What Your Body Water Numbers Can Tell You

Most people look at body fat first on an InBody scan and skip the water section. That section holds one number worth understanding: the ECW/TBW ratio.

Two places water lives

About 60% of an adult's body weight is water, and it sits in two main compartments.

• Intracellular water (ICW) is inside your cells. Most of it is inside muscle cells, so this number tracks closely with how much muscle you have.
• Extracellular water (ECW) is outside your cells: in blood plasma and in the fluid that bathes your tissues.

Total body water (TBW) is the two added together. The InBody separates them by using several current frequencies. Low frequency current travels mostly through the fluid outside cells, while higher frequency current also passes through cell membranes.

What the ratio means

ECW/TBW is the share of your body water that sits outside your cells. In a healthy, well hydrated adult the balance is fairly stable. InBody treats roughly 0.360 to 0.390 as the expected range, and readings above about 0.390 are flagged as higher than expected.

A higher ratio means relatively more water outside cells than inside them. There are two ways to get there.

More fluid outside the cells. Inflammation draws fluid into the tissue around it. So can a recent injury, surgery, or a very hard training block that leaves muscles sore and swollen. Medical conditions that cause fluid retention, involving the heart, kidneys or liver, can raise it too.

Less water inside the cells. Because intracellular water lives mostly in muscle, losing muscle lowers it. As people age and lose muscle, the ratio tends to creep up even without any swelling.

Segments add detail

The InBody also reports the ratio for each arm, each leg and the trunk. A single high segment, such as one leg reading well above the other, can point to local swelling from an injury or a circulation problem in that limb. A whole body rise suggests something more general.

How we use it

The ratio is not a diagnosis. It is a signal that prompts a question. A high reading in someone who just ran a hard race usually settles within days as the muscle recovers. A high reading that persists, or that comes with swelling in the ankles or shortness of breath, should be looked at promptly and may need lab work. A slow upward drift over a year in someone who is not training points back to muscle loss.

It also helps us interpret the rest of the scan. If the ratio is high, lean mass may be overstated by extra fluid, so we read that scan with some caution and look at the trend.

That is the kind of detail a monthly scan picks up. 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.