It takes energy to evaporate water, just like it takes energy to boil water on the stove. The energy to evaporate sweat comes from our bodies. Energy loss through the lungs by evaporation during breathing obviously increases as the respiratory rate increases. The risk of heat loss is increased by the use of drugs and alcohol, which affect circulation and blood flow. Impaired consciousness also increases risk, since the affected person may not feel cold or take precautions to prevent heat loss.
Exhaustion also leads to increased heat loss because of decreased metabolism and cardiac output.
Hunger obviously signals the lack of food to burn for energy, and anemia interferes with the metabolism and delivery of oxygen to burn fuel. Infants also cannot produce as much heat as adults through muscle activity. Metabolism decreases as we age, so elderly people have more difficulty maintaining body temperature through metabolism in cold climates. The body does not waste energy heating the arms and legs. Metabolism increases as the body tries to burn fuel faster to generate heat.