John Castellani, a researcher in the Thermal & Mountain Medicine Division of the U.S. Army's Research Institute of Environmental Medicine, takes on this question.
Body weight changes usually result from long-term changes in lean or fat body mass, but they can also result from acute changes in total body water. Significant changes in body weight due to climate usually take the form of weight gained rather than weight lost, especially once the body has become acclimated to high levels of activity in the heat.
Water accounts for about 60 percent of a person's weight. (For an average person, this represents 42 liters, or 11 gallons.) For most people, water turnover, or the total amount of water that is lost and replaced by the body, averages about two to three liters, or 0.5 to 0.8 gallon, per day. This amount can change depending on several factors, including exercise and environmental stress. Aside from normal daily water loss in the form of urine, the major source of body water loss is sweat.
A person's daily water needs are dependent upon their level of activity and the air temperature. Hotter temperatures and intense exercise increase sweating rates and, as a result, water requirements. Humid weather will also ratchet up the sweating rate—but in high humidity, sweat typically drips off the body rather than evaporating, thus providing no cooling effect. Even with high sweating rates, the total body water of a person fluctuates over an eight- to 24-hour period in a narrow range of about 0.5 percent in hot weather and only 0.25 percent in temperate environments.
This tiny range holds even when sweat losses are increased substantially with exercise, because humans replace most of their fluid losses at mealtime, allowing them to come back into fluid balance. For individuals who exercise hard, fluid will need to be replaced during exercise as well.
As the climate changes and it becomes hotter, the body adapts via a process known as heat acclimatization in order to reduce the negative effects of heat stress. By the second day of heat acclimatization, sweating starts sooner and takes place at higher rates, which improves evaporative cooling and reduces body heat storage and skin temperature. Thus, after heat acclimatization, fluid requirements will be higher due to increased sweating. Heat acclimatization also improves fluid balance by better matching thirst to water needs, increasing the blood volume and increasing total body water.
In the summer, body weight can go up by several pounds due to increased body water. This is accomplished through fluid-conserving hormones such as aldosterone, which allows the kidney to retain more fluid and reduces the amount of salt in sweat, a measure that also aids in water retention. The increase and stabilization of total body water can only be accomplished by continuing to exercise in hot weather and will not occur in people who spend most of their time indoors in air-conditioned environments.