Which individual factor significantly affects energy balance?

The correct answer is psychology. Psychological factors can significantly influence energy balance through various behaviors, emotions, and thoughts related to food and physical activity. For instance, emotional stress can lead to overeating or undereating, while attitudes and beliefs about health can motivate or inhibit physical activity and dietary choices. Psychological conditions, such as depression or anxiety, can also affect appetite and energy expenditure, thereby altering energy balance.

Age, gender, and genetics also play roles in energy balance, but they do so in different ways. Age affects metabolism and body composition, which can influence energy needs and expenditure. Gender often influences body fat distribution, metabolic rate, and hormonal profiles, affecting how energy is utilized and stored. Genetics can determine predispositions to body weight, fat distribution, and even individual responses to diet and exercise. However, the immediate and subjective nature of psychological factors makes them a more significant influencer of an individual's day-to-day energy balance.