While changes in our body weight typically occur as a result of long-term changes in lean or fat body mass, Castellani explains that acute changes in total body water can have the same effect. Any changes that manifest themselves as a result of variations in the climate regime tend to take the form of gained weight, particularly once the body has become acclimated to higher levels of activity in the heat.
Assuming that water accounts for approximately 60% of an average person's weight, normal water turnover, or the amount that is lost and replaced by the body every day, lies between 2-3 liters. Environmental stress and exercise, in addition to several other factors, can change this amount: for example, hotter temperatures or increases in air humidity would lead to higher sweating rates and, thus, higher rates of water loss.The combined effect of such temporary climate variations, over the span of an 8 to 24 hour period, would be relatively mild: fluctuations in the total body water content of about 0.5% in hot weather compared to only 0.25% in temperate environments.
However, as the climate changes and temperatures permanently increase, the body adapts by using a process known as heat acclimatization to reduce the negative effects associated with heat stress. Sweating begins much sooner and occurs at higher rates, which helps reduce body heat storage and skin temperature by improving evaporative cooling, and daily fluid requirements go up as a result.
The net effect is an increase in both the blood volume and total body water content. This effect becomes especially pronounced during the summertime, when body weight will jump up several pounds due to increased water content.
Fluid-conserving hormones such as aldosterone allow this to happen by making the kidney retain more fluid and reducing the amount of salt excreted in sweat. While this phenomenon will occur in individuals who continue to exercise outdoors in hot weather, it will not occur in those who spend most of their time in air-conditioned environments.