February 06, 2024

Are Body Composition Scales Accurate? What is BMI, BIA, and How to Use a Smart Scale Correctly?

BMI or the body mass index is simply a ratio of weight to height squared. BMI - not to be confused with BMR or basal metabolic rate (the number of calories a body needs to maintain life at rest) - was invented about 200 years ago by a Belgian-born sociologist, astronomer, and mathematician Lambert Adolphe Jacques Quetelet, who was looking for a tool to study populations, not individuals.

Various BMI calculators can be found online - for example this one by NHS - but generally, people with results below 18.5 are considered to be underweight, between 18.5 and 24.9 - healthy, from 25 to 29.9 - overweight, and over 30 - obese.

A BMI scale may be a useful screening tool, but it does not account for the body’s physical makeup or fat distribution. Because lean mass weighs more than fat, people like Dwayne Johnson and Arnold Schwarzenegger with BMI over 30 are incorrectly classified as obese. And as Bloomberg writes “Goodbye, Body Mass Index. You Won’t Be Missed” the medical community is now turning to alternative measures for diagnosing obesity and other health issues.

Bioelectrical impedance analysis or BIA is used in many modern smart scales for estimating body composition - body fat percentage, muscle mass, bone density, and a dozen other parameters. Such scales send low-level electrical currents through the user’s body, measure the resistance these currents encounter, and calculate the body composition parameters according to a formula. BIA-based scale manufacturers use different formulas and algorithms, but all require additional user information, such as gender, age, and height to be provided via a companion app.  

The accuracy of such formulas and estimations depends on the context. Studies show that body composition scales are less accurate in certain populations (ethnicity, age, fitness level matter) and are not accurate enough for people requiring precise body composition data. FDA recommends that DEXA (dual-energy x-ray absorptiometry) is used for measuring body fat and lean mass in patients with malabsorption, cancer, or eating disorders. Moreover, the accuracy of a body composition scale can be influenced by its calibration, the user’s hydration level and skin temperature, as well as their adherence to instructions.

What are the Most Accurate Ways to Measure Body Composition? 

The most accurate methods for measuring body composition are typically expensive, complex, and reserved for clinical or research settings: 

• DEXA is a widely accepted and precise method for measuring body composition. It uses X-rays to differentiate between bone, fat, and lean tissue as well as provides information on bone mineral density.
• Hydrostatic Weighing or underwater weighing involves submerging an individual in a tank of water and measuring their body density. This method is based on the principle that fat tissue is less dense than lean tissue.
• Air Displacement Plethysmography is like underwater weighing, but with air. It uses the BodPod machine made by Life Measurements Instruments.
• Medical-grade BIA devices are calibrated to higher standards, often use multiple frequencies and four electrodes (as opposed to two in consumer-grade BIA scales).  
• Skinfold Caliper Measurements focus on the thickness of skinfolds at various body sites. The sum of these measurements can be used to estimate body fat percentage. Precision is highly dependent on the skill of the person performing the test.
• Infrared Interactance is based on the different ability of fat and lean tissue to absorb infrared light. The accuracy of this method can be affected by skin pigmentation.
• Both MRI and CT scans can provide highly detailed information about body composition.

According to CDC (Centers for Disease Control and Prevention) anthropometry can also be used to assess body composition. The main anthropometric measurements are height, weight, head circumference, body mass index (BMI), body circumferences (waist, hip, and limbs), and skinfold thickness. To make the required measurements clinicians have to use well-calibrated and high-quality equipment: weight scale, stadiometer, knee and skinfold calipers, nonstretchable tape measure and others. 

So, Are Consumer-Grade Body Composition Scales Accurate? 

They are not accurate enough for medical purposes and are unlikely to accurately reflect the user’s actual body composition. However, smart scales still provide a certain level of insight and are good for tracking trends. In combination with a connected app, BIA-based smart scales can be effectively used to monitor changes and progress over time. For instance, users focused on building muscle can document their growth of muscle and protein mass with the help of a smart scale with a weight monitoring app, such as the Health Diary by MedM.

What Body Composition Parameters Can a Smart Scale Measure? 

Most smart scales will offer from six to sixteen body composition parameters, but not all are equally meaningful. Some provide more actionable insights than others. Below is a list of the most common body composition parameters supplemented by brief explanations of their significance:

1. Body Fat Percentage - the proportion of a person’s total body weight made up of fat tissue.
2. Muscle Mass - measures the weight of muscle tissue in the user’s body.  
3. Bone Density - represents the amount of mineral content in bones. Generally, the higher bone density – the better.
4. Water Percentage - indicates what proportion of the body weight is water and helps to assess the level of hydration.
5. Basal Metabolic Rate or BMR – is the amount of energy a body requires at rest, maintaining such basic functions as breathing and digestion.  
6. Body Mass Index or BMI – as discussed above is not directly measured, but rather calculated based on weight and height.  
7. Metabolic Age - is an estimate of the user’s overall metabolic health.
8. Protein Mass - accounts for the amount of protein, which is crucial for building and repairing tissues.
9. Body Water Mass – represents the total amount of water in the user’s body, including both intracellular and extracellular water.  
10. Subcutaneous Fat - is the fat located just below the skin.
11. Visceral Fat – is the fat surrounding internal organs and stored deep inside the body. This kind of fat is associated with increased health risks.

What Types of Fat Do Humans Have? 

Some types of fat are necessary for good health, and some are linked to higher health risks. This Harvard publication lists five types of fat in the human body: white, brown, beige, pink, and essential:

• Brown fat - helps to keep humans (especially babies) warm, it releases heat when stimulated by cold temperatures. The amount of brown fat does not generally increase with calorie intake, and people who are overweight tend to carry less of it. 
• White fat - mostly accumulates in the belly, thighs, and hips, secretes dozens of hormones, enzymes, and growth factors including leptin and adiponectin, which help to regulate blood sugar. When present in excess, however, the white fat cells can lead to insulin resistance and chronic inflammation.
• Beige fat - can be transformed into brown fat to also generate heat with exposure to cold temperatures or during exercise.
• Pink fat - present during pregnancy and lactation, responsible for producing and secreting breast milk.
• Essential fat - can be found in most muscles, organs, and in the brain, helps to regulate hormones, control body temperature, and assists in absorption of vitamins and minerals. When a person’s body fat falls below a certain level - about 5% in men and 10% in women - some of the body’s most essential functions may suffer.

How to Use a Smart Scale Correctly?

For getting the most accurate readings possible at home, we suggest users follow this simple guide:

• Placement - the scale should be placed on a flat and hard surface, so no carpets or uneven floors.
• Pairing and Setup - smart scales need companion apps. A compatible app is usually indicated in the user manual or can be found with the help of Google. Personal information, such as age, gender, and height will need to be provided at setup. Scales can have Bluetooth, cellular or Wi-Fi connectivity. MedM finds that Bluetooth connectivity gives users the most control and is the easiest to set up.
• Stepping on the Scale - electric currents are involved, remember? So no socks! Users need to step onto their smart scales with bare feet, avoid shifting and ensure that their weight is evenly distributed.
• Wait for the Measurement - smart scales need time to “run their magic” and calculate the results and transfer them to the connected app.
• Consistency - it is important to set up a weighing routine, stepping on the scale at roughly the same time each day. Experts agree that the optimal time for body weight monitoring is in the morning, right after a visit to the restroom.
• Calibration, Cleaning and Maintenance - recalibration, including zeroing or adjusting the scale may be required. The surfaces need to be cleaned and batteries replaced.

And to those who have read this blog post to the very end we say “thank you” and remind them, that MedM Health Diary is the most connected health monitoring app in the world, able to automatically capture data from 800+ smart health and wellness meters, including dozens of smart scales by different manufacturers.  

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