After a sweaty workout, salty drinks held on to water better than plain water. Sugar-free or sugary? A trial in 20 adults could not settle it, and the drinks' maker paid for it

Drinking after exercise is not the same as rehydrating. Much of a large drink of plain water can come straight back out as urine within a few hours, which is why oral rehydration solutions carry more salt than most sports drinks. Newer powders promise the same effect without the sugar. A crossover trial at Arizona State University's Hydration Science Lab, published in Nutrients on 30 October, put one of them up against a sugar-sweetened version and plain water.

Twenty recreationally active adults, 11 men and 9 women with an average age of 26, came in on three separate mornings. Each time they walked and cycled in a 31 °C room for up to two hours, until they had lost about 1.8% of their body weight in sweat. Then they had four hours to recover: 500 mL of the drink of the day in the first five minutes, and after that as much as they wanted. One drink had 26 g of glucose and sucrose per litre. The sugar-free one was sweetened with allulose, a rare sugar with almost no calories, and had the amino acids glutamine and alanine added. Both carried similar sodium, around 46 to 49 mmol per litre. The third was plain bottled water.

Four hours later, people had kept 24.8% of the water they drank, against 46.6% of the sugar-free drink and 39.7% of the sugary one; both salty drinks beat water. On the scale, the sugar-free drink had won back 0.83% of body weight against 0.16% for water, a gap the authors put at roughly 470 g for a 70 kg adult. The sugary drink's 0.49% was not clearly better than water. People drank a little more of both salty drinks than of water (1.8 to 1.9 litres against 1.6), but that difference could have been chance. Nobody was fully rehydrated by the end, whatever they drank.

The comparison many readers will care about, sugar against no sugar, is where the study goes quiet. Apart from less urine at the three-hour mark with the sugar-free drink, the two came out statistically level. The trial was sized to compare a salty drink with water, and the authors warn that 'no difference' between the two powders should not be read as proof they are equal. The drinks were also commercial products, Liquid I.V. Hydration Multiplier and its sugar-free version, and the company funded the study. The researchers could not tell whether allulose, the amino acids or the mix made the difference, and they did not measure either in the blood. They also point out that most people eat after exercise, and food changes how a drink is handled; the results matter most when eating is impractical, such as between back-to-back sessions.