Why Water Alone Won't Rehydrate You

Drink a litre of water after a hard session and a good portion of it never reaches the tissue that needs it. It goes to your bladder.

The problem is osmolality. Sweat is hypotonic — it contains less sodium than the plasma it comes from. So as you sweat, you lose proportionally more water than sodium and plasma becomes more concentrated-osmolality climbs. That rise is what drives thirst and antidiuretic hormone release, and it's the signal that tells your kidneys to hold onto fluid.1

So you drink plain water. Plasma sodium concentration falls quickly as it is diluted by the water, osmolality drops back toward baseline, and the drive to retain fluid switches off — while you are still in net fluid deficit. Thirst disappears before you're replete. Your kidneys detect an increased blood volume so urine output rises. This is involuntary dehydration, and it's a physiological mechanism

Three studies go into this in more detail, and they're worth discussing because they answer different questions.

Maughan and Leiper looked at sodium first. Six volunteers were dehydrated by around 2% of body mass, then drank a volume equivalent to 1.5 times their fluid loss, with sodium at 2, 26, 52 or 100 mmol/L. The proportion of fluid retained rose progressively with the sodium concentration of the drink.2 More sodium in the drink equalled more effective hydration.

Shirreffs and colleagues then varied volume as well. Twelve subjects, dehydrated by about 2% of body mass, drank 50%, 100%, 150% or 200% of their losses, at either 23 or 61 mmol/L sodium. The finding that matters is the interaction: you need to drink more than you lost and the drink needs adequate sodium for the most effective hydration.3

Shirreffs and Maughan later confirmed the dose-response using whole-body sweat collection, with subjects drinking 150% of their losses at 0, 25, 50 or 100 mmol/L sodium. Urine production over the six-hour recovery period was inversely related to the amount of sodium ingested.4 Lower sodium content of the drink led to higher urine production and worse hydration.

When Maughan and colleagues compared water vs thirteen other drinks, milk and oral rehydration solution held their fluid substantially better than water.5 These drinks both have a higher sodium content.

How much sodium you actually lose varies enormously between individuals. One review of sweat sodium concentration put the typical range at roughly 230–2000 mmol/L,  with mean values around 920 mmol/L.6 Combine that with sweat rates that commonly exceed a litre per hour under load, and a heavy, salty sweater can lose several grams of sodium in a single session. 

Practical position:

• Short, low-sweat sessions: water is fine. Don't overcomplicate it.

• Heavy sweat losses, heat, or back-to-back sessions: replace with sodium, not just fluid.2,3

• Aim for roughly 1.25–1.5× the fluid lost, start hydration half an hour pre-session and continue post session.

• Weigh yourself before and after a few hard sessions to get a sweat rate baseline (there’s a calculator to do this more accurately on the site). Every kilogram lost is roughly a litre of fluid. That's your sweat rate, and it beats any guideline.6 Remember hard work and high humidity/temperature will increase this rate.

What's in DRVN Threshold

One serving mixed to a litre:

For context on dose: a large share of drinks sit between 200 and 500 mg of sodium per serving. This may be sufficient for sports with a low sweat rate, but isn't optimal for athletes training hard. The salt forms in DRVN  were selected deliberately — sodium as chloride and citrate, potassium as citrate, magnesium as malate — rather than defaulting to the cheapest chloride salts throughout as these carry additional benefits.

References

1. Sawka MN, Burke LM, Eichner ER, Maughan RJ, Montain SJ, Stachenfeld NS. American College of Sports Medicine position stand. Exercise and fluid replacement. Med Sci Sports Exerc. 2007;39(2):377–90. doi:10.1249/mss.0b013e31802ca597. PMID: 17277604.

2. Maughan RJ, Leiper JB. Sodium intake and post-exercise rehydration in man. Eur J Appl Physiol Occup Physiol. 1995;71(4):311–9. doi:10.1007/BF00240410. PMID: 8549573.

3. Shirreffs SM, Taylor AJ, Leiper JB, Maughan RJ. Post-exercise rehydration in man: effects of volume consumed and drink sodium content. Med Sci Sports Exerc. 1996;28(10):1260–71. doi:10.1097/00005768-199610000-00009.

4. Shirreffs SM, Maughan RJ. Volume repletion after exercise-induced volume depletion in humans: replacement of water and sodium losses. Am J Physiol. 1998;274(5):F868–75. doi:10.1152/ajprenal.1998.274.5.F868.

5. Maughan RJ, Watson P, Cordery PAA, Walsh NP, Oliver SJ, Dolci A, Rodriguez-Sanchez N, Galloway SDR. A randomized trial to assess the potential of different beverages to affect hydration status: development of a beverage hydration index. Am J Clin Nutr. 2016;103(3):717–23. doi:10.3945/ajcn.115.114769. PMID: 26702122.

6. Baker LB. Sweating rate and sweat sodium concentration in athletes: a review of methodology and intra/interindividual variability. Sports Med. 2017;47(Suppl 1):111–28. doi:10.1007/s40279-017-0691-5. PMID: 28332116. PMCID: PMC5371639.