Most people have heard the notion that you need to drink eight glasses of water a day. But Canadian recommendations debunk that idea, saying all fluid counts, not just water. And the amount you need is more like 9–13 cups/day for adults, depending on your sex.
A British group decided to get even more specific and developed a hydration index, a way of measuring how long different fluids remain in your body (1). In other words, it's a way of measuring which beverages will keep you hydrated the longest.
In the British group's study, 12 beverages were compared to plain water. Water was given a value of 1.0 on the hydration index. Beverages that stayed in the body longer than water scored higher on the index, while beverages that were eliminated in the urine sooner, scored lower.
Milk scores higher than water
The beverages that scored the highest were whole milk, skim milk, orange juice, and an oral rehydration solution like the ones used to treat chronic diarrhea. All these beverages had values close to 1.5 on the hydration index. Contrary to what many believe, coffee and beer scored about the same as water, meaning they aren't acting as a diuretic even when consumed in amounts used in the study (1 L). A further surprise: sports drink scored about the same as water too. The study was conducted in healthy young men.
Similar results with children
This new research echoes work done in 2014 by Canadian researchers at McMaster University (2). Brian Timmons, a researcher in the Canadian group, notes that when children who are active in sports become dehydrated, it's very important they rehydrate well before going into a second round of a game. Other researchers have shown that even a 1% decline in hydration results in a 15% decrease in performance (3).
The McMaster group studied children aged 7–17 and measured their hydration status after exercising and drinking one of three test drinks. Their results show that milk is a more effective rehydration beverage compared to either a sports drink or water. It is thought that the combination of electrolytes and protein in milk helps to improve hydration.
1. Maughan R.J. et al (2016). A randomized trial to assess the potential of different beverages to affect hydration status: development of a beverage hydration index. AJCN 103, 717–23.
2. Volterman K.A., Obeid, J., Wilk, B. and Timmons, B.W. (2014). Effect of milk consumption on rehydration in youth following exercise in the heat. Appl Physiol Nutr Metab, 39(11), 1257–64.
3. McMaster University. (2011, August 23). Milk better than water to rehydrate kids, study finds. ScienceDaily. Retrieved July 7, 2016 from www.sciencedaily.com/releases/2011/08/110817142849.htm
Written by Sydney Massey, MPH, RD