as melatonin. This molecule is synthesized from a brain neuromediator, serotonin, which is produced in the central nervous system from the amino acid tryptophan. Although it is not possible to directly modify the concentration of brain serotonin, since this molecule does not cross the blood-brain barrier, it is possible to influence its production by controlling the entry of tryptophan into the brain (Silber and Schmitt, 2010). Some studies have backed up this hypothesis, showing that: a) the ingestion of tryptophan was accompanied by a 45% reduction in the time it takes to fall asleep (Hartmann, 1982); and b) that during nights following tryptophan depletion, sleep was more fragmented (Arnulf et al. 2002). It is interesting to note that one of the best food sources of tryptophan is milk (Heine et al. 1996). This composition explains why, as is popularly believed, drinking milk before going to bed can help favour sleep and improve its quality.

26Research carried out by Markus et al. (2005) tested this hypothesis indirectly, by studying the effect on concentration and attention the morning after consuming lactalbumin (i.e. milk protein) at dinner. The results showed that consuming milk was associated with a decreased feeling of tiredness and a higher concentration level the next morning. Additionally, in subjects suffering from sleep disorders, performance levels were enhanced. This suggests that consuming milk could be an efficient nutritional strategy to improve sleep quality. In addition, drinking a mug of sweetened hot milk potentially presents other advantages as well as ensuring a high tryptophan intake. Firstly, ingesting a hot beverage increases the core body temperature, inducing a thermolitic reaction favourable to sleep. Secondly, the ingestion of carbohydrates (in the form of lactose, possibly provided by milk) enhances sleep by stimulating the entrance of tryptophan into the central nervous system (Silber and Schmitt, 2010). Even though these results appear promising and suggest that milk has a positive effect on sleep quality, further research is required to confirm this relationship.

7. Conclusion

27All of the results gathered in the scientific data highlight a range of benefits athletes can derive from consuming skimmed cow’s milk. These benefits can also be reaped by those who are intolerant to lactose by drinking delactosed milk. The array of current scientific evidence shows that ingesting milk every day contributes to good bone health, post-exercise muscle regeneration (even increasing lean mass, when this is an objective) and helps reduce fat mass. Recent studies showed that, in promoting recovery after endurance exercise, milk was as efficient as commercialized energy drinks. In addition, the electrolytes contained in milk make it effective in optimizing post-exercise rehydration. Finally, some preliminary data suggest that milk has a good impact on sleep, although further research is required to confirm this hypothesis, especially in trained athletes for whom sleep disorders are one of the first symptoms of overload.

28Milk is a cheap drink and is readily available for all athletes. To achieve the recommended daily calcium intake (1000 mg) without exceeding the maximal tolerated amount (2500 mg), the best strategy is to consume no more than a litre of milk per day and to schedule its consumption at the most opportune moments, i.e. after intense training sessions or before bedtime, depending on the objective sought.

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