Evaluating CMR

Research and clinical tools to identify individuals with excess visceral adiposity and quantify their risk of diabetes and cardiovascular disease (global cardiometabolic risk).

Many epidemiological studies have shown that chronic metabolic diseases such as hypertension, diabetes, and cardiovascular disease (CVD) are more common in overweight/obese patients than in normal weight individuals. Accordingly, several clinical tools have been developed to assess body fatness in clinical practice, such as body mass index (BMI), skinfold measurement, bioelectrical impedance, hydrostatic weighing, and air-displacement plethysmography. The most commonly used index of relative body weight is BMI (i.e., the ratio of weight to height in kg/m²), and several cutoff values have been proposed in order to classify underweight, normal weight, overweight, and obesity. However, obesity is heterogeneous in its etiology and metabolic complications. In this regard, remarkable clinical observations made over 60 years ago drew attention to the fact that body fat distribution was more important than excess weight per se in obesity-related complications. These pioneering observations were later supported by findings from a wealth of studies. For instance, several prospective studies have shown that large amounts of upper body fat (abdominal obesity) increase the risk of type 2 diabetes, CVD, and attendant mortality. These prospective studies have all used simple anthropometric measurements such as waist circumference, waist-to-hip ratio, or sagittal diameter to estimate the absolute or relative amount of abdominal fat. It has been suggested that in order to assess the health hazard of overweight/obesity, a measure of overall adiposity (BMI) is required. However, it is also vitally important to take into account the presence of abdominal fat, which can be quantified in terms of waist circumference. Studies have shown that an elevated waist circumference can predict an increased risk of complications beyond simple BMI. Lastly, since the relationship of overweight/obesity to metabolic complications that increase the risk of type 2 diabetes and CVD varies based on ethnicity and sex, it has been proposed that sex- and ethnicity-specific waist cutoffs would make it easier to identify high-risk abdominally obese patients in all populations worldwide.

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In order to properly assess the risk of type 2 diabetes and cardiovascular disease (CVD), it is vital to consider regional body fat distribution in addition to overall obesity. Epidemiological studies have clearly shown that abdominal obesity is the form of overweight/obesity associated with the highest risk of complications. However, most epidemiological studies have evaluated body fat distribution using simple and inexpensive anthropometric tools such as waist circumference. The development of imaging techniques such as computed tomography and magnetic resonance imaging has enabled visceral and subcutaneous fat to be measured with a high degree of accuracy. Using these techniques, researchers have been able to determine—for any given amount of total body fat—that individuals with a selective excess of visceral fat are significantly more likely to develop cardiovascular or diabetic complications than subjects with excess subcutaneous fat. More importantly, the amount of visceral fat has been shown to predict the risk of type 2 diabetes and total mortality. Imaging techniques have therefore enhanced our understanding of the importance of visceral fat to the clustering abnormalities of the metabolic syndrome.

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Cardiovascular disease (CVD) is a leading cause of disability and death. Primary and secondary prevention measures help reduce cardiovascular events and improve the overall health of patients. In an attempt to understand the factors contributing to the development of CVD, several epidemiological and prospective studies have been conducted over the last 60 years. These studies have followed thousands of individuals over a number of years to pinpoint a first or recurrent cardiovascular event. One of the first studies was the Framingham Heart Study. This landmark U.S. study followed men and women who were initially free of CVD in order to gain initial insight on the major cause(s) of heart disease. It has enabled researchers to identify major CVD risk factors such as hypertension, smoking, elevated cholesterol or LDL cholesterol (bad cholesterol) concentrations, reduced levels of HDL cholesterol (good cholesterol), and type 2 diabetes. Many international prospective studies have confirmed that these risk factors have a significant impact on the development of heart disease. Because they were identified early on, these variables are referred to as “traditional” risk factors. Further analyses from the Framingham Heart Study have led to the development of a CVD risk prediction model based on these traditional risk factors: the Framingham risk score. Another well-recognized epidemiological prospective study—the PROCAM study—has also developed a risk prediction model that uses some of the risk factors included in the Framingham risk score along with other variables. The risk of subsequent CVD is categorized as low, intermediate, or high depending on the result obtained. Other organizations and groups have also developed CVD risk prediction algorithms. With the obesity and type 2 diabetes epidemics sweeping the world, it remains unresolved whether these global risk assessment tools fully capture the risk of abdominal obesity and the related abnormalities of the metabolic syndrome.

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The recognition of the metabolic syndrome as a risk factor for type 2 diabetes and cardiovascular disease (CVD) in the NCEP-ATP III and IDF guidelines has had important clinical/public health ramifications. Gaining ground is the notion that the most prevalent form of the metabolic syndrome is associated with abdominal obesity, especially when accompanied by excess visceral fat. This form of adiposity has been strongly linked to a cluster of diabetogenic and atherogenic metabolic abnormalities known as the metabolic syndrome, which substantially increases CVD risk, even in the absence of traditional risk factors. Because abdominal adiposity can be assessed through simple anthropometric measurements such as waist circumference, this approach is often used to identify abdominally obese patients with the most prevalent form of the metabolic syndrome. The current debate about the relevance of considering the metabolic syndrome in clinical practice highlights the fact that pathophysiology and clinical screening tools have often been confused. Much additional research will be needed to determine which key features of the metabolic syndrome further contribute to global CVD risk (as assessed by classical risk factors). Moreover, simple and effective tools must be developed for health professionals as there is an urgent need to identify these high-risk abdominally obese patients. So far, several organizations and groups have proposed clinical tools to identify patients likely to have features of the metabolic syndrome (hypertriglyceridemic waist, NCEP-ATP III, IDF, WHO, EGIR, AACE). Regardless of whether one recognizes that this syndrome really exists, there is compelling evidence that the abdominal obesity and type 2 diabetes epidemics represent a major threat to the cardiovascular health of most populations worldwide.

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