The truth about BMI: Debunking myths and understanding the science

BMI can not distinguish between fat, muscle or bone mass, nor does it indicate body fat distribution.

According to the World Health Organization [1], the prevalence of obesity or being overweight has more than quadrupled from 4% in 1975 to 18% in 2016 in children and adolescents. In adults, at least 2 billion are overweight, while 650 million are obese. This equates to 40% of women and 39% of men as overweight and 13% as obese.

Obesity is no longer a health problem in highly developed and industrialized countries. Dubbed a health epidemic, it is also high in low to middle-income countries. In 2030, at least one billion people are likely to be obese.

Research has shown that obesity is the single risk factor for developing type 2 diabetes mellitus [2]. Obese people are nine times more likely to develop hypertension than normal-weight people [2].

Hypertension or consistent high blood pressure could lead to several illnesses, including stroke and cardiovascular diseases. Since hypertension is a silent killer, it does not show symptoms until it causes cardiovascular diseases. Hence, it is better to prevent hypertension from developing than treating this health condition.

To measure obesity and overweight, the BMI or body mass index is calculated based on the person’s height and weight.

What is BMI?

The CDC, or Centers for Disease Control and Prevention, defines body mass index (BMI) as “a person’s weight in kilograms (or pounds) divided by the square of height in meters (or feet).” [3]. The CDC states that a high BMI indicates that an individual has high body fat.

Although BMI may screen for weight categories that could lead to health conditions, it does not diagnose a person’s health or body fat. BMI refers to weight categories for adults 18 years of age and older, which are the same for men and women of all ages and body types.

Interpretation of BMI

Below 18.5 – Underweight

18.5-24.9 – Healthy weight

25.0-29.9 – Overweight

30.0 and Above – Obesity

This year, the theme of World Obesity Day is to change perspectives by talking about obesity. However, BMI may not be the best tool for indicating total body fat and obesity.

The CDC has explained that a high BMI could only mean body fat. The CDC is referring to the general population and not to individual people. For example, an individual who is highly athletic but has a high BMI may have a different level of fat than an individual who is sedentary but has the same BMI.

The BMI could not measure an individual’s fat mass density and free fat mass. For example, an individual with well-developed muscle tone, strong bones, and low-fat mass may still have a high BMI. Similarly, an individual with low bone mass density, moderate fat mass and insufficient body density may still fall in the category of normal BMI.

What are some myths regarding BMI?

Myth: BMI is a reliable measure of the body’s total fat

Fact: Several healthcare practitioners use BMI to determine an individual’s weight category because it is non-invasive and inexpensive. However, it is not an accurate measurement of body fat since individuals with high bone density and muscle tone but low-fat mass may still fall into the obese category. Further, BMI could not distinguish between muscle and fat.

The truth about BMI: Debunking myths and understanding the science — Photograph: microgen/Envato

Myth: BMI is an essential indicator in helping people understand their health condition

Fact: Individuals categorized as obese or overweight based on their BMI may overestimate their risk for heart diseases and diabetes but underestimate their risk of breast cancer or colorectal cancer.

Myth: Measuring an individual’s fat is difficult

Fact: Currently, there are other ways to measure fat. One example includes the use of DXA or dual-energy x-ray Absorptiometry.

Myth: BMI is the best way to measure an individual’s healthy weight

Fact: The weight and height ratio only measures one’s body fat within the desired or targeted BMI range for healthy weight.

Myth: BMI is an accurate measure of one’s health

Fact: BMI does not distinguish between muscles and fats; hence it could not account for total muscle mass. It also does not provide information on an individual’s diet quality, sleep quality, mental wellness, fitness, and other metrics for health.

Myth: Weighing less and having a lower BMI is always better than having a high BMI

Fact: Although a lower BMI or a BMI within the healthy range is acceptable, some have high BMI due to bone density and muscle mass. A high BMI does not equate to increased body fat mass for highly athletic individuals. However, lowering one’s weight to relieve the extra strain on body joints such as knee joints could help prevent common joint problems.

Myth: Losing weight is the best way to get healthy

Fact: Weight loss reduces your risk of obesity and cardiovascular diseases, but it doesn’t increase your lifespan consistently.

Use of BMI and health concerns

A study [4] recently published in the PLoS One journal showed that physically active individuals reduced their risk of cardiovascular diseases despite a high BMI. The study’s lead author, Xiaochen Zhang, from the Division of Population Sciences at Ohio State University, and the rest of his colleagues concluded that “participation in any level of physical activity is associated with a lower 10-year CVD risk for overweight and obese adults.”

This team of researchers used data from the National Health and Nutrition Examination Survey from 2007 to 2016. A subset of the data on adults aged 30-64 years old with no CVD history (n=22, 476 participants) was included in the analysis.

Investigators in the study relied on this group’s self-reported physical activity level. They stratified them into sedentary (0 minutes of physical activity per week), inactive (1-149 minutes/per week), active (at least 150 minutes or more of physical activity per week), and moderate to vigorous activities. Zhang and his colleagues then relied on Framingham risk scores to classify participants as having low/intermediate (less than 20%) risk of 10-year cardiovascular disease risk or high risk (>20%).

Results of this study revealed that more than a third (33.6%) were overweight, while 35.7% were obese. The summary of the results indicated that amongst physically active overweight and obese individuals.

This includes their 10-year CVD risk was lower compared to sedentary individuals. Among normal-weight individuals, participation in moderate to vigorous physical activities every week was linked to lower CVD risk. This was compared to those who were inactive and had average weights.

The study’s authors observed that “when compared the joint effects of physical activity level and weight status, physical activity was associated with a larger magnitude of reduced odds of 10-year CVD risk than weight status.” These findings prove that physical activity level is more critical in reducing the risk of cardiovascular disease than weight status. Hence, individuals with higher or high BMI but are physically active would have improved overall health compared to those who are sedentary with normal weights.

What are more accurate ways to measure body fat?

The International Journal of Obesity cited the following measurements as more accurate than BMI in measuring body fats:

Hip circumference
Waist circumference (sometimes divided by height)
Waist-hip ratio
Bioelectrical impedance analysis: This type of analysis uses electrical signals to measure fats since fats are more resistant to electrical signals.
Skin folds fat: Thickness and skin are measured away from the individual’s muscles to determine fat levels.
Dual-energy X-ray absorptiometry (DEXA) uses a body scan to measure fat content and bone density.
Sagittal abdominal diameter: Measures the distance between the highest point of the abdomen and the back.

While the BMI is an easy and quick tool to measure body fat and is commonly used in healthcare settings, healthcare professionals must assess patients individually and not use BMI as a blanket tool to determine body fats.

Can I use BMI to measure wellness and health?

The main takeaway from studies and international organizations’ recommendations on BMI is that it should not be used to measure one’s health and wellness. Health encompasses not only physical, but also mental, spiritual and social.

Further, health is a culmination of behaviors and habits practiced for several years and on most days. These include eating a balanced diet such as a high intake of fruits and vegetables, lean protein and fish meat, drinking sufficient amounts of water, and managing stress levels. To be healthier, focus on small steps and erase habits that do not support mental and physical wellness.

Start doing small but incremental steps such as the following:

Skip one soda drink a day until you are soda free.
Cut down junk foods one day at a time until you no longer crave junk foods
Eat more fruits and vegetables.
Exercise at least 10 minutes a day by walking in nature and increase these to at least 30 minutes per day of moderate to vigorous intensity walking or aerobic exercise
Attend a yoga class.
Attend a Tai-chi class.
Practice mindfulness breathing to ease stress
Reduce stress by focusing on small goals.
Reduce stress by listening to music and doing artwork or gardening

The World Health Organization recommends participating in at least 150 minutes of moderate-intensity to vigorous physical activities per week to reduce the risk of obesity, cardiovascular diseases and other health conditions. You can begin your journey to health and wellness by starting small and incremental steps.

While BMI remains an inexpensive tool in determining your body fat, supporting BMI results with results of other assessment tools will help give you an idea of your current physical health.