Body mass index (BMI) is a tool researchers often use to track obesity at the population level.

Doctors and other healthcare professionals also use it to assess people’s health.

However, the medical community has raised concerns over using BMI as a measurement of obesity and health due to its significant limitations, particularly in assessing the health of Black and Hispanic people, among other populations.

Nevertheless, some health organizations recommend that people stay within a certain BMI range to reduce their risk of disease.

This article explains BMI and how it relates to women’s health.

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Adolphe Quetelet, a Belgian astronomer and statistician, developed BMI 180 years ago. However, BMI didn’t gain much popularity among researchers until epidemiologist Ancel Keys identified it as an effective way to track body fat proportions at a population level (1).

BMI uses a person’s height and weight to estimate their body fat.

Issues with BMI

Using BMI as a marker of health is problematic for a number of reasons.

Even though some health professionals use BMI to categorize individuals based on body fat, it doesn’t actually take body fat or lean body mass into account. This means BMI doesn’t recognize the difference between muscle and fat (2).

For this reason, BMI might categorize someone with a very high percentage of muscle mass as having excess weight or obesity, even if they have low body fat levels (2).

Another shortcoming of BMI is that it doesn’t indicate where fat is stored on the body. Therefore, it doesn’t account for variation in fat distribution between sexes or different body types. It also doesn’t consider age-related decline in muscle mass.

The system may classify a small-framed person in a healthy BMI range, even if they have a significant amount of belly fat. However, this person may actually be at risk because belly fat is significantly linked to many chronic diseases and early death (2, 3).

In other words, it’s possible that a person with a “healthy” BMI may actually be at significant risk of disease, surgical complications, and increased mortality (4).

Additionally, health is multidimensional. There are many factors a person should consider when assessing their overall health and risk of disease.

Experts argue that BMI is much too simplistic a measure to use as an indicator of health (1).

Summary

BMI doesn’t take body fat or lean body mass into account. It also doesn’t consider where fat is stored on the body. Because of these issues, it may provide misleading health information for some people.

Despite its significant shortcomings, BMI is one of the most effective, easiest, and cheapest options for researchers to track obesity at the population level, which is extremely important for medical research (1).

Additionally, a high BMI is correlated with higher body fat levels, and studies have shown BMI can be an accurate way to determine disease and mortality risk in individuals (5).

People whose BMI classifies them as having obesity have been shown to be at a significantly greater risk of early death and developing chronic diseases, including heart disease and kidney disease, than people within the “normal” BMI range (5, 6, 7, 8, 9).

People whose BMI classifies them as underweight are also more at risk of early death and developing disease (10, 11).

Plus, even though people should not view BMI as a reliable measure of health on its own, healthcare professionals can use BMI in combination with other diagnostic tools to assess a person’s health and monitor their risk of disease.

Other tools include waist circumference and laboratory values such as cholesterol levels.

Summary

BMI has significant flaws. However, it is an important tool for researchers, and healthcare professionals can use it in combination with other measures of health to assess a person’s risk of disease.

BMI does not distinguish between men and women. It’s a simple formula that uses height and weight to calculate a number meant to represent a person’s body fat levels.

These numbers fit within the following categories (12):

BMI rangeClassification
less than 16.5severe underweight
less than 18.5underweight
18.5–24.9normal weight
25.0–29.9overweight
30.0–34.9obesity class I
35.0–39.9obesity class II
40 or greaterobesity class III

White, Black, and Hispanic women

BMI does not inherently distinguish between people of different genetic backgrounds. However, as this article will discuss later, different BMI cutoffs have been developed for Asian and South Asian populations.

White, Black, and Hispanic women are considered to be at a “healthy weight” when their BMI lies between 18.5 and 24.9.

For example, according to the Centers for Disease Control and Prevention (CDC), a woman 65 inches (1.65 meters) tall would fall within a normal BMI range if she weighed 111–150 pounds (50–68 kg) (13).

However, women differ in body shape and body composition.

For example, research shows that Mexican American women tend to have a different body fat distribution than white or Black women. Additionally, Black women tend to have a higher amount of muscle mass than white or Mexican American women (14).

On average, Mexican American women have more body mass in their abdomen. One review found that Mexican American women may have 3–5% more body fat and a larger waist circumference than white or Black women with similar BMIs (14).

These differences between women of various ethnicities are based on data averaged over many people.

However, a person’s ethnicity does not determine their weight, body fat distribution, proportion of muscle mass, waist circumference, or other body metrics. Every person is different.

This means that even if two women have the same BMI, they can have very different regional body fat distribution and muscle mass (14).

Furthermore, researchers acknowledge that the link between BMI and mortality risk is much weaker among Black women than among white women (15, 16).

This is because BMI is a weaker indicator of body fat levels in Black women and because, on average, Black women have different fat distribution than white women. Plus, BMI is not associated with blood fat levels in Black women as it is in white women (15).

Studies have shown that healthcare professionals are more likely to misclassify Black women as having obesity because of their higher percentage of muscle mass (17).

Asian and South Asian women

Research shows that Asian and South Asian populations have more body fat relative to their body weight than white populations and that health conditions like type 2 diabetes occur at lower BMI levels in Asian populations than in white populations (17).

This has led to changes in BMI cutoffs for Asian populations.

Asian and South Asian women are in the overweight category when their BMI is 23–27.5 and are considered to have obesity when their BMI is over 27.5 (18).

However, several different BMI cutoffs for Asian populations have been suggested.

Bodybuilders and athletes

The BMI system often misclassifies women with significant amounts of muscle mass as having overweight or obesity, even though their body fat levels are low. This includes people such as bodybuilders and professional athletes (19, 20, 21, 22).

This is because BMI doesn’t differentiate between muscle mass and fat mass, and it is why people shouldn’t use BMI as a measurement of body composition.

Postmenopausal women

Older women tend to have lower muscle mass and more fat located in their midsection than younger women.

This is due to age-related changes in hormone levels, slowing of physical activity, and changes in torso length due to osteoporosis (14).

Even though an older woman may have the same BMI as a younger woman, the older woman will likely have less muscle mass and more body fat, which increase her risk of disease.

A 2016 study that included 1,329 postmenopausal women found that the BMI cutoff of 30 was not an accurate indicator of obesity in this population. The researchers pointed out that BMI may not indicate obesity in many postmenopausal women who do have obesity (23).

Weight gain recommendations during pregnancy based on BMI

Healthcare professionals recommend that women gain a certain amount of weight during pregnancy based on their prepregnancy BMI range (24, 25).

Here are the Institute of Medicine guidelines for weight gain during pregnancy:

Prepregnancy BMITotal recommended weight gainMean rate of weight gain in the second and third trimesters
underweight (under 18.5)28–40 pounds (12.5–18 kg)1 pound (0.51 kg) per week
normal weight (18.5–24.9)25–35 pounds (11.5–16 kg)1 pound (0.42 kg) per week
overweight (25.0–29.9)15–25 pounds (7–11.5 kg)0.6 pounds (0.28 kg) per week
obesity (30 or greater)11–20 pounds (5–9 kg) 0.5 pounds (0.22 kg) per week

Even though you can use these recommendations as helpful guidelines, it’s important to work with your healthcare provider. They can help you determine a healthy weight gain during pregnancy based on your specific needs and overall health.

Keep in mind that weight gain guidelines are different for women carrying more than one baby.

Summary

A “normal” BMI for women ranges from 18.5–24.9. However, BMI is not an accurate measure of obesity in many people. It tends not to be a good indicator in postmenopausal women, athletes, and Black and Hispanic women, in particular.

There are other, more appropriate ways to measure health than BMI.

Here are a few more accurate ways to measure body composition:

  • DEXA. Dual-energy X-ray absorptiometry (DEXA) is an accurate way to measure total body fat and muscle mass as well as bone mass. Talk to your healthcare provider if you’re interested in a DEXA scan (23, 26).
  • Body circumference measurements. Waist circumference is an important indicator of disease risk. A circumference over 35 inches (89 cm) in women on average, or over 31 inches (79 cm) in Asian women, is associated with increased disease risk (27, 28, 29).
  • Skinfold calipers. Skinfold calipers are another relatively easy way to measure body fat. However, they can be uncomfortable and do require training before you can take accurate measurements.

There are many other ways to measure body composition, including bioimpedance spectroscopy (BIS) and air displacement plethysmography. However, these are typically available only in medical settings or in certain fitness centers.

Even though these methods can measure body composition, there are many other factors to consider when it comes to overall health and well-being.

For example, getting your blood tested is extremely important when assessing your health and risk of disease.

For this reason, women should regularly see a healthcare professional, who can recommend blood work, such as cholesterol testing, based on individual background and health status.

A healthcare professional should also consider your dietary patterns, activity levels, genetics, sleep habits, mental health, stress, and socioeconomic status when assessing your physical and mental health (1, 30, 31).

Summary

Accurate ways to measure body composition include DEXA and body circumference measurements. There are a number of ways to measure health, many of which have nothing to do with body weight or body composition.

Scientists and healthcare professionals are well aware that BMI has significant flaws. However, it remains an important tool for assessing public health at the population level.

Individuals can also use it as a general health indicator, though this may not be accurate for some people. A BMI of 18.5–24.9 is considered normal or healthy for most women.

Even though healthcare professionals can use BMI as a screening tool, they shouldn’t use it as a method to evaluate a person’s body fat levels or health status (32).

Keep in mind that health is much more than body weight or body composition. To properly assess your overall health and well-being, a healthcare professional should consider many factors, including your blood test results, diet, activity levels, and sleep habits.