Your Waistline Guidelines

Summarized by Robert W. Griffith, MD
June 28, 2007

Summary

Measurement of the waist circumference has become recognized as being useful in predicting type 2 diabetes, coronary heart disease, and mortalities (all-cause and cardiac). A group of experts has analyzed our present knowledge on this topic, which is reviewed here.

Introduction

For the last fifteen years the body mass index (BMI) has been the accepted measure for classifying overweight and obesity in clinical studies. There are direct correlations between BMI and the risk for medical conditions, such as diabetes or cardiovascular disease, and mortality rates. A BMI of 30 or over is obese, and one between 25 and 30 is overweight, by definition.

Body fat distribution is also an important risk factor for obesity-related diseases. In particular, abdominal (or central or upper-body) fat is associated with cardiometabolic disease. The waist circumference is used to express the abdominal fat mass, and is one of the factors used in defining the metabolic syndrome.¹ It's likely that waist measurement will come to rival, or even supplant, the BMI as a faster and easier indicator of unhealthy body weight.

The distribution of body fat has been used to describe two distinct body shapes: gynecoid, or "pear shape", with accumulation of fat in the hips and thighs, vs. android, or "apple shape", with accumulation in the middle part of the body, i.e. the abdomen. Android obesity is associated with more adverse metabolic and cardiovascular risk factors. This has led to adaptation of the waist-hip ratio (WHR) by some scientists; however, the WHR is harder to measure accurately, and, in most cases, doesn't add useful additional information.

The organizations listed in the publication title convened an expert panel to obtain information on the importance of waist circumference measurement. They were charged with examining how, and to what extent, waist circumference can predict adverse cardiovascular outcomes, and its relation to BMI. Finally, (an easy question), should waist circumference be measured routinely in clinical practice?

What does waist circumference measure?

The actual measurement of the waist circumference has been done differently in the different relevant studies. The most commonly used ways were to measure at the level of: 1) the midpoint between the lowest rib and the iliac crest (the hip-bone at its highest); 2) the umbilicus; and 3) the narrowest circumference. These were used in 29%, 28%, and 22% of studies, respectively. There is clearly no uniformly accepted approach. What's important is that the patient (or the physician's office, or the lab) always uses the same 'point-of-reference' in making the measurement, so that changes reported are real changes, rather than differences in technique.

Most fatty tissue (adipose tissue) - about 85% - is located under the skin as the subcutaneous fat. Only about 15% is located within the abdominal cavity - the intra-abdominal fat. Magnetic resonance imaging (MRI) and computerized tomography (CT) are the best way to determine these two groups of adipose tissue.

How closely does waist circumference match the BMI? In general, waist measurements reflect the subcutaneous plus the intra-abdominal fat volumes, and are highly correlated with BMI scores. But the waist measurement provides information on fat distribution that cannot be obtained by calculating the BMI. Of course, it cannot provide the ratio between subcutaneous and intra-abdominal fat, which requires an MRI or CT scan.

Why is the waist circumference linked to cardiometabolic risk?

As stated, the waist circumference cannot determine if cardiometabolic risk is associated with subcutaneous fat, intra-abdominal fat, or both. However, there are several theories as to the relationships involved. For instance, it could be that the limited ability of subcutaneous fat deposits to store excess energy results in an 'overflow' of chemical energy to intra-abdominal sites, which can include fat accumulation in the liver. Excess liver fat can disturb its metabolic function, causing abnormal lipid profiles and resistance to insulin. Increased skeletal muscle fat can also lead to insulin resistance in muscles. In this example, increased intra-abdominal fat is the 'marker' for the fat-energy overflow problem.

Another theory proposes a direct effect of fat accumulation in the fat storage organs in the abdominal cavity (the omentum and mesentary) producing insulin resistance, changed lipoprotein metabolism, and raised blood pressure. Triglycerides and free fatty acids released from the omental and mesenteric adipose tissue enter the portal vein, traveling to the liver. And they can be accompanied by specific proteins and hormones, such as inflammatory cytokines, angiotensinogen, and corticosteroids.

Finally, stress might activate the central nervous system-adrenal axis, causing both preferential deposition of fat in the abdomen and the cardiometabolic disorders found in association with this.

How well does waist circumference predict cardiometabolic disorders?

Results from numerous clinical studies show that waist circumference data can:

• provide good prediction of type 2 diabetes, coronary heart disease, and mortality rates (all-cause and selected conditions).
• provide additional predictive value over and above that obtained from BMI scores for diabetes, heart disease, and death.

There is less agreement about the ability of waist circumference measurements to provide any additional information above that provided by BMI scores plus cardiometabolic risk factor values, such as blood sugar levels, lipid levels, and blood pressure readings. In other words, if your doctor is measuring your height, weight (and calculating your BMI), and blood pressure, and is getting labs for blood glucose and lipids, the waist measurement will not change his prognosis. However, having a record of it can help in monitoring your efforts at lifestyle changes - diet, exercise, etc.

What's the practical value of measuring waist circumference?

There are different levels for waist circumference, above which cardiovascular risks appear:
Caucasian men: 40 inches (102 centimeters)
Caucasian women: 35 inches (88 centimeters)
Asian men: 35 inches (90 centimeters)
Asian women: 31 inches (80 centimeters)
These levels are somewhat limited; obviously there should be better (or more numerous) cut-off points for different races, age, and other factors.

Waist circumference measurement can provide additional information to help doctors determine which patients should be examined for the presence of cardiometabolic risk factors, such as blood lipid abnormalities and raised blood sugar. It can also be used to monitor patients' responses to diet and exercise regimens - regular aerobic exercise can reduce both waist circumference and cardiometabolic risks without affecting the BMI. Further work will establish the proper place of the tape measure in the physical exam of patients.

Source

• Waist circumference and cardiometabolic risk: a consensus statement from Shaping America's Health: Association for Weight Management and Obesity Prevention; NAASO, The Obesity Society; the American Society for Nutrition; and the American Diabetes Association. S. Klein, DB. Allison, SB. Heymsfield,  et al., AM J Clin Nutr, 2007, vol. 85, pp. 1197--1202

Footnotes
1. The term "metabolic syndrome" is often used to describe cardiometabolic risk factors, primarily overweight, type 2 diabetes, high cholesterol, and high blood pressure. It's better to think of these factors simply as raising your cardiometabolic risk. There is some controversy as to whether these factors represent a "syndrome," and scientists don't even agree on what the various components of the "syndrome" are. The important thing to know is whether you have any of these risk factors, and if so to take active steps to improve them. Se also the first Related Link, below.

Related Links
Do You Have the Metabolic Syndrome?
Trends in Waist Circumference among US Adults
US Dept. H&HS: Weight and Waist Measurements

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