And the amount of fast food the researchers linked to health problems may surprise you. Just two burger patties a day and one daily diet soda substantially boost the risk of getting metabolic syndrome, researcher Lyn M. Steffen, PhD, MPH, RD, tells WebMD.
Metabolic syndrome, in turn, increases the risk of heart disease and diabetes. For a diagnosis of metabolic syndrome, three of five criteria must be present, including a large waistline, elevated blood pressure, elevated fasting blood sugar, elevated fasting triglycerides, or reduced levels of HDL "good" cholesterol.
Western Diet vs. 'Prudent' Diet
"The Western diet increased risk by about 18% overall of getting metabolic syndrome over nine years [of follow-up]," says Steffen, an associate professor of epidemiology at the University of Minnesota, Minneapolis, who followed more than 9,000 people for the research. She divided them into those who ate a Western diet with those who ate a more prudent diet.
Those who ate two or more servings of meat a day, or about two burger patties, boosted their risk by 26% compared with those who only ate meat twice a week, she found. "Diet soda, one can a day, increased risk by 34%," she says. Regular soda didn't significantly boost risk of getting metabolic syndrome in this study, but Steffen notes that another recent study has linked it to metabolic syndrome.
Fried foods also boosted the risk of getting metabolic syndrome, she says, with those eating the most fried foods at 25% higher risk of getting the syndrome than those eating the lowest amounts.
At the nine-year mark, about 40%, or nearly 4,000 participants, had three or more criteria for metabolic syndrome, although none was diagnosed with it at the study start.
Diet and Metabolic Syndrome: Study Details
Steffen and her colleagues evaluated the diet habits of the more than 9,500 men and women who were part of the Atherosclerosis Risk in Communities Study by looking at a 66-item food questionnaire. Participants were 45 to 64 when the study started and were evaluated every three years over the nine-year follow-up.
The Western diet followers ate refined grains, processed meat, red meat, fried foods, eggs, and soda and not much fish, fruit, vegetables, or whole-grain foods.
The prudent diet followers ate more fruits, vegetables, fish, seafood, poultry, whole grains, and low-fat dairy foods.
The researchers grouped the participants in each diet group into segments, from those who ate the least amount of the foods studied to those who ate the most servings. They also looked individually at how eating fried foods, sweetened beverages such as regular soda, nuts, coffee, and diet soda affected the risk of metabolic syndrome.
The study is published in the journal Circulation, an American Heart Association publication.
Diet and Metabolic Syndrome: More Results
Overall, eating the prudent diet did not decrease the risk of getting metabolic syndrome, which surprised Steffen. But when she looked at individual foods in the prudent diet, they found that three servings of dairy products lowered the risk of metabolic syndrome by 13%.
The prudent diet may not have produced a lot of benefit, she tells WebMD, because on average the participants didn't meet the five or more recommended servings a day of fruits and vegetables.
"The finding that a Western diet, with high intakes of processed red meat and fried foods, would lead to the development of metabolic syndrome is in accordance with our traditional thinking," says Peter Sheehan, MD, an endocrinologist and member of the board of directors for the American Diabetes Association, who reviewed the study for WebMD.
Exactly why isn't known, says Sheehan. In food processing, a "common denominator" substance produced might be to blame, he says. Or it "may be people who have a tendency to [be] overweight drink diet soda."
Industry Weighs In
The link found between diet and metabolic syndrome is an association, not cause and effect, cautions Roger A. Clemens, DrPH, professor of molecular toxicology at the University of Southern California School of Pharmacy, Los Angeles, who also reviewed the study for WebMD.
And that association was found between those who consumed the most servings, adds Clemens, a spokesman for the Institute of Food Technologists. "These results really emphasize the importance of dietary moderation and appropriate exercise," Clemens tells WebMD.
"I think diet sodas have a role in our lifestyle," he says. "They help reduce calories." The problem is, many people also eat a large order of fries with their diet sodas, he says.
"Certainly fried foods are a huge culprit here," says Sheehan. "When cooking, try more gentle forms of cooking, like steaming. Limit consumption of processed foods and fast foods." Turn to water instead of other drinks, he adds.
Steffen advises a back-to-basics strategy. Instead of ordering the traditional burger, fries, and soda at your favorite fast-food place, "Order a baked potato, water, and [grilled] fish," suggests Steffen.