I sit around a ~178 max, so 70% is about 125 — I aim at 120 as one number I can lock mid-session while staying in Zone 2, where fat oxidation peaks at Fatmax.

I protect a floor of two 40-minute Zone 2 sessions a week; I try to beat that when recovery allows.

How did lean mass hold?

Lean held on the Withings chart because I lifted about three days a week and kept protein high on a cut — a deficit with muscle protected, not work harder empty.

Is the treadmill's fat-burn zone right?

The easy-effort "fat-burn zone" is true on a percentage chart and it is the wrong question; I care about grams of fat oxidized per minute, which peaks at Fatmax around the top of Zone 2.

Why forty minutes per session?

Forty minutes is the floor I protect — long enough to spend real time at Fatmax, short enough that two sessions a week actually happen; a 150-minute hero session is how you skip Thursday, and the week is the unit, not one long grind.

What is Fatmax heart rate?

Fatmax is the exercise intensity where your body burns the most fat per minute — harder than a stroll, still easy enough to talk. Researchers pin it down with a lab test that measures your breathing while the effort steps up (Achten, Gleeson & Jeukendrup, 2002). I have not done that test, so my 120 bpm is an estimate from the age-based tables on this page, not a measured Fatmax.

How do you do Zone 2 for fat loss?

Easy walking or cycling I can do while talking, at least twice a week, stacked on the protein, lifting, and food habits already spelled out above — cardio supports the cut; it does not replace them.

Protocol · Fat loss

How I lost the fat without melting muscle.

People keep asking how I lost fat so fast. I wasn’t melting muscle — lean mass held, so almost all of the scale drop was fat. It accelerated once I stopped chasing the wrong “fat-burn zone” chart and ran a simple formula:

Protect muscle (protein + lift) + a sustainable weekly deficit + enough Zone 2 cardio in the Fatmax plateau that I can actually repeat every week.

This describes my own routine; it is not medical advice.

Quick answers

Why 120 bpm?: My max is around 178, so 70% is about 125. I aim for 120: one number I can hold mid-session, still in Zone 2, near where Fatmax should be.
How often?: I protect a floor of two 40-minute Zone 2 sessions a week; I try to beat that when recovery allows.
How did lean mass hold?: My lean mass didn't drop on the Withings chart. I lifted about three days a week and kept protein high while cutting.
Is the treadmill's fat-burn zone right?: The easy-effort "fat-burn zone" is true on a percentage chart and it is the wrong question; I care about grams of fat oxidized per minute, which peaks at Fatmax around the top of Zone 2.
Why forty minutes per session?: Forty minutes is the floor I protect — long enough to spend real time at Fatmax, short enough that two sessions a week actually happen; a 150-minute hero session is how you skip Thursday, and the week is the unit, not one long grind.
What is Fatmax heart rate?: Fatmax is the exercise intensity where your body burns the most fat per minute — harder than a stroll, still easy enough to talk. Researchers pin it down with a lab test that measures your breathing while the effort steps up (Achten, Gleeson & Jeukendrup, 2002). I have not done that test, so my 120 bpm is an estimate from the age-based tables on this page, not a measured Fatmax.
How do you do Zone 2 for fat loss?: Easy walking or cycling, at a pace I can still talk, at least twice a week. It sits on top of protein, lifting and food. Cardio helps the cut. It doesn't replace them.

N=1 Withings proof, April to August 2026: weight 175 to 160 lb, fat 23% to 11.7%, lean mass 77% to 86.7%. — Withings, April → end of August 2026. Same story as Protocol Results. Lean held, so almost all of the scale drop was fat.

Weight
175 lb160 lb−8.6%
Fat
23%11.7%−49.1%
Lean mass
77%86.7%+12.6%

Withings, April → end of August 2026. Same story as Protocol Results.

The formula

Three interventions. That is the whole stack.

01 · Cardio
Fatmax at 120 bpm
One number I can lock. Two 40-minute sessions a week as the floor.
02 · Muscle
Protein + lift
Hold lean while I cut. Three strength days. That is why the drop looked fast without costing capacity.
03 · Food
A sustainable weekly deficit
Eat a bit less than I burn. Macros in ratio. Nothing heroic. The week is the unit, not the session.

01 · Cardio

Fatmax at 120 bpm.

What I’m doing

Aim: 120 bpm — one number, not a range. Easier to lock mid-session than a band I have to babysit.
Floor: 2× / week × 40 min. That is the minimum I protect. I try to beat it when recovery allows.
Extras: A weekend session if there are no family plans. Sometimes after strength if I still have energy.
How: Low-impact so I can think. Podcasts every time. Treadmill walk by default, elliptical about 25% of the time, stairmaster sometimes.
Constraint: I would rather walk or jog outside, but there are no sidewalks here and it is flat, so BPM will not hold. I am looking at a weighted vest for outdoor Zone 2.

Why 120. Pick your number.

Here is the research path that got me to 120 — and a simple table so you can pick yours. Try it 3–4 weeks and tell me how it goes.

The treadmill still labels a “fat-burn zone” at an easy effort because a bigger share of those calories come from fat. That percentage chart is true, and it is the wrong question. At a stroll you might be mostly fat-fueled and barely working. The honest chart is grams of fat oxidized per minute. That peaks at moderate intensity — Fatmax — around the top of Zone 2.

Teaching sketch: treadmill fat-burn percentage versus grams of fat oxidized, peaking in Zone 2. — Teaching sketch, not my N=1. The left chart is what machines love. The right chart is the question I actually care about.

As effort climbs, carbohydrate takes a larger share of the mix. Zone 2 is the plateau where fat is still the main fuel and total work is high enough that the grams add up — and low enough that I can do it again this week. The lab version of that mix is respiratory exchange ratio: about 0.7 when you are oxidizing fat, about 1.0 when you are on carbohydrate.

Teaching sketch of fat versus carbohydrate fuel mix across effort, with Zone 2 Fatmax plateau highlighted. — Teaching sketch. The gold line is fat grams, not the fat percentage. It peaks in the Zone 2 plateau, then falls as carbohydrate takes over.

In-session fat grams keep rising if you stay on the machine for two hours. Recovery does not. A 150-minute hero session is how you skip Thursday. Forty minutes is the floor I protect: long enough to spend real time at Fatmax, short enough that two sessions a week actually happen.

Teaching sketch of session length versus repeatable weekly fat-loss benefit, with a 40-minute floor and a 150-minute hero session. — Teaching sketch. Forty minutes is the floor I protect. A 150-minute hero session is how Thursday disappears.

Pick your number.

Estimated max HR = 220 − age. Zone 2 is 60–70% of that. The target I use is 70% — then I lock one number. Mine sits around a 178 max, so 70% is about 125. I aim at 120. That is my best estimate of where Fatmax sits for me — not a lab-measured number.

Table A · by age

Zone 2 heart-rate targets by age using 220 minus age
Age	Est. max HR	Zone 2 (60–70%)	Target (70%)
30	190	114–133	133
35	185	111–130	130
40	180	108–126	126
45	175	105–123	123
50	170	102–119	119
55	165	99–116	116
60	160	96–112	112

Table B · if you know max HR

Zone 2 heart-rate targets from a known maximum heart rate
Max HR	Zone 2 (60–70%)	Target (70%)
170	102–119	119
175	105–123	123
~178 (mine)	107–125	120
180	108–126	126
185	111–130	130
190	114–133	133

~178 max → I aim 120. One number I can see on a watch without doing math mid-stride. It still sits in Zone 2.

Sources

Heart rate training zones
The 60–70% Zone 2 band, and why max HR estimates are a starting point.
Zone 2 training
Aerobic work, fat as principal fuel, and the usual 60–70% max-HR definition.
Achten, Gleeson & Jeukendrup, Fatmax (2002)
Open PubMed abstract: the intensity where fat oxidation peaks, often near the top of Zone 2.
Respiratory exchange ratio
RER ~0.7 is mostly fat. RER ~1.0 is mostly carbohydrate.

02 · Protect muscle
Protein + lift.
What I’m doing
Lift on the Base Protocol schedule — strength about three days a week. Keep protein high enough to hold lean while cutting. The exact grams stay on the protein + food pillar; this page is the reason that pillar exists on a cut.
Daily burn above intake, without building or protecting muscle, can pull from lean tissue. The goal is not “work harder empty.” It is a deficit with muscle protected. That is why the scale drop looked so fast without melting muscle.
Concept, not a lab printout. Daily burn above intake with nothing telling the body to keep muscle is how lean disappears with the fat.
Sources
- Strength training
  The signal to keep — and later build — lean mass while you cut.
- Weight loss
  Scale weight is a mix. Fat and lean do not have to move together.
03 · Weekly deficit
Sustainable. Nothing heroic.
What I’m doing
Eat a bit less than I burn. Macros in ratio, a small deficit — the same story as the nutrition pillar. Less junk. I do not live at 150-minute sessions to “earn” fat loss.
Session fat grams are not the scale. The week is. Zone 2 supports the deficit; food is still the main lever. Protect the 40-minute floor so the cardio actually repeats. Do not turn one long session into the whole plan.
When the deficit is food-first, planning around protein and macro ratios helps — try Macro Friendly Meals.
Sources
- Calorie deficit
  The week is the unit. Session fat grams are not the scale.
- Food energy
  Energy in versus energy out, without turning it into a religion.

Try your number.

If you want a starting point, take 70% of max HR — from the table, or from a number you actually know — and lock one bpm you can hold. Try it 3–4 weeks. Tell me how it goes. This is not a prescription. It is the formula that moved my N=1.

The rest of the subset, the products, and the six-month charts live on Protocol. The outcome I am holding is Lose fat, on Health goals. Please sign up for the daily newsletter of helpful information and tips at Immortals.

Last reviewedOctober 24, 2026