Final answer:
A positive energy balance in the body means more energy is consumed than expended, leading to fat storage and an increase in body mass. A negative energy balance results in weight loss as the body utilizes stored fat. BMR adjustments and exercise play pivotal roles in managing this energy balance for weight control.
Step-by-step explanation:
When the body experiences a positive energy balance, this means that it is gaining more energy from food than what is being expended through heat transfer, doing work, or cell repair and growth. This excess energy is then stored as fat. A positive ΔU, which represents internal energy change, leads to an increase in body mass, not a decrease. If ΔU is consistently positive due to overeating, fat accumulation will occur. Conversely, a negative energy balance indicates that the body is using more energy than it is receiving from food intake, leading to weight loss as the body metabolizes stored fat to make up for the energy deficit. This principle underlies how calorie counting and dieting can lead to weight loss, demonstrating the 'thermochemistry' of weight management.
It's important to understand that the basal metabolic rate (BMR) plays a critical role in this process. The BMR can adjust to compensate for over-eating or under-eating by increasing or decreasing, respectively, which can make weight loss more challenging after initial diet efforts. Exercise can combat this adjustment by not only burning calories directly but also by increasing your metabolic rate even when at rest.