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Energy Costs & Basal Metabolic Rate

Many people try to follow a calorie controlled diet. To lose, gain or maintain weight you must have an idea of your daily calorie requirement. To lose weight, the calories taken in from food and drink must be less than the calories expended. To gain weight, the calories taken in from food and drink must be more than the calories expended. Finally, to maintain weight, there has to be a balance between the calories consumed and the calories expended. Whether you are a lightweight competitor or overweight and trying to reduce, knowing your daily energy costs is an important part of your weight management programme.

Energy costs can be divided into three main groups, the basal metabolic rate, (BMR) which is solely the cost of running your life support system, heart, lungs, digestion etc. Daily expenditure, which is what you do when awake up and about and finally the energy costs of exercise.

You can find out your BMR, and all the other calculations in this article, from our BMR Calculator. Below is a worked example as well as advice on why knowing your BMR is useful, and how you can use it to maintain a correct calorie balance between the three main food groups.

BMR is related to age and gender (with women holding a proportionally higher level of fat and less muscle tissues than men. Muscle tissue is metabolically more active than fat tissue therefore requires more energy to maintain). To calcuate your BMR by hand, you need the following formulas:

W = Bodyweight (kg)

As an example: a 22 year old female who weighs 65 kgs. Her BMR is 1449 kCals (14.8x65+487). To estimate your average daily expenditure you need to take your BMR and multiply it by a factor, which is determined by your activity level. If we assume that our example is very active during the day then the factor is 2.0, so the BMR is doubled to 2898 kCals to give the estimated daily expenditure. If she follows this with a day of relaxation where she is mainly sedentary, mostly seated or just standing then you multiply the BMR by a factor of 1.4, so her energy expenditure falls to 2028 kCals. On another day she is moderately active, regular brisk walking or manual work then the factor is 1.7 and her energy cost rises to 2463 kCals.

To determine the cost of training we have to find the hourly BMR, which is done by simply dividing your BMR by 24. If we use the BMR in the example then the hourly BMR is 1449/24=60.3

For the different rowing intensities there is a Physical Activity Ratio (PAR) and to find the energy costs of training the formula is, hourly BMR x PAR x duration.

Continuing to use our example, the energy cost for some typical training sessions are:

• 30 minutes @ 2:20 = 60.3 x 9.0 x 0.5 = 271 kCals
• 24 minutes @ 2:00 = 60.3 x 14.2 x 0.4 = 343 kCals
• 12 minutes @ 1:50 = 60.3 x 18.4 x 0.2 = 222 kCals

If the young woman carries out a 30 minute session at a 2:20 pace on an active day then on that day the total energy costs is 3166 Cals.

To embark on a calorie controlled diet with no idea of your daily calorie requirement is not only a waste of time but can present a health risk. Many diets seem to pluck a total daily calorie requirement out of the air and it is not unusual to see a figure of 1,000 Calories for women. In this worked example of a small woman two things are apparent: the difference from day to day and that her BMR is almost 1,500 kCalories.

Although in order to lose weight it is necessary to take in less calories than you expend, it should not fall below your BMR. If it does then your body goes into "Famine mode." In this condition it is not necessarily fat that is lost faster but lean body mass and water, and the metabolism slows down to balance the reduced calorie intake. The consequence is loss of strength, lack of energy, mood swings, tiredness, sluggishness, not to mention hunger. Your immune system will also be affected and you will become vulnerable to infection.

The process outline here is a guide to your energy output and is a good place to start if you are interested in weight management. The other aspect is knowing the calorie balance. Energy will be provided by three food groups, carbohydrates, protein and fats. The relative balance between these groups is 60% carbohydrates, 17% protein and 23% fats. Going back to our example, on the day where the energy expenditure is 3106 kCals then the estimated dietary requirement is 433g of carbohydrate, 80g of fat and 123g of protein. This is because each gram of carbohydrate and protein will provide 4.3 kCal while fat provides 9 kCal. All of this maths is worked out for you in the BMR Calculator.