On hot days, cold drinks may not be the answer to effective rehydration.
In Chinese medicine, hot and cold water are functional foods that play a role in our health, with each used only in the right situations. In general in China, warm and hot water is considered good for health, and it’s been suggested that having some in the morning kick-starts digestion.
Japanese researchers who investigated the effect of food temperature on gastric emptying had interesting results. They discovered that when liquid and solid meals were served at a higher temperature (60°C), they appeared to be emptied from the stomach more quickly than if the same foods had been served at 4°C or 37°C. So, food eaten hot, rather than cold or at room temperature, increases the rate of stomach emptying.
However, other studies looking at hot versus cold beverages have found that temperature has little effect, with warmer drinks possibly speeding up stomach emptying for about 10 minutes. Thereafter, though, the process returned to normal, as (it’s presumed) the stomach contents shifted to reflect our core body temperature.
Interestingly, in a study dating back to 1978, hot water and hot chicken soup helped induce slight decongestion in the nasal passages of healthy adults, more so than drinking cold water. So, if you’ve got a cold and a blocked nose, hydrating yourself with hot water may help.
But what about on hot days? Which is better for cooling off, a cold or a warm drink?
When a group of dehydrated cyclists, who had just pedalled in a hot, humid environment, drank water served at different temperatures, it produced different rates of voluntary liquid intake and different sweat responses. Remember that sweating is a useful response to being hot. If there is a breeze and relatively low humidity to evaporate our perspiration, sweating is a highly efficient way to cool down.
Researchers found that the dehydrated cyclists who were given water at 16°C drank more and produced more sweat than those who were given 5°C water. The researchers suggested, then, that drinking 16°C water (cool, rather than refrigerated) is more effective when we are dehydrated.
Canadian researchers took things a step further. They assessed the effects of water served at 1.5, 10, 37 and 50°C on males taking part in a 75-minute cycling session to assess what effect the water had on their core body temperature and heat loss on their skin.
The research showed that in conditions that allowed full sweat evaporation (an important step in cooling the body), body-heat storage was lower when the cyclists drank the warm (50°C) water rather than the room-temperature or cold water.
So, it seems warmer water is a more effective way to cool our body, if we’re able – and willing – to sweat. But that depends on one key thing: drinking the warm water.
Water will hydrate and improve gut motility, thereby preventing constipation, only if we actually drink it. If you don’t enjoy warm water, then you’re not likely to drink a lot of it. So, the answer therefore is likely to be: drink water at a temperature that you prefer.
This article was first published in the March 9, 2019 issue of the New Zealand Listener.