The math behind fat loss.


Let's express overeating using a few simple equations. Given 1 lb of fat = 3500 calories if calories in > TDEE you store the remainder as fat. So if your TDEE is 2000 calories but you eat 2500 calories a day you'll have 500 extra calories that your body needs to store as fat.

500 calories excess * 7 days = 3500 calories excess in a week = +1 lb of fat a week


You're stressed and binge eat a pizza every evening for a week. If one pizza is 2000 calories excess that's 2000 calories excess * 7 days = 14,000 calories excess

14,000 calories / 3,500 calories = +4 lbs of fat a week.

That means you would gain 4 lbs of fat that week by overeating by a pizza every night! This is an example I've unintentionally validated in my life before.


If TDEE > calories in your body taps into fat storage to come up with the extra energy. Up to a point! Then it starts using your body's muscle aka protein as energy as discussed before.

So if you want to lose 1 lb of fat in a week you would need to under eat by 500 calories a day:

7 days * 500 calorie deficit = 3500 calorie deficit = 1 lb of fat used

You can calculate the maximum amount of fat your body can mobilize using this equation. That effectively puts a cap on how fast you can lose fat. Here's the equation:

body fat % / 20 * current weight * 7 = maximal lbs you can lose per week

e.g.: For someone who is 25% body fat and weighs 200 lbs:

0.25 / 20 * 200 lbs = -2.5 lbs a week maximum

The Protein Trick

As we discussed, eating at least 1g of protein per lb of weight is crucial so your body doesn't start breaking down your muscle to use in other places. It's also critical to lift 2-3 times a week to signal to your body you still want your muscle. But there's a really cool trick when it comes to protein we can exploit.

Your body knows how to use protein as fuel if necessary. This can be bad if it's breaking down your muscles to get protein to use as fuel. But it's totally fine if there's excess protein in your bloodstream after eating and your body has finished building/repairing itself with it.

Your body doesn't like using protein as fuel because it has to expand a lot of energy to convert it to energy. That means every 10 calories of protein you eat about 3 calories are needed just to convert it to energy. This can be exploited with a high protein diet. If you eat 1000 calories of protein a day only 700 calories of energy are left after processing it. That means you get to feel like you ate more while not incurring the caloric cost.

I exploit this property of protein myself by eating 300g of protein a day. 1g of protein contains 4 calories according to the label. That means I get 1200 calories of energy daily from protein. But because it takes 30% of energy just to convert protein to calories, 300g of protein is actually only 300g*3 calories=900 calories. That's an extra 300 calories I get to eat a day. 🤤

There is a maximum amount of protein you can consume per day before it becomes harmful but it's usually well beyond what a typical person consumes. For me, a 175 lb, 5'11" male the max is 360g of protein a day. I struggle to consume 300g of protein most days though because I feel so full even though I'm in a maximal deficit cutting.

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