Endurance sports and activities are generally considered to be sub-maximal activities that can be performed for more prolonged periods of time. These activities are also characterized by continuous exercise/activity that is highly dependent on oxidative metabolism as a source of energy and usually involve large muscle groups. This dependence on oxidative metabolism and involvement of a large muscle mass leads to higher rates of total substrate turnover and under certain conditions, depletion of muscle glycogen in the active muscles. However, there can be considerable differences in the responses observed depending on the intensity of the exercise performed.
From a nutritional standpoint, there are three primary times when nutrition is considered for endurance based activities; before exercise, during exercise, and after exercise. Each time period of nutritional concern has varying objectives. Prior to exercise, nutritional goals are to ensure that the athlete is well fuelled and that any nutritional intake will not interfere with the normal physiological responses to the activity. The goal of nutritional intake during exercise is to provide exogenous substrates in an attempt to delay the depletion of endogenous substrates, and to provide fluids to offset fluid losses due to sweating. Finally, the nutritional objectives of post-exercise nutrition are to promote muscle recovery and adaptations, fuel resynthesis in muscle, and fluid replenishment. With specific regard for the role of milk as a nutritional option for endurance activities, there is limited research into the possible benefits of milk and it is also difficult to extend findings into any consistent recommendations because of differences in design and methodology. However, a number of recent studies suggest that there is significant potential for expanded research in this area, especially in the area of recovery from endurance exercise.
What is evident is that when milk is compared to carbohydrate based sports drinks, similar responses in many physiological variables are observed during the exercise. There have been some minor differences reported such as increased concentrations of essential amino acids [17
], based on the protein content of milk. Interestingly, when milk is consumed during prolonged exercise, following completion of the exercise a reduction in whole body protein breakdown and protein synthesis was observed with a simultaneous increase in protein oxidation [18
]. The authors speculated that the decrease in whole body protein synthesis might have been due to preferential oxidation of the ingested protein during the exercise, leaving less amino acids available for synthesis after the exercise [18
]. Another difference that has been reported is that participants reported greater feelings of stomach fullness with milk as compared to water or carbohydrate based beverages [19
]. The increase in stomach fullness possibly suggests that the rate of fluid intake may have been greater than gastric emptying [19
], which would not have been unexpected as the rate of gastric emptying declines with increasing energy density of the fluid consumed [20
]. Despite these reported variations in physiological responses and stomach fullness, there were no reported differences in actual performance measures. For example, when participants rode at a set intensity until exhaustion, milk and carbohydrate based sports drinks resulted in similar times to exhaustion, suggesting that milk is just as beneficial as commercially available sports drinks at delaying the onset of fatigue under these conditions [19
]. Clearly, additional research is required to further establish the efficacy of milk as an endurance exercise supplement beverage. Future research should include time trial based performance measures, as they are more realistic to endurance sport performance. Furthermore, future research should also strive to develop a greater understanding of the metabolic influence of milk consumption during prolonged exercise, as there is very limited research in this area.
The use of milk as a recovery beverage after endurance exercise has also been investigated to a limited extent. The main goal of any post exercise nutritional intervention is usually to promote muscle glycogen resynthesis, and fluid recovery. In regards to glycogen resynthesis, there is very limited direct research into the efficacy of milk consumption to replenish muscle glycogen levels. However, there is some performance based data that suggests that chocolate milk is as effective as a commercially available sports drink in facilitating recovery [21
] (See Table for drink composition). A group of trained endurance athletes were used to compare the effectiveness of varying endurance recovery beverages, one being chocolate milk, following a series of glycogen depleting intervals [21
]. The chocolate milk drink and the carbohydrate recovery drink were controlled for based on carbohydrate and energy content. After four hours of recovery and consumption of the different beverages, the participants performed a ride to exhaustion. The time to exhaustion and total work performed during the performance trial was the same when the participants consumed chocolate milk or a common commercially available sports drink [21
]. Based on these findings one could speculate that chocolate milk may be as effective as more commonly used sports drinks at promoting glycogen resynthesis. However, to date there have been no well controlled studies that have directly measured the efficacy of milk to promote muscle glycogen recovery following prolonged endurance exercise. Therefore, future research should directly quantify rates of muscle glycogen resynthesis following prolonged endurance exercise and compare the efficacy of milk to other commonly used recovery beverages.
Composition of chocolate milk drink used by Karp et al. 2006
The other previously stated goal of a post endurance exercise beverage is to promote rehydration due to the excessive fluid loss that occurs as a result of sweating. To date there has been one well controlled study that investigated the effectiveness of low-fat milk as a rehydration beverage [22
]. In this study they compared the effectiveness of low-fat milk alone, low-fat milk with additional sodium chloride, a sports drink, and water at restoring fluid balance after exercise in a hot environment (1.8% loss in body mass). The volume of each drink consumed was 150% of the volume of fluid lost during the exercise. The intake of 150% of fluid lost is a common recommendation for rehydration after exercise. The different recovery beverages were separated into four equal amounts and were given to the participants every 15 minutes during the recovery period. Urine output and net fluid balance was determined during 4 hours of recovery. Urine output increased during the first two hours of recovery in all groups but the increase was attenuated in the two milk groups [22
]. Furthermore, by the end of the 4 hours of recovery both milk groups were in a net positive fluid balance, while both the sports drink and water conditions remained in a net negative fluid balance [22
]. The authors concluded that low-fat milk was an effective beverage for promoting rehydration following exercise induced dehydration, and that the low-fat milk was superior to a commercially available sports drink in promoting rehydration due to lower total urine output during recovery.
The ability of milk to effectively act as a rehydration beverage likely relates to the composition of milk. Milk naturally has high concentrations of electrolytes (133 mg Na+
and 431 mg K+
in a 250 mL serving) which aid in fluid retention when consumed. Another factor that has been speculated to contribute to the ability of milk to be an effective post-exercise rehydration beverage is the rate at which it empties from the stomach [22
]. Energy dense fluids empty from the stomach much more slowly, leading to a slower absorption into the circulation [23
]. This slower absorption attenuates the large fluctuations in plasma osmolality that can occur with consumption of large volumes of water or sports drinks. Subsequently, the large fluctuations in osmolality (decreased osmolality) would result in increased clearance rates by the kidneys, similar to those observed by Shirreffs et al [22
], resulting in large increases in urine output.
In summary, milk shows great promise as an alternative endurance exercise recovery beverage. The limited literature that does exist suggests that milk is as effective as commercially available sports drinks at facilitating recovery for additional performance, suggesting that it may be an effective beverage for promoting glycogen recovery. Furthermore, milk is also a very effective beverage at promoting fluid recovery following dehydrating exercise in the heat. More research is need to better understand how milk promotes recovery after exercise and to better understand the physiological mechanisms through which it acts.