Marialaura Bonaccio, lead author of the new study and a researcher at IRCCS Istituto Neurologico Mediterraneo Neuromed, an Italian Clinical Research Institute, said in an email that this same problem -- in which people from different income levels get different results from the same diet -- may also be true for other diets.
The reason? Diets "focus on quantity, rather than on quality" of the food, she said.
Bonaccio and her co-authors randomly recruited over 18,000 people living in the Molise region of southern Italy between March 2005 and April 2010. The Pfizer Foundation, which helped fund enrollment costs, did not influence the analysis or interpretation of results, Bonaccio noted.
She and her team calculated total physical activity, body mass index (BMI), smoking status and health history, including cardiovascular disease, diabetes and cancer. The data available for each participant also included education, household income and marital status.
Using the Mediterranean Diet Score, Bonaccio and her colleagues assessed participants' food intake, examining the variety of fruits and vegetables, meat and fish consumed. They scored participants' cooking methods, detailing whether they're using healthy methods such as boiling and stewing or less healthy methods such as frying, roasting and grilling. Vegetables were categorized as organic or not, bread as whole-grain or not.
Over an average followup period of about four years, participants experienced a total of 5,256 cardiovascular disease events, including incidents of heart failure, diagnoses of coronary heart disease (a buildup of plaque in the arteries) and strokes.
Analyzing the data, the researchers found that a Mediterranean diet effectively reduced cardiovascular disease risk, but only among a select group of participants: those with higher income or more education.
"We found heart advantages were limited to high socioeconomic status groups, even if groups showed the same adherence to the Mediterranean diet," Bonaccio wrote. No benefits occurred for participants in the low income and low education group.
Differences in food quality
Surprised by this result, the researchers dug into the data more deeply and unearthed a possible reason for the difference: The same Mediterranean diet adherence score still included slight differences in food consumption.
"For example, as compared to less advantaged counterparts, people with high socioeconomic status tended to consume fish more frequently," Bonaccio wrote. She added that, beyond diet adherence, participants in the most advantaged category reported a higher quality diet, which included higher consumption of organic products and whole grain foods.
"Let's give that two persons follow the same diet, that is equal amounts of vegetables, fruits, fish, olive oil etc. every day so that they report the same adherence score to Mediterranean diet," Bonaccio said. "It might be that, beyond quantity, differences in quality may exist. For example, in olive oil."
She said it's unlikely that a bottle of extra virgin olive oil with a price tag of 2 to 3 euros has the same nutritional properties as one costing 10 euros. Given that it is reasonable to assume higher-income participants are more likely to buy the 10-euro bottle compared with lower-income participants, "our hypothesis is that differences in the price may yield differences in healthy components and future health outcomes," Bonaccio said.
How the food is cooked or prepared might also contribute to differences in results, according to Bonaccio, though she said the differences in cooking procedures -- "a kind of marker of the numerous differences still persisting across socioeconomic groups" -- probably did not "substantially account" for the disparities in cardiovascular risk.
Similar results in US?
Mercedes Sotos-Prieto, an assistant professor and visiting scientist at Harvard Chan School of Public Health, said evidence, including from her own research
, shows that a Mediterranean diet is "one of the best choices to improve health."
Sotos-Prieto, who was not involved in the new research, wrote in an email that the new study, which relied on self-reported data, does not prove that socioeconomic status caused the health benefits seen; it shows only a relationship between income and/or education and health outcomes.
"Previous studies have already showed a socioeconomic gradient regarding adherence to diet quality," Sotos-Prieto wrote. Because of this, a similar difference in health results depending on socioeconomic status may also be occurring in the United States among those who follow a Mediterranean diet, she said.
Dr. Barbara Berkeley, who specializes in weight management and practices medicine in Beachwood, Ohio, said "one caveat in interpreting studies like this is that they are based on diet recall. It is generally very difficult for people to keep accurate food records and there is a tendency for participants to record their diets in the best possible light."
Berkeley, who was not involved in this research, agreed with the hypothesis of the authors.
"A good diet is undoubtedly more than just a shopping list," she said. "Quality, freshness, variety and purity of production may truly differentiate diets even when they appear to be the same."
Berkeley noted that "food deserts" in lower-income areas means both quality and variety of fresh foods may be limited, while organic produce may be unavailable or too expensive.
"A healthy diet is likely not the sum of its parts but the quality of its elements," Berkeley said.
Maria Korre, a research fellow at Harvard Chan School of Public Health, noted that "among the most important perceived barriers to healthy eating are the time and cost of shopping." Korre, who did not contribute to the new study, added that "we need to work toward identifying ways ... to overcome these barriers."
"As a result of the worldwide epidemics of obesity and diabetes, we witness a strong and renewed interest in the traditional Mediterranean diet," Korre said. Yet the appeal of this diet extends well beyond proven health benefits.
With its wide range of colorful foods, the diet provides "delicious meals" and "because of its emphasis on limited consumption, rather than abstention from red meat and sweets" plus its inclusion of moderate drinking of alcoholic beverages, "the Mediterranean diet represents a healthy yet indulgent and appealing lifestyle that can be sustained over long periods of time," Korre said.
According to the study authors, people of high socioeconomic status may actually be selecting foods that are higher in both polyphenols (plant-based micronutrients) and antioxidants (a nutrient found in fruits and vegetables that helps repair damage in our bodies). Such daily choices could result in health advantages unseen by those who make different selections.
"This hypothesis could be only tested by a direct measure of such natural compounds in biological samples, e.g., blood levels or urinary polyphenol excretion," Bonaccio said. She said her groups' future research and analysis will test this theory.