Body Mass Index (BMI) uses a person’s weight and height to estimate body fat. The calculation involves dividing weight in kilograms by the square of height in meters. This number serves as an inexpensive and widely used screening tool. At a population level, it helps correlate weight with potential health issues and provides a simple way to categorize weight status.
Standard Adult BMI Classifications
The most common BMI classifications for adults aged 20 and over are established by organizations like the World Health Organization (WHO). These categories are based on the statistical relationship between BMI and health risks observed across large populations. For most adults, these classifications are applied regardless of age, sex, or race, providing a standardized framework for initial health screenings.
A BMI below 18.5 is categorized as underweight, while the range from 18.5 to 24.9 is considered a healthy or normal weight. These values are associated with the lowest rates of disease risk in broad population studies. Maintaining a weight within this range is recommended for general health.
The classification for overweight begins at a BMI of 25.0 and extends to 29.9. A BMI of 30.0 or greater indicates obesity, which is further subdivided. Class I obesity is defined by a BMI from 30.0 to 34.9, Class II from 35.0 to 39.9, and Class III, often termed severe obesity, includes any BMI of 40.0 and above.
Adjustments for Different Populations
Standard BMI cutoffs are not universally applied, as body composition and growth patterns can vary. For children and teenagers between ages 2 and 19, BMI is interpreted using BMI-for-age percentile charts. Their BMI is compared to the values of other children of the same sex and age, which generates a percentile ranking that places them into a weight status category.
Using these charts, a child with a BMI below the 5th percentile is considered underweight, while a BMI between the 5th and 85th percentiles is a healthy weight. The overweight category spans from the 85th to just below the 95th percentile, and a BMI at or above the 95th percentile is defined as obesity.
Research has also shown that the relationship between BMI, body fat, and health risks can differ among ethnic groups. For many people of Asian descent, the risk of conditions like type 2 diabetes and cardiovascular disease increases at lower BMI values. In response, some public health bodies propose alternative cutoffs for these populations, suggesting a BMI of 23 as an indicator for overweight and 27.5 for obesity.
Associated Health Risks by Category
A BMI in the underweight range is associated with risks such as malnutrition, anemia, and a weakened immune system, which increases susceptibility to infection. It can also be linked to hormonal abnormalities, delayed wound healing, and a higher risk for osteoporosis.
As BMI increases into the overweight and obese categories, the risk for a range of chronic diseases rises. These conditions include:
- Cardiovascular problems like high blood pressure, coronary artery disease, and stroke
- Type 2 diabetes
- Certain types of cancer (including breast, colon, and kidney)
- Gallbladder disease
- Osteoarthritis
Fat cells are metabolically active and can release substances that cause low-grade inflammation, which contributes to the development of these chronic conditions. The risk for health complications is continuous and increases as BMI rises further into the obese classifications. This correlation is why BMI is used as an initial screening method to identify individuals who may need further health evaluation.
Context and Limitations of BMI
While BMI is a useful screening tool, it has limitations because its calculation is based only on height and weight. A primary drawback is its inability to distinguish between lean muscle mass and fat mass. Since muscle is denser than fat, an individual with high muscle mass, such as an athlete, could have a BMI that classifies them as “overweight” despite having a low percentage of body fat.
Conversely, BMI can misrepresent the health status of other individuals. An older adult who has experienced age-related muscle loss might fall into the “normal” BMI range while having a high percentage of body fat. This scenario can mask potential health risks, highlighting that a “normal” BMI does not always equate to being metabolically healthy.
Another detail BMI does not account for is the distribution of fat in the body. Visceral fat, stored around the internal organs, is more strongly linked to health problems like heart disease and diabetes than subcutaneous fat, which lies just under the skin. Two people with the same BMI can have different fat distribution patterns and, therefore, different health risk profiles. For these reasons, healthcare providers use BMI as one part of a comprehensive assessment that includes other measurements like waist circumference.