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Body Mass Index (BMI): What It Tells You — And What It Doesn’t

By Raghuveer Vedala, MD, FAAFP, DABOM, DABLM, Board-Certified Family Medicine and Obesity Medicine Physician HealthDay Reporter

Medically reviewed by Carmen Pope, BPharm. Last updated on May 4, 2025.

SUNDAY, May 4, 2025 — Body Mass Index, or BMI, is a number that reflects the relationship between your height and weight. It's widely used in healthcare and public health to categorize body size and estimate potential risk for weight-related diseases.

While it isn’t perfect, BMI can be a helpful first step in identifying whether someone might be at higher risk of chronic conditions like heart disease, type 2 diabetes or certain cancers. However, it doesn’t always give you the full picture of a person’s health.

How is BMI calculated?
The formula for BMI is simple:

• Metric: BMI = weight (kg) ÷ height (m²)

• Imperial: BMI = (weight in pounds ÷ height in inches²) × 703

So, for example, if someone weighs 180 lbs and is 5 feet 6 inches (66 inches) tall:

• BMI = (180 ÷ 66²) × 703 = 29.0

This puts them in the overweight category.

What does your BMI tell you?
BMI is divided into the following categories:

• Underweight: Less than 18.5

• Normal (healthy) weight: 18.5 – 24.9

• Overweight: 25.0 – 29.9

• Obesity class 1 (mild obesity): 30.0 – 34.9

• Obesity class 2 (moderate obesity): 35.0 – 39.9

• Obesity class 3 (severe obesity): 40.0 or higher

These categories give a rough estimate of body fat and health risk. In general, as BMI increases above the normal range, so does the risk of chronic diseases such as high blood pressure, high cholesterol, type 2 diabetes, heart disease, fatty liver, joint pain, sleep apnea and even some cancers.

Is BMI a useful health tool?
Yes — but it’s a screening tool, not a diagnostic test. BMI is useful for:

• Tracking weight trends in individuals and populations.

• Raising awareness of potential health risks.

• Starting conversations between patients and providers.

But BMI doesn’t tell the whole story. For example, someone with high muscle mass — like an athlete — might have a high BMI but low body fat. On the flip side, someone with a normal BMI could still carry dangerous levels of visceral (belly) fat and have poor metabolic health.

What’s a ‘healthy’ BMI?
A BMI between 18.5 and 24.9 is generally considered the healthiest range for most adults. But that number alone doesn’t guarantee good health. Lifestyle, genetics, muscle mass, diet and activity level all play major roles. Think of BMI as one puzzle piece — not the full picture.

What if your BMI is too low?
Being underweight (BMI < 18.5) is linked with these risks:

• Weakened immune function

• Nutrient deficiencies

• Fertility problems

• Osteoporosis and bone fractures

• Muscle wasting

This is especially important for older adults, in whom low muscle mass can lead to frailty, falls and hospitalization.

What are the risks of higher BMI?

With rising BMI — especially obesity class 2 or 3 — risks escalate. These may include:

• Type 2 diabetes

• Cardiovascular disease

• Stroke

• Fatty liver disease

• Osteoarthritis

• Sleep apnea

• Certain cancers (like breast, colon, and pancreatic cancer)

However, even individuals in the overweight or class 1 obesity range can have high health risks if they have a large waist circumference or high visceral fat. Visceral fat is the deep fat surrounding organs in the abdomen. That’s why BMI alone is not enough.

Beyond BMI: Better ways to assess health risk

1. Waist circumference

Abdominal fat, especially visceral fat around your organs, increases risk more than fat on the hips or thighs.

• High risk:

  • Men: Waist > 40 inches

  • Women: Waist > 35 inches

Even with a BMI in the “normal” range, a high waist circumference can signal increased risk for metabolic disease. Think of it as a red flag. Waist circumference is easy to measure and very helpful.

2. Body composition analysis

Scales like SECA or InBody give a clearer picture by breaking weight down into:

• Fat mass

• Muscle mass

• Water weight

This helps identify if someone is “skinny fat” (normal BMI but high body fat) or muscular with little fat.

3. DEXA scan

The Dual-Energy X-ray Absorptiometry (DEXA) scan is considered the gold standard. It shows:

• Precise body fat percentage

• Fat distribution (abdominal versus limb)

• Bone density

It’s particularly useful for athletes, people with unusual weight patterns or those undergoing weight loss treatment. However, it is not always the most practical test, due to cost and most insurers’ lack of coverage for evaluating muscle mass or body fat percentage. The test is generally used to evaluate bone density and screen for Osteoporosis.

4. Metabolic health markers

Lab work helps assess what BMI can’t, including:

• Blood pressure

• Fasting glucose or HbA1c

• Lipid panel (cholesterol)

• Inflammation markers

A “metabolically unhealthy normal weight” person may have normal BMI but poor lab numbers. Conversely, someone with a slightly high BMI but excellent labs and fitness may be at lower risk.

BMI and ethnicity: Why one size doesn’t fit all

Ethnic differences matter.

• Asian populations, including South Asians, face higher risks of diabetes and heart disease at lower BMIs. This can lead to underestimation of health risk. In this population, some guidelines consider:

  • Overweight: BMI 23+

  • Obese: BMI 25+

• African American individuals often have more lean mass and less visceral fat at the same BMI compared to white counterparts. This can lead to overestimation of health risk if BMI is used alone.

• Hispanic and Indigenous populations may also carry higher metabolic risks at lower BMIs due to genetic predispositions.

That’s why ethnicity-specific BMI cutoffs are increasingly being used in clinical care.

So, should you care about your BMI?

Yes — but not in isolation.

BMI is a starting point, not the finish line. Think of it as a screening tool that helps start the conversation about your health. It tells doctors where to look deeper — at your lifestyle, fat distribution, lab values, and fitness levels.

If your BMI falls outside the “healthy” range, it’s worth talking to your healthcare provider. But don’t panic — and don’t make assumptions based on BMI alone. Ask about your waist circumference, body composition and lab tests. These provide a clearer, more complete picture.

In summary:

• BMI is useful but not perfect.

• Waist size and body fat location matter.

• Muscle, fat percentage and fitness also count.

• Ethnic background can affect risk.

• Health is more than a number.

Raghuveer Vedala, MD, FAAFP, DABOM, is a board-certified family medicine physician practicing at Norman Regional’s Primary Care – South OKC clinic. Dr. Vedala also holds board certifications from the American Board of Obesity Medicine and the American Board of Lifestyle Medicine.

Disclaimer: Statistical data in medical articles provide general trends and do not pertain to individuals. Individual factors can vary greatly. Always seek personalized medical advice for individual healthcare decisions.

Posted May 2025

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