"The research is preliminary but offers new evidence that soybean and other plant extracts containing sterols can increase the cholesterol-lowering benefits of a healthy low-fat diet," said Floyd P. Horn, Administrator of the Agricultural Research Service. "People who want to reduce their cholesterol through diet may see better results by including low-fat foods having added sterols as part of their low-fat diet."
The study's lead researcher, chemist Joseph T. Judd of the USDA’s Agricultural Research Service in Beltsville, MD., presented the findings on April 18th 2008 at the Experimental Biology 2000 meeting in San Diego. Judd led the study at ARS' Belstville Human Nutrition Research Center in 1999.
Horn said cholesterol reductions nearly doubled in the study's 53 men and women volunteers when their low-fat diet included two daily servings (4 tablespoons total) of salad dressing containing soybean sterols. The volunteers consumed the sterols--2.2 grams or about ½ teaspoon--daily for three weeks of the six-week study period.
Plant sterols are ingredients in a number of plants and fat-based foods on the market including salad dressings. Potential health benefits of plant sterols, including cholesterol reduction, have been studied for decades. Judd said the Beltsville study was unique in examining plant sterols as part of a tightly controlled low-fat diet because most studies have looked at sterol effects in diets higher in fat. In other words, the effects of plant sterols would be expected to make some impact on a diet high in fat and where cholesterol levels were high as a result. The Beltsville study demonstrated the power of plant sterols more dramatically because it showed that the soybean extracts could lower cholesterol of people already consuming a heart-healthy diet.
Sterol esters like the ones used in the study lower cholesterol in two possible ways: By limiting its intestinal absorption, and by binding to bile in the intestines and causing bile to be excreted, thus forcing the liver to have to use up more cholesterol in order to make more bile.
The volunteers began the study with their levels of "bad" (LDL) cholesterol in the mildly elevated range. For six weeks, they ate all their meals at the Beltsville center. For three of those weeks, their daily diet included the addition of 2.2 grams of soybean sterols as an ingredient in salad dressing. On the low fat diet alone, without plant sterols, the volunteers' total and "bad" cholesterol levels dropped 7.3 and 8.4 percent, respectively. With the sterols, the reductions nearly doubled: 14.1 and 18.2 percent respectivelly.
Overall fat intake in the study diet amounted to 32 percent of total calories.
Of note, five of the 53 volunteers lowered their cholesterol only during the part of the study that included sterol esters. "Many people with high cholesterol," Judd explained, "do not respond to a low-fat diet alone and rely on cholesterol-lowering drugs. The question is, could dietary plant sterols also help these kinds of people?"
Judd conducted the study with chemist Beverly Clevidence, who leads the center's Phytonutrients Laboratory, along with physiologist David Baer of the Diet and Human Performance Laboratory, and nutrition scientists Shirley Chen and Gert Meijer of Lipton.
The plant sterols used in the study are present in low concentrations in many raw and refined vegetable-based foods including vegetable oils. A typical Western diet provides approximately 0.25 g of plant sterol per day.
"It would be impractical to try to consume 2.2 grams a day of sterols from refined oils or other foods," Judd noted. A more practical way to get the levels of plant sterols used in the study is to supplement.