How Many Calories Can You Cut Before You Start to Lose Muscle?

You’re better off losing nothing than you are losing muscle.

Muscle is metabolically active tissue. There’s no point losing weight if your metabolism drops because you’re going to gain all the weight back (and probably more) by losing muscle.

We want to lose as much fat as possible whilst holding onto as much lean mass as possible. You want to be aggressive but not too aggressive because it starts to create more harm than good.

The entire theory is based around this study:

A Limit On The Energy Transfer Rate From The Human Fat Store In Hypophagia
https://pubmed.ncbi.nlm.nih.gov/15615615/

Basically, how much energy can your body fat provide from its fat reserves when you’re eating below maintenance calories (hypophagia)?

This is possibly one of the most important studies for those who are interested in losing body fat because it tells you exactly the maximum amount of food you can eat each day in order to maximise fat loss without losing muscle mass.

In other words, your fat reserves provide you with only a limited amount of calories per day and you have to provide the rest, otherwise you will lose lean tissue.

The study actually came from the Minnesota Starvation Experiment Data, which was titled The Biology of Human Starvation.

They had 36 volunteers who were given 1570 calories per day to eat for six months, and they lost 25% of their body weight.

Using this data, they created the following mathematical model:

“A limit on the maximum energy transfer rate from the human fat store in hypophagia deduced from experimental data of underfed subjects maintaining moderate activity levels and is found to have a value of (290+/-25) kj/kgd. A dietary restriction which exceeds the limited capability of the fat store to compensate for the energy deficiency results in an immediate decrease in the fat-free mass (FFM). In cases of a less severe dietary deficiency, the FFM will not be depleted.”

What this is saying is:

You will lose body fat if you eat enough calories to allow your body fat to fuel you.
You will lose fat and muscle if you do not eat enough total calories.

If you know exactly how much body fat you have, then you can know how much energy it can provide for you each day. If you make up the difference, you’ll lose fat without losing muscle. If you don’t, you’ll start losing muscle tissue instead.

Based on this study, every pound of fat can provide between 29–34 calories of energy per day. To simplify calculations, we’ll round it off to 30 calories per pound of fat.

You don’t need to guess your calorie target anymore. You can know it.

You do, however, need to know how many pounds of body fat you currently have, as well as your maintenance calories or total daily energy expenditure (TDEE).

Use these two calculators:

Let’s Look at Some Real World Examples

300lb man with 40% body fat
150lb woman with 30% body fat
200lb man with 12% body fat

Example 1: 300lb Man with 40% Body Fat

300lb x 40% = 120lb fat
120 x 30 = 3600 calories available from fat
TDEE (sedentary) = 2560 cal/day

This man’s fat reserves can fully fuel his daily needs. He could fast for a day and still have more than enough energy from stored fat alone. He is a great candidate for alternate day fasting.

Example 2: 150lb Woman with 30% Body Fat

150lb x 30% = 45lb fat
45 x 30 = 1350 calories from fat
TDEE = 1678 cal/day
1678 – 1350 = 328 calories minimum from food

She should eat at least 328 calories per day to avoid muscle loss. While that’s not a recommended daily intake, it shows she can still benefit from low-calorie days or a 5:2 intermittent fasting model.

Example 3: 200lb Man with 12% Body Fat

200lb x 12% = 24lb fat
24 x 30 = 720 calories from fat
TDEE = 2513 cal/day
2513 – 720 = 1793 calories from food needed

He must eat at least 1800 calories daily to avoid muscle loss. Alternate day fasting isn’t ideal here; a time-restricted feeding protocol is better.

So What About You?

Determine your body fat percentage using this calculator
Find your maintenance calories with this TDEE calculator
Multiply your fat in pounds by 30 to get your daily fat-derived calorie supply
Subtract that number from your TDEE to find your minimum daily calorie intake

If your result is zero or negative, you’re a candidate for alternate day fasting.

If your result is a positive number, eat at least that many calories each day.

Closing Thought

Fat loss is not about suffering—it’s about strategy. The more lean mass you preserve, the stronger your metabolism stays, and the longer your results last.

When you learn to understand your body—not just by how it looks, but by how it functions—you move from guessing to mastery. The goal isn’t just to lose weight. The goal is to lose fat—and never find it again.

Approach your transformation with intelligence, not desperation. Because the smartest fat loss strategy is the one that lets you keep your strength, your confidence, and your results for life.