Working in the heat: fixed-volume hydration may exceed water losses

Two environments, the same amount of water

The researchers compared a hot, dry environment at 40.6 °C and 20 % relative humidity with a less hot but humid environment at 30 °C and 80 % relative humidity.

The 20 participants, including 10 women, had an average age of 24 years. Each completed both conditions in randomized order, with 120 minutes of moderate-intensity intermittent walking. They drank 237 mL of cold water every 20 minutes.

This amount corresponds to the strategy studied, not a personal prescription. According to the abstract, NIOSH recommendations call for approximately 237 mL every 15 to 20 minutes while working in the heat.

No substantial weight loss, but a fluid surplus

No participant exceeded the threshold of 2 % body mass loss. By this measure and under these conditions, the strategy therefore achieved its goal.

Average sweat rates nevertheless differed: 0.53 L per hour in the hot, dry environment, compared with 0.35 L per hour in the hot, humid environment. The same amount of water consumed therefore did not match the same losses.

Average net fluid balance was positive in both conditions: 0.70 L in the humid environment compared with 0.40 L in the dry environment. Average body mass gain reached 1.1 % and 0.6 %, respectively. The authors interpret these findings as intake exceeding requirements under the conditions tested.

This finding questions fixed volumes, not the value of staying hydrated

The study shows that avoiding substantial weight loss and matching intake to losses are two distinct goals. The tested strategy met the first while producing an average fluid surplus.

Hydration biomarkers and perceived thirst did not differ significantly between the two environments. However, the abstract reports no clinical consequences related to the positive fluid balance: weight gain during this trial does not, on its own, demonstrate harm.

The authors suggest taking greater account of the environment and individual needs. They also mention the possibility of drinking freely, but this alternative was not directly compared with the fixed-volume strategy in the trial.

What we can take away

In this trial, the fixed volume prevented body mass loss greater than 2 % but exceeded average fluid losses. This supports studying more tailored recommendations, not systematically reducing water intake during exertion in the heat.