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Fat tissue is 13% lean mass
The lean part of fat
The lean part is the structural protein of the adipose tissue, plus some water. A person who gets a DEXA or InBody scan after losing 20 pounds and sees 2 pounds of lean mass gone has not necessarily lost muscle.
Water and organs
Body water loss registers as lean mass too, and the splanchnic tissues of the organs can shrink a little. As a result, it is normal for the average person to lose 25% or 30% of the weight they lose as lean mass, which doesn't mean skeletal muscle.
Who loses more lean mass
The share depends on starting body fat. As body fat goes down, the percentage of weight loss from lean mass goes up. Someone who is very obese has so much adipose tissue to draw on that there is very little reason for the body to metabolize lean tissue, while a lean person with higher-than-normal lean mass has far more of it at stake.
How does a DEXA scan estimate lean mass?
A DEXA scanner sends X-rays at two different energy levels through the body. Bone, fat, and lean tissue absorb the two energies in different ratios, and software uses those ratios to sort tissue into three compartments. The method assumes lean tissue holds a roughly constant share of water. Changes in hydration can therefore move the lean mass reading.
How do InBody scales measure body composition?
They use bioelectrical impedance. A weak electrical current travels more easily through water-rich tissue like muscle than through fat. The device measures resistance to the current and estimates body water, then converts it to lean and fat mass using equations built from reference populations. Drinking, eating, or sweating before a reading changes the result.
Why does the share of lean loss change with body fat?
Gilbert Forbes, an American pediatric researcher, described this relationship in the 1980s. His analysis of weight change data showed that the fatter a person was, the smaller the fraction of weight change that was lean tissue. The relationship is known as Forbes's rule. It follows from the larger store of energy available to be burned.
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