Modelling your body and where fat comes from.

Before reading this section I recommend projecting the maximum speed at which you could physiologically lose fat, according to science, using this free Leanrr widget.

  • A reasonable sub-goal is cutting 1% of fat which is how the result is laid out.

  • Note how may pounds you could lose in ~30 days by adding up the duration column.

The Mechanics Of Fat

Your body needs a certain amount of fuel aka food to function daily. It also needs materials to build and repair with. It prefers that your fuel consists of carbohydrates (e.g.: potatoes) and fats (e.g.: avocados) while structural materials primarily consist of protein (e.g.: chicken).

Fuel is converted to energy to power moving, thinking, pumping blood, processing toxins in the liver, and much more. The unit of measure for energy is a calorie. Food is labeled with the amount of energy it contains:

pure protein bar = 200 calories

egg = 80 calories

Energy Excess

If you ingest more than the necessary amount of fuel and structural material for the day the excess gets stored in fat cells. Specifically, excess carbs/proteins in the bloodstream are converted to fat. And excess fat in the bloodstream is then stored inside fat cells. The key conversion is then:

3500 calories = 1 lb of fat

If you're under the influence of alcohol, any fat in the food you eat is immediately stored because your body prioritizes processing the toxins from alcohol rather than utilizing fat. Try to avoid fatty foods in those cases.

Energy Deficit

If you consume less than the necessary amount of fuel for the day your body taps into your fat stores to mobilize energy. This is fat loss. If your body can't get energy quickly enough from your fat it starts breaking down your muscle and converting it into energy. You don't want that as it gives the infamous skinny fat look and is generally not good for your body as it perceives it as stress.

Even if there is enough fuel but not enough structural material aka protein your body will break down muscle to use it in other places like your brain. That's why it's important to ensure your deficit never exceeds the amount of fuel your body can mobilize from fat and to always eat at least 1g of protein per pound of weight at a minimum. We'll walk through how to calculate the maximum amount of fat your body can utilize in a later section.

Lastly, if you're in a perfect caloric deficit so only fat stores are utilized as energy, eventually your body can decide to preserve fat and switch to using muscle as fuel instead. This is evolutionary biology. When your body senses a deficit of energy for prolonged periods of time and a lack of heavy exertion, it decides you'd have a higher likelihood of survival if you lost some of your muscle. It's able to leverage muscle as fuel while storing extra fat. On top of that, it takes extra energy to maintain muscle so it kills two birds with one stone by eliminating it. To avoid this fate you need to lift regularly to signal to your body not to down-regulate muscle.


  • Create a deficit to lose fat.

  • Lose 1 lb of fat per 3500 calorie deficit.

  • Make sure the deficit is not big enough to result in muscle loss.

  • Lift to signal to your body during prolonged deficits that you still need your muscle.

Approximating Fat Loss

Let's say you evaluate your body fat percentage and decide your goal is to lose 40 lbs of fat to hit your goal body fat percentage. That means you have to burn: 40 lbs * 3500 calories per lb = 140,000 calories

So you'll need to create a 140k calorie deficit in your body over some period of time for your body to mobilize this energy from your fat stores and utilize it.

Cutting Too Fast

As mentioned before, your body can only mobilize a limited amount of fat per day. I'll use a gross oversimplification to explain the intuition behind this. Let's say you have 100 fat cells. Then your body can mobilize about 100 units of fat for energy. But let's say you only have 10 fat cells. Then your body can only mobilize 10 units of fat.

The pounds of fat you have in your body roughly correlates with the number of fat cells you have. The more fat you have the more fat your body can mobilize in a day. So the higher your body fat percentage is the faster you can lose fat. The leaner you are the less fat you can lose per day.

If you exceed the amount of fat your body can mobilize as energy it works through a couple of other energy sources and eventually starts tapping into your muscle looking for fuel. You don't want that. This should only be a concern when you're in the lean and healthy ranges of <20% bf for men and <30% bf for women. Above that your body can usually generate enough energy for the day even if you didn't eat.

We don't use unconventional methods for losing weight such as fasting more than 16 hours a day in the Leanrr methodology since the idea is to make losing or maintaining weight indistinguishable from regular eating.

But a famous example of only using fat stores as fuel for a prolonged period of time is Angus Barbieri's 392 day fast. He had a really high body fat percentage so his body was able to cover his daily energy needs using his fat stores. Obviously I'm not recommending doing this but it's an interesting case study.

Cutting Too Slow

On the flip side, if you don't cut as much fat as possible per day you'll be unnecessarily prolonging the amount of time you have to eat in a deficit which is not fun. The results you have weekly will also not be as noticeable meaning you won't have an important positive feedback signal. It's the same amount of work to cut at 50% speed in one day and 100% speed. So if you have to do 100 days of work cooking and maintaining a deficit versus 50 days why not just go at max speed?

You can approximate the max amount of fat you can lose using the equations in the following sections or you can use Leanrr which combines Bluetooth scale weigh ins, bioimpedance measurements, Fitbit sensor data, and any available DEXA scans to calculate exactly what the tipping point for you is on a daily and weekly basis.

Cutting Just Right

You can project how fast you could go from your current body fat to a goal bf% using this free Leanrr widget. The math to calculate this yourself is covered in the next section:


Total Daily Energy Expenditure (TDEE)

Total Daily Energy Expenditure (TDEE) is all the energy you use in one day to stay alive and move around expressed in calories. You breath out the byproducts of that energy after it's used when you exhale.

Everything you consume in excess of your TDEE is stored as fat in your body. If you don't eat enough energy to power your day you end up in a deficit and your body "eats" your fat to generate additional energy to compensate. Your TDEE can vary greatly day to day depending on how active you are that day. Estimating TDEE is tricky business but we'll cover the best methods to do this.

A clear differentiator of using the Leanrr app versus this guide is there's a powerful algorithm that's constantly calibrating your daily TDEE by processing all incoming streams of data from your weigh ins, Fitbit sensors, and food you track. This allows for realtime feedback so you can make decisions about how much more to exercise or how much you need to eat to hit max fat loss that day. Obviously this is not something I can show you how to recreate in this guide because it required machine learning, data science, and programming to create but it is available in the app.

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