By Dr. Asnat Raziel, bariatric surgeon specializing in obesity treatment, metabolic health and healthy longevity; Director of the Multidisciplinary Center for Obesity Treatment, Assia Medical, Assuta Ramat HaChayal, Tel Aviv
Tracking body composition: why weight alone isn’t enough
Bioimpedance body composition testing helps you understand what a change in weight is made of: fat, muscle or fluid. The scale shows only one number. A body composition test, on the other hand, separates fat tissue from lean mass. The most common method in the clinic is bioelectrical impedance, or bioimpedance. It’s quick, inexpensive and noninvasive, but you need to know how to measure and how to read it.
What the scale doesn’t tell us
Let’s say you lost three kilograms in a month. Some of it may be fat, some fluid and some muscle. Each of these affects your health differently. Losing fat, and especially the visceral fat in the abdomen, improves your metabolic health. Losing muscle, on the other hand, hurts strength, function and metabolism.
That’s why I explain to my patients that body composition matters more than weight. Also, two people at the same weight can be in completely different health. One may be muscular and active, while the other has little muscle and a lot of fat.
How a bioimpedance body composition test (BIA) works
In a BIA test (bioelectrical impedance analysis), the device sends a very weak electrical current through the body. You can’t feel it. Each tissue resists the current to a different degree. Lean tissue contains a lot of water and conducts well. Fat tissue, by contrast, conducts poorly.
It’s important to understand what the device actually does. It doesn’t see fat or muscle. It measures resistance to the current and uses it to estimate the amount of water in the body. Then it uses a formula to calculate lean mass, and it calculates fat as the difference from your weight. According to the ESPEN review of the test’s principles, the calculation is reliable only when you use a formula validated for the right population. It also holds only for people without a fluid or electrolyte imbalance.
Lean mass isn’t just muscle
Lean mass includes muscle, but also water, internal organs and bone. So a drop in “lean mass” on the report doesn’t necessarily mean you’ve lost muscle. It can also reflect a loss of fluid.
Bioimpedance body composition: what throws off the result
Because the test relies on water, any change in fluids changes the result. In a study published in the International Journal of Medical Sciences, 95 healthy young people drank 600 ml of an isotonic drink before a repeat measurement. After half an hour, average body fat percentage rose from 21.9% to 22.4%. The visceral fat estimate also went up, from 55.4 to 58.1 cm². In other words, a single drink changed the report, even though the body hadn’t changed.
The researchers concluded that you shouldn’t drink for at least 90 minutes before the measurement. They also noted standard rules for preparing for the test:
- Don’t eat or drink for about four hours before the measurement.
- Don’t exercise in the 12 hours before the test.
- Don’t drink alcohol in the two days before the test.
- Empty your bladder about half an hour before the measurement.
Moreover, the ESPEN guidelines for clinical use state that in severe obesity or abnormal fluid states, you shouldn’t rely on the test routinely. Tracking over time is possible within a BMI range of 16 to 34, without a fluid disorder, but even then you need to interpret it with caution.
BIA, DXA and calipers: what’s the difference?
There are several ways to estimate body composition, and each one measures something different:
- Bioelectrical impedance (BIA): fast and widely available, and well suited to repeat measurements. It’s sensitive to your fluid status.
- DXA: a low-dose X-ray scan at two energy levels. It distinguishes between fat, lean tissue and bone, and researchers use it as a reference method.
- Calipers: a measurement of skinfold thickness. It estimates only the fat under the skin, and it depends heavily on the examiner’s skill.
There are also other imaging methods. For example, I’ve written about measuring body composition with ultrasound after sleeve gastrectomy. The key rule is simple: compare only measurements taken with the same method, and preferably on the same device.
Bioimpedance body composition tracking during obesity medication
New obesity medications lead to substantial weight loss. However, some of the weight lost isn’t fat. According to a review on preserving muscle during drug treatment, lean mass, including skeletal muscle, made up 20% to 50% of total weight loss in the studies. After one to one and a half years, that comes to about 6 kg, or roughly 10% of lean mass.
That’s why tracking body composition is especially useful during treatment. It shows whether the loss is coming mainly from fat. If lean mass is dropping quickly, that’s a sign to strengthen your diet and your workouts. I’ve covered this in more detail in my article on lean body mass on GLP-1 drugs.
The two main tools for protecting muscle are a protein-rich diet and regular strength training. It’s worth reading up on protein intake during weight loss. At an older age, this matters even more, which is why I’ve also written about building muscle after 60 and about sarcopenia and obesity.
How to measure correctly, step by step
First, pick a fixed time, preferably in the morning. Second, come fasting and with an empty bladder, without exercising the day before. Next, stand barefoot on the device, with clean, dry feet. Finally, don’t measure when you’re sick, after a night of heavy drinking or after a heavy, salty meal.
Beyond that, women should note when the measurement falls in their menstrual cycle. That makes it easier to understand an unusual jump in fluids, if there is one.
Bioimpedance body composition: how to read the numbers
A single measurement says little. What matters is the trend over weeks and months. So it’s worth measuring every four to eight weeks, under the same conditions. A small change between two measurements may come from fluids alone.
Here’s how I read a good report: weight goes down, body fat percentage goes down, and lean mass barely changes. On the other hand, if weight drops and lean mass plunges, it’s time to review the diet, the protein and the workouts. In any case, you should interpret the numbers together with waist circumference, strength and function.
A dietitian works in our clinic. In addition, when needed, I refer patients to a multidisciplinary mind-body team: dietitians, psychologists, holistic coaches, naturopaths and complementary medicine practitioners, as well as Pilates and yoga instructors and experts in stress-reduction techniques.
Bottom line
Tracking body composition with bioimpedance is a useful tool, as long as you understand its limits. The test estimates water, and from that it calculates lean mass and fat. That’s why drinking, eating and exercise change the result. Measure under the same conditions, on the same device, and follow the trend rather than a single number.
The information in this article is general and does not replace personal medical advice.
Questions and answers
Is a bioimpedance body composition test accurate?
It provides an estimate, not a direct measurement. In healthy people with normal fluid balance, and with an appropriate formula, it works well. In severe obesity or fluid disorders, accuracy drops.
How often should I track my body composition?
Usually once every four to eight weeks is enough. Daily measurements are confusing, because fluid shifts affect them.
Why did my body fat percentage go up even though I lost weight?
Sometimes it’s a fluid fluctuation, for example after drinking or exercising. If the trend continues, you may have lost lean mass, and it’s worth getting advice.
Is a home scale that measures body fat enough?
A scale like that works on the same principle of bioelectrical impedance. It can show a trend, as long as you measure under the same conditions. However, don’t compare its numbers with a test on a different device.
Sources
Bioelectrical impedance analysis – part I: review of principles and methods
Bioelectrical impedance analysis – part II: utilization in clinical practice
