Potential role for protein in assisting post-exercise rehydration
Ms Suzane Leser, R Nutr, Nutrition Manager – Lifestyle Ingredients, Volac, Volac House, Orwell, Royston, Hertfordshire SG8 5QX, UK.
The latest International Olympic Committee Consensus Statement on Sports Nutrition 2010 concluded that when athletes must compete in several events in a short time-period, strategies to enhance recovery of fluid and fuel are important. In fact, all athletes and recreational exercisers might benefit from effective rehydration strategies, as rapid rehydration is not limited only to optimal subsequent performance. Rehydration also regulates cell function in favour of the adaptive processes and improvements in body composition, which take place during recovery. The composition of a fluid consumed soon after exercise has an important impact on body water restoration and should be considered if rapid rehydration is a goal. Typically, guidelines recommend using sports drinks or foods and fluids that contain carbohydrate for replacement of glycogen stores and electrolyte sodium, which promotes greater fluid absorption and retention. However, more effective restoration of body water and plasma volume have been observed in some studies when more nutrients and food compounds are consumed. It suggests a role for other nutrients, such as protein, in the strategy to enhance rehydration. Emerging research looking into milk proteins, whey and casein, points to a role for protein in assisting post-exercise fluid retention. The most obvious mechanisms are enhanced sodium and water absorption from the gut, and increased plasma protein synthesis resulting in higher osmotic pressure exerted by plasma proteins. This article reviews current strategies to enhance post-exercise recovery of fluid balance, with a focus on protein.
Rehydration during prolonged exercise has been the focus of much attention, as loss of fluid and reduction of the body's carbohydrate stores are the two major causes of fatigue (Maughan et al. 1993). After exercise, rapid rehydration is generally recommended in order not to limit subsequent performance (Watson et al. 2008). The International Olympic Committee (IOC), in its latest Consensus Statement on Sports Nutrition 2010, advised that ‘during recovery from exercise, rehydration should include replacement of both water and salts lost in sweat’. In addition, the Committee concluded that ‘when athletes must compete in several events in a short time-period, strategies to enhance recovery of fluid and fuel are important’ (IOC 2010).
In fact, all athletes and recreational exercisers might benefit from effective rehydration strategies. Although it is possible to replace fluid losses with normal food and fluid intake when the interval between sessions is greater than 24 hours (Casa et al. 2000; ACSM 2007), the benefit of rapid rehydration is not limited only to optimal subsequent performance. Most importantly, rapid rehydration regulates cell function in favour of the adaptive processes and improvements in body composition, which take place shortly after exercise (Keller et al. 2003). Alongside timing and volume of fluid ingested, the concentration of electrolyte sodium is a key factor when rapid rehydration and sustained positive fluid balance are the goals after exercise (Ray et al. 1998; Shirreffs et al. 2004). Most recently, the presence of additional nutrients, such as carbohydrate and protein, have been suggested to also influence fluid absorption and retention (Seifert et al. 2006; Sharp 2007).
This article will review current strategies to enhance post-exercise recovery of fluid balance. It will focus on emerging research, exploring ways to improve the formulation of sports drinks for enhanced body fluid restoration, with a potential role for milk proteins.
For rapid recovery of fluid balance, the American College of Sports Medicine (ACSM) recommend that sports people drink, in practice, around 1.5 l of liquids for each kg of body weight lost, or 150% of the body weight lost, induced during exercise (ACSM 2007). After exercise, urine volume is largely governed by the amount of metabolic waste products to be excreted. Thus, if the intake of fluids is low, the body at first gives up its own water for obligatory urine losses, even when dehydrated (Sharp 2007). Therefore, it is always important to correct any water deficit as quickly as possible after exercise in a volume greater than the volume of sweat that has been lost (Sharp 2007; Evans et al. 2009a).
However, simply drinking a volume in excess of sweat losses may not be sufficient to sustain prolonged positive fluid balance after exercise (Shirreffs et al. 2007). Receptors in the body respond to changes in body water (or plasma volume) and blood pressure, but are strongly influenced by blood concentration (or osmolality) (Baylis 1987). When blood osmolality decreases quickly, as a consequence of fast hyperhydration, fluid is lost to urine formation to restore normal blood osmolality. As blood osmolality increases, signals are sent to the kidneys to retain fluid (Seifert et al. 2006). Therefore, typical guidelines recommend taking sports drinks or foods and fluids that contain electrolyte sodium before, during and after exercise (ACSM 2007). Sodium promotes fluid absorption and retention, but more effective restoration of plasma volume after exercise has been observed when more nutrients and food compounds are consumed compared with traditional sports drinks. Although authors of these studies suggested that the increased restoration of body fluids was caused by the additional sodium from foods consumed (such as soup) (Maughan et al. 1996; Ray et al. 1998), there is also a potential role for other nutrients, such as protein, in the strategy to enhance rehydration (Ray et al. 1998).
Strategies to enhance rehydration
Post-exercise rehydration is often looked at as a singular issue, but it is composed of three interrelated components: gastric emptying, intestinal absorption and ultimately fluid retention. The IOC (2010) suggests that athletes and their nutrition advisors look at strategies to enhance recovery of fluid. Rehydration strategies which avoid a large fall in plasma osmolality will be of most benefit for sustained positive fluid balance after exercise, because it attenuates loss of fluid to urine production (Evans et al. 2009b).
It is well known from sports hydration studies that the ingestion of plain water after significant sweat losses can cause a higher than usual dilution of blood, with significant decrease in blood osmolality, and subsequent large urine output (Costill & Sparks 1973). Rehydration with plain water has been demonstrated to retain around 50% of the volume ingested, normally failing to maintain positive fluid balance throughout 3 hours of exercise recovery period and delaying the complete restoration of body water (Seifert et al. 2006).
Electrolyte sodium is the most commonly found osmotically active particle in rehydration drinks (Shirreffs 2009). Before and during exercise, water-based, commercially available carbohydrate-electrolyte drinks, or ‘sports drinks’, deliver mainly water and electrolyte sodium to replace sweat losses and promote retention of an increased proportion of the fluid ingested. Such drinks aim to minimise dehydration, while generally containing carbohydrates for energy provision and glycogen restoration. The combination of carbohydrate and sodium is also designed to speed up fluid absorption and optimise flavour to encourage voluntary drinking. However, sports people usually lag behind on adequate fluid intake during exercise to match sweat losses, replacing no more than 75% of fluid losses (Ray et al. 1998; Hew-Butler et al. 2006). As sweat losses generally exceed fluid intake, a dehydrated state is normally expected at the end of the exercise (Greenleaf 1992; Maughan et al. 2004). However, this is not ideal for recovery and needs to be corrected (Sharp 2007).
A number of studies have investigated the effectiveness of sports drinks for post-exercise rehydration. Maughan et al. (1996), were among the first to investigate the use of sports drinks for restoration of fluid balance after exercise-induced dehydration. A commercially available sports drink was compared with a standard solid meal and water, both matched for water intake at 150% of the exercise-induced body mass loss. The composition of both interventions is explained in Table 1. Rehydration with standard meal and water provided 20 mmol additional sodium, 14 mmol additional potassium and approximately 50 g protein, compared with rehydration with sports drink. Both interventions acutely recovered body fluid balance. However, the peak in hyperhydration observed after the fast absorption of the sports drink led to rapid return to negative fluid balance, within 2 hours after rehydration. The standard meal with water was able to maintain positive fluid balance for 4 hours after rehydration. At the end of the 6 hours of rehydration period, retention of total fluid ingested was only 51% when using the sports drink, similar to that of plain water. Similar findings were reported by Ray et al. (1998) (Table 1), whose study observed that a commonly used sports drink did not restore plasma volume more effectively than water at the end of a 2-hour rehydration period. Most recently, Watson et al. (2008; Table 1) and Shirreffs et al. (2007; Table 1) also demonstrated that post-exercise rehydration with sports drinks was not able to maintain long-term positive fluid balance. At the end of a 3-hour and 4-hour recovery period, respectively, participants were in negative fluid balance when using a sports drink, but were able to effectively replace fluid losses and keep hydrated when drinking milk caused by decreased urine output.
Table 1. Human studies investigating the use of water, sports drinks, energy-dense meals and milk as post-exercise rehydration strategies
|Maughan et al. (1996)||Eight physically active individuals. (5 men, 3 women). Mean age 31 years.||2.1 ± 0.0% induced by 50 ± 7 min intermittent cycle ergometer exercise in the heat.||1. SD: commercially available sports drink (64 g/l carbohydrate; 21 mmol/l Na; 3.4 mmol/l K; 12 mmol/l Cl). |
2. Meal: standard meal (rice, beans and beef) + flavoured water (63 kJ/kg; 53% carbohydrate; 28% fat; 19% protein*; 0.118 mmol/kJ Na**; 0.061 mmol/kJ K*** + drink: 1 mmol/l Na; 0.4 mmol/l K; 1 mmol/l Cl).
* For a 70 kg individual: ∼50 g protein.
** Mean total Na intake with Meal was 63 mmol (20 mmol greater than with SD, at 43 mmol).
*** Mean total K intake with Meal was 21 mmol (14 mmol greater than with SD, at 7 mmol).
In a randomised crossover design, each subject completed two trials with SD alone and one trial with Meal. Total fluid intake on all trials was 150% of body weight loss (average 2050 ± 131 ml). Fluid balance was monitored for 6 h post-rehydration.
|• At the end of 6 h, subjects were euhydrated with Meal. With SD, there was a median negative fluid balance of approximately 350 ml. |
• Meal retained 67% of the fluid ingested, and SD retained 51–52%.
• Retention of sodium and potassium was greater with Meal, but cumulative urine sodium and potassium output was higher.
|Ray et al. (1998)||30 physically active college-age individuals (15 men, 15 women).||2.5 ± 0.1% induced by alternating periods of 20 min light exercise on a cycle ergometer and 10 min of sauna exposure, which lasted for 90–120 min.||1. W: water. |
2. CE: carbohydrate-electrolyte drink (65 g/l carbohydrate; 16.0 mmol/l Na; 3.3 mmol/l K; 359.3 mOsmol/kg H2O).
3. CB: chicken broth (0.8 g/l carbohydrate; 16.5 g/l protein; 1.2 g/l fat; 109.5 mmol/l Na; 25.3 mmol/l K; 16.5 g/l protein; 1.2 g/l fat; 306.5 mOsmol/kg H2O).
4. Soup: chicken noodle soup (93 g/l carbohydrate; 30.0 g/l protein; 14.0 g/l fat; 333.8 mmol/l Na; 13.7 mmol/l K; 270.5 mOsmol/kg H2O).
In a randomised crossover design, subjects ingested 175 ml at the start of a 2 h rehydration period and 20 min later. H2O was given every 20 min thereafter for a total volume equal to, bw loss (average 1836 ± 53 ml).
|• At the end of 2 h rehydration period, plasma volume returned to levels significantly closer to pre-dehydration in both CB and Soup (−1.6 ± 1.1% and −1.4 ± 0.9%, respectively). But in both W and CE, plasma volume remained significantly below pre-hydration levels (−5.6 ± 1.1% vs. −4.2 ± 1.0%, respectively). |
• Urine output was lower with CB (188 ± 20 ml) than with CE (310 ± 30 ml).
• Sodium excretion in urine was higher with Soup and CB than with CE and W.
|Shirreffs et al. (2007)||11 physically active individuals (5 male, 6 female). Mean age 24 years.||1.8 ± 0.2% induced by intermittent cycle exercise in the heat (total exercise time not reported).||1. W: water (0.3 mmol/l Na). |
2. CE: carbohydrate-electrolyte sports drink (60 g/l carbohydrate; 23 mmol/l Na; 2 mmol/l K; 283 mOsmol/kg).
3. M: low-fat milk (50 g/l carbohydrate; 36 g/l protein; 38.6 mmol/l Na; 45.2 mmol/l K; 299 mOsmol/kg).
4. M + Na: low-fat milk + 20 mmol/l NaCl (50 g/l carbohydrate; 36 g/l protein; 58 mmol/l Na; 47 mmol/l K; 345 mOsmol/kg).
In a randomised crossover design, subjects ingested volumes equal to 150% bw loss (average 1790 ± 420 ml) during 60-minute, followed by a 4 h exercise recovery period.
|• At the end of the 4 h recovery period, the fraction of drink retained was higher for M (69 ± 10%) and M + Na (72 ± 4%) than for W (36 ± 10%) and CE (38 ± 16%). |
• On both M and M + Na, subjects were in positive fluid balance at the end of the recovery period. On both W and CE, subjects were in negative fluid balance.
• The greatest urinary electrolyte loss was observed during M + Na trial.
|Watson et al. (2008)||7 physically active males. Mean age 23 years.||2.0 ± 0.1% induced by around 36–37 min intermittent cycle exercise in the heat.||1. CE: carbohydrate-electrolyte sports drink (60 g/l carbohydrate; 23 mmol/l Na; 1.6 mmol/l K; 280 mOsmol/kg). |
2. M: low-fat milk (50 g/l carbohydrate; 33 g/l protein; 32 mmol/l Na; 42 mmol/l K; 278 mOsmol/kg).
In a randomised crossover design, subjects ingested volumes equal to 150%, bw loss (approximately 2270 ± 245 ml) during 60 min, followed by a 3 h exercise recovery period.
|• At the end of the 3 h recovery period, the proportion of drink retained was greater for M (77 ± 6%) than for CE (62 ± 17%). |
• On trial M, subjects were in positive fluid balance at the end of the recovery period. On trial CE subjects were in negative fluid balance.
• Despite a significant greater amount of sodium ingested in trial M than in trial CE, in neither trial was sufficient sodium consumed to replace losses.
The potential ability of skimmed milk to outperform sports drinks in aiding post-exercise fluid recovery has been attributed to its carbohydrate content (similar to that of commercially available sports drinks), the protein composition and its naturally high concentration of electrolytes (Roy 2008). Although a greater total electrolyte content is found in milk as sodium, potassium, chloride, calcium and magnesium, its typical sodium range is similar to that of sports drinks, at around 20–25 mmol/l (Shirreffs 2009; The Dairy Council). Studies investigating the effects of different levels of sodium in enhancing rehydration are shown in Table 2. Shirreffs et al. (2007) tested the hypothesis that fluid retention after exercise-induced dehydration is directly related to the amount of sodium in the drink. Low-fat milk with additional 20 mmol/l sodium chloride (NaCl) was investigated. There was no further benefit gained to fluid balance over that of commercially available milk. Merson et al. (2008) investigated how differing moderate NaCl concentrations affected rehydration after exercise. They observed that the fraction of fluid retained 4 hours after drinks ingestion was 39, 50, 60 and 64% of the volume consumed for the 1, 31, 40 and 50 mmol/l NaCl drinks, respectively. No significant difference was observed between 40 and 50 mmol/l NaCl. However, even at such optimal intakes, study participants returned to negative fluid balance at 3 hours into recovery. Regarding rate of fluid delivery, Jeukendrup et al. (2009) found no difference between 6% carbohydrate drinks of different sodium levels (20–60 mmol/l). These studies suggest a limited role for increased sodium levels above those naturally found in milk.
Table 2. Human studies investigating drinks with different sodium levels for enhanced body fluid absorption and retention
|Shirreffs et al. (2007)||11 physically active individuals (5 male, 6 female) . Mean age 24 years.||1. W: water (0.3 mmol/l Na). |
2. CE: carbohydrate-electrolyte sports drink (23 mmol/l Na).
3. M: low-fat milk (38.6 mmol/l Na).
4. M + Na: low-fat milk + 20 mmol/l NaCl (58 mmol/l Na).
In a randomised crossover design, subjects ingested volumes equal to 150% bw loss (average 1790 ± 420 ml) during 60 min, followed by a 4 h exercise recovery period.
|• At the end of the 4 h recovery period, no further benefit gained to fluid retention by adding 20 mmol/l NaCl to low fat milk. |
• W and CE resulted in marked diuresis 2 hours after drinking.
• Greatest urinary electrolyte loss observed during the M + Na trial.
|M: 38 mmol/l (88 mg/dl).|
|Merson et al. (2008)||8 healthy male. Mean age 25 years.||1. Trial 1: 2% carbohydrate, 1 ± 1 mmol/l Na. |
2. Trial 30: 2% carbohydrate, 31 ± 1 mmol/l Na.
3. Trial 40: 2% carbohydrate, 40 ± 1 mmol/l Na.
4. Trial 50: 2% carbohydrate, 50 ± 1 mmol/l Na.
In a randomised Latin square design, subjects ingested volumes equal to 150% bw loss (average 2253 ± 311 ml) during 60 min, followed by a 4 h exercise recovery period.
|• At the end of the 4 h recovery period, more of the test drink ingested was retained as the sodium concentration of the drink increased: 39 ± 14%, 50 ± 13%, 60 ± 14%, 64 ± 11% for trials 1, 30, 40 and 50 respectively. |
• Significantly more fluid was retained in trials 40 and 50, with no significant difference between trials.
• 4 h after drinking, subjects were in negative fluid balance on all trials, but tended to be in greater net negative fluid balance as the sodium concentration of drink decreased.
|Trial 40: 40–50 mmol/l (92–115 mg/dl).|
|Jeukendrup et al. (2009)||20 healthy male. Mean age 21 years.||1. G0: water + 20 mmol/l Na. |
2. G3: 3% glucose + 20 mmol/l Na.
3. G6: 6% glucose + 20 mmol/l Na.
4. G9: 9% glucose + 20 mmol/l Na.
5. Na0: 6% glucose.
6. Na20: 6% glucose + 20 mmol/l Na.
7. Na40: 6% glucose + 40 mmol/l Na.
8. Na60: 6% glucose + 60 mmol/l Na.
Subjects were divided into two groups of 10 (Carbohydrate and Na). Each subject undertook four trials under their respective groups. At rest, subjects ingested a fixed volume of 500 ml (enriched with deuterium oxide tracer) and remained seated for 2 h
|• At the end of 2 h, the sodium content in the range investigated (20–60 mmol/l) did not affect fluid delivery. |
• Compared with water, rate of fluid delivery was reduced when increasing the glucose content of the beverage above 6%.
• The presence of 6% glucose in the beverages may have masked any effect that sodium has upon fluid delivery.
Results from Shirreffs et al. (2007), Ray et al. (1998) and Maughan et al. (1996) suggest that additional sodium and electrolytes are largely expelled by the kidneys. Whole body fluid restoration seems to be related to the sodium content of the beverage, but only up to a certain level. Increasing the sodium concentration of sports drinks above typical levels is unlikely to significantly enhance the effectiveness of these drinks and might compromise palatability and voluntary intake. Studies with milk suggest that the superior effects observed over sports drinks in promoting rehydration might be explained by its higher energy density (Shirreffs et al. 2007).
One factor speculated to contribute to restoration of fluid balance is the energy density of the food or drink ingested, and consequently, the rate at which it empties from the stomach (Leiper 2001). It is long recognised that high-moisture foods, like pasta, rice, fruits and vegetables, soups and milk, that have a relatively higher energy density than traditional sports drinks, contribute not only with fluid and electrolytes, but also with macronutrients and compounds which may assist in regulating hydration, by affecting fluid absorption, urine volume and fluid retention (Maughan et al. 1996; ACSM 2007; Sharp 2007; Ghigiarelli et al. 2009). A slower absorption into the circulation caused by higher energy density may attenuate the large fall in blood osmolality, responsible for large increases in urine output. Studies demonstrating the increased effectiveness of including foods, and thus, increasing energy density in post-exercise rehydration strategies are those of Maughan et al. (1996) and Ray et al. (1998; Table 1), although it should be noted that the authors of these studies suggested that including foods had improved rehydration because of their electrolyte content. It is also possible that other nutrients present in these foods, like carbohydrate, fat and protein, may have affected fluid retention by increasing plasma osmolality.
The role of carbohydrates
The well-established role of carbohydrate in exercise rehydration is based partly on observations that at low concentrations it can increase the rate of water uptake in the small intestine (Evans et al. 2009b). However, evidence regarding the role of carbohydrate in subsequent body fluid retention is mixed. Lambert et al. (1992) showed that the addition of carbohydrate (100 g/l) to rehydration drinks, consumed over a 4-hour period after exercise-induced sweat losses at approximately 4% of body mass, demonstrated no effect on the restoration of plasma volume or on total urine output (Table 3). Evans et al. (2009a), when supplementing a drink with carbohydrate only, found no difference between 10% hypertonic and 2% hypotonic carbohydrate-electrolyte solutions in maintaining hydration after exercise, in a situation of voluntary fluid intake (Table 3). When the same authors prescribed a fixed fluid volume, a significant difference between trials was observed (Evans et al. 2009b). Fluid retention for the 10% hypertonic drink was 46 ± 9% of the volume ingested, compared with 40 ± 14% for the 2% hypotonic drink (Table 3). However, such fraction of fluid retention for the 10% hypertonic drink is similar to that observed for traditional sports drinks at 6% carbohydrate concentration (Maughan et al. 1996; Shirreffs et al. 2007; Watson et al. 2008). Nevertheless, Evans et al. (2009b) suggest that hypertonic solutions may be effective for restoration of exercise-induced water losses and maintenance of hydration status, because subjects remained hydrated for a longer period after ingestion of the hypertonic 10% carbohydrate solution, compared with the hypotonic 2% carbohydrate solution or the energy-free solution. Also, carbohydrate must be consumed to recover muscle glycogen stores.
Table 3. Human studies investigating the role of carbohydrate in post-exercise fluid retention
|Lambert et al. (1992)||8 physically active males. Mean age 28 years.||4.12 ± 0.02% induced by cycling at 50% VO2max in the heat for 128 ± 3 min.||1. CK: carbonated 10% carbohydrate (glucose and fructose). |
2. CNK: carbonated non-carbohydrate.
3. NCK: non-carbonated 10% carbohydrate (glucose and fructose).
4. NCNK: non-carbonated non-carbohydrate.
In a cross-over randomised design, drinks were ingested at 15 min intervals at a total volume equal to, bw loss (approximately 3270 ml) during a 4 h recovery period.
|• At the end of the 4 h recovery period, the quantity of urine produced was not different between treatments (397 ± 49, 463 ± 54, 428 ± 49 and 480 ± 73 ml for CK, CNK, NCK and NCNK, respectively. |
• The 10% carbohydrate solutions were as effective as the non-carbohydrate solutions with regard to fluid retention.
• Rehydration was similar but incomplete with the four treatments.
|Evans et al. (2009a)||9 physically active individuals. (6 men, 3 women). Mean age 23 years.||1.99 ± 0.07% induced by 50 ± 16 min intermittent cycle exercise in the heat.||1. 0% glucose (74 ± 1 mOsm/kg; 31 ± 1 mmol/l Na, 0.6 ± 0.1 mmol/l K; 27 ± 1 mmol/l Cl). |
2. 2% glucose (188 ± 3 mOsm/kg; 31 ± 1 mmol/l Na, 0.6 ± 0.1 mmol/l K; 26 ± 1 mmol/l Cl).
3. 10% glucose (654 ± 4 mOsm/kg; 31 ± 1 mmol/l Na, 0.6 ± 0.1 mmol/l K; 27 ± 1 mmol/l Cl).
In a randomised crossover design, ad libitum total fluid intake was around 143–165% of, bw loss (average 2173 ± 252 ml – 2539 ± 436 ml), with total fluid intake and urine output not different among trials.
|• At the end of the 5 h rehydration period, the fraction of fluid retained was 48 ± 20%, 49 ± 13%, 57 ± 15% during the 0%, 2% and 10% glucose trials respectively, but not significantly different between trials. |
• Total urinary Na and Cl excretion was greater at the end of the experimental period on the 0% glucose than on the 10% glucose trial. Excretion of K was the same during all trials.
• Subjects were in negative electrolyte balance at 3 h into the rehydration period.
|Evans et al. (2009b)||6 physically active male. Mean age 26 years.||1.9 ± 0.1% induced by 45 ± 8 min intermittent cycle exercise in the heat.||1. 0% glucose (79 ± 4 mOsm/kg; 32 ± 1 mmol/l Na, 0.5 ± 0.1 mmol/l K; 27 ± 2 mmol/l Cl). |
2. 2% glucose (193 ± 5 mOsm/kg; 32 ± 1 mmol/l Na, 0.4 ± 0.1 mmol/l K; 26 ± 3 mmol/l Cl).
3. 10% glucose (667 ± 12 mOsm/kg; 31 ± 0 mmol/l Na, 0.4 ± 0.1 mmol/l K; 27 ± 2 mmol/l Cl).
In a cross-over randomised design, drinks were ingested in four equal aliquots over a period of 60 min at a total volume equal to 150% of, bw loss (1962 ± 247 ml) (one subject drank 130% of, bw loss).
|• At the end of the 6 h rehydration period, the fraction of fluid retained was 27 ± 13%, 40 ± 14%, 46 ± 9% during the 0%, 2% and 10% glucose trials respectively, and was significantly different between trials (P = 0.046). |
• Urine output, although not significantly different, was greater during the 2% glucose trial than the 10% glucose trial in five of the six subjects.
• Subjects remained euhydrated for 1 h longer in the 10% glucose trial than in the 2% glucose trial, i.e., returned to negative fluid balance 4 h after rehydration.
Protein-added sports drinks
Regardless of a potential role in enhancing fluid retention, the primary function of protein after exercise is to promote protein synthesis. The well-established role of protein in contributing to the health of the general population has been recently supported by the European Food Safety Authority (EFSA) in its scientific opinion on the substantiation of health claims related to protein, as part of European Commission Regulation 1924/2006 on nutrition and health claims made on foods (EFSA Panel on Dietetic Products, Nutrition and Allergies 2010). Protein has been found to maintain bone health through the promotion of calcium absorption, and to be important for the maintenance and growth of muscle mass, as the loss of muscle at any age will reduce muscle strength and power. This is of particular importance, as a growing proportion of the older population is increasingly taking part in mass sport and exercise. Data from the 2008 London Marathon have revealed that almost 60% of the 34 500 runners were over 35 years old, of which nearly 3% (over 1000 participants), were aged over 60 years (Lucozade Sport 2008).
Early-stage research focused on investigating the benefits of adding protein to sports drinks in an attempt to enhance glycogen resynthesis during recovery. However, this response appears to be replicated through the ingestion of additional carbohydrate, sufficient to meet optimal energy intake (Gibala 2007; Hawley et al. 2007; Watson et al. 2008). However, the co-ingestion of carbohydrate and protein can be advantageous when sufficient carbohydrate is not consumed (Burke et al. 2004). The protein energy contribution appears to be at least as good as that of carbohydrate, with additional short-term benefits for the exercise recovery (Millard-Stafford et al. 2008).
Although the role of fat and energy density cannot be dismissed, the main apparent compositional difference between low-fat milk and traditional sports drinks is the milk protein content (Roy 2008). Emerging science looking into milk proteins, whey and casein, points to a role for protein in significantly assisting post-exercise rehydration (Seifert et al. 2006; James et al. 2011). Seifert et al. (2006) demonstrated that by adding 1.5% whey protein to the traditional formulation of a sports drink, water retention was improved by 15% over the base sports drink formulation and 40% over plain water. Because there was no difference in plasma volume restoration, the differences in retention have to be found inside tissues and cells (Table 4). It should be noted however, that this study has received some criticism as it did not follow ACSM recommendation in its rehydration strategy (subjects ingested drinks equal to body weight lost during exercise, not 150% of body weight lost as recommended by the ACSM), and the drinks were not matched for calorie content, making it difficult to separate the effects of adding extra protein from the effect of the increase in energy density. Most recently, James et al. (2011) isolated for the first time the effect of protein on post-exercise fluid retention. By substituting 2.5% of a 6% carbohydrate drink with milk proteins, fluid retention was significantly increased by an average of 12%. It resulted in less negative fluid balance, 4 hours after exercise. Although fluid balance for the carbohydrate-milk protein solution was still negative, it was not found to be statistically different from pre-exercise levels (Table 4). In this study, parameters have been set at an equivalent calorific value for both carbohydrate-only and protein-carbohydrate formulations, meaning that in the latter, the carbohydrate levels have been decreased to allow for the addition of proteins. Had protein been added on top of the carbohydrate concentration, the increased energy density could have exerted a different effect on fluid balance. Furthermore, fluid ingestion was controlled at 150% of body mass lost during exercise. It is uncertain if the results observed would persist with voluntary fluid intake.
Table 4. Human studies investigating protein-added sports drinks as post-exercise rehydration strategies
|Seifert et al. (2006)||13 experienced endurance athletes (8 men and 5 women). |
Age range 20–28 years.
|2.4 ± 0.1% induced by cycling at 80% of maximal heart rate at 25°C. Exercise duration ranged from 60–75 min for males and 75–105 min for females.||1. CWP: carbohydrate-whey protein drink (1.5%) (60 g/l carbohydrate; 15 g/l protein; 53 mg/100 ml Na; 18 mg/100 ml K; 305 mOsm/kg). |
2. CHO: carbohydrate-only drink (60 g/l carbohydrate; 46 mg/100 ml Na; 12.5 mg/100 ml K. 280 mOsm/kg).
3. WA: plain water; 2 mOsm/kg.
In a randomised crossover design, subjects ingested volumes equal to, bw loss during a 3-hour recovery period.
|• At the end of the 3 h recovery period, fluid retention was significantly greater for CWP (88.0 ± 4.7%) than CHO (74.9 ± 14.6%) and WA (53.2 ± 16.1%). |
• Fluid retention for CWP was 15% greater than CHO and 40% greater than WA.
• There was no difference in plasma volume restoration. The differences in retention have to be found in the intracellular and interstitial spaces.
|James et al. (2011)||8 healthy male individuals. Mean age 21 years.||1.9 ± 0.2% induced by intermittent cycling exercise in the heat, lasting 82 ± 10 min.||1. C: carbohydrate drink (65 g/l carbohydrate; 0.8 g/l fat; 7 mmol/l Na; 5 mmol/l K; 247 mOsm/kg). |
2. CMP: carbohydrate-milk protein drink (40 g/l carbohydrate; 0.8 g/l fat; 25 g/l protein; 7 mmol/l Na; 4 mmol/l K; 229 mOsm/kg).
In a randomised crossover design, subjects ingested volumes equal to 150%, bw loss during 60-minute, followed by a 4-hour exercise recovery period.
|• At the end of the 4 h recovery period, fluid retention was significantly greater for CMP (55 ± 12%) than C (43 ± 15%). |
• Body fluid balance was negative for both trials CMP and C (−0.26 ± 0.27 l and −0.52 ± 0.3 l, respectively), but only significantly so following ingestion of drink C.
• Ingestion of a 6% carbohydrate solution resulted in significant fluid deficit. Partially replacing carbohydrate with milk protein retained 281 ± 312 ml of the fluid ingested.
The most obvious mechanisms of action for proteins in assisting rehydration are enhanced sodium and water absorption from the gut and increased plasma protein synthesis resulting in higher osmotic pressure exerted by proteins (oncotic pressure) (Hellier et al. 1973; Okazaki et al. 2009). Protein-derived amino acids are absorbed from the intestine by multiple transport systems. The ‘sodium dependent amino acid co-transporters’ are separate from the ‘sodium-glucose co-transporters’. When both are activated, more sodium can be moved across the intestinal wall. The end result may be a greater osmotic gradient by combining protein, glucose and sodium than with just glucose and sodium. The combination of carbohydrate, amino acids and sodium may provide separate, but additive, mechanisms to enhance water absorption, which is at least as good, or better than that of just the sodium-glucose transport system in a traditional sports drink (Seifert et al. 2006).
Similar to muscle protein synthesis, the rate of albumin synthesis (plasma protein) is increased in the liver for several hours after intense exercise. A rapid increase in plasma albumin content draws fluid into the vascular space so that plasma albumin concentration remains constant. Such oncotic pressure results in more electrolytes and water retention in the intravascular space and increased plasma volume (Okazaki et al. 2009). The study of Okazaki et al. (2009) in Japan hypothesised that such exercise-induced responses in increased albumin synthesis and plasma volume were enhanced in young and older individuals when a protein-carbohydrate supplement (0.18 and 0.55 g/kg protein and carbohydrate, from whey protein and sugar, respectively) was given strategically soon after exercise, compared with a placebo (0 and 0.11 g/kg protein and carbohydrate, respectively), and in addition to a diet meeting the recommended dietary allowances for a moderately physically active Japanese population (1.01 g/kg protein, 30% energy from fat and 3.7 mg/kcal NaCl). Results suggested that protein-carbohydrate supplementation immediately after exercise enhanced albumin synthesis, accompanied by increased plasma volume, in the old and in the young. This study suggests that the lower increase in albumin plasma concentration, normally observed after training in older subjects, might be caused by substrate supply inefficiency. Therefore, the timing of protein and carbohydrate intake soon after exercise might be a key condition to increasing albumin content, in addition to meeting the total daily protein requirements in the diet, supporting sustained prolonged positive fluid balance after exercise.
As demonstrated by James et al. (2011), gram-for-gram, milk proteins can be more effective at augmenting fluid retention than carbohydrate. However, maintaining the drinks' optimal carbohydrate levels at 6% is important for glycogen resynthesis. Therefore, a sports drink which does not substitute carbohydrate but is enriched with protein, or a milk drink enriched with carbohydrate, may be preferable strategies for enhanced rehydration soon after exercise.
Rehydration, substrate metabolism and performance
Significant improvements in recovery of fluid balance have been observed in some studies with the intake of whole foods, milk and most recently, protein-added sports drinks. However, short-term improvements in exercise performance are not normally observed (Merson et al. 2008; Watson et al. 2008). Instead, recovery of fluid balance is useful to enhance exercise-induced plasma volume expansion which, in the medium- to long-term, increases thermoregulatory capacity during exercise (Okazaki et al. 2009). Recovery of fluid balance also results in modest changes of blood osmolality, which induce acute alterations in cell volume, and have been reported to modify cell function (Häussinger et al. 1994). Keller et al. (2003) examined for the first time the ‘cell volume’ hypothesis in humans, from which learnings could be extrapolated to explain the health benefit of always correcting fluid deficit after exercise. As a consequence of dehydration, increased blood osmolality leads to ‘cell shrinkage’, while decreased blood osmolality with rehydration leads to ‘cell swelling’. Cell swelling and shrinkage lead to opposite effects on metabolism. A significantly dehydrated shrunk cell favours catabolism, promoting glycogen breakdown and possibly protein breakdown. On the other hand, a hydrated swollen cell counteracts glycogen breakdown, increases whole-body fat breakdown, which favours the reduction of body fat stores, and significantly decreases whole-body leucine oxidation rate (marker of irreversible protein catabolism), that translates into protein sparing. Factors leading to ‘cell swelling’ are not only hormones and elevated blood insulin, but also protein-derived amino acids glutamine, glycine and alanine, which could explain another role of protein intake in assisting post-exercise rehydration (Ray et al. 1998; Keller et al. 2003). It should be noted that excessive swelling of cells caused by over-hydration and/or hyponatraemia has dangerous side effects and is not desirable.
Nutritional strategies that enhance recovery of fluid are important in the metabolic and physiological processes leading to training adaptations. They may lead to desirable alterations in body composition and plasma volume expansion, which might arguably result in long-term improvements in exercise performance.
Effective strategies to enhance post-exercise rehydration are those which result in sustained positive fluid balance during the recovery period. Most attention has been centred on the role of electrolyte sodium in retaining ingested fluid, partly because it is the most abundant electrolyte lost in sweat, but also because of its well-known role in body water retention. However, traditional sports drinks containing sodium have been shown to maintain positive fluid balance for only a short period of time, up to 1–2 hours after exercise. Increasing the sodium concentration in sports drinks above optimal levels is likely to compromise palatability and may result in larger sodium losses in the urine. A small number of studies, with generally a small number of subjects have emerged that suggest that milk and liquid meals of higher energy density exert better capacity to retain fluid and sustain long-term positive fluid balance up to 3–4 hours after exercise, compared with water and traditional sports drinks. However, it is not possible to draw conclusions from these studies regarding which aspect (either its nutritional composition or the rate at which it empties from the stomach) is having an effect on hydration status. Most recently, only two studies have looked directly into the potential role for the addition of milk proteins to sports drinks. One study suggests an independent role for milk proteins in enhancing body fluid retention, but this study is not without limitations. While the body of evidence underpinning the effectiveness of sodium and carbohydrate to rehydration is well established more research on the potential role for protein is needed. A recovery solution that provides significant amounts of fluids, electrolytes, carbohydrate and protein may represent the most preferable strategy to not only enhance and maintain post-exercise rehydration, but also to aid other aspects of recovery, such as glycogen replenishment and protein synthesis for training adaptations.
Conflict of interest
At the time of writing this article, the author is employed by Volac. Volac is a manufacturer and supplier of whey protein ingredients for application in food and beverages targeting the lifestyle market.