How much Fat can I Lose Before I Lose Muscle?

max rate of fat lossThis is a question I see fairly frequently on the fitness boards I visit. It’s definitely an integral part of the cut and bulk cycle, where lifters eat over their caloric requirements to gain muscle — which also results in fat gain — and then diet to lose the fat while trying to retain as much of the muscle as possible. It’s a tried and true method for getting more muscular, but it requires discipline and planning.

Bulking is easy: just eat more. To do it right, you want to health nutritious food, but as long as you get enough protein and excess calories, and are training effectively, your muscles will grow. But once you’ve reached that point where you look in the mirror and think I need to cut down, how do you go about it?

You can lose fat only so fast before you start losing muscle too. But how fast is that?

Let’s figure it out….

Losing Fat, not Muscle

Losing weight isn’t complicated (which is not to say it isn’t hard). You lose fat by eating fewer calories than you use. That works for everyone, unless you have some sort of medical condition. The more you restrict calories, the quicker you’ll lose weight. But if you restrict calories too much, you risk losing your hard earned muscle as well.

If you’ve done any reading online about cutting, you’ve probably heard the general guideline to reduce daily calories by 500. Since a pound of fat is 3,500 calories, that would result in about a one pound a week fast loss.

While that 500 caloric daily deficit might work, is it the best? Sticking to a general number for everyone ignores each individual’s specific circumstances. It doesn’t take into consideration you gender, or, more importantly, how much fat and muscle you’re carrying. Someone how’s obese can lose much more than than one pound a week, while a lean person, such as a body builder prepping for a contest, might not be able to lose that much without losing muscle to..

So how can you figure out the maximum rate of fat  loss for you?

The 20% Method

One way to figure out your caloric deficit is based on your Total Daily Energy Expenditure (TDEE), so the first thing you would need to do is figure out your TDEE. Your TDEE is the nuber of calories you would need to eat in order to maintain your current weight.

Obviously, the TDEE for a six foot tall 200 pound man would be different that the TDEE for a five-and-a-half foot tall 130 pound female.

  • A six foot tall 200 pound 25 year old male who is lightly active has a TDEE of around 2,711
  • A five-and-a-half foot tall 130 pound 25 year old female who is lightly active has a TDEE of around 1,912

So right away you can see there’s a big difference based on gender and size.

Using the 20% method, we can calculate the caloric deficit a person should use to lose weight based on their TDEE. That would be a daily deficit of 540 calories for the male and 380 for the female.

This number would be adjust as weight loss occurs.

Also remember that TDEE is only an estimate. Eat at TDEE for a week or two, observe the results, and adjust accordingly.

The Body Weight Method with 20% Method Combined

Since your exact TDEE can be hard to pin down, another method is to calculate based on your body weight.

Maintenance calorie intake for almost everyone works out to be 14 – 15 calories per pound of body weight. Using the same examples above we get:

  • A six foot tall 200 pound male would be 2,800 – 3,200
  • A five-and-a-half foot tall 130 pound female would be 1,820 -2,080

Pretty close to the number generated by the linked TDEE calculator.

Now we apply the 20% method to get a daily caloric deficit of 560 – 640 for the male and 364 – 416 for the female, pretty close to the numbers above.

Conversion of Fat to Energy

There’s actually a maximum rate that the body can convert stored fat to energy. In order to use this method, you need to have an accurate reading of your body fat level, since you need to know your lean body mass and your body fat mass.

Here’s some of the scientific blather from the scientific study:

…the maximal rate at which fat stores can provide energy to the body is 290 +- 25 kj/kg which is approximately 31 kcal/lb of fat per day.

In English, that means the maximum rate the body can convert fat to energy is 31 calories per pound of fat per day.

From that perspective, lets look at our example cases at a couple different levels of body fat:

  • A six foot tall 200 pound male at 10% body fat = 20 pound of fat
    • max caloric deficit would be 20 x 31 = 610 calories a day
  • A six foot tall 200 pound male at 20% body fat = 40 pound of fat
    • max caloric deficit would be 40 x 31 = 1,220 calories a day
  • A five-and-a-half foot tall 130 pound female at 20% body fat
    • max caloric deficit would be 26 x 31 = 806 calories a day
  • A five-and-a-half foot tall 130 pound female at 30% body fat
    • max caloric deficit would be 36 x 31 = 1,209 calories a day

What those numbers represent is the maximum amount of energy the body can derive from fat stores daily. In theory, this means that is the maximum amount of energy your body can get from fat before it affects lean muscle mass.

What’s also important to note is that as weight loss happens your fat stores shrink, which means your deficit has to decrease. And, as your total body mass drops, your maintenance calories drop as well.

In easily understandable terms, this means as you lose fat and get leaner, fat loss slows due to metabolic changes. You either have to cut calories further or increase activity, and both can cause muscle loss.

Calorie Restriction versus Exercise

It’s not necessary to use caloric restriction as the sole means of achieving a caloric deficit. Exercise is a great way to add to the deficit. Some quick searches on the Internet suggests that men try to get 25% of their caloric deficit from exercise, while women aim for 50%.

So if you’re a male and you deficit for losing weigh is 610, the recommendation is that you do it through reducing caloric intake by about 457 calories and then burning an additional 153 calories through exercise. This helps retain muscle mass.


So there are a few ways to figure out your maximum sustainable daily caloric deficit from calorie restriction.

They all work, although the best is t use the calculation of figuring out your body fat percentage and then multiply how much fat you have in pounds by 31.

Remember that as you lose weight and get leaner, you’ll have to recalculate your daily calories.

As always, it comes down to where the rubber meets the road. If you’re quickly losing strength, your deficit is too big regardless of  what numbers you calculated and you need to bump it up.

It’s always going to take a bit of trial and error to dial everything in.


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