DOES SITE MATTER?
It is evident that there is confusion as to the clinical utility of each particular WC measurement site. Two main issues to be reconciled before determining the best site for WC measurement are 1) which site has the greatest intratester and intertester reliability? and 2) which site provides the best estimate of VAT and therefore cardiometabolic risk? Several studies haveshown that reliability ofWC is generally good across all WC measurement sites (10,14,18,28). This means that, whether using the ACSM-recommended minimal waist or AHA-recommended iliac crest, trained professionals taking the measurement on the same client should produce similar results. These findings also demonstrate that WC measurements are reliable when performed repeatedly by a single trained clinician. Despite this reliability, it is an unfortunate fact that WC results will vary greatly depending on which site is selected for the measurement. Measurement site selection is particularly important in women with values observed at different sites leading to disparate conclusions about risk (18). Recognizing how site selection impacts WC measurement is critical to subsequently interpret cut point values and ultimately determine health risk status.
Although the WC measurement site suggested by major health organizations differs, there currently is only one set of values widely used as cut points for health risk classification. These were established in 1995 by Lean et al. (16) in Glasgow, Scotland, while measuring WC at the midpoint between the lowest rib and iliac crest. They determined that a man should not gain more weight if WC > 94 cm and a woman should not gain more if WC > 80 cm (i.e., “increased” risk). A man should lose weight with a WC > 102 cm and WC > 88 cm for a woman (i.e., “high” risk). These cut points for WC were based on BMI, with BMI > 25 kg/m2 considered “increased” health risk, whereas a BMI > 30 kg/m2 is indicated as a “high” health risk category. Lean et al. (16) used only BMI and did not consider VAT or other cardiometabolic risk factors (e.g., hypertension, high cholesterol) when establishing the WC cut points. Even though using BMI alone provides a fallible indicator of overall cardiometabolic risk, the WC cut points determined by Lean etal. (16) were widely adopted by major health organizations. NHLBI (1) suggests measuring WC superior to the iliac crest but uses cut points of Lean et al. (16) developed for measurement between the lowest rib and the iliac crest. Using cut points this way can lead to misclassification of health risk (28).
In a study comparing WC measurement sites, while using metabolic screening and CT scans, it was demonstrated that 54% more men and 68% more women were at risk for cardiometabolic disease using the umbilicus compared with the minimal waist measurement (29). In other words, a lot more people are classified as “at risk” when WC is measured at the umbilicus. Recognizing this, several researchers proposed new cut points based on measurement site (18) and BMI (2). Ethnicity (4) and age (23) also have been considered in the development of cut points because they may affect fat deposition patterns. For example, African Americans tend to have less VAT deposition, whereas Asians tend to have more, than whites with the same WC (4,19,24). We also know that, with age, abdominal fat mass typically increases, leading to different meanings for the same WC in a 25- versus 75-year-old (9). New cut points are being developed to address site and population specifics (Table 2), but it should be noted that none of these values are widely adopted.
In a further attempt to determine how measurement site affects cardiometabolic risk classification, Caitlin Mason, Ph.D., and Peter T. Katzmarzyk, Ph.D., compared WC across four anatomic sites on 520 people (19). They took the WC at the umbilicus, midpoint between the lowest rib and the iliac crest, superior to the iliac crest, and at the minimal waist. Risk factors examined were resting blood pressure, fasting total cholesterol, high-density lipoprotein cholesterol, triglycerides, and glucose. They defined elevated cardiometabolic risk as two or more risk factors. Sensitivity and specificity of each WC measurement site were examined using the cut points developed by Lean et al. (16). Note that sensitivity is a measure of a test’s ability (e.g., WC) to predict the presence of a condition (e.g., metabolic syndrome), whereas specificity is the test’s ability to predict the absence of the condition. Mason and Katzmarzyk (19) reported that the measurement site chosen significantly affected prediction of cardiometabolic risk. In another study, they also reported (18) that the various WC measurement sites led to classification of high health risk caused by abdominal obesity (>102 cm) in as few as 23% (minimal waist) or as many as 34% (umbilicus) of 223 men studied. Site choice led to more dramatic differences in the women studied. Of 319 women, 31% were classified with a high health risk caused by abdominal obesity (>88 cm) using the minimal waist, but 55% were classified with a high health risk caused by abdominal obesity when umbilicus was the site (18). These findings highlight the importance of having a single accepted WC measurement site because disease risk prediction will vary depending on the site chosen.
WHICH IS THE BEST SITE?
There is currently no consensus on the best site for WC measurement, but emerging information is promising. New research suggests that measuring WC below the lowest rib may be best for identifying cardiometabolic disease risk based on VAT deposition patterns (24,29). The “lowest rib” site is the most anatomically superior WC site (i.e., anatomically located above minimal waist, umbilicus, and other recommended WC measurement sites), and more metabolically active VAT is found in the upper than lower abdomen (6,15). The suggestion to use a more superiorly located abdominal site for WC measurement is further strengthened by CT imaging studies of VAT at L1 to L2 (15) and at L2 to L3 of the lumbar spine (5,8). In contrast, the site just above the iliac crest has been shown to have the greatest correlation with total percent body fat, but this is not proven to be predictive of cardiometabolic risk (28). Ironically, an expert panel of the AHA recently recommended in a consensus statement that the iliac crest is the preferred WC measurement site (4).
In an effort to determine the best measurement site, Bosy-Westphal et al. (3) compared WC measures below the lowest rib, above the iliac crest, and midway between the lowest rib and the iliac crest. They examined 528 people and also followed 75 participants after dietary intervention for weight loss. MRI was used to determine VAT and SAT volumes, and metabolic risk factors (e.g., blood pressure, lipids, glucose) also were screened. This study allowed comparison of cardiometabolic risk, SAT, and VAT volume for each WC measurement site. Measuring WC below the lowest rib was the only site able to predict weight loss associated with decreased VAT in women. WC measurements at the iliac crest had a very low association with cardiometabolic risk factors and VAT in women. On the other hand, the site below the lowest rib had a strong correlation (r = 0.70 in women, r = 0.74 in men) with total VAT and cardiometabolic risk. The lowest rib is a fixed landmark (unlike the umbilicus) that will not move if body composition changes, and it is easily identified by a clinician. In summary, there is good logical and scientific reason to consider the site below the lowest rib the preferred WC measurement site.
IS WC A USEFUL CLINICAL MEASUREMENT?
Despite complications, research generally agrees that WC is avaluable clinical measure. WC appears to be a better indicator of cardiometabolic risk than solely using BMI (12,13,27) or WHR (22,23). However, when used in conjunction with BMI, WC measurements can effectively identify cardiometabolic disease risk within a BMI (kg/m2) category, particularly for those of a normal weight (22). In fact, ACSM (1) in its book ACSM’s Guidelines for Exercise Testing and Prescription (GETP) (see Table 4.1 in reference 1) encourages the use of WC together with BMI to determine disease risk. As an example, this table in the GETP (1) classifies a woman with a BMI of 30 to 34.9 as “high” risk if her WC is less than 88 cm but “very high” risk if her WC is more than 88 cm. Therefore, combining use of WC and BMI is valuable; however, the simple measurement of WC alone remains an effective means to identify risk for cardiometabolic disease.
Beyond identifying cardiometabolic risk, WC also is a low-cost measure that can be used to assess the effects of an intervention (e.g., altering diet or exercise habits). As Després et al. (6) noted, “weight loss… has been reported to induce a substantial mobilization of abdominal and visceral adipose tissue among high-risk abdominally obese patients.” BMI (or WHR) may not account for body composition changes that would lessen the risk of cardiometabolic disease in a client who has lost weight (12). WC, when consistently measured, may allow identification of risk-related changes that may otherwise only be seen using specialized imaging equipment or metabolic screening. WC changes are proven to be positively related to diet or exercise-induced reductions in VAT (26), and an NHLBI expert panel recommended WC measurement be used to follow changes in adiposity with weight loss (20). In conclusion, WC is a simple measure that is a potentially important predictor of abdominal obesity and cardiometabolic risk.
HOW DO WE USE WC MEASUREMENTS IN PRACTICE?
WC measurements are best used for repeated comparisons on a given client to assess if interventions such as exercise programming (or nutritional counseling) are effective at impacting VAT and related cardiometabolic risk. Using WC in your facility requires choosing a single measurement process. Train and ascertain that all professional staff consistently can apply this measurement through careful practice and evaluation. At this point in time, we recommend measuring WC below the lowest rib because the research described above (3,15,24,29) leads us to believe that it is the most clinically relevant measurement site. In your facility, however, you may choose to apply the ASM-recommended minimal waist or the ACSM recommended superior to the iliac crest as used in the large-scale national (i.e., NHANES III) survey that found this site to be effective at predicting metabolic risk. In any case, WC measured at a single site, and applied with careful/consistent measurement technique, should provide valuable information for your clients. Understand that choosing a measurement site for your facility can make your assessments less (e.g., minimal waist) or more (e.g., umbilicus) conservative, leading to less or more obesity classifications, respectively. Clinicians are urged to vigilantly watch for changing standards. For example, we might see major international organizations agree on measuring WC at a single site (maybe below the lowest rib) in the near future. However, there are currently no cut points specifically designated for the “lowest rib” site. For now, we recommend using Lean etal. (16) values as cut points for all WC sites because these values are the only standards widely accepted. Future research should be directed at developing acceptable cut points across ethnicities, age, and gender for WC measurement sites.
It helps us, as clinicians, to understand the prevailing limitations of the health assessments we use. Despite shortcomings (as seen with most health assessments), WC is a simple, affordable, and practical clinical tool that should be widely used. Hopefully, standardized protocols for WC measurement and appropriate cut points will be adopted by major organizations and be available for our clinical use in the near future. Until then, judiciously apply the valuable measure of WC for the benefit of your clients.
CONDENSED VERSION AND BOTTOM LINE
Waist circumference (WC) measurement is widely recognized as a screening tool for cardiometabolic risk. It is a better indicator of visceral adipose tissue (VAT) and metabolic risk than body mass index or waist-to-hip ratio. Despite this, major health organizations have not come to a consensus on an appropriate WC measurement site or values for risk cut points. WC measured below the lowest rib may be the best indicator of VAT and health risk compared with other measurement sites. Future research efforts should focus on standardizing a measurement protocol and developing appropriate and accepted values for cut points. In the meantime, we can still carefully use WC as a clinical tool and estimate of cardiometabolic risk. Clinicians can determine effectively if an intervention (e.g., an exercise or weight loss program) is having an impact on WC (and cardiometabolic risk) by establishing a consistent measurement protocol for use in their facility.
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Keywords:© 2013 American College of Sports Medicine.
Anthropometric Measures; Abdominal Adiposity; Risk Stratification; Visceral Fat; Health Assessment