Body Mass Index, BMI, calculated by dividing weight in kilograms by height in meters squared, is the most widely used clinical measure for classifying weight status. It is simple, inexpensive to calculate, and correlates reasonably well with health risk across large populations. It is also, at the individual level, a genuinely imperfect tool with meaningful limitations that every patient deserves to understand.
At Dr. Maryam Khan’s primary care practice in Lutherville-Timonium, MD, BMI is one data point among several, used as a screening tool but never as the complete picture of a patient’s weight-related health.
What BMI Measures and What It Doesn’t
BMI is a mathematical ratio of weight to height. It measures neither body fat directly nor its distribution. Two patients with identical BMIs can have dramatically different metabolic and cardiovascular risk profiles depending on how their weight is distributed and what it is composed of.
| BMI Range | Classification |
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity Class I |
| 35.0 – 39.9 | Obesity Class II |
| 40.0 and above | Obesity Class III (severe obesity) |
The limitations of BMI are well-documented:
• Muscle vs fat: A muscular athlete and a sedentary person of the same height and weight have the same BMI, but very different body compositions and health risk profiles. The athlete’s higher weight reflects lean muscle mass; the sedentary person’s reflects adipose tissue.
• Fat distribution matters more than total weight: Visceral fat, fat stored around the abdominal organs, sometimes called ‘central’ or ‘abdominal’ obesity, is metabolically active in ways that subcutaneous fat (fat under the skin) is not. Visceral fat drives inflammation, insulin resistance, and cardiovascular risk. Two patients with the same BMI may have very different amounts of visceral fat.
• Ethnic variation: The health risks associated with a given BMI differ by ethnicity. Asian patients face elevated cardiometabolic risk at lower BMIs, and professional organizations recommend screening for obesity at BMI ≥23 rather than ≥25 in patients of Asian descent.
• Age and sex effects: The proportion of body fat for a given BMI increases with age and is higher in women than men. Older adults with ‘normal’ BMIs may have relatively high body fat percentages.
What Clinicians Use Alongside BMI
Waist Circumference
Waist circumference is a direct measure of central adiposity and is one of the most clinically meaningful single measurements for cardiovascular and metabolic risk. Elevated risk thresholds are: above 40 inches (102 cm) in men and above 35 inches (88 cm) in women. In patients of Asian descent, thresholds are lower: above 35 inches in men and above 31 inches in women.
A person with a BMI of 27 and a waist circumference of 44 inches carries more cardiometabolic risk than a person with a BMI of 32 and a waist circumference of 35 inches, even though the second person has a higher BMI. Waist circumference tells a different story.
Waist-to-Height Ratio
An increasingly supported metric: waist circumference divided by height. A ratio below 0.5, ‘keep your waist to less than half your height’, is a practical target that correlates well with cardiovascular risk across a wide range of populations and is less sensitive to ethnic variation than BMI cutoffs.
Metabolic Markers
Fasting glucose, A1C, lipid panel (particularly triglycerides and HDL cholesterol), liver enzymes, and uric acid provide insight into the metabolic consequences of adiposity. A patient who is overweight by BMI but has a normal lipid panel, normal blood sugar, and normal blood pressure carries a very different risk profile from an overweight patient with pre-diabetes, elevated triglycerides, and hypertension.
Blood Pressure
Hypertension is one of the most weight-sensitive cardiovascular risk factors. Even a modest weight reduction of 5 to 10 pounds produces measurable blood pressure decreases in overweight patients with hypertension.
The ‘Metabolically Healthy Obese’ Concept
Some patients have a BMI classified as obese but have normal blood pressure, normal blood sugar, favorable lipid profiles, and no metabolic syndrome criteria. This phenotype is sometimes called ‘metabolically healthy obesity’ and carries lower short-term cardiovascular risk than metabolically unhealthy obesity. However, longitudinal research suggests that metabolically healthy obesity is often a transient state, metabolic abnormalities tend to develop over time even in initially healthy obese individuals. It is not a safe harbor that eliminates the case for weight management.
What Dr. Khan’s Assessment Actually Looks Like
In a weight management consultation at Dr. Khan’s Timonium practice, the evaluation includes BMI calculation, waist circumference measurement, blood pressure assessment, and a metabolic laboratory panel, providing a multidimensional view of weight-related health rather than a single number. This informs both the urgency of the clinical situation and the selection of the most appropriate treatment approach.
The clinical goal is not achieving a specific BMI. It is reducing metabolic and cardiovascular risk, improving functional capacity, and supporting long-term health, goals that are defined differently for each patient.
| For a comprehensive weight management assessment in Timonium, MD, contact Dr. Maryam Khan, MD at (443) 577-4010. Located at 1205 York Rd, Suite 11, Lutherville-Timonium, MD 21093. Accepting new patients. |