Fluid Loss in Masters Swimmers – Effects of Water Temperature


Most experienced and competitive masters swimmers have trained or competed in many different pools, lakes, rivers and oceans, and under many different climatic conditions and water temperatures. We’ve learnt that hard sessions in warm water can work up a thirst. While little research has been done on fluid loss in swimmers, here is some recent Italian research that used older open water swimmers as their subjects. The results suggests the level of dehydration, the sweat rate, and the body temperature all increase with increased water temperature as we move from cool to warmer water.

The Research

The purpose of the study was to evaluate the effects of three different water temperatures (23, 27 and 32 ̊C) on physiological responses (dehydration, sweat rate, urine output, rectal temperature, plasma electrolytes and fluid balance) to a “simulated” race of 5 km in competitive athletes in an indoor 25m pool. Nine competitive male master swimmers ranked in the top 5 of category in open water (1.5–10 km) Italian races, were studied (age: 34.6 ± 14.4 years, height: 172.1 ± 9.8 cm, mass: 72.7 ± 8.5 kg, body fat: 12.7 ± 3.5%). The subjects trained five to six times per week (3–8 km per training session) in 25- and 50-meter swimming pools (water temperature about 27 ̊C). Each swimmer completed three experimental trials, separated by 7 days, in a 25-meter indoor swimming pool; they swam 5 km with water at the temperatures of 23, 27 and 32 ̊C. The swimming speed of all athletes in each trial was as close as possible to their personal lactate threshold speed (race pace). No food or fluid was used during the tests. The sport scientists measured body weight, rectal temperature, urine output, and blood electrolytes (sodium, potassium and magnesium before and after each swim.

The Results

Sweat rate increased and body weight loss (%) decreased with increased water temperature (Table 1).


23 ̊C 27 ̊C 32 ̊C

Rectal temperature ( ̊C)

37.2 37.9 38.0
Body weight loss (%) 0.9 1.3


Sweat rate (L/hr) 0.48 0.76


Urine output after each swim was no different between the trails. Body temperature only increased in the 27 and 32 ̊C trials. The sodium level in the blood only increased in the 32 ̊C swim and most probably due to the amount of fluid lost. The researchers concluded that dehydration, sweat rate and body temperatures simultaneously increase with the rise of water temperature during the shortest open water swimming event distance (5 km) performed at race intensity.

So What?

This unique study confirms that as water temperature increases we need to be more cautious about ensuring our hydration status is good. Drink fluids before a pool session and ensure, particularly in hard sessions in warmer water (its 31 ̊C in our Uni pool at present!), that you replace fluids regularly during your session. The hotter the water, the more fluids you need to drink. The harder and longer the session, the more fluids you need to drink. If you are a once a day swimmer doing sessions under an hour, water is all you need. Twice a day and/or doing sessions longer than an hour and/or hard, the more important sports drinks become. For more on temperature regulation and fluid guidelines for masters athletes, see Chapter 11 of my book The Masters Athlete – Exercising in the Heat and Cold.

Source: Macaluso et al. (2011) Effects of three different water temperatures on dehydration in competitive swimmers. Science in Sports, 26: 265-271.