Few people in the United States would argue that we aren't an overweight nation. Most are familiar with the statistics that suggest 1/3 of the population is obese and another 1/3 is at least overweight. These statistics mainly come from height and weight data, which is used to compute body mass index levels for these individuals. Body mass index (BMI) is one of the most commonly used measures for determining whether individuals are at an appropriate level of body fatness, though it does not directly measure body fat levels at all. In fact, as you’ll see, the BMI data we use likely underestimates the number of people living with unhealthy body fat levels. That’s right – our reliance on this inaccurate assessment of one’s estimated level of body fat may be making us look healthier than we actually are!
History of Body Mass Index
Body mass index (BMI) has been in use since the 1800s. It was originally put into use by a mathematician from Belgium named Adolphe Quetelet, who named his calculation the Quetelet Index of Obesity.[i] It wasn’t until the 1980 release of the Dietary Guidelines for Americans that BMI became heavily promoted in the United States. At that time, it was introduced as height and weight recommendations and was gender-specific. The ideal BMI levels kept changing for the next 20 years, making it confusing to use. In 2000, the BMI recommendations were again updated and reflect today’s non-gender-specific levels:[ii]
- 18.5-25.0 kg/m2: Healthy weight
- 25.0-29.9 kg/m2: Overweight
- ≥30.0 kg/m2: Obese
Calculating Body Mass Index
Determining BMI is simple. BMI is equal to an individual’s weight in kilograms divided by their height in meters squared. Since most people in the United States don’t use the metric system, you can use the calculator below from the CDC to easily determine your BMI.
For researchers looking at large populations of people, BMI allows for an efficient way of determining possible levels of body fatness. Unfortunately, measuring BMI does not account for the two biggest factors in whether an individual is really at risk for health problems – lean mass and body fat.
Support for Body Mass Index
As mentioned above, using BMI to make assumptions about a large population of people and their level of body fatness is efficient. The only numbers needed are height and weight. Since much of the data about our current levels of overweight and obesity are based on self-reported data, there is a possibility people will lie. However, if they did lie, most often it would be to under report their weight, which would mean the population is more overweight than we may account for.
The majority of the population is not athletic, so they either have normal, or even below normal levels of lean body mass. A recent meta-analyses of data from more than 30,000 people showed BMI was very accurate in indicating people over the 25 kg/m2 were overweight (or overfat) and over 30 kg/m2 were obese. The athletic population often disregards BMI markers since they have more muscle, which means their weight will be higher. However, athletes are such a small percentage of the population that they do not affect the accuracy of BMI in identifying overweight and obese people when they are included among the general population. [iii]
So, again, when looking at large groups of people who are a good subset of the average population, BMI is reasonably accurate in saying almost all people with a BMI greater than 25 kg/m2 carry too much body fat.
Problems with Body Mass Index
The greatest issue in the use of BMI is when it is used on an individual rather than a large population. There are three types of individuals for whom BMI is a poor indicator of physical health.
The first type of individual is an athlete. Imagine Minnesota Vikings' running back Adrian Peterson doing a phone consultation with a physician. The doctor might only have Adrian’s height and weight to look at. Peterson is 6’1” and 217 pounds, making his BMI 28.6 kg/m2, which puts him in the overweight category. Being one of the leanest, most muscular players in the National Football League, no one would tell him his body fat level is too high or that he should lose some weight (other than maybe the defense from his opposing teams). For the small percentage of the population that is athletic, BMI measures can make them appear less healthy than they actually are. However, this is such a small percentage of the population that it has little effect on national statistics.
The second type of individual makes our reliance on BMI data of greater concern. Many individuals who have a “healthy weight” BMI (<25) actually have high levels of body fat. According the article in the International Journal of Obesity,[iii] a significant percentage of the population has body fat levels that would categorize them as overweight or obese, even though their body weight seems to be normal for their height. Simply put, these people have low levels of lean body mass. They are “skinny fat” or “normal-weight obese.” They may appear thin or normal when wearing street clothes, but the percentage of their weight that comes from body fat is still very high. Often in these individuals, their arms and legs might seem normal in size, but when you squeeze their thighs, biceps and triceps, they’re very soft and squishy.
Individuals with sarcopenia (muscle loss), osteoporosis (loss of bone density), or those who are simply inactive can have very low levels of muscle tissue. With lack of use, muscle becomes fattier. These people may not get appropriate levels of healthcare as their body weight does not suggest they are at risk as much as those who have a BMI greater than 25. Based on these findings, we may still have 2/3 of the population considered “overweight” but an even greater percentage of the population considered “overfat.” In fact, those who do have higher levels of body weight might be at an advantage as they could get appropriate medical testing sooner than those who appear to be at a healthy weight.
A recent investigation in the Archives of Internal Medicine looked at the percentage of each of the three BMI categories who displayed more than one metabolic abnormality, such as elevated blood pressure, elevated triglycerides, high fasting glucose, low HDL cholesterol and high C-reactive protein. They found 49% of overweight individuals and 68% of obese individuals displayed metabolic abnormalities. Interestingly, they also found 24% of “healthy weight” individuals were metabolically abnormal![iv]
A final issue with BMI is that even when people are accurately identified as being overweight, BMI alone cannot say where the excess body fat is being stored. It is clear that carrying extra body fat around the stomach, called visceral fat, is much more a concern than anywhere else on the body. Without a more personal assessment of body composition, individuals who are at the greatest risk of health problems are not as easily identified.
Alternatives to Body Mass Index
There are a variety of other methods for determining body fat levels, including skinfold measurements, bioelectrical impedance (BIA), DEXA scans, underwater weighing and infrared scanners. Out of convenience, skinfold measurements are often the method of choice. The major importance with skinfold measures is having them done by an experienced professional, as human error is the most common cause of variability. Also, if you carry a large amount of your body fat as visceral fat, it is difficult to measure as it is found under the stomach muscles. Visceral fat, or belly fat, is the worst type of fat.
If you don’t have any of those methods readily available, the mirror can be a good judge as well. You don’t need to have a six-pack to be considered at a healthy body fat level, but you should have some visible definition throughout your body. If your stomach, thigh and shoulder muscles are invisible, it’s likely your body fat level is too high, or your lean body mass is too low to be considered healthy.
[i] Quetelet A. A Treatise on a Man and the Development of His Faculties. 1842. Reprinted by Burt Franklin. New York. 1968
[ii] Kuczmarski RJ, Flegal KM. Criteria for definition of overweight in transition: background and recommendations for the United States. Am J Clin Nutr. 2000;72(5):1074-1081
[iii] Freedman DS, Sherry B. The Validity of BMI as an Indicator of Body Fatness and Risk Among Children. Pediatrics. 2009;124:S23-S34
[iv] Wildman RP, Muntner P, Reynolds K. The Obese Without Cardiometabolic Risk Factor Clustering and the Normal Weight With Cardiometabolic Risk Factor Clustering. Arc Int med. 2008:168(15):1617-1624