Lean Body Mass Calculator (Boer, James, Hume)

Real-world use cases

• Set a protein target that ignores body fat

  You weigh 90 kg but carry real fat, so prescribing 2 g of protein per kg of total weight would push you to 180 g a day. Compute lean body mass first, get say 68 kg, and 2 g per kg of LEAN mass is 136 g. That is the figure sports dietitians actually use, and it stops a heavier lifter from over-eating protein on the theory that all of their weight needs feeding.

• Check a weight-based medication dose

  Several anaesthetic and chemotherapy agents are dosed to lean mass, not total weight, because fat tissue barely takes up the drug. The Boer LBM is the exact input the carboplatin Calvert formula expects. Punch in height, weight and sex, read the Boer number, and you have the lean-mass figure to sanity-check what a dosing tool produced before it goes near a patient chart.

• Tell muscle loss from fat loss during a cut

  You dropped 4 kg over a diet phase and want to know whether it was fat or hard-won muscle. Enter your before and after numbers with your measured body-fat percentage each time. If lean body mass barely moved while total weight fell, the cut worked; if LBM dropped two or three kilograms, you cut calories too hard and are burning muscle.

• Compare three formulas before trusting one

  A single LBM number hides how uncertain the estimate is. For a very muscular or very lean build, Boer, James and Hume can disagree by several kilograms. Seeing all three at once tells you when the spread is small enough to trust the average and when it is wide enough that you should go measure body fat directly with a scan.

Common pitfalls

• Using total body weight for protein math. Protein guidelines of 1.6 to 2.2 g per kg are written against lean mass, not total weight. A 100 kg person at 30% fat has 70 kg of lean mass; feeding 2 g per total kg gives 200 g instead of 140 g, a needless 60 g overshoot every day.

• Picking the wrong sex. The male and female coefficients differ in every formula because women carry more essential fat. Selecting male for a female body inflates LBM by one to three kilograms, which then distorts a protein target or a medication dose downstream.

• Trusting one formula at the extremes. For a very lean or very heavy build, Boer, James and Hume can disagree by four kilograms or more. Reading only one number hides that uncertainty. When the spread is wide, measure body fat directly with DEXA or a BodPod instead of guessing.

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