What is a body composition analyzer (BIA scale)?

In one line

Body composition analysis by BIA, the common gym scale test, sends a weak current through the body and uses how much the current is resisted to estimate what the body is made of.

What is a body composition analyzer (BIA scale)?
The machine you stand on barefoot and hold
At the gym you step barefoot onto a machine, hold the handles with both hands, wait a moment, and a result sheet comes out. This is body composition analysis, often called by a brand name in Korea. How does the machine find out what is inside your body?
In one line
A test that estimates body parts with a weak current
Body composition analysis splits your weight into parts such as fat, muscle and water. These gym machines send a weak electric current through the body to estimate how much of each you have. This method is called bioelectrical impedance analysis, or BIA.
How it works, part 1
Water carries current; fat does not
Body water has salts dissolved in it, so current flows through it easily. The non-fat part of the body, such as muscle, is about 73% water, so current passes well. Fat holds almost no water, so current does not flow through it well.
How it works, part 2
It measures how much the current is blocked
The machine sends a very weak current through metal plates touching your hands and feet and measures how much the current is blocked as it passes through. This blocking is called resistance (impedance). The more water and muscle a body has, the lower the resistance.
From resistance to numbers
A formula does the math
What the machine actually measures is resistance. A formula that combines the resistance with height, weight, age and sex works out body water and fat-free weight. Fat-free weight is subtracted from body weight to get the amount of fat, and that share is the body fat percentage.
So it is sensitive to water
Change body water and the numbers change
This method is very sensitive to the amount of water in the body. So the basic rule is to measure in a similar state each time. That means measuring in the morning after waking and before eating, and the machine used in a US public health survey called for 6 hours without food before testing.
Compared with other methods
Quick and easy, but an estimate
Other ways to measure body composition include a low-dose X-ray scan (DXA) and weighing the body underwater. Today research uses the X-ray scan as the reference. The electric method takes a few minutes and costs little, so it is widely used at gyms and at home.
When it is not used
People with electronic devices inside the body
People with an electronic device inside the body, such as a pacemaker, do not take this test. The reason is the risk that the current interferes with the device. In a US public health survey, pregnant people and people with metal implants were not tested either.
A common myth
Are the numbers on the sheet measured directly?
No. The machine does not see fat. It measures electrical resistance and puts that value into a formula to work out fat and muscle. So for the same person, the numbers change when body water changes or when the machine uses a different formula.
Summary
Body parts worked out from electrical resistance
Body composition analysis by BIA is a test that sends a weak current through the body and uses the resistance to estimate fat and muscle. It does not see fat; it calculates it, so numbers are compared only when they are measured under the same conditions. Related terms: body fat percentage, skeletal muscle mass, BMI (body mass index).