Is BMI useful as a measure of health risk?

Sue Heikkinen, MS, RDN, CDCES, BC-ADM, ACE-PT - Registered Dietitian Nutritionist and Certified Diabetes Care and Education Specialist

Body Mass Index (BMI) is a common screening tool, but is it useful in what it tells you? Learn what BMI means, how it is used, and the disadvantages of this measure.

What is BMI, and how useful is it?

Chances are your BMI report came up during your last doctor’s visit. Healthcare providers use BMI because it is a simple weight assessment. BMI is calculated by dividing your weight (in kilograms) by your height (in meters) squared.

The Centers for Disease Control (CDC) provides these BMI categories for adults over age 20:

Under 18.5: Underweight
18.5-24.9: Normal
25-29.9: Overweight
30 and above: Obese

High BMI correlates with an increased risk of health conditions such as diabetes, high blood pressure, and heart disease. So, how useful is knowing your BMI?

What BMI doesn’t tell you

Looking only at your BMI presents disadvantages because you won't have a complete picture of your current or future health status. Here are some things a BMI cannot show you.

Your body composition

Although there is a correlation between BMI and body fat, BMI cannot directly measure how much body fat you have.

Some muscular people with a BMI in the “overweight” range don’t have an increased health risk due to favorable body composition. Likewise, some people with a BMI in the “normal” range have a high percentage of body fat, thus greater health risk. Such a consideration is important for older people who tend to have lower bone density and muscle mass.

BMI also doesn’t account for where you carry excess body fat. An individual with excess abdominal fat will have more significant health risks than someone with more fat in the hips and thighs. A simple waist measurement can help assess the risk from excess abdominal fat.

How fit and healthy you are

BMI is best used to gauge population health and screen for potential individual health risks. Your health is more nuanced than BMI can measure. A conversation with your healthcare provider about your lab results, blood pressure, family history, and eating and exercise habits is essential to making an accurate health assessment.

What you “should” weigh

Looking at a BMI chart and using the “normal” range for your height as a goal may feel unrealistic if your current weight is far above that range. Your personal weight goal should be individualized. Consider that 5-10% weight loss lowers health risk, even if you do not achieve a BMI in the “normal” range.

Racial, ethnic, and gender differences

Because body fat distribution can differ among racial and ethnic groups, BMI standards may not apply to all populations. For example, Asian people tend to have an increased risk of diabetes compared to their white counterparts with the same BMI, indicating a lower BMI cutoff may be appropriate for this population. On the other hand, some experts have proposed a slightly higher BMI cutoff to define obesity for Black women. Even if these differences are minor, they provide another reason to avoid solely relying on BMI as a health measure.

Monitor your progress with MyNetDiary

If you find knowing and following your BMI over time useful and motivating, you can find it listed on the “Weight” screen in the MyNetDiary app. When you update your weight, the app will recalculate your BMI. Make sure to include other indicators such as body composition and waist measurement.

Most importantly, focus on the eating and activity plan that helps you feel your best and keeps you in good health!