Energy requirements differ from one person to the next because of genetic predisposition, build, gender, age, metabolism, environment and amount of regular physical activity. An individual’s energy requirements can also differ from one day to the next and as we grow older. For example:(kilojoule and calorie explained here )

• Young children and adolescents require high amounts of energy to fuel their growth and development.
• Women need more energy during certain stages of their reproductive lives, such as pregnancy and breastfeeding. It is thought that daily energy needs increase on average by about 1,800kJ for pregnant women and around 2,500kJ during breastfeeding.
• Muscle tissue has a big appetite for kilojoules. The more muscle mass you have, the more kilojoules you will burn.
• Men generally have higher energy requirements than women because they have more muscle tissue.
• As we age activity levels are often reduced, which causes a loss of muscle tissue, and so our energy requirements tend to decrease. Various other age-related changes to the metabolism also contribute to the reduced energy requirements. It’s not clear how much of the muscle lost during ageing is a result of the ageing process or due to reduced activity. Strength and resistance training in older adults (even the very old and frail) seems to help reduce or prevent the decline in muscle mass generally observed with ageing.