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BMI vs Body Fat: Is There a Better Way to Measure?


Article at a Glance

  • Body mass index (BMI) is commonly used to determine people’s overall health but is often an inaccurate representation of health on an individual level.
  • Body fat is a more accurate determinant of a person’s health than BMI.
  • Certain body fat measurements can be done at home, and others require specialized equipment.

People’s relationships with their body fat can be complicated, both for health and social reasons. Fat is an essential component of the body—we cannot survive without any body fat. Fats help store some crucial nutrients, including vitamins A, D, E, and K, and they are also involved in making hormones. But how much fat is the desirable amount? And what’s the best way to make that determination?

For years, researchers have used many metrics to determine if a person is underweight, healthy, overweight, or obese. Body mass index, or BMI, has been one of the most popular metrics…but should it be?

BMI as a Metric

BMI vs body fat: Sandwich with measuring tool on it

A person’s BMI is a metric that measures a person’s relative height to weight ratio. According to the CDC, a healthy BMI is between 18.5 and 24.9. A BMI between 25 and 30 is overweight, over 30 is obese, and under 18.5 is underweight [R][R].

BMI is useful in understanding population-based studies, but by itself does not paint a clear enough picture of an individual’s health.

Four Flaws of Using BMI

Using BMI as a marker of health has come under fire because it’s a poor indicator of health for the reasons below.

1. Inaccuracy For Athletes and Fit People

You may have read that muscle weighs more than fat. This idea is untrue—one pound of muscle weighs the same as one pound of fat [R]. The difference between the two is that muscle is denser than fat—i.e. one pound of muscle takes up less space than one pound of fat does.

Consequentially, different people can be at the same height, body weight, and thus BMI with very different body compositions.

A large amount of muscle on a person can increase their BMI based on the nature of the equation. The BMI may suggest, for example, that a very fit person is obese when their body fat percentage is in fact quite low. A large amount of muscle does not make a person unhealthy—in fact, the strongest and most fit people tend to live the longest [R]. Thus, BMI can lead to misconceptions about what an ideal weight should be in those with larger than average muscle mass.

2. Location of Fat

BMI also fails to detail the location of fat in the body, which has significant health implications. Generally, fat deposited in the abdomen and upper-body area indicates a higher risk of health complications than fat deposited in the lower body.

A study in women, for example, found that fat deposited in the thighs did not correlate with metabolic diseases like diabetes. However, abdominal fat was closely correlated with metabolic issues [R].

Similarly, a long-term study on men showed that a large waist circumference (representing abdominal fat) is closely related to an increased risk for diabetes [R]. Abdominal fat is also linked with several other serious health risks, including heart disease [R].

3. Differing Types of Fat

Similarly, BMI doesn’t quantify certain dangerous types of fat that can still be dangerous even at low levels relative to the overall fat mass. Visceral and liver fat, for example, are both predictors of metabolic disease but represent significantly less mass than subcutaneous fat [R][R]. A person can have a high level of visceral fat but little subcutaneous fat, and thus have the same BMI as a healthy person.

4. The Elderly

BMI guidelines may turn out to be inapplicable to the elderly, with some researchers suggesting they be scrapped for this age group.

First, BMI doesn’t take into account age-related fat distribution in elderly people. Second, studies show that being overweight by BMI standards was actually protective to the elderly. A BMI that was technically overweight or even mildly obese was associated with a lower morbidity rate [R].

Four Ways to Measure Body Composition

BMI vs body fat: Person holding measuring tape around stomach

There are better ways than BMI to determine if your amount of fat versus muscle is in a healthy range. Certain tests can help you get a good understanding of your body composition and help you figure out the best strategy for your health.

1. Bioelectrical Impedance Analysis

Bioelectrical impedance analysis (BIA) has proved to be a useful way of measuring overall body fat. BIA sends electrical signals through the body, which are conducted by water. Fat acts as an electrical resistor, so BIA can give a fairly accurate idea of how much total body fat a person has in certain populations.

A study showed that BIA is effective in measuring fat levels in people with a stable weight, but may be inaccurate in measuring the amount of fat a person has during a weight loss program [R]. Additionally, BIA may be inaccurate in measuring the total fat percentage in obese people [R].

Various BIA devices are available on the market, though ones used in a professional setting are likely to be higher quality than consumer models.

2. X-Ray Analysis

Dual-energy X-ray absorptiometry (DEXA) is useful in determining overall body composition, including fat composition.

DEXA is particularly useful because it can give specific information about individual parts of the body [R]. So, in addition to measuring overall body fat, DEXA gives a clearer picture of just where a person’s body fat is located. The scan takes less than 15 minutes on average.

Previously, individuals who were obese were limited by the size of the scanners and only half of the body could be scanned at a time. However, a study showed that two half-body scans were similarly accurate when compared to one whole-body scan [R].

3. Underwater (Hydrostatic) Weighing

Underwater, or hydrostatic, weighing is a method that measures body volume, body weight, and residual lung volume. It relies on the concept of displacement to determine a person’s body composition. Since muscle mass is denser than fat, a person with high levels of muscle mass will weigh more in the water relative to a person with a higher level of fat.

Hydrostatic weighing can face significant bias due to complications like air remaining in the lungs and improper utilization of the equipment [R]. Underwater weighing requires the subject to be fully submerged and exhale completely, which is difficult for some. Air in the lungs adds to buoyancy and mimics fat’s buoyancy effect. Newer methods have added extra measurements to reduce the incidence of skewed results.

4. Calipers

Calipers get mixed reviews. In general, calipers appear to lead users to underestimate the amount of body fat they have compared to more accurate assessments.

One study suggested this discrepancy was due to unmeasured amounts of non-subcutaneous fat [R]. Another study shed more light on the downside of calipers: In the study, using calipers led people to underestimate their body fat percentage by 21-45% [R].

Calipers are, however, the least expensive means of testing body fat, and if nothing else, can be good for tracking progress over time in a weight-loss or fat-loss program if the same calipers and test administrator are used for each measurement.

Moving Forward

BMI vs body fat: Nuts sitting in vintage scale

BMI is useful in large-scale epidemiological studies but falls short where individuals are concerned. Fortunately, progress marches ever forward and gives us new ways to understand ourselves and our bodies.

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