Posted by Dayne Hudson in Weight Loss
Estimated reading time: 6mins
The best way to answer this question is to start at the very beginning; we must know how many calories we burn per day.
The amount of calories we burn a day is referred to as our Total Daily Energy Expenditure (TDEE). We work it out like this:
Step 1 = Discover your Basal Metabolic rate (BMR). This is the number of calories your body needs to stay alive for organ functioning. You'll work this out by multiplying your body weight by 20.
For example, Robyn is 75 kg.
75 kg x 20 = BMR of 1500 calories.
Step 2 = Work out your Thermic Effect of Feeding (TEF). This is the number of calories we burn daily from the food we eat, given the energy generated by the body to digest it. So, multiply your BMR by 0.1 to get your TEF.
Robyn's BMR = 1500 calories.
1500 calories x 0.1 = 150 TEF
1500 BMR + 150 TEF = 1650 calories.
Step 3 = Work out the Exercise Energy Expenditure (EEE). As the name suggests, that's how many calories you burn during exercise. (Note: If you're not doing any exercise at all, skip this step entirely). A smartwatch can help you get this totally correct, but generally, an hour of light exercise is around 250 calories burned, and an hour of intense exercise (training legs, for example) is about 500 calories burned per hour. Let's say Robyn is doing resistance training five times per week; we'll add 500 calories.
1500 BMR + 150 TEF + 500 EEE = 2150 calories.
Step 4 = Work out your rate of Non-Exercise Activity Thermogenesis (NEAT). NEAT is the calories burnt daily that aren't from exercising. It's things like tapping your foot to music, chewing gum, maintaining posture sitting at your desk, and walking to the bathroom. A desk job employee burns about 250 calories from NEAT, whilst a parking inspector delivery driver, building site worker, etc, burn about 500 calories daily from NEAT.
Let's say Robyn has a desk job. So, our numbers now look like this:
1500 BMR + 150 TEF + 500 EEE + 250 NEAT = 2400 calories.
Therefore, Robyn's TDEE is 2400 calories.
Armed with this information, Robyn must now decide how quickly she wants to lose fat.
Let's say she wants to lose fat in time for an event in three months. These are Robyn's options:
The third option is obviously the most aggressive. I recommend a moderate deficit, but for the sake of this example and Robyn's event coming up in just three months, let's look at the "large deficit."
So, 25% of a TDEE of 2400 is 600 calories.
So, we take this number and minus it from Robyn's TDEE: 2400 - 600 = 1800 calories.
So, Robyn must eat 1800 calories a day to be in a 25 per cent aggressive deficit.
After 4 weeks, given a deficit of 7,700 calories equates to about a kilogram of body weight lost, Robyn would lose 2.1 kilograms, which will definitely be evident!
And just how we work out the below macronutrient breakdown (protein, fat and carbohydrates) is beyond the scope of this article, Robyn's sensible diet approach would look like this:
Protein: 150 grams (600 calories)
Fat: 70 grams (630 calories)
Carbohydrates: 143 grams (572 calories)
Total = 1802 calories.
If Robyn can commit to this for a month, she'll lose about 2.1 kilograms before her event.
Under Robyn's 25% daily deficit approach, let's say Robyn wants to lose 4 kilos.
Given we've declared that 7,700 calories equate to a kilogram of body fat, then 4 kilograms x 7,700 calories gives us 30,800 calories; the cumulative deficit Robyn has to achieve.
Under the 25% deficit, Robyn is in (a 600 calorie deficit daily to be precise), we simply divide the required deficit of 30,800 calories by her daily deficit of 600.
This turns out to be 51.3 days, or 7.3 weeks, For Robyn to lose 4 kilos.
Here's a trap a lot of people fall into; NOT reducing their calories as they go in order to keep losing weight!
When you're lighter, you must make adjustments.
So, when fat loss slows to below 0.5 kilos a week, Robyn must reduce her calorie intake further by 10%.
This way Robyn's weight loss won't stall!
Now, this is very important: the notion that a "7,700 calorie deficit equates to a lost kilogram of body fat" is sometimes seen as an oversimplification, but it's still a strong yardstick for us to use.
This is also because Robyn could experience some muscle loss, making the equation not totally accurate. But for the sake of simplicity, this is what weight-loss experts and nutritionists continue to use.
You can work out how long it will take to lose a certain amount of weight based on the principle that a 7,700 calorie deficit equates to a kilogram of body fat. But you should work that out only after you've discovered how many calories you burn a day, how aggressive you want your calorie deficit to be, and then what your number of calories per day should be with this all in mind.
You can reach your goal number of fat loss faster by choosing a more aggressive deficit, but that might not be ideal given it can cause more weight loss problems than it solves (ie, severe hunger!).