Researchers from Canada and the United States have shown that diets low in saturated fatty acids and high in unsaturated fatty acids, including monounsaturated and polyunsaturated fatty acids, all similarly reduce high-density lipoprotein cholesterol (HDL-C), which is associated with reduced obesity by boosting the efflux of cholesterol. The findings were presented by Xiaoran Liu, PhD candidate, Pennsylvania State University, University Park, Pennsylvania, USA.

HDL-C concentration has been inversely associated with the risk of cardiovascular disease in epidemiologic studies. But this relationship has not been evident in clinical trials, suggesting that HDL-C function, rather than concentration, may be the important factor [Rosensen RS et al. Circulation 2013]. In the arterial wall, lipid-laden macrophages drive the formation of atherosclerotic plaques, while HDL can promote the efflux and subsequent washing away of cholesterol [Rader DJ, Puré E. Cell Metabol 2005].

The Canola Oil Multicentre Intervention Trial [COMIT; NCT01233778] was undertaken in part to assess the effects of various edible oils on cardiovascular health in subjects with risk factors for metabolic syndrome. Specifically, COMIT assessed the effects of the selected oils on the efflux of cholesterol and the relationship with visceral adiposity.

The 101 participants (49 men, 52 women) satisfied the following eligibility criteria: age, 20 to 65 years; waist circumference ≥90 cm (men) and ≥80 cm (women); and at least 1 of the established metabolic risk factors of metabolic syndrome: elevated blood glucose, HDL-C, triglycerides, or blood pressure ≥130/85. Subjects were not taking cholesterol- or glucose-lowering medications. Blood pressure medications were permitted.

The participants consumed diets elevated in canola oil, high oleic canola, high oleic canola supplemented with docosahexaenoic acid (DHA), corn/safflower, and flax/safflower (Table 1). The order of each 4-week diet was randomized. There was a 2- to 4-week washout period in between each diet. The efflux of cholesterol from THP-1 macrophages was measured by an in vitro fluorescence assay following the addition of serum obtained from the subjects during the study.

Baseline characteristics of the subjects are summarized in Table 2.

Ms. Liu then reported the results from the trial. Overall, the levels of HDL-C and apolipoprotein Al were higher in the diet containing DHA and lower in the flax/safflower diet (Table 3).

All diets significantly decreased total cholesterol, low-density lipoprotein cholesterol, apolipoprotein Al, and apolipoprotein B compared with baseline. All diets reduced triglycerides compared to baseline except the high-oleic canola diet. All diets significantly and similarly increased cholesterol efflux from macrophages compared to baseline. The levels of HDL-C were higher after consumption of the high oleic canola + DHA diet compared with the flax/safflower diet. Cholesterol efflux was negatively correlated with waist circumference at baseline (n = 101; r = −0.25, p = .0115) and with abdominal fat mass (n = 54;r = −0.33, p = .0173).

The results suggest that HDL-C plays a role in central obesity and atherosclerotic health, and they lend more evidence to the view that HDL-C concentration may not be reflective of HDL-C function.