"The take-home, public-health message is this: It's further evidence to support the need to aggressively reduce the amount of saturated fat consumed in the diet," said lead author Stephen Nicholls, now a cardiologist at Cleveland Clinic in Ohio.
The results coincide with increasing attention on the types of fat in food products, with the UK Food Standards Agency continuing to press for revision of the European directive that governs the content and format of nutrition labels on foods marketed in the UK and other European countries, so that these fats are labelled.
The FSA believe that mandatory addition of the content of saturated fat and trans fatty acids to nutrition labels would enable consumers to make healthier food choices that could lower LDL concentrations and reduce the risk of coronary heart disease and other vascular events.
Previous studies have reported that elevated levels of saturated fat, found in a wide range of common foods, including meat products, hard cheese, cream and palm oil, can increase the amount of cholesterol in the blood, which increases the chance of developing heart disease.
The new research, published on-line in the Journal of the American College of Cardiology (doi:10.1016/j.jacc.2006.04.080), expands the understanding of the action of saturated fats by indicating that high consumption may not only raise levels of the so-called 'bad' cholesterol (LDL-cholesterol) but may actually inhibit the positive action of HDL ('good') cholesterol.
The researchers recruited 14 adults and asked them to eat a meal containing either a polyunsaturated or a saturated fat on two different occasions. The meals contained the same number of calories. Six hours after the meal blood samples were taken and the action of HDL on the production of different molecules associated with inflammation.
Chronic inflammation, brought about by an over-expression or lack of control of the normal protective mechanism, can lead to a range of inflammatory related disease, particularly cardiovascular disease, the leading cause of death in Europe.
The researchers looked at endothelial levels (the cells that line the inner wall of blood vessels) of intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1) both of these molecules are involved in the immune system response and raised levels are associated with inflammation.
The HDL-cholesterol collected after eating the saturated fat meal were found to be less effective at inhibiting the expression of both ICAM-1 and VCAM-1, than HDL isolated from fasting plasma. Moreover, HDL collected after eating the polyunsaturated meal was found to have an increased inhibitory activity than even the HDL collected from fasting plasma.
Lead researcher, Dr. David Celermajer from the Heart Research Institute, Sydney, told Reuters Health that he and his team were "a little surprised" by the results.
"Most everyone concentrates on the amount of 'good' cholesterol. This study shows that its quality may be very important too, in determining its protective ability," he said.
"Most people also measure cholesterol and its fractions with people fasting - but it seems that its quality after a meal (and most of us spend a lot of time in the post-absorptive state) may be important to consider too," Celermajer told Reuters.
The researchers also looked at blood flow by measuring so-called flow-mediated dilation (FMD) before, and at three and six hours after eating. FMD measures changes in the diameter of blood vessels that are associated with increased blood flow.
Blood flow, as measured by FMD, was found to have decreased by 2.2 per cent after eating the saturated fat meal, while consumption of the polyunsaturated fat meal reduced blood flow by only 0.9 per cent.
"Consumption of a saturated fat reduces the anti-inflammatory potential of HDL and impairs arterial endothelial function. In contrast, the anti-inflammatory activity of HDL improves after consumption of polyunsaturated fat," wrote Nicholls.
"These findings highlight novel mechanisms by which different dietary fatty acids may influence key atherogenic processes."
Dr. Nicholls said that although the stduy was small the findings have broad implications because diet and exercise are the foundations for preventing heart disease.
Robert Vogel, from the University of Maryland Medical Center, did not take part in the study, but said the research helped to expand on just why we should limit consumption of saturated fats.
"Traditionally, we think of unhealthy foods as raising cholesterol or raising blood pressure, but this demonstrates that depending on what you eat, you can actually change the effect of HDL - typically thought of as 'good' cholesterol - from protective to detrimental. This opens up new insights and avenues for research," he said.