Which statement best describes the energy balance equation and weight implications?

• A. Energy intake is always greater than expenditure
• B. Energy balance has no relationship to body weight
• C. Energy intake equals energy expenditure; a positive balance leads to weight gain, and a negative balance leads to weight loss
• D. Energy expenditure is determined only by basal metabolic rate

Answer: (C)

The main concept here is that body weight changes when calories in differ from calories out. The energy balance equation describes how the change in stored energy corresponds to this difference: energy intake minus energy expenditure. When intake is greater than expenditure (positive balance), excess energy is stored, mostly as fat, leading to weight gain. When expenditure exceeds intake (negative balance), stored energy is used, leading to weight loss. When intake and expenditure are equal, weight remains stable.

The option that best matches this idea—linking energy balance to weight change with positive balance causing gain and negative balance causing loss—captures the core relationship between calories consumed and calories burned and their effect on weight. The other choices either misstate the relationship or overlook the role of the different components of energy expenditure, such as physical activity and the thermic effect of food, not just basal metabolic rate.