Is BMI an Accurate Measure of Health?

We are a society obsessed with health metrics but, in recent years, body mass index (BMI) has become a controversial health assessment tool. While it’s still commonly used by health professionals, is BMI an accurate measure of health? Here are the facts you need to know.

What is BMI?

BMI, or Body Mass Index, is a calculation of a person’s weight relative to their height used to estimate their level of body fat. The higher the number the more body fat a person is estimated to have, the lower the number the less body fat a person is estimated to have.

BMI is used as a medical assessment tool as a predictor of risk factors for serious health problems. According to the data, both a high and low BMI can increase a person’s risk of serious health conditions; the higher a person’s BMI the greater their chances of developing conditions such as cardiovascular disease, high blood pressure, and diabetes, and the lower a person’s BMI the greater their chances of developing conditions such as malnourishment, bone loss, decreased immune function, and anemia.

The standard BMI formula is intended for use in adults only, specifically individuals over 20 years old, as kids and teenagers are in a constant state of growth. More recently, BMI-for-Age Growth Charts were developed to calculate BMI for ages 5 to 19 to help healthcare providers track growth and optimize care for children and adolescents.

How is BMI calculated?

BMI is calculated by dividing weight by height and can be calculated by metric or imperial measurements (1).

• The metric BMI formula is: Weight (kg) / [Height (m)]2
• The imperial BMI formula is: Weight (lb) / [Height (in)]2 x 703

The calculation of your BMI will classify you into one of the following weight status categories (2):

• Underweight: BMI is less than 18.5
• Normal Weight: BMI is 18.5 to 24.9
• Overweight: BMI is 25 to 29.9
• Obese: BMI is 30 or more

The obese category is further divided into Class I Obesity (BMI of 30-34.9), Class II Obesity (35-39.9) and Severe Obesity (40+).

Uses of BMI

BMI is primarily used as a health screening tool commonly used by doctors, personal trainers, and other health care professionals. It is used to gauge body fat, assess health risks, and even used to determine eligibility for certain medical procedures and surgeries.

Scientific researchers in large-scale population research tend to use BMI as a quick and easy way to calculate body fat, as it is less costly and invasive than other methods such as calipers and hydrostatic weighing. Insurance companies also use BMI as a metric to determine the risk level associated with insuring a person for different types of care.

While BMI does provide broad data for estimating body fat percentage at a population level, it does have limitations at the individual level.

Limitations of BMI

Here are some of the biggest concerns and problems with BMI:

1. Doesn’t account for body composition.

While BMI uses body weight as a metric, it doesn’t account for body composition, or the proportion of fat, muscle, and bone that make up body weight. BMI does not discriminate between muscle mass, which we want to build and maintain for health and longevity, and fat mass, which we generally want to keep low for optimal health (3, 4, 5, 6).

An arguably better metric for assessing obesity, although not perfect, is body fat percentage, which can be tested via more accurate methods such as skinfold calipers, dual-energy x-ray absorptiometry (DXA), hydrostatic weighing, or air displacement plethysmography (Bod Pod) (7).

While there isn’t necessarily an “ideal” body fat percentage, having too much body fat, particularly subcutaneous fat, increases your risk of various health complications (4, 5, 6, 8). A normal, healthy body fat percentage in the general population is 18-24% in adult men and 25-30% in adult women (9). These percentages are based on population-level statistics; however, they can vary at the individual level based on lifestyle factors.

2. Doesn’t account for fat distribution.

Body fat distribution is the pattern of fat deposits throughout the body, which can be a key indicator of health risk, and is not accounted for in BMI.

In addition to essential body fat, the minimal amount of fat necessary for normal physiological function, fat is primarily stored as central adiposity (around the trunk and upper body) and peripheral adiposity (around the hips and lower body)(10). Central adiposity includes visceral fat, the fat surrounding the abdominal organs, and abdominal subcutaneous fat.

While the majority of body fat is subcutaneous, the fat that you can pinch on your belly, thighs, and arms, visceral fat, stored deep within the abdominal cavity, surrounding the heart, liver, intestines, and other organs, is the most dangerous. High levels of visceral fat increase your risk for health issues such as diabetes, heart disease, stroke, and some cancers (11, 12, 13).

Recent research has suggested that abdominal adiposity, defined by waist-to-hip ratio and waist-to-thigh ratio, is more predictive of cardiovascular disease than BMI measurements (14). Moreover, a meta-analysis confirmed that visceral fat is a strong predictor of negative health outcomes, but a reduction in total abdominal fat may occur without any change in BMI (15).

Not only does this mean that certain individuals can fall into the “obese” category due to high muscle mass and low visceral fat mass, but that individuals can fall into the “normal weight” range due to low body weight but with high visceral fat mass and have an increased risk of health problems that have the potential to be overlooked.

3. Doesn’t account for sex or age.

BMI doesn’t account for sex, as it was developed from data on non-Hispanic white men, and is therefore not a completely reliable data point for women.

Women naturally have a higher body fat percentage than men with different storage regions: men are more prone to develop visceral body fat, while women predominantly have subcutaneous body fat (16). Estrogen levels in women cause increased fat storage in the hips, thighs, and breasts, and the prominent role of estrogen influencing fat composition is further seen in women as hormone changes after menopause cause an increase in abdominal adipose tissue.

Moreover, even if body weight remains the same, age tends to cause considerable changes in body composition. Young men and women generally have more muscle mass than older adults who tend to have more body fat and less muscle. While maintaining muscle mass is recommended for optimal health and longevity, this natural decline in muscle mass and bone density is not accounted for in BMI, which can underestimate body fat in the elderly.

4. Doesn’t account for ethnicity.

Large variations in body composition by ethnic groups have been found, as both body fat percentage and muscle mass can vary widely by race and ethnicity.

Studies have found that black individuals have less body fat and more lean muscle mass than other ethnicities with the same BMI, while individuals of Asian descent tend to have less muscle mass and higher central body fat distribution (17, 18). This leads to an overestimated risk in black individuals and an underestimated risk for those of Asian descent.

5. Doesn’t account for other risk and lifestyle factors.

BMI does not account for health risk factors such as high blood pressure, high blood sugar, family health history, poor diet, inactivity, and tobacco and alcohol use.

Not only is possible to be within the “normal weight” range and have many healthy risk factors, but it is also possible to be in the “overweight” range with no risk factors.

Is BMI accurate for athletes?

Due to the lack of consideration of body composition, BMI is not necessarily a reliable metric for assessing strength athletes, muscular individuals, or bodybuilders. These individuals can have a high BMI due to greater muscle mass, despite having low body fat and not considered overweight or obese (19, 20).

Is BMI accurate for women?

Overall, the physiological differences in men and women are not accounted for in BMI calculations. Women naturally store more body fat than men and post-menopausal women have an increase in visceral body fat mass compared to pre-menopausal women. Moreover, Hispanic and black women had significantly higher average BMI than white women despite having lower body fat (21).

So, is BMI accurate?

Although BMI is useful to assess population-level trends, it isn’t an accurate metric of health risk at an individual level on its own.

While it is a quick, accessible, and low-cost measure of obesity, and if you are obese according to BMI, you are most likely obese according to body fat percentage, there are better methods to measure body fat percentage overall (22).

Although BMI can be used as a health screening tool, it should not be used it should in isolation to determine overall health status or as a determinant of healthy weight. BMI suffers from both overclassification and underclassification of weight and health as it does not account for body composition, fat distribution, age, sex, ethnicity, and other risk and lifestyle factors.

While the World Health Organization (WHO) still recommends BMI as a health metric, in recent years the American Medical Association (AMA) has adopted a new policy that BMI alone is not enough to assess health and weight, and that it be used in conjunction with other measures of risk (23).