Energy homeostasis, commonly referred to as the homeostatic control of energy balance, is a biological process that entails the coordinated homeostatic regulation of food intake (energy inflow) and energy expenditure (energy outflow). Three main factors make up daily energy expenditure: thermic effect of food (TEF), thermic effect of activity (TEA), and resting metabolic rate (RMR). RMR is the energy required to maintain the majority of bodily functions and makes up 60–75% of daily energy consumption. Energy balance refers to the situation, when the amount of calories spent equals the amount of calories consumed, while energy imbalance reflect either excessive calories, or undernutrition. The fundamental tenet of thermodynamics, according to which energy can only be created, lost, or stored by an organism, forms the basis of the idea of energy balance. Energy balance is the condition that is reached when energy intake and energy usage are equal. Energy density, which is typically expressed as the number of calories in a gram (kcal/g), is the quantity of energy or calories in a specific weight of food. Calories per gram are less abundant in foods with a lower energy density than those with a higher energy density. A person can eat more food with a lower energy density for the same number of calories than of a food with a greater energy density. According to study by Pontzer et al (2021), a low density diet consists mainly of fruits, vegetables, whole grains, lean meats, and low-fat dairy products lowers daily calorie consumption. To achieve energy imbalance is usually simpler than maintain energy balance (Westerterp, 2018). The basic principle of energy imbalance is to consume less calories than you burn, or eat more calories that you burn. Both of those cases are energy imbalance – the first one leads to losing weight, and the second causes gaining weight.
Reference
Pontzer, H., Yamada, Y., Sagayama, H., Ainslie, P. N., Andersen, L. F., Anderson, L. J., & IAEA DLW Database Consortium §. (2021). Daily energy expenditure through the human life course. Science, 373(6556), 808-812.
Westerterp, K. R. (2018). Exercise, energy balance and body composition. European journal of clinical nutrition, 72(9), 1246-1250.