Lean mass
Lean mass is everything in a body that is not fat, muscle, bone, organs and water, and it is the part a deficit is meant to keep while fat is lost. IZeat records no lean mass; the two things it does record that bear on it are the day's protein and the day's calories, against the targets the customer set.
Lean mass is measured by the same scans and estimates as body fat, with the same margins. What a person can do about it sits on the intake side, protein high enough and the deficit not too deep, and that is what IZeat shows: grams of protein per day with their sources, and the calorie remainder. The body's answer is measured elsewhere.
The fear of losing muscle is what makes people watch protein, and protein is where a food record's sources matter most: an estimated chicken breast and a weighed one are different facts. IZeat names the source on each item so a customer at 150 g of protein knows how much of it was labelled or weighed. Whether the muscle stayed is for the scan or the mirror.
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