With warm weather and/or physical activity you lose perspiration, and the evaporation of the sweat from your skin cools the body. Before long, that sweat loss needs to be replaced. Water is good. You know from previous articles both coffee and tea can rehydrate the body. Sports drinks are specifically designed for rehydration, especially for athletes during sport.
After sweating did you ever consider a glass of milk for rehydration? Well, it is 90% water, just like fruit juice, so theoretically it should be a useful drink. Or, if you prefer, soy milk which is also 90% water. One Australian research group has just published their results on rehydration drinks.
Drink more fluid than you lose
If you perspire, say 1 litre (34 oz), it makes sense to drink the same volume to make up for the loss. Only problem is that the kidneys still want to make urine, so you might drink 1000mL of water and yet produce 350mL of urine, still leaving you 350mL short of the 1000mL replacement. This is why you hear coaches and sports dietitians encouraging athletes to drink more, 150% more, than they lost as sweat.
So what is happening here? If you replace sweat losses with just water it actually dilutes the blood because you sweat both water and salt, then replace it with just water and next-to-no salt. Not a problem for normal sweating as you will get your salt from the next snack or meal. If you sweat heavily, like in this study, then drinking just water dilutes the blood and the kidneys act to concentrate the blood by creating some pee. This is why sports drinks have some added salt – it helps you to retain more water absorbed from the gut, with less pee, after sweating.
The power of milk
This is where milk comes in. This was a study of 15 young males who were exercised until they lost about 2% of their body weight as sweat in a session on the exercise bike (that’s 1.5kg in a 75kg person; 3.3lb in 165lb). To rehydrate they were given either milk, soy milk, Sustagen Sport (a milk-based nutrition supplement), or the sports drink Powerade. The volume given was 150% of the amount lost as sweat, in other words, more than sufficient to replace sweat losses, on paper at least. The milk and soy-based drinks were chosen as their natural salts, sugars and protein all work to help retain the fluid in the body, meaning you produce less pee.
Sustagan Sport takes the title
After drinking 1.5 litres of drink for every 1 litre lost as sweat the athletes were tracked for another four hours, taking blood and pee samples, while checking body weight. The least amount of urine production was after drinking Sustagen Sport, the flavoured milk-based supplement that is fortified with extra vitamins and minerals, and very popular with Australian and Kiwi athletes. Put another way, more fluid was retained from a flavoured milk supplement drink, making it very viable as a rehydration fluid.
In second place came milk and soy milk, with little to separate them. The sports drink was last, suggesting you need to drink much more than 150% of losses, probable twice as much, when replacing losses with a sports drink.
What does it all mean?
Remember, this is a study of fluid replacement after you have finished your workout. Let’s be very clear, water or a sports drink are still very good choices to have while you are perspiring. After the gardening, brick-laying, half-marathon or cycle training has finished there are other options to help replace the sweat.
What this study shows is that a milk-based drink will do an excellent job of replacing your fluid losses when the work is finished. After a long bike ride, I put the bike back in the shed, open the fridge and enjoy a coffee-flavoured milk (pictured). I’m getting:
- Carbs to replace my glycogen
- Protein to help repair any muscle micro-damage; and
- A great flavour
- And a nice change after 2-3 litres of water on the bike.
Try it and see what you think.
Desbrow et al (2014) Applied Physiology, Nutrition & Metabolism 39: 1366-1372