Abdominal Fat Changes by CT Question Value of Risk by BMI: Framingham Cohort

Marcia Frellick

October 03, 2016

FRAMINGHAM, MA – Higher volumes of abdominal fat and decreased density of the fat, as measured by computed tomography (CT) imaging, are associated with a worse cardiovascular risk profile "above and beyond" risk prediction based on body mass index (BMI) or waist circumference, an analysis based on the Framingham Heart Study suggests[1].

Also, changes in abdominal fat volume and density, the latter measured as signal attenuation at CT imaging, correlated with changes in the linked cardiovascular risk factors, which included hypertension, raised LDL cholesterol, and elevated triglycerides.

The findings were based on 6-year follow-up of 1106 members of the renowned longitudinal study's third-generation cohort and were published September 26, 2016 in the Journal of the American College of Cardiology, with lead author Dr Jane J Lee (National Heart, Lung, and Blood Institute, Framingham, MA).

The study adds to evidence that imaging fat volume and quality gives important clues to cardiometabolic risk that BMI may miss, according to an accompanying editorial from Drs Ian J Neeland and James A de Lemos (University of Texas Southwestern Medical Center, Dallas)[2].

They note that BMI can give mixed messages. While it can identify those at greater risk for mortality, people who are overweight or mildly obese may have lower or similar mortality risk compared with normal-weight people. Also about one-third of obese adults are metabolically healthy (a 0 or 1 cardiometabolic risk score) and some people with high BMI fall into an "obesity paradox," showing lower morbidity and mortality than normal-weight peers, the editorialists write.

BMI "is a crude measure," observed Neeland for heartwire from Medscape. CT or MRI can locate the fat, which may be benign in some parts of the body, harmful in others, and may be beneficial in the buttocks and hips in protecting against disease, he explained. "CT and MRI are the best tools we have right now for measuring the 'bad fat.' "

In the current study, the group underwent CT measurement of abdominal fat volume, encompassing both subcutaneous adipose tissue (SAT) and visceral adipose tissue (VAT) and their density by signal attenuation in Hounsfield units (HU).

During 6.1 years of follow-up, subjects' total abdominal fat volume rose a mean of 602 cm3 for SAT and 703 cm3 for VAT. Attenuation dropped an average of 5.5 HU for SAT and increased just 0.07 HU for VAT. At the same time, mean weight rose 2.4 kg, BMI increased by 1.1 kg/m2, and average waist circumference expanded by 3.7 cm.

Odds Ratio (OR 95% CI) for Cardiovascular Disease Risk Factors for Changes in Fat Volume and Attenuation

CV Risk Factor

SAT OR (95% CI)

VAT OR (95% CI)

Each 500-cm3 increase in fat volume

   

Hypertension

1.21 (1.03–1.41)

1.32 (1.11–1.57)

Hypercholesterolemia

1.10 (0.96–1.26)

1.40 (1.20–1.63)

Hypertriglyceridemia

1.14 (1.00–1.30)

1.59 (1.35–1.87)

Metabolic syndrome

1.42 (1.21–1.66)

1.77 (1.47–2.13)

Each 5-HU decrease in fat attenuation

   

Hypertension

2.89 (1.61–5.19)

1.23 (0.80–1.88)

Hypercholesterolemia

1.82 (1.11–2.98)

1.29 (0.88–1.90)

Hypertriglyceridemia

1.87 (1.15–3.06)

1.32 (0.89–1.96)

Metabolic syndrome

1.31 (0.75–2.29)

2.49 (1.52–4.08)

Adjusted for baseline abdominal fat attenuation, BMI, waist circumference, standard CV risk factors, age, sex, current smoking, alcohol intake, physical activity, postmenopausal status, and hormone replacement. Fat-attenuation change further adjusted for fat volume.

"Our findings highlight the importance of quantitative and qualitative aspects of adipose tissue for a better understanding of CVD risk," the authors write. They acknowledge that one limitation of the study is that it included mostly white participants, so it is unclear whether results would be the same in other races. Also, because it is an observational study, causal information is lacking.

"Most of the literature shows that the visceral fat is really the bad actor," according to Neeland. "To my knowledge, no one is using an assessment of visceral fat yet to specifically tailor someone's therapy." But with that information, he said, physicians in the future would have a better idea of whether surgery, medications, or diet and exercise will be most effective for individual weight loss.

However, scanning fat with CT and MRI adds cost, scan time, and radiation, he added. But new modalities such as dual-energy X-ray absorptiometry (DXA), which is currently used for measuring bone density and has lower radiation levels and cost, may be more feasible clinically in measuring fat.

"For the general person, BMI is doing the job," Neeland said. "But understanding the limitations of BMI is very important."

The study leaves several questions, he says, including: "How can we tell if the fat is dysfunctional in an individual and incorporate that into risk assessment?" Also to be resolved is how to incorporate the imaging into clinical use with less cost and more ease.

The study was supported by the National Heart, Lung and Blood Institute's Framingham Heart Study. Lee reports no relevant financial relationships. Disclosures for the coauthors are listed in the article. Neeland reports support from the National Institute of Diabetes and Digestive and Kidney Diseases of the National Institutes of Health and by the Dedman Family Scholarship in Clinical Care from UT Southwestern Medical Center. de Lemos has reported no relevant financial relationships.

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