B. Obtain Height and Weight; Calculate Body Mass Index (BMI)

objective

Screen all adults for overweight or obesity.

background

Though there are several ways to estimate body fat (e.g., skin-fold calipers, hydrodensitometry, dual energy X-ray absorptiometry, and bioelectrical impedance), most are not readily available or convenient in the clinical setting.  The body mass index (BMI) is recommended as a practical screening tool to determine overweight and obesity in adult populations due to its ease of obtaining and use. 

In this guideline, overweight and obesity are defined according to the 1998 NHLBI classification (see Table 1).  The classification is based primarily on the associations between BMI, chronic disease, and mortality.  The relation between BMI and disease risk varies among individuals and among different populations.  For example, individuals who are short in stature or who have a relatively high muscular mass may fall into the overweight category by BMI but may not be at increased risk of obesity-associated conditions.  Therefore, this classification must be viewed as a broad generalization.

For the BMI Calculation Chart, see Appendix B.  Additional BMI calculators and tables can be accessed at: http://www.cdc.gov/nccdphp/dnpa/bmi/.

When obtaining weight and height, healthcare providers should be sensitive to the needs of obese patients.  Many obese patients require appropriate sized blood pressure cuffs, wide-based armless chairs, and scales that measure individuals greater than 350 pounds.

recommendations

1. Adult patients should have their BMI calculated from their height and weight to establish a diagnosis of overweight or obesity.  [B]
2. Obese patients (BMI ≥ 30 kg/m ² ) should be offered weight loss treatment.  [B]
   (See Module B: Treatment for Weight Loss and Weight Maintenance)
3. Overweight patients (BMI between 25 and 29.9 kg/m ² ) or patients with increased waist circumference (> 40 inches for men; > 35 inches for women) should be assessed for the presence of obesity-associated conditions that are directly influenced by weight, to determine the benefit they might receive from weight loss treatment.  [B]
4. Normal weight patients (BMI between 18.5 and 24.9 kg/m ² ) should be provided with education regarding healthy lifestyle behaviors, advised of their BMI and their weight range margins, and instructed to return for further evaluation should those margins be exceeded.  [Expert Opinion]

discussion

Presently, there is no precise clinical definition of obesity, based on the degree of excess body fat that places an individual at increased health risk.  General consensus exists for an indirect measure of body fat, called the weight for height index or body mass index (BMI).  The BMI is an easily obtained and reliable measurement for overweight and obesity and is defined as a person’s weight (in kilograms) divided by the square of the person’s height (in meters).  If weight is measured in pounds and inches, the BMI is calculated as [weight (in pounds)/height (in inches)²]x 703 (McTigue et al., 2003; NHLBI, 1998, Qeutelet, 1869).  Obesity cut-offs based on mortality risk are defined in body mass index units of kilograms per meter squared (kg/m ² ).  (WHO, 2000)

Although BMI is commonly used to identify obesity, there are questions regarding how accurately BMI can determine body composition and identify obese from non-obese individuals.  In a study by Frankenfield et al. (2001) obesity was defined as body fat of at least 25 percent of total body mass for men and at least 30 percent for women.  Obesity based on body fat was always present in subjects with a BMI of at least 30 kg/m ² .  However, 30 percent of men and 46 percent of women with a BMI below 30 kg/m ² had obesity levels of body fat.  The greatest variability in the prediction of percentage of body fat and body fat divided by height (m ² ) from regression equations using BMI was at a BMI below 30 kg/m ² .  In conclusion, using impedance derived body fat mass as the criterion, people with a BMI of at least 30 kg/m ² are obese.  However, significant numbers of people with a BMI below 30 kg/m ² are also obese and thus misclassified by BMI.  These results suggest that evaluation of body fat by measurement of WC may be a more appropriate way to assess obesity in people with a BMI below 30 kg/m ² .  (See Annotation C)

Whereas little evidence exists from prospective studies showing that weight loss improves long-term morbidity and mortality, strong evidence suggests that obesity is associated with increased morbidity and mortality and that weight loss in obese persons reduces important disease risk factors (NHLBI, 1998).  In adults, disease risk increases independently with increasing BMI and excess abdominal fat.  Cardiovascular and other obesity related disease risks increase significantly when BMI exceeds 25 kg/m ² .

Obesity-- Overall, mortality begins to increase with BMI levels greater than 25 kg/m ² and increases most dramatically as BMI levels surpass 30 kg/m ² .  An almost linear relationship between BMI and mortality is found in adults with a BMI of 30 kg/m ² or above (obese) (WHO, 2000).  A largely linear relationship is found between body weight and conditions such as coronary heart disease (CHD), hypertension, and type 2 diabetes mellitus (Must et al., 1999; WHO, 2000).  Based on these clear relationships, all adults with a BMI of 30 kg/m ² or above should be offered weight loss treatment.

Overweight-- For adults with a BMI of 25 to 29.9 kg/m ² (overweight), the relationship between body weight and mortality is less clearly defined (Heiat, 2003; Heiat et al., 2001; Strawbridge et al., 2000).  Furthermore, some adults with a BMI lower than 30 will have a disproportionate amount of abdominal fat which increases their cardiovascular risk despite their low BMI (NHLBI, 1998). Waist circumference (WC) measurements greater than 40 inches (102 cm) in men and 35 inches (88 cm) in women do indicate an increased risk of obesity related comorbidities.

Most overweight individuals are considered at increased risk for developing obesity-associated morbidities such as hypertension and type 2 diabetes (Must et al., 1999; WHO, 2000).  Thus, the decision to refer overweight patients for weight loss treatment should be made in the context of assessments of obesity-associated conditions that are known to increase health risks and patient preferences. (NHLBI, 1998). (See Annotation D)

While there is evidence that the BMI level associated with increased disease risk differs between ethnic groups, (Fernandez et al., 2003; Mozumdar & Roy, 2004; Tzamakoukas, et al., 1994) more data are needed to generate clear ethnic group specific cut-points for treatment of overweight (NHLBI, 1998).

Notably, there is also an on-going debate surrounding the mortality implications for overweight in those over 65 years of age.  Numerous studies have demonstrated that overweight individuals over 65 years of age do not have a higher risk of death than their normal weight peers (Heiat et al., 2003).  Thus, among those over 65 years of age, the relationship between BMI and mortality risk is best described as a ‘‘U-shaped curve, with a large, flat bottom and a right curve that does not begin to rise significantly until BMI is greater than 31 to 32 kg/m ² ’’ (Heiat et al., 2001).

Normal weight--  In general, the lowest mortality risk is associated with a BMI between 18.5 and 24.9 kg/m ² (normal weight).  These individuals should be advised to maintain their current body weight since weight gain, even within the normal range, may be associated with increased risk of chronic medical conditions (WHO, 2000).