January 2014 Issue
Assessing Weight Status — Is BMI the Best Tool?
By Sharon Palmer, RD
Vol. 16 No. 1 P. 30
As a dietitian, you’ve probably encountered many people who on paper, using the BMI calculation, rank as overweight or obese. But once you see them, you realize that because of body musculature, their weight is healthy. This is the classic scenario that has many health care experts grumbling about BMI, a calculation based on only height and body weight.
The National Heart, Lung, and Blood Institute calls BMI a useful measure of overweight and obesity, an estimate of body fat, and a good gauge for risk of diseases that can occur with increased body fat. The institute reports that the higher a person’s BMI, the higher his or her risk of heart disease, high blood pressure, type 2 diabetes, gallstones, breathing problems, and certain cancers. However, it also concedes that BMI has limitations.1
So how can you accurately assess your patients’ and clients’ weight status? Let’s start at the beginning of the BMI story.
“BMI is almost 200 years old, but it wasn’t used in medicine until the 1980s,” says Francisco Lopez-Jimenez, MD, MSc, FAHA, FACC, a professor of medicine at the Mayo College of Medicine and the director of the cardiometabolic program and preventive cardiology fellowship program who’s an expert in the field of body weight, weight loss, body fat, and obesity, publishing several research papers on these topics. “The main reason why we used BMI for the past 30 years is that epidemiological evidence relates BMI to bad outcomes,” he says.
BMI originated in the 19th century, when a Belgian statistician named Adolphe Quetelet, who pioneered cross-sectional studies of human growth, devised the Quetelet Index of Obesity in 1832. He observed that “the weight increases as the square of the height”; thus, his index measured obesity by dividing a person’s weight in kilograms by the square of his or her height in meters.2
It may seem as though obesity really didn’t hit our radar until the past few decades, but obesity as a disease—with well-defined complications—dates back 100 years. During much of human history, high body fat was considered positive—a sign of “good health” and “plentiful food.” But then, in the early 20th century, the insurance industry documented the relationship between weight and cardiovascular disease and mortality, which subsequently was investigated by epidemiologists.2
Life insurance policies created tables of “normal” weights, which were based on average weights they gathered for heights. They attributed the wide range of weights to body “frame,” adding accommodations for “small,” “medium,” and “large” body frames to the weight tables. The average weights on the tables were first referred to as “ideal,” which was soon changed to “desirable”; undesired weight was considered 20% to 25% above the desirable range for a given body frame, and morbid obesity was considered 70% to 100% above the desirable range for a given body frame.2
In the 1960s, the Quetelet Index was used as a practical gauge for weight status in clinical studies. The Framingham Study in 1970 was one of the first to confirm its validity; Ancel Keys, PhD, also confirmed the Quetelet Index’s validity, renaming it BMI in 1972. BMI became an international standard for obesity measurement in the 1980s, and it became part of the public dialogue in the late 1990s when the US government launched healthful lifestyle initiatives.2 Today, the National Institutes of Health recommends physicians include BMI in their routine patient assessments.1
The Best of BMI
So how well has BMI performed since its adoption in the 1980s? According to the Centers for Disease Control and Prevention (CDC), calculating BMI is one of the best methods for assessing overweight and obesity in the population; since the calculation requires only height and weight, it’s inexpensive and easy to use for clinicians and the general public, and it allows people to compare their own weight status with that of the general population.3
“Overall, BMI can be a useful tool to start. It also can be helpful to clients. When some clients learn they’re ‘obese,’ it motivates them to start making changes to their diet and lose weight,” says Ruth Frechman, MA, RDN, CPT, a spokesperson for the Academy of Nutrition and Dietetics (the Academy) and the author of The Food Is My Friend Diet.
The National Obesity Observatory, now part of Public Health England, reports that BMI correlates reasonably well with most people’s level of body fat.4 The CDC also notes that the correlation between the BMI number and body fatness is fairly strong, though the correlation varies by sex, race, and age.3
Nevertheless, obesity essentially boils down to excessive levels of body fat, not weight. The World Health Organization identifies the most basic definition of overweight and obesity as having too much body fat—so much so that it “presents a risk to health.”5 According to the National Obesity Observatory, excess body fat is linked to both current and future morbidity.4
“BMI is useful in two ways. When it’s 30 or higher, unless they’re a body builder, there’s no doubt that that person has some health risks; they must have excessive amounts of fat,” Lopez-Jimenez says. “In that sense, to identify those in that extreme range is extremely useful and simple. There’s no other measurement needed because that person certainly has a lot of body fat.
“The second way that BMI is useful is in the obese who already engage in a weight-loss program; in this case, BMI may reflect improvement, particularly if the weight-loss program was based only on diet and not on intense exercise,” he adds.
Tying BMI to Health Outcomes
The World Health Organization relates high BMI to 21% of the cases of ischemic heart disease, 23% of ischemic stroke, 58% of type 2 diabetes, and 39% of hypertension. Obese BMI levels also are linked to increased cancer risk as well as other concerns, including joint problems and infertility.6 This trend also is true for mortality.7,8
For most people, when they fall into high BMI categories or gain weight, it’s due to increased levels of body fat, not body muscle. Studies have shown that as people gain weight, their disease risk rises. Data from the Nurses’ Health Study and the Health Professionals Follow-Up Study found that middle-aged women and men who gained 11 to 22 lbs after the age of 20 were up to three times more likely to develop heart disease, high blood pressure, type 2 diabetes, and gallstones than those who gained 5 lbs or fewer. Those who gained more than 22 lbs had an even greater risk.9-13
In a new study from King’s College, London, patients with higher BMIs were found to be at risk of multiple chronic conditions, and young obese patients had disease profiles similar to older nonobese people.14 The Prospective Studies Collaboration, which included nearly 900,000 participants, found that mortality was lowest when BMI was about 22.5 to 25. At 30 to 35 BMI, median survival was reduced by two to four years, and at 40 to 45 BMI, it was reduced by eight to 10 years, which is comparable to the effects of smoking.15
However, the evidence that BMI is a good predictor of disease risk isn’t as conclusive as some experts may hope. According to Lopez-Jimenez, the body of evidence relating BMI to disease risk is controversial at best. In a systematic review of cohort studies, which he coauthored, looking at body weight and total mortality and cardiovascular events, people with low BMI had an increased risk of total mortality and cardiovascular mortality; overweight BMI had the lowest risk of total mortality and cardiovascular mortality; and obese BMI had no increased risk of total mortality or cardiovascular mortality compared with people who had a normal BMI. Patients with severe obesity (BMI of 35 or higher) didn’t have an increased risk of total mortality, although they had the highest risk of cardiovascular mortality. The researchers concluded that these findings may be explained by the lack of discriminatory power of BMI to differentiate between body fat and lean mass.16
A cross-sectional study of adults assessed by BMI and dual-energy X-ray absorptiometry (DXA), which measures bone density, found that 39% of subjects were misclassified as nonobese by BMI but were found to be obese by DXA.17 And in a new systematic review and meta-analysis of all-cause mortality related to BMI, overweight was associated with significantly lower all-cause mortality, and grade 1 obesity (BMI of 30 to less than 35) wasn’t linked with higher mortality, though grades 2 and 3 obesity (BMI of 35 or higher) were associated with significantly higher all-cause morality compared with normal weight.18
The conflicting research indicates that BMI has its drawbacks. “BMI is only a ratio for weight and height; it’s not a body fat indicator,” Frechman says. Factors such as fitness or muscle mass and ethnic origin can alter the relationship between BMI and body fatness. It also doesn’t provide any indication of the distribution of body fat and doesn’t fully adjust for the effects of height or body shape.4
“The fact that BMI doesn’t differentiate between fat, muscle, or bone mass can be a limitation. It may lead to inaccurate assumptions in certain patients,” says nutrition consultant and weight-loss expert Christopher R. Mohr, PhD, RD, of Mohr Results. “There are times that a BMI is calculated and shows a patient as overweight, but the individual has excess muscle rather than fat. BMI also doesn’t provide any information on the distribution of the excess weight.”
“The differences in BMI when people are near normal BMI can be due to bone mass or muscle mass,” Lopez-Jimenez says. “In that group, it’s a measurement intended to measure something bad, which is fat, but it also measures things that are good, which is muscle and bone. When you measure something bad at the same time you measure something good, you end up mixing the good and the bad in the end result.
“In people with BMI in the normal or overweight range, BMI isn’t telling the whole truth; people might still be at a very high risk of diabetes or cardiovascular disease on the basis of fat distribution and how much body fat exists, regardless of BMI,” he continues. “BMI can give a false sense of reassurance; a person may not be obese based on the BMI, but they may be at risk due to body fat. Using only the BMI, we’re not identifying everyone at risk of unhealthful fat distribution. And for people in a lifestyle-change program, where they’re incorporating a serious exercise program, including stretching, weight lifting, plus diet, BMI or weight may not change much. BMI may not reflect all of the good changes that are happening in the body, such as lower total body fat, increased muscle mass, and better fat distribution.”
Lopez-Jimenez describes the results of his recent study, which investigated people who enrolled in a wellness center.19 “After follow-up, many barely saw any difference in BMI; the weight didn’t change much. But we saw a significant amount of people who had favorable changes. Up to 30% had no weight change, but they had increased lean mass and decreased body fat. Even among those who gained a few pounds, there was a one-third increase in weight due to increase in lean mass, but fat didn’t go up. We’re talking about people who otherwise would have been labeled in a program that was a waste of time, yet significant changes did occur.”
According to Frechman and Lopez-Jimenez, there are other tools that may provide a better assessment of body fat, including waist circumference and waist-to-hip ratio. The National Institutes of Health agrees, suggesting that because of BMI’s possibility for error, physicians should add waist circumference to their assessment tools.1
Other body fat tests include skin-fold measurements, where a fold of skin is pinched to measure the subcutaneous fat layer just beneath the skin); DXA; or bioelectrical impedance, which measures the opposition to a flow of electrical current through the body.3 Additional methods include the Bod Pod (based on air displacement plethysmography) and CT and MRI scans. These techniques may be more precise, but a trained medical professional must administer them. Moreover, they may be more expensive, are difficult to perform on large populations, and lack as much clinical research support as BMI.3
Putting It Into Practice
Given the latest research on BMI, how should dietitians best assess obesity? Jessica Crandall, RDN, CDE, a national spokesperson for the Academy says, “BMI is a simple, inexpensive way to assess my patients. I generally calculate it for all of my patients. Sometimes it’s just for my own knowledge; however, if a patient asks, I’m happy to share what it means. I do use other tools as well, such as a scale that calculates body fat percentage. I also recommend that my clients track their weight loss with body measurements in addition to the scale. I have patients that don’t see the pounds coming off when they step on the scale, but their waist circumference or bicep measurement has decreased. This can encourage them and help them to stick to their diet plan.”
“There are limitations,” Mohr says, “but BMI is a tool nonetheless and can be a useful piece to the overall health puzzle. Of course, if most of your clientele are athletes, BMI won’t’t be quite so useful, as it doesn’t differentiate between lean body mass and fat. For simple measurements, I really like waist circumference; it’s easy, it’s inexpensive, and some data suggest it’s a better marker for overall health than BMI. This also gives a tangible number. Like BMI, the scale is just a tool. I think using a few different tools to measure success are important, with the most important one being how you feel.”
“It’s not that BMI is totally useless; that’s not fair or accurate,” Lopez-Jimenez says. “In the same way, it would be wrong to say that BMI is the answer and best way to detect obesity-related risk. Use BMI for patients who are above 30; for those that aren’t above this BMI, use waist-to-hip ratio, and if that’s normal, then that person has no obesity-related risks.”
— Sharon Palmer, RD, is a Los Angeles-based foodie, the author of The Plant-Powered Diet, the editor of the Environmental Nutrition newsletter, and a contributing editor at Today’s Dietitian.
Body Mass Index
Formula and Calculation
Kilograms and meters (or centimeters)
Formula: weight ÷ (height)2
With the metric system, the formula for BMI is weight in kilograms divided by height in meters squared. Since height commonly is measured in centimeters, divide height in centimeters by 100 to obtain height in meters.
Example: weight = 68 kg, height = 165 cm (1.65 m)
Pounds and inches
Formula: [weight ÷ (height)2] x 703
Calculate BMI by dividing weight in pounds by height in inches squared and multiplying by a conversion factor of 703.
Example: weight = 150 lbs, height = 5'5" (65")
— Source: Centers for Disease Control and Prevention
Interpreting Body Mass Index
18.5 to 24.9
25 to 29.9
30 and above
1. Assessing your weight and health risk. National Heart, Lung, and Blood Institute website. http://www.nhlbi.nih.gov/health/public/heart/obesity/lose_wt/risk.htm. Accessed October 15, 2013.
2. Eknoyan G. Adolphe Quetelet (1796–1874)—the average man and indices of obesity. Nephrol Dial Transplant. 2008;23(1):47-51.
3. About BMI for adults. Centers for Disease Control and Prevention website. http://www.cdc.gov/healthyweight/assessing/bmi/adult_bmi/index.html. Last updated September 13, 2011. Accessed October 15, 2013.
4. National Obesity Observatory. Body Mass Index as a Measure of Obesity. London, England: Public Health England; 2009.
5. Obesity. World Health Organization website. http://www.who.int/topics/obesity/en/. Accessed October 15, 2013.
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19. Cruz P, Johnson BD, Karpinski SC, et al. Validity of weight loss to estimate improvement in body composition in individuals attending a wellness center. Obesity (Silver Spring). 2011;19(11):2274-2279.