How an InBody Scan Works and How to Get an Accurate Reading

An InBody scan takes about a minute. You stand on a platform, hold two handles, and a small electrical current you cannot feel passes through your hands and feet. From that, the machine reports body fat percentage, lean mass by arm, leg and trunk, a visceral fat level and your body water. Here is how a current turns into those numbers.

Water conducts, fat resists

The method is called bioelectrical impedance. Impedance is how much a tissue resists the flow of a current. Water with dissolved salts conducts electricity well. Lean tissue is roughly 70 to 75% water, so current moves through it easily. Fat holds very little water, so it resists.

By measuring that resistance, the machine estimates how much water is in your body. Because lean tissue holds water in a fairly steady proportion, total body water leads to an estimate of lean mass. Subtract lean mass from your weight and what remains is fat mass.

Two details make the InBody more useful than a basic bathroom scale with sensors:

• Segments. Electrodes at both hands and both feet let it measure each arm, each leg and the trunk separately, so you can see lean mass by region.
• Multiple frequencies. Low frequency current travels mostly through the water outside your cells. Higher frequency current also passes through cell membranes. Comparing them separates extracellular from intracellular water, which is where the ECW/TBW ratio comes from.

Why preparation matters

Since the whole method reads water, anything that shifts your water shifts the result. A large meal, a hard workout, a sauna, a big glass of water or dehydration can all move the numbers. That is the main weakness of bioimpedance, and it is manageable.

For an accurate reading:

• Scan at the same time of day each time, ideally in the morning.
• Do not eat for about 2 to 3 hours beforehand.
• Do not exercise or use a sauna before the scan.
• Use the restroom first.
• Stand for a few minutes before stepping on, so body fluids settle.
• Remove watches, rings and other metal.

For women, scanning at a similar point in the menstrual cycle each time avoids the water retention that comes before a period.

One safety note: people with a pacemaker or another implanted electronic device should not have a bioimpedance scan. During pregnancy the readings are not reliable, so it is not a time to scan.

What it is good for

InBody is less precise than DEXA for a single snapshot, but it is quick and inexpensive, so it is well suited to frequent tracking. Under consistent conditions, a monthly scan shows whether fat mass is falling and lean mass is holding. That trend is what we use to adjust a plan.

Same conditions, same machine, about once a month. That is how we get numbers worth acting on. You can pick your time right after you buy.

Book an InBody scan at HYBRD Health ($39)

This article is educational and is not medical advice.