What Does Body Mass Index (BMI) Measure?

BMI or body mass index is a calculation that is sometimes used in healthcare settings as an indirect method to determine a person's body fat category. It is not a diagnostic tool nor is it a measurement of body fat percent. A high BMI may be an indicator of high body fatness, but it doesn't necessarily mean that a person is overweight or obese and it is not a direct indicator of health.

In some populations BMI has been found to be a fairly reliable indicator of body fat measures. But the calculation is less effective in other populations, such as bodybuilders and the elderly. There are other methods that are more accurate in estimating body fat percent.

BMI can be calculated easily, using your height and weight. It can be a starting point for understanding the way that your body fat may impact your overall health. You can use the number along with other health measurements to begin a conversation with your healthcare provider about ways to reduce your risk for disease and improve overall wellness.

There are several different methods of assessing the percent of fat and lean mass of an individual. These methods are referred to as body composition analysis. Some of the most common measurements include skinfold thickness, underwater weighing, and bioelectrical impedance.

• Underwater weighing or hydrodensitometry is complex and complicated, so most experts use simple skinfold thickness measurements to determine body fat percent.
• Bioelectrical impedance is another common method of assessing body fat percentage. This method determines total body weight, the percent and amount of body fat, muscle mass, water, and even bone mass. While the readings can be affected by hydration level and other factors, they provide fairly accurate readings over time. Body fat scales are available for home use that employ this method of measurement.

Metric BMI Formula: Formula: weight (kg) / [height (m)]²

Example: Weight = 68 kg, Height = 165 cm (1.65 m)
Calculation: 68 / (1.65) ² = 24.98

English BMI Formula: weight (lb) / [height (in)]² x 703

Example: If your weight is 150 lbs and height is 5’5” (65")
BMI Calculation: [150 / (65) ²] x 703 = 24.96

You can interpret your BMI result using this simple chart. For adults, BMI results are interpreted as follows:

• BMI below 18.5 = Underweight
• BMI 18.5 – 24.9 = Normal weight
• BMI 25.0 – 29.9 = Overweight
• BMI 30.0 and Above = Obese

Note that BMI is interpreted differently in children. Growth charts and percentiles are used. If they are at or above the 95th percentile of children their age they are considered obese.

Even though there is a fairly strong correlation between BMI and body fat measurement, there are some limitations based on an individual's gender, age, and athletic ability. These limitations include the following:

• Women tend to have more body fat than men.
• Older people tend to have more body fat than younger adults.
• Highly trained athletes often have a high BMI due to higher levels of muscle mass which increases their body weight measurement, rather than higher body fat causing a higher weight measurement.

Athletes who have higher levels of muscle mass need to be somewhat leery of the BMI calculation. Because the BMI number cannot distinguish the different components that make up total body weight, an athlete is better served by using a direct measurement of body composition and body fat than using a simple formula.

The reason the BMI is used for screening the health of the general population is due to the correlation between being overweight or obese and having certain health problems. People who are overweight or obese have an increased risk for the following health conditions:

• Hypertension
• Type 2 diabetes
• Coronary heart disease
• Stroke
• Osteoarthritis
• Some cancers
• Sleep apnea and respiratory problems

It is important to understand, however, that body fatness is not the only predictor of health. Researchers are trying to understand the "obesity paradox." Specifically, experts have found that despite the potential adverse impact of overweight and obesity, evidence demonstrates that being overweight or mildly obese is associated with improved survival.

Researchers have also found that being fit negates the adverse effects of body fatness, as well as other traditional cardiovascular risk factors, including overweight/obesity, metabolic syndrome, type 2 diabetes mellitus, and hypertension.

The bottom line is that while body mass index can be a useful tool in some settings for some people, it is just one number that should be balanced out with other evidence. Talk to your healthcare provider about the best way to understand your BMI in a comprehensive plan for good health and longevity.