To start losing weight right away, you’ll need to know the scientific principles behind weight loss. Let’s start by learning how many calories you actually need each day.
Your basal metabolic rate, or BMR, dictates how many calories your body needs to perform its functions for the day. This number is the bare minimum; it tells you how many calories your body needs to simply survive and carry out natural processes. If you’re an active person, you will require more calories than your BMR suggests. The sum of your BMR plus the calories you burn during daily activities is your Total Energy Expenditure, or TEE.
There are various formulas you can use to help you figure out an appropriate number of calories for weight loss. The most widely used is the Harris-Benedict formula, which considers your age, gender, height, and current weight to come up with a suitable number of calories. Here is an example of this formula in action:
Men: BMR = 66 + (13.7 X wt in kg) + (5 X ht in cm) – (6.8 X age in years)
Women: BMR = 655 + (9.6 X wt in kg) + (1.8 X ht in cm) – (4.7 X age in years)
(For calculation purposes, 1 inch = 2.54 cm and 1 kilogram = 2.2 lbs.)
A 35 year old female who stands 5’4” (163 cm) tall and weighs 130 pounds (59 kilos) would use the following equation:
655 + 566 + 293 – 165 = 1,349.
The woman in the example would need 1,349 calories a day just to lie around in bed and keep her organs functioning. If that number seems small, it’s because we haven’t yet taken this woman’s activity level into consideration.
Activity multipliers work as such:
Sedentary people (office jobs, no real exercise) multiply their BMR by 1.2.
Lightly active people (exercise 1 – 3 times per week) multiply their BMR by 1.35.
Moderately active people (exercise 4 – 5 times per week) multiply their BMR by 1.55.
Highly active people (sports or hard exercise most days) multiply their BMR by 1.725.
Extremely active people (hard exercise every day) multiply their BMR by 1.9.
Let’s revisit the woman in our example. We’ll assume she works a desk job and is sedentary most days of the week. Even at such a low activity level, she would expend extra calories walking around, driving to and from work, typing, and fidgeting at her desk.
Her BMR (1,349) multiplied by her sedentary activity level (1.2) reveals that she needs about 1,619 calories each day to maintain her current weight. If she started exercising just 2 or 3 days each week, her daily calorie requirement would increase to 1,821.
So how do these numbers help you lose weight? Easy: To lose a pound of fat each week, you must burn 3,500 more calories than you consume. In order to lose a pound of fat, the woman in our example would need to eat 1,119 calories a day if sedentary, or 1,321 a day if lightly active.
As you can see, it’s much easier to lose weight through diet and physical activity than through diet alone.