Answer to Question 1
The first law of thermodynamics applies to energy balance in humans. This law states that energy can be transformed from one form to another, but cannot be created or destroyed. In humans, the amount of energy we consume from food equals the amount of energy the body has available for growth, physical activity, other body functions, and energy stores. Unless you are currently losing or gaining weight, the number of calories you need is the number you usually consume in your diet. Adults who maintain their weight are in a state of energy balance. Because they are not losing weight (using fat and other energy stores) or gaining it (storing energy), their bodies' expenditure of energy and its intake of energy are balanced.
When energy intake is less than the amount of energy expended, people are in negative energy balance. In this case, energy stores are used and people lose weight. When a positive energy balance exists, weight and fat stores are gained because more energy is available from foods than is needed by the body. Small increases in calorie intake above calorie need may not amount to noticeable gains in body weight on a day-to-day basis, but they can add up to significant weight gain over the course of months or years.
Sometimes a positive energy balance is a healthy and normal circumstance. For example, a positive energy balance is normal when growth is occurring, as in childhood or pregnancy, or when a person is regaining weight lost during an illness.
Answer to Question 2
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