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Water not the best beverage for hydration, say Scottish scientists

Surprisingly, milk is better than plain old fashioned water for hydrating the body, say researchers after a study that compared the hydration responses for several different drinks.

The team from Scotland’s St Andrews University found that while water — both still and sparkling — does a pretty good job of quickly hydrating the body, beverages with a little bit of sugar, fat or protein do an even better job of keeping us hydrated for longer, reports CNN.

The reason relates to how our bodies respond to beverages, said Ronald Maughan, a professor at St. Andrews’ School of Medicine and the study’s author. One factor is the volume of a given drink: the more you drink, the faster the drink empties from your stomach and gets absorbed into the bloodstream, where it can dilute the body’s fluids and hydrate you.

Milk better than water

The other factor affecting how well a beverage hydrates has is even more hydrating than plain water because it contains the sugar lactose, some protein and some fat: all help to slow the emptying of fluid from the stomach and keep hydration happening over a longer period.

Milk also has sodium, which acts like a sponge and holds on to water in the body and results in less urine being produced.

The same can be said for oral rehydration solutions used to treat diarrhoea. Those contain small amounts of sugar, as well as sodium and potassium, which can also help promote water retention.

Sugar in moderation

“This study tells us much of what we already knew: electrolytes — like sodium and potassium — contribute to better hydration, while calories in beverages result in slower gastric emptying and therefore slower release of urination,” said Melissa Majumdar, a registered dietitian, personal trainer and spokeswoman for the Academy of Nutrition and Dietetics, who was not involved in the study.

However, beverages with more concentrated sugars, like fruit juices or colas, are not necessarily as hydrating as their lower-sugar cousins. They may spend a little more time in the stomach and empty more slowly than plain water, but once they enter the small intestine their high sugars concentration dilutes during a physiological process called osmosis.

This, in effect, “pulls” water from the body into the small intestine to dilute the sugars contained by these beverages. And technically, anything inside the intestine is outside your body.

Juice and soda are not only less hydrating, but offer extra sugars and calories that won’t fill us up as much as solid foods, said Majumdar. If the choice is between soda and water for hydration, go with water.

After all, our kidneys and liver depend on water to eliminate toxins from our bodies, and it also plays a key role in maintaining skin’s elasticity and suppleness.

While staying hydrated is important — it keeps our joints lubricated, helps prevent infections, and carries nutrients to our cells — in most situations people don’t need to worry too much about how hydrating their beverages are.

“If you’re thirsty, your body will tell you to drink more,” Maughan said. But for athletes training seriously in warm conditions with high sweat losses, or for someone whose cognitive function may be negatively affected by working long hours without beverage breaks, hydration becomes a critical issue.

Hydration from beer and lattes?

Alcohol acts as a diuretic, causing you to pass more urine, so when it comes to alcoholic beverages hydration will depend on the drink’s total volume. “Beer would result in less water loss than whisky, because you are ingesting more fluid with beer,” Maughan said. “Strong alcoholic drinks will dehydrate, diluted alcoholic drinks will not.”

When it comes to coffee, how well your java hydrates you will depend on the amount of caffeine you consume. A regular coffee with about 80 milligrams of caffeine would be about as hydrating as water, according to Maughan’s research.

Consuming more than 300mg of caffeine, or about 2-4 cups of coffee, could cause you to lose excess fluid as the caffeine causes a mild, short-term diuretic effect. This is more likely to happen with someone who doesn’t typically consume caffeine, and it could be offset by adding a tablespoon or two of milk to your cup of joe.

The article was originally published in 2019 but has been updated.

Study details

A randomised trial to assess the potential of different beverages to affect hydration status: development of a beverage hydration index

Ronald J Maughan, Phillip Watson, Philip AA Cordery, Neil P Walsh, Samuel J Oliver, Alberto Dolci, Nidia Rodriguez-Sanchez, Stuart DR Galloway

Published in The American Journal of Clinical Nutrition in March 2016.


The identification of beverages that promote longer-term fluid retention and maintenance of fluid balance is of real clinical and practical benefit in situations in which free access to fluids is limited or when frequent breaks for urination are not desirable. The postingestion diuretic response is likely to be influenced by several beverage characteristics, including the volume ingested, energy density, electrolyte content, and the presence of diuretic agents.

This study investigated the effects of 13 different commonly consumed drinks on urine output and fluid balance when ingested in a euhydrated state, with a view to establishing a beverage hydration index (BHI), i.e., the volume of urine produced after drinking expressed relative to a standard treatment (still water) for each beverage.

Each subject (n = 72, euhydrated and fasted male subjects) ingested 1 L still water or 1 of 3 other commercially available beverages over a period of 30 min. Urine output was then collected for the subsequent 4 h. The BHI was corrected for the water content of drinks and was calculated as the amount of water retained at 2 h after ingestion relative to that observed after the ingestion of still water.

Total urine masses (mean ± SD) over 4 h were smaller than the still-water control (1337 ± 330 g) after an oral rehydration solution (ORS) (1038 ± 333 g, P < 0.001), full-fat milk (1052 ± 267 g, P < 0.001), and skimmed milk (1049 ± 334 g, P < 0.001). Cumulative urine output at 4 h after ingestion of cola, diet cola, hot tea, iced tea, coffee, lager, orange juice, sparkling water, and a sports drink were not different from the response to water ingestion. The mean BHI at 2 h was 1.54 ± 0.74 for the ORS, 1.50 ± 0.58 for full-fat milk, and 1.58 ± 0.60 for skimmed milk.

BHI may be a useful measure to identify the short-term hydration potential of different beverages when ingested in a euhydrated state. 


Journal of Clinical Nutrition article – A randomised trial to assess the potential of different beverages to affect hydration status: development of a beverage hydration index (Open access)


CNN article – Which drink is best for hydration? Hint: It isn’t water (Open access)


See more from MedicalBrief archives:


Hydration may slow cardiac decline and reduce heart failure risk – NIH study


Plant extract combo may relieve hangover symptoms; dehydration causation challenged


Eight glasses of water daily might be excessive for most people – Japanese study






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