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ADDING A NEW TECHNIQUE TO ASSESS VISCERAL OBESITY TO YOUR REPERTOIRE

Sagittal Abdominal Diameter

Van Guilder, Gary P. Ph.D.; Kjellsen, Alicia M.S. ACSM-EP

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doi: 10.1249/FIT.0000000000000543
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Abstract

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Visceral obesity, reflected by increased fat tissue deposition in and around abdominal organs (i.e., intra-abdominal or belly fat), is a strong predictor of early death from cardiovascular diseases (1–3) and type 2 diabetes (4). Owing to their convenience and ease of use, many anthropometric techniques, particularly BMI, waist circumference, and waist-hip ratio are widely used in clinical settings to estimate visceral obesity (5). An alternative method to measure visceral obesity is to quantify the height of the abdomen in the sagittal anatomical plane, referred to as sagittal abdominal diameter (SAD). The sagittal plane runs parallel to the long axis of the body, separating it into left and right sections. With SAD, we are observing (measuring) the height of the abdomen along this axis.

An alternative method to measure visceral obesity is to quantify the height of the abdomen in the sagittal anatomical plane, referred to as sagittal abdominal diameter (SAD).

In addition to other common measures of abdominal obesity, including waist circumference, waist-hip ratio, and BMI, SAD is a good predictor of chronic disease risk, including risk of death from cancer (6–8). Generally, as the amount of abdominal fat mass increases—whether estimated by waist circumference, waist-hip ratio, or SAD—the risk of disease tends to increase. However, many studies have demonstrated that SAD is independently and more strongly linked with heart disease risk factors (e.g., high blood pressure [BP], increased cholesterol, etc.) and incident heart disease than other common field measures of obesity (6,8–13). It is important to point out, though, that not all studies support this notion (14). For example, in normal weight women, waist-hip ratio and waist circumference were better predictors of heart disease than SAD (15). By contrast, in normal weight men, SAD was a better predictor of heart disease than waist circumference and waist-hip ratio. A more recent study reported that SAD provided no additional benefit to other obesity measures in predicting incident severe liver disease (16). Taken together, because of the inconsistencies in the literature, the health and fitness professional should not use SAD in clinical practice to estimate disease risk. It should only be used to estimate visceral fat mass. Studies are currently being conducted to better identify specific SAD values that predict increased disease risk.

One important feature of SAD is that it estimates the amount of visceral fat better than other common field measures of obesity. This is partly due to the fact then when a subject lies in the supine position, gravity allows the subcutaneous fat in the abdomen (located superficially beneath the skin) to shift downward and laterally. The displacement of subcutaneous fat away from the longitudinal axis of the sagittal plane allows the technician to isolate more visceral fat. By eliminating some of the impact of subcutaneous fat on the height of the abdomen, a more accurate estimation of visceral obesity is obtained. Many clinical studies indicate that SAD, whether assessed directly with computed tomography or via measurement of abdominal height, more precisely estimates the amount of visceral fat compared with waist circumference (17–19), even after taking into account total body adiposity and heart disease risk factors. In fact, visceral fat mass rather than subcutaneous fat better identified overweight and obese individuals with more severe metabolic risk profiles— important information on risk stratification that was not provided by waist circumference or BMI (20). This finding is critical because it implies that estimations of visceral fat via a simple and reliable assessment of SAD may reveal more individuals with a risky body fat distribution. Such individuals might benefit more from the implementation of SAD into risk factor assessments. Certainly, from a community health perspective, traditional anthropometric screening using only waist and BMI measurements may misclassify risk in some people. This is important because certain individuals, particularly the elderly and obese, possess relatively more abdominal visceral fat for the same waist circumference (21). Therefore, the utility of waist circumference to estimate visceral fat may wane with age-related increases in total body adiposity. However, using only SAD as a screening tool may also misclassify risk. A comprehensive assessment method that uses a variety of anthropometric techniques to estimate overall body fat mass, as well as visceral fat mass, is recommended. The message that we want to convey in this article is to use SAD along with other common anthropometric techniques to better quantify abdominal obesity.

…Certain individuals, particularly the elderly and obese, possess relatively more abdominal visceral fat for the same waist circumference.

To further improve the comprehensiveness of a health assessment in obese persons with elevated abdominal fat mass, it is essential that health and fitness professionals also include measures of cardiorespiratory fitness. We now know that regular physical activity (22,23) and higher cardiorespiratory fitness (24–26) alleviate the unfavorable influence of obesity on all-cause and cardiovascular disease mortality. In other words, fitness provides a substantial level of health protection, even in obese persons, and should be included in clinical health assessments, as suggested in the American Heart Association’s position stand on the importance of assessing cardiorespiratory fitness in clinical practice (27). Collectively, it would be pragmatic to include SAD in health screenings to better identify clients/patients with excess visceral fat who require a more individualized health/exercise programming treatment approach. Without an accurate anthropometric assessment of visceral adiposity per se, the extent of cardiometabolic risk may be under/overestimated in some people and the potential for health and exercise promotion programs to reduce the burden of chronic disease risk may be limited.

Despite its potential to identify people with elevated disease risk and utility in predicting visceral fat mass, SAD has not been widely adopted in clinical practice or in community-wide health promotion programs. To address this concern, SAD is currently included as part of the U.S. National Health and Nutrition Examination Survey (NHANES 2011–2012) to determine its relation to disease risk. NHANES is a population-based survey designed to collect information on the health and nutrition of the U.S. household population. NHANES data are used to inform policy decisions regarding health programs and services. Currently though, the health and fitness community does not have a standardized measurement protocol for SAD. Here we provide a straightforward diagram with detailed step-by-step instructions for the health and fitness professional to apply the use of SAD as an alternative to estimate visceral fat mass. In turn, as part of healthy lifestyle interventions, greater emphasis on documenting and tracking visceral fat mass with a variety of anthropometric techniques, including SAD, will play an important role in improving the assessment of health risks related to increased body weight in clinical practice. Lastly, at present the clinical applications of SAD are limited by the lack of normative reference values and cut points to categorize risk in a representative U.S. population. In this article, we provide evidence-based SAD normative reference values derived from recent NHANES epidemiological data to identify those with elevated visceral fat mass.

Why Is Too Much Visceral Fat Bad?

Not only is it important to reliably estimate the amount of visceral fat, but it is equally critical to convey to a client or patient why too much visceral fat is unhealthy. In general, visceral fat accumulation (aka visceral obesity) refers to increased deposition of fat tissue in and around major organs and tissues such as the liver, heart, spleen, pancreas, and stomach. Other terms commonly used are “central obesity,” “abdominal obesity,” and “ectopic (abnormal) obesity.” Visceral obesity is well recognized to be a primary factor contributing to chronic disease, especially heart disease. In fact, excess visceral fat is an independent predictor of cardiovascular death resulting from heart attacks. Interestingly, although the cardiovascular and metabolic problems associated with visceral obesity are due in part to accompanying risk factors (i.e., high BP, elevated bad cholesterol, high blood glucose, etc.), they are often independent of overall body fat mass. This is important because it implies that too much fat located viscerally is unhealthier than fat located subcutaneously (see Figure 1). But how does excess visceral fat contribute to an unhealthy scenario? Current research is still trying to answer this question, but we do know some important factors.

Figure 1
Figure 1:
Process by which visceral obesity is thought to contribute to cardiometabolic disease. A critical feature of visceral obesity is the accumulation of fat in and around major organs. This “ectopic fat” is metabolically dysfunctional, contributing to altered fat and cholesterol metabolism and elevated bood sugar. Over time, function of the major organs deteriorates, leading to elevations in cholesterol, triglycerides, BP, and eventually vascular disease. Modifed with permission from SpringerNature in partnership with the Copyright Clearance Center’s RightsLink® service (32).

As shown in Figure 1, unhealthy lifestyle behaviors such as physical inactivity, smoking, and overeating combined with an unfavorable genetic background that shifts fat distribution from normal subcutaneous locations to undesirable intra-abdominal locations. One would not expect fat to pack in and around one’s major organs. In this situation, sedentary people are unable to store extra energy in their subcutaneous locations. Fat then accumulates in the visceral regions, a phenomenon described as ectopic fat deposition, as shown in Figure 1. A critical feature to appreciate is that ectopic fat is a live and active endocrine organ, albeit a metabolically dysfunctional and unhealthy organ. In contrast to healthy levels of subcutaneous fat where ectopic fat is low (and people more often present with normal cardiometabolic features), visceral fat is characterized by abnormal fat metabolism, increased fat released into the blood, liver dysfunction, and elevated blood sugars. As a result, the SAD will be greater. Over time, too much fat released into the blood leads to an altered metabolic profile characterized by increased cholesterol, elevated BP, increased triglycerides, and insulin resistance. In turn, these risk factors can damage the vascular system and contribute to heart disease.

A critical feature to appreciate is that ectopic fat is a live and active endocrine organ, albeit, a metabolically dysfunctional and unhealthy organ.

STANDARDIZED PROCEDURE FOR MEASURING SAD IN ADULTS

Equipment

  • Cosmetic pencil
  • Alcohol swabs
  • Measuring tape
  • Treatment table
  • Sliding-beam, abdominal caliper (Figure 2)

Not only is it important to reliably estimate the amount of visceral fat, but it is equally critical to convey to a client or patient why too much visceral fat is unhealthy.

Figure 2
Figure 2:
Sliding-beam caliper. A sliding-beam caliper is needed to correctly measure adominal height. The caliper is used to measure the distance between a patient’s lower back and the front of their abdomen while lying down on a medical table. A typical caliper will have two arms, one that slides up and down, and a stable reference arm that is placed under the patient’s lower back. The technician will hold the vertical shaft and record the measurement that is displayed on a centimeter scale. Sliding-beam calipers have a measurement range of 10 cm to 50 cm.

NORMATIVE VALUES

In addition to documenting the values of SAD that you obtain, consult the table to identify the percentile ranking that corresponds to the value. The table provides evidence-based SAD normative reference values derived from recent NHANES epidemiological data to identify those with elevated visceral fat mass. These data were collected in 2011 to 2012 from 4,817 U.S. adults representative of the demographic makeup of the American population (28). The average SAD value in this sample of people was 22.5 cm. Generally, the average SAD values are higher in men compared with women and increases with age. However, in contrast to waist circumference for example, it is important to note that there are no established values for SAD that can be used in clinical practice to estimate disease risk. Therefore, the table should not be used to estimate disease risk. Nevertheless, we can review published data to obtain a ballpark SAD value that is linked to disease. Risérus et al. (29) indicated that SAD levels above 22 cm for men and 20 cm for women is associated with elevated disease risk. Other data from the 15-year longitudinal Paris Prospective study reported that an SAD of 21 cm was associated with increased cancer mortality (30). In addition, Empana et al. (31) showed that the multivariate adjusted relative risk of sudden death was increased in middle-aged men with elevated SAD levels (>23 cm), regardless of whether they were normal weight (i.e., BMI < 25 kg/m2) or overweight (BMI ≥ 25 kg/m2). Altogether, these estimates correspond to the 50th percentile for men and women as shown in the Table 1. For example, for a 56-year-old woman with an SAD value of 21.8 cm, the normative reference table indicates that this patient would be in the 50th percentile. This means that half of the population would have a SAD value above her 21.8 value, and half of the population would have a value below 21.8. Lastly, to reiterate, the health and fitness professional should not use the SAD percentiles shown in the table to categorize disease risk in clinical practice. The table is only meant to provide values of SAD that represent the average American adult in the supine position.

TABLE 1
TABLE 1:
Normative Reference Values of SAD in U.S. Adults, from NHANES 2011–2012

Recommended Script When Performing Sagittal Abdominal Height Measurements

“Now I am going to take a measurement of your abdominal height. Measuring the height of your abdomen allows us to obtain a better indicator of belly fat stored around your organs. Too much belly fat can lead to chronic diseases such as heart disease and diabetes.

I want you to lie down on the treatment table with your knees bent and your feet resting flat. Place your arms crossed over your chest. Now I am going to make a mark on your skin near your right hip and your belly button.

I am going to perform at least two measurements. I am going to slide this caliper under your lower back. Please raise your hips just a little bit. Take a gentle breath in and slowly exhale. At the end of your exhalation, I want you to pause and relax while I obtain the reading. Great, now we will repeat the measurement again.”

SPECIAL CONSIDERATIONS

There are certain instances and special considerations that the health and fitness professional should be aware of when implementing SAD into practice. First, while the images in Figure 3 in a normal weight individual are intended to depict the procedures for measuring SAD, you may find it more difficult in clinical practice when applying the technique to a person with a larger waist. Given the challenge in identifying anatomical landmarks in persons with increased body fat, it is recommended to practice SAD several times in a variety of people with different waist sizes before implementing its use into regular practice. In this case, a larger caliper may be required. A typical caliper will provide a measurement range of 36 cm, but larger calipers with ranges up to 50 cm are available. Second, a routine exam table may not be readily available to the practitioner. Should you encounter this scenario, a reasonable approach would be to obtain a measurement of abdominal height in the standing position. Position the client/patient standing erect with feet together and arms at their sides. Apply the standardized measurement procedures described in Figure 3 but modify accordingly. For example, instead of moving the caliper arm up and down vertically, you will need to move the caliper in the horizontal plane, or in other words from side to side when the patient is standing. In addition, be sure to hold the reference arm stable against the lower back as you perform a measurement. Complete SAD measurements at the end of a normal expiration. It is important to understand that obtaining SAD in the standing position will result in higher values than the supine position. This is because measuring SAD in the standing position will include more subcutaneous fat. Unfortunately, the normative reference table referred to in this manuscript should not be used for standing SAD measurements because those data were obtained in the supine position.

Figure 3
Figure 3:
Standardized measurement procedures for SAD. Follow these six steps to perform a standarized measurement of abdominal height in the supine position.

Another special consideration is that SAD can be measured in children ages 8 and above to be consistent with the anthropometric procedures established by NHANES. SAD should not be measured in pregnant women. Lastly, when you have completed SAD measurements, remove the cosmetic pencil marks with baby oil or an alcohol swab. Wipe down the exam table and clean the caliper’s arm with wipes.

BOTTOM LINE

SAD measures the height of the abdomen. Some, but not all studies, show that SAD provides a more accurate estimation of visceral fat mass compared with waist circumference and BMI. Higher levels of SAD are associated with increased risk of heart disease, diabetes, and some cancers. In addition to other common field- and laboratory-based anthropometric techniques, we encourage the health and fitness professional to incorporate SAD into their health assessment repertoire. In turn, greater emphasis on documenting and tracking visceral fat mass with a variety of anthropometric techniques, including SAD, will play an important role for improving the assessment of health risks related to increased body weight in clinical practice.

BRIDGING THE GAP

SAD is superior to waist circumference in estimating intra-abdominal visceral fat mass. A growing body of research shows that increases in SAD are more closely linked to heart disease risk factors (e.g., high BP, increased cholesterol, etc.) and incident heart disease than other common field measures of obesity, including waist circumference. However, SAD has not been widely adopted in clinical practice in the health and fitness field. After having read this article, the health and fitness professional can apply standardized measurement procedures for SAD to estimate visceral obesity. In addition, the latest sagittal abdominal height population reference values derived from National Health and Nutrition Examination Survey are provided.

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Recommended Readings

Centers for Disease Control and Prevention. Department of Health and Human Services. National Health and Nutrition Examination Survey. 2019. Available from: https://www.cdc.gov/nchs/nhanes/index.htm
    Centers for Disease Control and Prevention. Department of Health and Human Services. National Health and Nutrition Examination Survey (NHANES) Anthropometric Procedures Manual [Internet]. 2017. Available from: https://wwwn.cdc.gov/nchs/data/nhanes/2017-2018/manuals/2017_Anthropometry_Procedures_Manual.pdf
      Centers for Disease Control and Prevention. Department of Health and Human Services. Anthropometric Reference Data for Children and Adults: United States, 2011–2014 [Internet] https://www.cdc.gov/nchs/data/series/sr_03/sr03_039.pdf
        Keywords:

        Abdominal Height; Cardiometabolic Disease; Intra-abdominal Fat; Standardized Measurement Procedure

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