Reading Your First Scan

Lean mass is everything in you that is not fat: muscle, organs, skin, bone, and the water in all of them. Your report may call it lean mass, lean body mass, or fat-free mass. A DEXA report often lists bone on its own line; an InBody may print skeletal muscle mass separately as well. On the example it reads 128.0 lb.

It leads for two reasons. It is the part of you worth keeping: muscle tends to shrink with age when nothing asks it to work, and a diet that cuts too hard can take some of it along with the fat. And lean mass is where most resting-rate formulas start, so it sets the rest of your calorie math.

Read one lean mass number gently. It includes water, and water moves. A salty dinner, a hard workout, or a scan at a different hour can shift it by a pound or more, mostly on bioimpedance machines, with no change in muscle at all. What matters is the change between two scans taken the same way.

Body fat percentage is fat mass divided by total weight. It is the number people remember, and the one that varies most between machines.

Each scanner measures one thing and estimates the rest. DEXA passes two low-dose x-ray beams through the body and sorts what it sees into bone, fat, and lean tissue. InBody and other bioimpedance devices send a small electrical current through the body, estimate body water from how it travels, and work out lean mass and fat from that. The Bod Pod measures the air your body displaces in a sealed chamber, which gives your volume and density, and an equation turns density into body fat.

So the same person can read several points apart on two machines on the same morning, and neither machine has to be broken. The rule that follows is simple: compare a scanner only with itself. If your first scan was a DEXA, read your next DEXA against it, not the InBody at the gym.

The percentage also moves when the rest of you moves. Gain a little lean mass and body fat % drops slightly, even if your fat did not change. That is one reason to follow fat mass as well.

Fat mass is the same fat, written in pounds: 44.0 lb on the example. It is often the clearer line to follow over time, because it does not depend on what the rest of you weighs.

Picture a second scan, 90 days later, that reads 172 lb again. On a bathroom scale, nothing happened. Now say fat mass reads 42.5 lb and lean mass 129.5 lb. The scale missed 1.5 lb of fat lost and 1.5 lb of lean mass gained, and a weight-only plan would see neither. Those are example numbers; yours could move either way, or not at all.

A mark is an estimate until the rescan.

Some reports also print visceral fat, the fat around the organs, as an area, a weight, or a level. Each scanner estimates it its own way, so compare it only with the same scanner. If yours is flagged, bring it up with your doctor.

Resting rate, often printed as RMR or BMR, is roughly what your body burns in a day lying still. Many reports print one, and most of those are estimated: the scanner takes your lean mass and runs it through a published formula. That is why two people of the same weight can get very different resting rates.

A measured resting rate comes from a separate test called indirect calorimetry. You lie still and breathe into a mask or under a clear hood while a machine reads how much oxygen you use. If you had one, the report will say so, and a measured number is a better starting point than any estimate.

On the example, the Katch-McArdle formula turns 128.0 lb of lean mass into about 1,620 calories a day. Your total daily burn is higher, because you also move, digest, and stand. The calculator multiplies resting burn by an activity level to estimate that total, often called TDEE.

A mark is an estimate until the rescan.

Treat resting burn as a floor. Eating below it for weeks at a time makes lean mass harder to keep, which is why a FitNoggin calorie target never goes under it.

A second scan is only worth its cost if it can be compared with the first. These habits make that likely:

• Same scanner, and the same location if you can manage it.
• Same time of day. Early morning, before breakfast, is the easiest to repeat.
• Same eating and drinking the evening before: no big meal, no alcohol, no extra salt.
• No hard workout in the day before the scan.
• Use the bathroom just before.
• Light clothing with no metal, the same kind each time.

Then give it time. Lean mass changes slowly, and many people rescan about 90 days after the first scan. Sooner than that, a scan mostly measures water and timing.

Before you put the report away, write your baseline down: the date and hour, the scanner, and the five lines from this guide. That page is what your next scan gets read against.

This is education, not medical advice. Talk to your doctor before a big change to how you eat or train, and about anything on your report that worries you.