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The energy balance equation is the relationship between the energy consumed – measured in calories and the energy expended – also measured in calories. Maintaining a healthy weight requires a balance between energy in and energy out. Too much energy in or too little energy out leads to excess energy being stored as fat. Too little energy in or too much energy out leads to weight loss. Typically, men require 2,500 Kcal per day and women 2,000 Kcal but these general values are affected by four main factors:

• age – in general, adults consume more than children but calorie consumption declines in later life
• gender – in general, men consume more than women
• height – the taller the person, the more that person will need to consume
• energy expenditure – the more active a person is, the more calories are required to maintain weight

Energy in and energy out do not have to balance every day. The energy equation needs to balance over time for people to maintain a healthy weight.

Understanding food – knowing what makes a healthy diet – helps people to manage the energy balance, and people's levels of physical activity are as important to the energy balance as their diet.