Does the body tend to retain more fluids in the warmer weather, resulting in weight gain? I thought I'd read/heard this somewhere. Also, can working out aerobically for 40 minutes, followed by 60 minutes of moderate to heavy weight training, result in a weight gain on the scale (again, the body retaining more fluids)?
I assume that this question is pertaining to exercising in warmer weather, so my response will be answered based on that assumption. Weight gain is not necessarily directly correlated with warmer weather as a standalone dependent variable. Warmer weather by itself does not cause weight gain. However, a series of factors could lead to weight gain in athletes during exercise in warmer weather.
One of the likely causes of weight gain during exercise can be traced more to nutrition by the condition known as hyponatremia. Hyponatremia is a fluid-electrolyte balance disorder that results in an abnormally low plasma sodium concentration. This can lead to death if not recognized or treated properly, but relative to the question, there is body weight gain suggestive of an absolute increase in total body water. Signs of hyponatremia include rapid post-workout weight gain, bloating, swollen hands and feet, vomiting, signs of disorientation, dizziness, severe fatigue, throbbing headache and lack of coordination, to name a few.
There are four postulated causes of hyponatremia:
- Increased total body water caused by excessive water consumption or IV therapy.
- Reduced urine output caused by exercise and/or heat exposure.
- Inadequate sodium intake caused by a low sodium diet and/or inadequate sodium consumption during exercise.
- Large sodium loss due to a high sweat rate, a high sweat sodium concentration and/or poor heat acclimatization.
It is likely that hyponatremia is caused by a combination of these factors and not one as a standalone cause. Heat index and the duration of exercise are both factors, but hyponatremia is more prevalent when duration is longer and the heat index is higher. The main goal of preventing hyponatremia experienced with exercise is to avoid excess fluid retention as seen by weight gain, during or after exercise. Refer to the four common causes of hyponatremia listed above for methods of preventing it and associated weight gain.
Regarding the second question, it would somewhat depend on what type of aerobic exercise is prescribed and the environmental conditions. If the exercise is high intensity in a warmer climate with higher humidity, the athlete would be expected to lose weight unless a copious and ridiculous amount of fluid is consumed during this time. It is not uncommon for athletes to lose upwards of two to six pounds per hour of moderate to moderately intense exercise. Therefore, in this example, the athlete would enter the strength training workout fluid depleted, dehydrated and possibly hyponatremic. It is likely that the athlete would show a net weight loss on the scale unless, again, s/he consumed too many fluids and sodium.
Any weight gain that is observed during this relatively short time frame of one hour and 40 minutes would be surely associated with fluid retention.
Warm environments are merely one factor in the weight gain/weight loss puzzle with athletes and exercise. Look at your clients’ fluid and food intake prior to and during their workout and the duration and intensity of their workouts to determine causes of weight shifts.
- Hyponatremia in Athletes. Gatorade Sports Science Institute. Sports Science Exchange 88, Volume 16, 2003, Number 1.
- Hew-Butler, T. et al. (2005). Position Statement: Consensus Statement of the 1st International Exercise-Associated Hyponatremia Consensus Development Conference. Clin J Sports Med; 15, 208-213.