To reap the benefits of training and maintain good health, the required amount of energy (i.e. calories) must be consumed. Failure to consume the required energy that we need can lead to muscle loss, reduced performance, slower recovery, increased risks of fatigue, injuries and illnesses, and hormone disruption in females.
The number of calories we need daily is affected by several factors including gender, age, weight, body composition, daily energy expenditure, and genetics. Fortunately, if we know our body-weight and physical activity level we can calculate the daily calorie requirements using the three-step process below:
Step 1: Calculate your Basal Metabolic Rate (BMR)
During rest, the systems of the body are still working so that the body can stay alive and healthy. The BMR is the number of calories your body burns to keep these systems going during rest and usually accounts to around 60-75% of the total calories consumed. Since men usually have a higher muscle percentage, their BMR is usually higher than women. We can calculate the BMR using the equation below.
· Males: BMR (kcal / day) = 10 x weight (kg) + 6.25 x height (cm) – 5 x age (y) + 5 (kcal / day)
· Females: BMR (kcal / day) = 10 x weight (kg) + 6.25 x height (cm) – 5 x age (y) – 161 (kcal / day)
So, a woman weighing 60 kg, is 163 cm tall and 27 years of age, her BMR would be:
BMR = 10 X 60 + 6.25 X 163 – 5 X 27 – 161
= 600 + 1018.75 – 135 – 161
= 1322.65 kcal/day
There are many different equations to predict BMR including the Harris-Benedict, Mifflin-St Joer, Owen and the World Health Organization/ Food and Agriculture Organization/ United Nations University. However, the Mifflin-St Joer equation (the one used above) has been found to be the most accurate equation (Frankenfield et al. 2005)
Step 2: Calculate the Daily Energy Expenditure
Once you worked your BMR, the value is multiplied with the Physical Activity Level (PAL), an estimated ratio of your physical activity to your BMR. It is important to note that physical activity does not include only planned exercises, but also all physical activities during the day including housework, walking your dog, playing with your children, etc. The more intense and the longer the activity is, the more calories you burn. Your weight is also another factor that affects the calories burnt during physical activity as the more you weigh, the more energy you need to move. The ratios are presented in table 1.
So, if the same woman from step 1 trains three times a week, we would multiply the BMR with 1.55:
Daily Energy Expenditure = BMR x PAL
= 1322.65 x 1.55
= 2050.11 kcal/day
Step 3: Add the number of calories expended during exercise
If you have an activity tracker such as a watch that measure the heart-rate and calories expanded, you can easily measure the number of calories expended during exercise. If not, you can calculate how much calories you burn using table 2:
After estimating the calories burned during a whole week, divide the amount by 7 to get a daily average and add it to the daily energy expenditure. So, if the woman from previous examples joins a circuit class with minimal rest for three times a week, we would multiply 472 calories (the closest value to her weight for circuit training) by three (as she trains three times a week), and then divide it by seven.
Total calories expended during exercise = 472 calories x 3 = 1416 kcal
1416 kcal ÷ 7 = 202.29 kcal/day
To calculate the daily calorie needs, we would then add the daily energy expenditure for step 2 and the total calories expended during exercise from step three:
Daily calorie needs = Daily energy expenditure + Total calories expanded during exercise
= 2050.11 + 202.29 = 2252.4 kcal/day
This value is simply the number of calories you need daily to maintain your current body weight. If you eat lower than this amount, you will lose weight, if you eat more than this amount you will gain weight. If you manage to accurately measure the calorie intake you consume daily you can gradually adjust the intake until it matches. This may take quite some time to get used to. If your goal is to reduce weight, reduce your daily calorie intake by 15%, whilst if your goal is to add weight, increase the calorie intake by 20%.
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Frankenfield, D., Roth-Yousey, L., Compher, C., & Evidence Analysis Working Group. (2005). Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. Journal of the American Dietetic association, 105(5), 775-789.
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Thanks for reading, and as always stay fit!