Olive oil is a cornerstone of Mediterranean diets, which are renowned for being good for the heart. Many nutritionists have attributed that benefit to the oil's high proportion of monounsaturated fatty acids. However, a European study suggests that olive oil's fatty acid makeup is only part of the story.
The study indicates that lightly processed olive oils—the virgin types common in Mediterranean diets—offer additional ingredients with a cardiovascular advantage: abundant antioxidants known as polyphenols.
When healthy men incorporated a virgin olive oil especially rich in these polyphenols into their diets, characteristics of their blood changed in many beneficial ways. Before eating polyphenol-rich oil, the men had consumed a diet low or devoid of the olive antioxidants.
María-Isabel Covas of the Municipal Institute for Medical Research in Barcelona and her colleagues report their findings in the Sept. 5 Annals of Internal Medicine. The researchers conclude that olive oil "is more than a monounsaturated fat." The new data are "evidence to recommend the use of polyphenol-rich olive oil—that is, virgin olive oil"—beyond the benefits provided by its fatty acids.
Nor is this the only newfound benefit from olive oil. Another European research team reported this month on a test-tube study showing that compounds in oil from the skin of olives trigger the death of human-cancer cells.
What is a Mediterranean diet?
Epidemiologic studies have shown that many people from Mediterranean countries can eat fatty diets and yet not have a high risk of heart disease.
Spaniards José Mataix of the University of Granada and Francisco Javier Barbancho of the University of Extremadura in Cáceres observe in a new book that the Mediterranean diet "is based on products derived from wheat, olive, and grape, these constituting the Mediterranean triad of bread, oil, and wine."
In fact, Covas' group points out that several additional features characterize diets common to the region. Among them: several daily servings of vegetables and fruits, only a small serving (100 to 150 grams) of red meat per day, few or no carbonated drinks, at least three weekly meals including shellfish or legumes, few commercially prepared pastries or sweets, servings of fowl or rabbit instead of pork or other red meats, and plenty of peanuts and other nuts.
Some studies have pointed to the alcoholic component as a substantial contributor of such diets' cardiovascular benefits. However, others have found that people who drink as much wine and other alcoholic beverages as do Mediterranean eaters fail to get as many health benefits if they don't follow the rest of the diet.
For instance, a separate team of all-Spanish researchers that included Covas recently reported such data in early results from the PREDIMED Study. This trial is ultimately slated to track for 4 years some 9,000 men and women at elevated risk of heart disease. Each volunteer is being randomly assigned to eat a low-fat diet or a higher-fat diet in which much of that fat, Mediterranean style, comes from either olive oil or nuts, which also are rich in heart-healthy monounsaturated fats (SN: 11/21/98, p. 328).
In the July 4 Annals of Internal Medicine, Ramon Estruch of the Hospital Clinic in Barcelona and his colleagues report results from 770 recruits during the first 3 months of their participation in PREDIMED. People eating either version of the Mediterranean fare had lower blood glucose concentrations, lower blood pressures, lower LDL cholesterol concentrations, and fewer markers of inflammation than did people eating the low-fat diet. All these factors indicate a reduced risk of heart disease.
Acknowledging that the trial so far is too small and the findings too preliminary to validate that Mediterranean diets lower heart-disease incidence, the authors nevertheless point out that data such as these already support the Mediterranean diet "as a useful tool in managing individuals who are at high risk for coronary heart disease."
What polyphenols do
In the more-recent Annals of Internal Medicine study, Covas and her team recruited 200 healthy men, ages 20 to 60. Each man ate the same diet, except for getting differing types of olive oil for cooking or adding to their food.
The men were randomly assigned to receive 25 milliliters (about 5 teaspoons) a day of one of three different olive oils: a virgin olive oil containing 366 milligrams of polyphenols per kilogram of oil, a highly refined olive oil with a polyphenols concentration of just 2 mg/kg, or a mixture of these two oils with a polyphenol concentration of 164 mg/kg. During each segment of the trial, the men received a different oil, so that by the end, all had consumed each oil for one 3-month period. A 2-week "washout" period, during which the recruits were asked to avoid olives or olive oil, separated trial segments.
The researchers conducted the trial, financed by the European Union, in six research centers in five European countries: Spain, Denmark, Finland, Italy, and Germany. Its implications, therefore, shouldn't be confined to any one ethnic group, say the researchers.
The men's good, or HDL, cholesterol was modestly different—varying by some 1.75 milligrams per deciliter of blood—in the three trial segments, but showed a clear correlation: highest when they took in the most olive-oil polyphenols and lowest when they took in the least. Covas calls this trend "the most striking" of the study.
The new report notes that previous studies by others have shown even a 1 mg/dl increase in HDL could translate into a 2 to 3 percent reduction in cardiovascular risk across a treated population.
The polyphenols also appeared to reduce the chemical oxidation of bad, LDL cholesterol. "Oxidation of LDL is considered a risk factor for coronary heart disease," notes Covas, a clinical biochemist who focuses on nutrition as a means to fight heart disease. In this study, the higher the polyphenol content of the oil, the lower the signs of LDL oxidation in the men's blood.
Concentrations of triglycerides—fats in the blood—decreased with all olive-oil types, compared with values in the men prior to the trial. This suggests that this particular effect is attributable to something in olive oil other than polyphenols.
Earlier research by Covas' group had also indicated that polyphenols reduce LDL oxidation. In a study published in the Feb. 15 Free Radical Biology and Medicine, the researchers recruited a dozen men in their early 20s and had them eat a breakfast of bread and 40 ml of olive oil on each of three occasions 10 days apart. At each breakfast, the olive oil was different—with a low, medium, or high concentration of polyphenols. Blood sampled in the hours immediately after each meal was evaluated to assess whether the LDLs' chemistry and composition changed from one test day to the next.
The blood tests showed that the men's LDLs picked up olive polyphenols in proportion to the amounts present in the oil. Moreover, the researchers found that oxidative stress, as indicated in the men's blood, increased after the low- and medium-polyphenol breakfast, but not after the breakfast featuring the high-polyphenol oil.
Olives vs. cancer
Olive oil's benefits appear to extend beyond heart health. For instance, M. Emília Juan of the University of Barcelona and her colleagues report finding that triterpenes—waxy hydrocarbons that coat olive skins—shut down the runaway proliferation of human colon cancer cells.
Juan's team notes that epidemiologic studies have suggested that regular consumption of olive oil diminishes cancer risk. Although many scientists had suspected that the fatty acids in olive oil were responsible for the effect, Juan's team wondered about the possible role of triterpenes.
The researchers isolated the triterpenes from oil and other compounds by immersing olives for a minute in a solvent. They then purified the compounds and added them to the growth serum for test-tube colonies of cancer cells.
The extracted chemicals didn't poison the cells, but they did inhibit proliferation of the cells by triggering them to undergo a natural process of cell death—something cancer cells typically avoid—the researchers report in the October Journal of Nutrition.
Moreover, Juan and her colleagues note that concentrations of triterpenes needed to achieve this effect appear comparable to what people would obtain from the daily intake of olives and olive oil typical of Mediterranean diets—i.e. about 10 medium-size olives and about 6.6 teaspoons of oil.
On several grounds, virgin olive oil appears to represent a healthy dietary fat. However, all fats are high in calories and a potential source of oxidative stress to the body. That's why Covas—and nutritionists generally—recommend that olive oil be substituted for other fats in the diet, not added to them.
Although virgin olive oil, which is usually the product of cold-pressed olives, is the most flavorful, it can be bitter enough to deter people from using it. Another group of Spanish researchers has been working to counter this problem. They've shown that giving olives a 3-minute hot bath prior to pressing converts strongly bitter oils to ones that are only slightly bitter (see The Sweet Benefit of Giving Olives a Hot Bath).
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Lipids and Cardiovascular Epidemiology Unit
Municipal Institute for Medical Research
Carrer Dr. Aiguader, 80
M. Emília Juan
Departament de Fisiologia
Facultat de Farmàcia
Universitat de Barcelona
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