Ask a Doctor: Should I check my BMI to find out my healthy weight?

Q: How do I find what my healthy weight should be? BMI charts are everywhere, but I’ve also read that they’re more accurate at examining populations than individuals.

A: Everyone should pursue health at any weight, regardless of their body mass index (BMI).

BMI, a screening tool, has been criticized because it can sometimes suggest that a healthy, fit person with greater muscle mass has obesity. Also, some people may develop weight-related health problems even when they have a lower BMI.

Most of the time, though, BMI is useful because it is easy to calculate and is reproducible, correlates well with body fat, especially at BMIs greater than 30, and is clinically helpful in predicting the likelihood of obesity-related diseases.

BMI is calculated from weight in kilograms divided by height in meters squared. BMI is not a direct measure of body fat, but there is a good correlation between a BMI greater than 30 and body fat assessed by direct measures of body composition, such as CT scans, MRI or dual X-ray absorptiometry.

If your BMI is greater than or equal to 30, you should be screened for risk factors such as blood pressure, lipids, liver function and fasting glucose associated with weight-related diseases.

Using BMI to diagnose obesity

BMI is widely used as a screening tool to diagnose the disease of obesity. Online calculators for BMI are available from groups such as the Centers for Disease Control and Prevention, NIH, the American Cancer Society and others.

On a population basis, BMI has been used to establish cutoff points for the diagnosis of obesity. In adults:

-Underweight: BMI less than 18.5

-Healthy weight: BMI between 18.5 and 25

-Overweight: BMI between 25 and 30

-Class 1 obesity: BMI between 30-35

-Class 2 obesity: BMI between 35-40

-Class 3 obesity (also known as severe obesity): BMI greater than or equal to 40

Higher levels of BMI are associated with an increased likelihood of obesity-related diseases such as heart disease, Type 2 diabetes, liver disease and 13 different cancers, including liver, ovarian, esophageal and kidney cancers. Also, both extremely low and increased BMIs are associated with premature deaths.

Widespread stigma and bias are directed at people with increased BMIs. Many people with obesity have internalized that bias to blame themselves for their weight.

It is critical to recognize that obesity is rarely a choice, but rather a disease that results from complex interactions between your genetic predisposition, how you are wired metabolically and an environment that promotes overconsumption of food and inactivity.

If I have a BMI greater than 30, can I still be healthy?

People with a BMI of 30 or greater are highly likely to have increased body fat.

Groups such as professional athletes or weightlifters with BMIs of 30 or above, however, are in the obesity range, but they do not have excess body fat. Their increased muscle and bone mass is apparent on clinical examination.

Likewise, a substantial number of people with a BMI between 25 and 30 may be overweight but not overfat. Again, clinical observation and judgment of the whole person is essential to the distinction.

One of the ongoing controversies regarding obesity is whether some people with obesity are metabolically healthy. The answer depends on the criteria used to identify metabolic health. Most studies have used criteria such as blood pressure, fasting blood glucose and lipids such as triglycerides and HDL cholesterol.

A person who has metabolically healthy obesity, however, may still have or develop obesity-related diseases. And some people with metabolically healthy obesity may develop metabolically unhealthy obesity.

Is BMI relevant to all races and ethnicities?

BMI was developed via studies of men, and that lack of diversity has raised questions regarding its validity in women and people of different races and ethnicities. As a result, some consider BMI to be a racist measure.

Social determinants, race, ethnicity and age may modify the risk associated with a given BMI. For example, Black adults have more lean body mass (nonfat mass) at the same BMI as White adults, and Asian populations develop complications of obesity at lower BMIs than other race and ethnic groups.

At a BMI of 30 and above, however, these variations have a limited effect on the risks of obesity-associated diseases across racial ethnic groups, sex and age.

Is there a better measure than BMI to assess my health risks?

BMI is the best screening measure for obesity, but the distribution of body fat also affects the risk of associated diseases. For example, waist circumference as a measure of abdominal fat provides additional information regarding the risks of obesity.

Abdominal fat consists of subcutaneous and intra-abdominal visceral fat. Visceral fat is of particular concern because it releases a variety of proteins that cause inflammation, and that inflammation may lead to many negative consequences of obesity.

In men and women, the cutoff points for increased waist circumferences are a BMI greater than or equal to 35 and 40 respectively.

Avoiding the risks of obesity-related diseases

You can reduce your risk of weight-related diseases by lifestyle changes. Physical activity increases muscle mass and reduces fat, particularly visceral fat. A healthy diet rich in fruits and vegetables and low in ultraprocessed foods can also help.

How should physicians talk to patients about weight?

Obesity is a fraught issue, and because the term has such a pejorative connotation, many providers do not use the term, but talk about people in bigger bodies or people with increased BMIs.

The first step for a provider is to ask a patient if it is okay to discuss their weight.

Most patients with significant obesity know they have a problem and have experienced stigma and bias. The issue is not whether they recognize that their weight has increased, but whether they consider their weight a problem, or trust the provider enough to begin to engage around treatment.

William H. Dietz is a pediatrician and chair of the Redstone Global Center for Prevention and Wellness at George Washington University School of Public Health. He is a former director of the Division of Nutrition, Physical Activity and Obesity at the Centers for Disease Control and Prevention. He is also a consultant to the National Academy of Medicine’s Roundtable on Obesity Solutions, and director of the STOP Obesity Alliance at GWU.