Calculating basal metabolic rate
Energy is not a nutrient but it is essential for our body. We get our energy mainly from our protein, fat and carbohydrate consumption. The energy requirement varies and it takes into account our gender, age and whether or not we are active or sedentary. The unit of energy can be measured using kilocalorie or kilojoule, whereby 1 kilocalorie =4.18 kilojoule.
BASAL METABOLIC RATE (BMR)
You see, our body needs energy to function at rest. At any given time, for example, while you are reading this and sitting down, your body is working to sustain life. So, without you knowing, your body is burning energy to perform many essential functions such as breathing, pumping of the heart, cellular metabolism, enzyme and hormone synthesis, digestion, brain activity and maintaining your body temperature.
Among our organs, our liver produces the highest BMR (27%), followed by our brain (19%), skeletal muscles (18%), kidneys (10%), heart (7%) and others at 19%. The total energy required to run these functions is called the basal metabolic rate (BMR) and it takes up 45-70% of your body energy expenditure already.
The body metabolic rate will go on regardless whether you are active or sedentary, whether you are sitting down, sleeping or working. That is the reason why we need energy and eating the right food so as to provide our body the ability to function efficiently on a daily basis.
Basal metabolic rate however can be influenced by several factors. The BMR may be increased if we are at our growing spurt, recovering from or fighting an illness, eating and exercising. Hence it is very important that we eat enough calories to maintain healthy weight and balance energy levels.
CALCULATING YOUR ENERGY NEEDS PER DAY
We need information regarding our BMR and our Physical Activity Level (PAL) to calculate our energy needs in a day. There are many equations available to calculate your BMR and your energy requirement. Here, I will use the Schofield equation as it is simple to understand. There are 3-steps involved:
Step 1: Calculating your Basal Metabolic Rate using Schofield equation.
The information needed are gender, age and your weight in kilogram. Then you can use the formula in table 1 to determine your BMR. For example, say if you are a female aged 30 years old and weigh 70kg, your BMR will be 8.126 x (70) + 845.6 = 1,414.42 Kcal/day
Table 1: Schofield Equation
Age (Years) Basal metabolic rate / 24 hours
10 – 17 17.686 x (weight in kg) + 658.2 13.384 x (weight in kg) + 692.6
18 – 29 15.057 x (weight in kg) + 692.2 14.818 x (weight in kg) + 486.6
30 – 59 11.472 x (weight in kg) + 873.1 8.126 x (weight in kg ) + 845.6
≥ 60 11.711 x (weight in kg) + 587.7 9.082 x (weight in kg) + 658.5
Step 2: Determining your Physical Activity Level (PAL) Factor
Then we determine our physical activity level from table 2, which will tell us the physical activity factor. For example, if you are a female and does light activity, your PAL factor would be 1.5
Table 2: The activity levels to be used along with the BMR derived from the Schofield equation
Physical Activity Levels (PAL) Descriptions Physical Activity Level (PAL) Factor
Sedentary Little to no exercise. 1.3 1.3 Inactive in both work and leisure.
Lightly active Intensive exercise for at least 20 1.6 1.5 minutes once or twice per week or daily routine includes some walking. Example: student. Generally you do not exercise regularly, but you maintain a busy life style that requires you to walk frequently for long periods.
Moderately active Intensive exercise for at least 20 to 45 1.7 1.6 minutes 3 to 4 times per week or a job with a lot of walking, or a moderate intensity job.
Very active Intensive exercise for 60 minutes or 2.1 1.9 greater 5 to 7 days per week. Labor-intensive occupations also qualify for this level. Labor-intensive occupations include construction work (brick laying, carpentry, general labor, etc.). Also farming, landscape worker or similar occupations.
Extremely active Exceedingly active and/or very demanding 2.4 2.2 activities: Examples include: athlete with an almost unstoppable training schedule with multiple training sessions throughout the day or a very demanding job, such as shoveling coal or working long hours on an assembly line. Generally, this level of activity is very difficult to achieve.
Step 3: Calculate you daily calories per day
Now, to get our calorie needs in a day, you only now need to multiply your BMR and your PAL factors.
Equation: Calories need in a day = BMR x PAL factor
= 1,414.42 x 1.5
= 2,121.62 Kcal/day
For a female, aged 30 weighing at 70kg
BMR = 8.126 x (weight in kg (70) ) + 845.6 =1,414.42
If she is lightly active, her PAL factor is 1.5
Calories/day = BMR x PAL = 1,414.42 x 1.5
= 2,121.63 Kcal/day
I list here the links to the calculators and perhaps you may want to try out all of them and compare the results for your personal usage:
Harris-Benedict Equation. http://www.globalrph.com/harris-benedict-equation.htm
Revised Harris-Benedict Equation. http://www.globalrph.com/revised-harris-benedict-equation.htm
Resting Metabolic Rate (RMR). http://www.globalrph.com/resting_metabolic_rate.htm
Schofield Equation (BMR). http://www.globalrph.com/schofield_equation_bmr.htm
Institute of Medical Equation. http://www.globalrph.com/estimated_energy_requirement.htm