Mr Z

AI Prompt for Protein from Lean Mass: By a Coach

Francesco Zen, founder of ZTraining · Published 9 July 2026 · Updated 9 July 2026

The prompt

Copy-paste prompt, built from 18 years of coachingAct as my coach for setting my protein target correctly. Before giving me anything, ask me for: my height, current weight, a rough body-fat estimate (or an honest description of my body if I do not know it), my training level (bodybuilding-level, structured general gym training, active but casual, or mostly inactive), and my age. Once you have that: 1. Calculate my lean body mass: weight x (1 - body fat % / 100). Show me this number clearly. 2. Set my protein target from that lean mass figure, not my total weight, using roughly 0.7g per lb of lean mass as a baseline, scaled up for more advanced or structured training and down if I am mostly inactive or over 50. 3. Explain in one sentence why lean mass is the correct basis, not total bodyweight, so I understand the reasoning, not just the number. 4. Only count complete protein sources toward this target: meat, fish, eggs, dairy, whey, EAA. Do not credit incomplete sources like bread, rice, oats, or legumes, even though they contain some protein. 5. Tell me roughly how to spread that total across my meals so I am not trying to eat it all at once. 6. Remind me this number should be recalculated if my weight or body fat estimate changes meaningfully. Ask me the setup questions now, before giving me a target.

Why this prompt works

Fat tissue does not use dietary protein the way muscle tissue does, which is the entire reason lean mass is the correct basis for a protein target and total bodyweight is not. A formula based on total weight quietly assigns the same protein needs to someone at 15% body fat and someone at 35%, despite one of them carrying far more of the tissue that actually requires it.

The gap this creates is not small. At higher body-fat percentages, a total-bodyweight formula can overstate the real protein requirement meaningfully, which is expensive, unnecessary, and often what makes a nutrition plan feel harder to sustain than it needs to be.

A real-world example

Take someone at 100kg and 32% body fat. Their lean mass is well under 70kg, a large gap from the number on the scale. A protein target calculated from lean mass lands in a genuinely different place than one calculated from the full 100kg, and it is the lean-mass number that reflects what their body can actually use.

This is precisely the calculation most generic AI prompts and diet apps skip, defaulting to total bodyweight because it does not require a body-fat estimate.

Where a single prompt hits its ceiling

This calculation depends entirely on the accuracy of your body-fat estimate, and most people estimate it by eye. A meaningful misjudgement there shifts the lean mass figure and everything calculated from it, an error a one-off prompt has no way to catch.

It also freezes the moment you close the chat. As your body composition changes, this target should move with it, but nothing here tracks that unless you come back and recalculate manually.

Common questions

Should protein be based on total weight or lean mass?

Lean mass. Muscle tissue uses dietary protein, fat tissue largely does not, so basing the target on total weight overstates protein for anyone carrying meaningful body fat.

How do I estimate my lean body mass?

Multiply your weight by one minus your body-fat percentage. The harder part is getting an accurate body-fat estimate, since the lean mass figure is only as reliable as that input.

Is more protein always better?

No. Protein needs scale with lean mass and training demands, not with ambition. Beyond a certain point, additional protein does not add benefit and just displaces other useful calories.

Francesco Zen is the founder of ZTraining and ZLIFE, with 18 years of coaching experience working with executives across the UK, US, and UAE. He built Mr Z, an AI coach trained on his methodology to bring the same approach to a wider audience.