How much energy do you need in your diet?

One’s body requires a certain amount of energy every day to keep your metabolism ticking over, which means that your organs function correctly and the reactions of life in your cellular material can take place. This energy is recognized as your basal metabolic rate (BMR) and is measured when you are at complete rest. It is calculated from the temperature alterations that result as the human body gives out heat over a period of time or days in a heatproof place (another type of calorimeter). In babies and children the BMR is proportionately higher than in adults because they use more energy in growth.

The BMR is related to the entire body mass and the lean body mass. People with a high proportion of muscle will have a higher BMR, because muscle tissue requires more energy for preservation compared to weight. This really is one particular reason why men normally have higher BMRs compared to women, because they tend to have a higher proportion of muscle to fat. As people age, not only is their tissue replaced less often but they also tend to lose muscle and so the BMR has a tendency to fall with age. The BMR makes up, on average, about 75% of the metabolic needs of the body. But if you are very active then your BMR may make up 50% or less of your daily energy requires, because you are doing so much exercise as well.

Measurements of BMR show that a common man needs to take in about 7500 kJ/day, and a common women needs about 5850 kJ/day. This reflects the energy needed if the person concerned lies on a bed all day and night and expends no energy above that needed to breathe and excrete not even to feed! You can see from this that the BMR is of relatively little use on its own in assessing the energy in take needed in a healthy diet. To make the measure more useful, the physical activity level (PAL) must also be taken into account. Multiplying the BMR by a factor that reflects the PAL gives the estimated average requirment (EAR) for enery. The EARs for energy of people of different ages are shown in table.


AgeEAR (kj/day) (males)EAR (kj/day) (females)AgeEAR (kj/day) (males)EAR (kj/day) (females)
0-3 months2280216011-14 years92707720
4-6 months2890269015-18 years115108830
7-9 months3440320019-50 years106008100
10-12 months3850361051-59 years106008000
1-3 years5150486060-64 years99307990
4-6 years7160646065-74 years97107960
7-10 years8240728075+ years87707610