Until now, no clear benefit has been demonstrated for higher levels of either PUFAs or MUFAs, according to the researchers, but there has been greater interest in using MUFAs to manipulate total fat in the diet due to concerns over the effect of PUFAs on cardiovascular risk factors, such as oxidative stress.
But the results of the Penn State study, published in the July issue of the Journal of the American Dietetic Association, did not show that high levels of PUFAs increase measures of oxidative stress, leading the researchers to conclude that the optimum dietary fat is neither one nor the other, but a proper balance of both.
According to the American Heart Association, 50.7 percent of American adults had total cholesterol of 200 mg/dL or higher in 2002, and for 18.3 percent it was 240ml/dL.
In the same year, 70.1 million Americans suffered from some form of cardiovascular disease (CVD) - that is, 34.2 percent of the population - and it was the cause of 0.9 million deaths.
The new research could be significant in reducing the incidence of high cholesterol as health care professionals are increasingly recommending a dietary approach to controlling cholesterol over prescription drugs such as statins, which may have serious side effects.
Dietary sources of MUFAs include olive, canola oils, avocados, peanuts, nuts and seeds. One of the main forms of PUFAs is omega-3, commonly derived from fish, which have been shown to deliver considerable benefits for cardiovascular health and cognitive function. Omega-6 is also essential for human health, found in grains, most plant-based oils, poultry, and eggs.
The object of the study was to evaluate the effects of a new MUFA source that is also a good PUFA source, on lipid levels, lipoproteins and measures of oxidative stress. The source in question was NuSun sunflower oil, produced by the National Sunflower Association.
NuSun contains the same amount of oleic acid as olive oil, but more PUFAs and less saturated fatty acids. Its fatty acid profile means it is highly stable, so hydrogenation, a process that creates trans fatty acids, is not required.
The study involved 31 individuals (12 men and 19 women) with moderate hypercholesterolemia, aged between 25 and 64 years. It set out to compare the effects of three different diets on their lipid and lipoprotein levels and oxidative stress.
The two experimental diets each provided 294mg of cholesterol each day and a total of 30 percent fat. In the 'olive oil' diet, 8.3 percent was saturated fatty acid, 17.2 percent was MUFA, 4.3 percent was PUFA.
In the 'NuSun' diet, 7.9 percent was saturated fatty acid, 14.2 percent was MUFA and 7.7 percent was PUFA.
The control diet was an average American diet consisting of 34 percent fat - 11.2 percent saturated fatty acid, 14.9 percent MUFA and 7.8 percent PUFA.
The participants followed each diet for a four-week period, with a two-week compliance break between each.
The researchers found that the NuSun diet decreased total cholesterol by 4.7 percent and low-density lipoprotein cholesterol by 5.8 percent compared with both the olive oil diet and the control diet.
This result was what the researchers were expecting, but other outcomes were not foreseen:
"The surprise was the fact that the olive oil diet, which is also low in saturated fat, did not lower cholesterol levels compared with the average American diet," said Penny Kris-Etherton, distinguished professor at Penn State's Nutritional Sciences Department and one of the study's authors.
The team attributed this outcome to the olive oil lower PUFA content compared with NuSun, since both diets contained approximately the same amount of saturated fat.
Kris-Etherton added: "Also surprising was that the greater percentage of PUFAs in the NuSun sunflower oil diet did not increase LDL oxidation products that are risk factors for atherosclerosis."
The conclusion drawn was that a mixture of both mono- and polyunsaturated acids seems to provide the greatest health benefits within the context of a moderate-fat diet.
But beyond lowering cholesterol, PUFA-MUFA balance has also been linked with prevention of Parkinson's disease. In a Dutch study published in last month's Neurology (2005;64:2040-2045) indicated that higher consumption of both types of unsaturated fat may help stave off the degenerative disease.