Very rapid changes in body weight are usually caused by fluctuations in body water status. For instance, water losses via sweating and/or poor fluid consumption can reduce body weight by 2 pounds (1 kilogram) for each lost liter. This mild dehydration is common and triggers thirst, the principal prompter of fluid consumption. In fact, you might not perceive thirst until your body weight has been reduced by 1 percent from water losses. Also it should be recognized that when a person does not eat for an entire day, more than half of weight loss they experience would be attributable to water loss. On the contrary, there are certainly times when we may hold a little extra water in our tissue. Women certainly know this to be true at certain points in their menstrual cycles.
On the other hand, more significant changes in body weight and composition over time are more attributable to regular over-consumption or under-consumption of calories as well as the type of diet we eat and the exercise we perform. In general, the effects of these factors are relegated to specific hormones and other signals. The handling of energy nutrients being absorbed from the digestive tract is primarily influenced by insulin. In contrast, glucagon, cortisol, and epinephrine largely control the handling of stored body nutrients during fasting or exercise. In addition, serious exercise leads to additional signals in muscle to adapt and possibly get bigger (thus influencing body composition).
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