What is a calorie deficit?
The First Law
Energy can neither be created nor destroyed. It can only be transformed.
To lose weight, you need to expend more energy than you consume. To gain weight, you need to consume more energy than you expend.
The unit of measurement used is a calorie. It doesn't matter what that means or how it is calculated or whether it is actually a kilo calorie. You can call it whatever you like.
From the First Law of Thermodynamics, we realise the constraint.
You have a lot of stored energy
Let us use an example. 70 kg male at 20% body fat. Goal is to get to 15% body fat.
The goal is thus 3.5 kilos (5% of 70 kilos) of fat loss. One kilo of fat is 7700 calories [see reference]. Multiplying that by 3.5 gives us ~27,000 calories.
For simplicity's sake, we will assume that all loss in weight that we see will be fat. Which is practically impossible and the most one can generally aim for is a generous skewing of the ratio towards fat vs muscle.
So, to lose weight, our imaginary guy has to, over the course of an agreed upon timeframe, eat 27000 calories lesser. If the stars align, these 27000 calories come directly from the stored energy source in the body aka fat.
What is maintenance?
Your body has an energy requirement.
To do its bodily functions. For you to sit and stand and walk around. To breathe. To digest food. To play football. To work at your computer. To watch TV.
The energy required by the body to neither lose weight nor gain weight is its maintenance number.
A 6-month timeframe for 3.5 kilos
Yes, you can lose weight a lot faster than this. But this is my example, my world.
Dividing 27000 calories by 180 days gives us 150 calories/day. Thus, we arrive at the calorie deficit needed to lose 3.5 kilos over a 6-month timeframe for our 70kg male friend.
A calorie deficit is essentially the difference between what the body's expenditure vs consumption is. Since the body still requires the extra energy to subsist and survive, that deficit has to come from somewhere. Which is all the excess fat you have in your body.
Energy can only be transformed. Energy that we consumed in the days, weeks, months, years before this moment - that's what we are trying to lose when we say
weight loss.
In essence, our calorie deficit today is simply balancing our calorie excess from earlier days.
How do you put on muscle?
Using the same example, if you want to add more weight to your body, you need to have a calorie surplus.
If you want to put on 3.5 kilos over 6 months, you need to eat 150 calories/day more than what is needed for maintenance.
Just like it is impossible to lose ONLY fat during a fat-loss programme, it is impossible to gain ONLY muscle during a muscle-gain programme. Important fact to know but irrelevant for now.
Where does this leave you?
There are accurate methods to calculate body fat percentage. A DEXA scan or a skinfold test, or a quicker (less accurate) method. With this and your weigh scale, you can make an estimate of how many kilos you want to lose. And from there, the calorie deficit you need to have over a day, across how many days.
Before we proceed further, you should know that ~5% body fat is essential for men and ~13% is essential for women.
Start by knowing some of the following.
- your weight.
- your body fat percentage.
- your goal - lose weight (fat) or gain weight (muscle).
- your maintenance energy requirement. Based on your age, your activity levels, using the Goog or by strapping on a HRM for an entire day - whatever you'd like.
And you can arrive at your calorie delta i.e. the deficit or surplus that you need to maintain, along with the timeframe.
And finally, a caveat
There are insensible methods to do both - losing and gaining weight. Don't do them. Or at least, I do not subscribe or prescribe them. But then, I am not the guy to talk to for single-digit body fat percentages as well.
- Starvation is not the answer (for fat loss). Feeling queasy and constantly full is not fun either (for mass gain).
- Nutrient density should be high. This will ensure that you do not starve or even come close. Try eating 5 cups of vegetables and 3 cups of protein, and let me know if you are still hungry.
- Sanity is high. If you are snapping at your friends and family, what is the point of your dream body?
- Calorie counting is NOT a method I prescribe. It is a useful tool to use occasionally. Given that there are a lot of errors in calculation, differences in absorption and the tediousness of the actual counting and consumption of foods, it fails my sanity check.