As discussed in the presentation entitled ‘Measurement of food’, the amount of energy made available to the body can be classified in either calories or kilojoules.

Total daily energy expenditure (TDEE), otherwise known as ‘Calories Out’, largely determines daily energy requirement from food i.e. ‘Calories In’. Ideally the two should match. However, as discussed later, the two are far from independent, and therefore Calories In can also determine TDEE

TDEE equals basal metabolic rate (BMR) plus the thermogenic effect of food, plus non-exercise activity, plus exercise. The majority of TDEE is made up of the basal metabolic rate – probably as much as 95%. The BMR equates to basic bodily functions such as maintaining the functions of the vital organs such as the brain, heart, kidneys, liver and lungs and maintaining body temperature.

As previously discussed BMR accounts for probably 95% of TDEE. Importantly however the TDEE can be increased by for example 30 minutes of cardiovascular exercise, in the form of a 5km run, in the region of 400 – 500 calories.

The basal metabolic rate of an individual depends on numerous factors including gender, weight, height, age, genetics and external temperature.

Another important determinant of basal metabolic rate is the number of calories that have been consumed ie ‘Calories in’. There is very good evidence that demonstrates reducing ‘Calories In’ has a significant effect on the BMR in that the body automatically reduces it. And this is something that you would expect from an evolutionary standpoint. The body has evolved to survive in the face of adversity. So, if food is suddenly unavailable then the body uses its in-built mechanism to cope with this, i.e. it automatically senses that the available energy is restricted and compensates by reducing the BMR accordingly.

As a consequence, you feel cold, tired, slow, hungry, irritable and generally miserable. Understanding this concept is an essential part of appreciating why calorie-restricting diets fail in the long term, as discussed in the presentation entitled ‘Calorie Restricted Diets’. Fundamentally Calories In and Calories Out are not independent variables. Quite the converse - they are very dependant as just highlighted.

The TDEE for the average male is approximately 2500 calories and for the average female 2000 calories. However as previously discussed these figures are variable. It is not essential to know your own individual BMR but is certainly helpful to have an idea. Silmilarly, whilst calorie-counting on a daily basis is not necessary, it is once again good to have an idea about the what you’re eating.