Milk as an Effective Post-Exercise Rehydration Drink
Title Shirreffs SM et al. Milk as an effective post-exercise rehydration drink. Br J Nutr (2007): Pg 1-8
Abstract The effectiveness of low-fat milk, alone and with an additional 20 mmol/l NaCl, at restoring fluid balance after exercise-induced hypohydration was compared to a sports drink and water. After losing 1·8 (SD 0·1) % of their body mass during intermittent exercise in a warm environment, eleven subjects consumed a drink volume equivalent to 150 % of their sweat loss. Urine samples were collected before and for 5 h after exercise to assess fluid balance. Urine excretion over the recovery period did not change during the milk trials whereas there was a marked increase in output between 1 and 2 h after drinking water and the sports drink. Cumulative urine output was less after the milk drinks were consumed (611 (SD 207) and 550 (SD 141) ml for milk and milk with added sodium, respectively, compared to 1184 (SD 321) and 1205 (SD 142) ml for the water and sports drink; P,0·001). Subjects remained in net positive fluid balance or euhydrated throughout the recovery period after drinking the milk drinks but returned to net negative fluid balance 1 h after drinking the other drinks. The results of the present study suggest that milk can be an effective post-exercise rehydration drink and can be considered for use after exercise by everyone except those individuals who have lactose intolerance.
My comments In addition to glycogen replenishment and the promotion of recovery and adaptation to training, the issue of re-hydration after exercise (endurance training more so than resistance training) is also of importance and finding ways to optimally rehydrate the body following dehydration is a critical aspect of sports nutrition. During endurance exercise, fluid loss usually exceeds fluid intake and athletes end up slightly dehydrated at the end of the bout and even small amounts of dehydration can negatively impact on exercise performance (i.e. at the next training session).
While re-hydration when training once per day usually isn’t too big of an issue, many athletes train more frequently than this and finding ways to optimally rehydrate (again, in addition to issues of recovery, etc.) is important. Previous work had found that the addition of both sodium and potassium to fluids was a key in re-hydration, thus the popularity of drinks such as Gatorade and Powerade both of which also provide carbohydrate for glycogen replenishment. With the exception of one study which examined the role of a whole food meal on re-hydration, most studies have used various experimental solutions, so this week’s study set out to examine a more commonly found beverage (low fat milk) in terms of its effects on rehydration following exercise.
The study recruited eleven healthy male volunteers who were physically active but described as not being accustomed to exercise in a warm environment. Following a familiarization trial, subjects performed an exercise trial in a warm/humid room consisting of 10 minute bouts at 2 w/kg workload. Body weight was measured in-between bouts with the exercise stopped when the subjects had lost 1.7% of their starting body weight. One of four drinks was provided starting at 20 minutes after exercise. The drinks were 2% milk, 2% milk with added sodium, water, or Powerade. The total drink volume given was equal to 150% of the total weight lost in four equal amounts (every 15 minutes for an hour); subjects were monitored for an additional 4 hours. No food or drink was allowed and urine production was measured every hour by having the subjects pee.
Subjective measurements of hunger and thirst were made every hour as well (in addition to before and immediately after the exercise bout). All subjects lost roughly 1.8 kg of weight during the exercise bout and the total amount of fluid given over the hour of recovery drinking was 1.8 l (slightly under half a gallon). In terms of urine production (a measurement of the amount of ingested fluid retained by the both), both milk trials showed significantly less urine output compared to either water or Powerade with no real difference between the milk and milk plus sodium drinks.
Similar results were seen looking at net fluid balance, due to decreased urine output, the milk groups reattained fluid balance after 4 hours while the water/Powerade groups were still slightly dehydrated. Finally, subjective rating of thirst went up after the exercise bout but decreased with re-hydration, with no difference between drinks. Feelings of hunger also increased throughout recovery although both milk groups and the Powerade reduced hunger more than water. Subjects also reported that the Powerade was sweeter and slightly more palatable compared to the milk drinks which were reported as being saltier and more bitter. No other differences were seen.
The researchers concluded that milk (with or without extra sodium) was superior to either water or Powerade at rehydration although the slight differences seen at the end of the study are unlikely to significantly impact on exercise performance in temperate climates. They suggest that at least part of this is due to the quantities of both sodium and potassium in milk; as mentioned both electrolytes are important in helping the body retain the fluid consumed following exercise. They also note that the digestion rate of milk is going to be slower than either water or Powerade due to the presence of protein and fat in addition to the carbohydrate; this might have affected the body’s utilization of the milk for re-hydration compared to the other drinks.
In keeping with this, subjects in the milk group reported greater fullness, probably due to the length of time it took for the milk to be fully absorbed. So this study adds to previous data showing that milk can be useful for recovery (in terms of protein synthesis) following either resistance training or endurance training, as I’ve discussed in previous newsletters. At the same time, the amount of fluid consumed (nearly a half gallon of milk) is significant and some people might not find milk terribly appealing following an exhaustive exercise bout.
It would be interesting to see if a protein/carbohydrate drink containing either dextrose/maltodextrin and whey or milk protein isolate (with similar amounts of electrolytes to what is found in milk) promoted the same level of re-hydration following exercise. I suspect that it would and this might provide an easier way to promote both training adaptations and re-hydration following exercise. This would probably be easier than trying to drink 16-32 oz of milk following exercise. An additional issue, of course, is that of lactose intolerance but the availability of lactose reduced or removed milk (cf. Lactaid/DairyEase) should make this less of an issue.