August 7, 2006 -- The anti-inflammatory activity of high-density lipoprotein (HDL) cholesterol appears to decrease after consumption of saturated fat, but improves on consumption of polyunsaturated fat, a new study shows. The findings imply that the cardioprotective effects of HDL may depend not only on HDL level, but also on its behavior in the body in response to meals or other stimuli.

"This is the first demonstration that the quality of HDL can be changed by what you eat," senior author on the study, David S Celermajer, MBBS, PhD, explained in an interview with heartwire . "Eating alters the amount and behavior of cholesterol and its subfractions. What we're saying is that it's not just the amount of HDL, it's how active it is, or how good it is. And its 'goodness' can be profoundly influenced by either a diet high in polyunsaturated fat, or in saturated fat."

The study, led by Stephen J Nicholls, MBBS, PhD, at the Cleveland Clinic in Cleveland, Ohio, appears in the July 21, 2006, JACC Online issue and will appear in the August 15, 2006, print issue of the Journal of the American College of Cardiology.

Good Cholesterol, for Better or Worse

For the study, 14 adults consumed a meal rich in saturated or polyunsaturated fat, followed by a second meal, 1 month later, rich in the alternate type of fat than the type they would have eaten during the earlier meal. Both meals were identical -- a slice of carrot cake and a milkshake -- except that one was prepared with safflower oil (a polyunsaturated fat) and the other with coconut oil (a saturated fat). All meals were prepared in such a way that each volunteer consumed 1 g of fat per kilogram of body weight.

Nicholls and colleagues report that levels of plasma triglycerides, insulin, and nonesterfied fatty acids rose after both meals, but the HDL collected 6 hours after the saturated fat meal was accompanied by a higher level of endothelial cell expression of intercellular adhesion molecule-1 and vascular cell adhesion molecule-1, both markers of inflammation. HDL collected after the polyunsaturated fat meal, by contrast, had significantly lower expression of both molecules. Inflammation of endothelial cells is believed to be an important process in the development of atherosclerosis, Celermajer told heartwire , while HDL is hypothesized to play a role in modulating inflammation in the endothelium. As a secondary finding in the study, the authors report that microvascular flow was also increased to a greater degree in adults who had eaten the polyunsaturated diet, while flow mediated dilation was decreased to a greater degree following the saturated meal.

Of note, the volunteers in the study by Nicholls and colleagues were between the ages of 18 and 40 years with no existing cardiovascular disease. Whether similar results would be seen in subjects with atherosclerosis or other risk factors are unknown, the authors point out.

Future Directions

The possibility of modifying HDL behavior should reinvigorate the field of HDL research, Dr. Celermajer says. "Now that we've established that the good cholesterol is not a fixed thing, it's a dynamic thing, it opens up a whole new area of study of what can influence the quality of HDL. Will different kinds of exercise make your HDL more protective, what is the best diet for your HDL, what do certain medications that we use frequently do to your HDL?"

Dr. Celermajer also suggests that ongoing research should evaluate HDL behavior when meals are being digested.

"Almost all of the studies of cholesterol and fats are done in people after an overnight fast. Those of us who live in countries like the US or Australia spend about half our lives in the post-absorptive state -- the three or four hours after we've eaten something. What this work highlights is that to understand the relationship between cholesterol and heart health better, we need to start to look at what cholesterol looks like, not just after an overnight fast, but after we eat."

J Am Coll Cardiol. 2006;48:715-720.

Published online August 8, 2006.

The complete contents of Heartwire , a professional news service of WebMD, can be found at www.theheart.org, a Web site for cardiovascular healthcare professionals.