14 DAY TRIAL // We tend to believe that bodyweight variations are strictly linked to the changes in fatty body mass. Still, they can be also the result of acute changes in total body water.
Body weight changes triggered by climate usually take the form of weight gain rather than weight loss, especially when the body acclimates to intense activity in heat conditions.
Water makes about 60 % of the human body (for a 70 kg men, this means 42 liters, or 11 gallons.) Water turnover (the water amount lost and replaced by the body) is on average approximatively 2-3 liters (0.5 to 0.8 gallon) per day.
The water turnover can be biased by factors like exercise and environmental stress (air temperature). Water is lost through urine and sweat.
Higher temperatures and intense exercise raise the sweating speed but high humidity impedes water evaporation, inducing no cooling effect through sweating.
The highest sweating rates will affect the total body water of a person in a 24-hour just by 0.5 % in hot weather and only 0.25 % in a temperate ambiance. This small variation is maintained because even the highest sweat losses, boomed by exercising, are replaced by the body with water from the food, balancing the fluid equilibrium.
When exercising hard, the water must be replaced even during the exercising. When it is getting hotter, the body experiences heat acclimatization for coping with the heat stress.
Beginning with the second day of heat acclimatization, sweating installs sooner and is more abundant, increasing body's mechanism of cooling by sweat evaporation, that occurs by sucking body heat storage and lowering skin temperature.
Due to increased sweating, fluid requirements will also greatly increase.
Heat acclimatization makes a better correlation between the thirst and the water needs, raising the blood volume and total body water. This translates into a higher body weight by several pounds due to supplementary water amount. The body manages to do this by fluid-conserving hormones like aldosterone, which change kidney's activity, retaining more water and salt.
Salt keeps the water inside the body and the summer sweat is less salty (it's just more abundant). The increased stabilized total body water will be reached just by keeping on exercising in hot weather and will be hampered in people who stay mostly indoors in air-conditioned ambiances.