"The kind of fat matters," said lead study author Dr. Hanna Bloomfield of the Minneapolis Veterans Affairs Medical Center in Minnesota.

"In the clinical trials, participants were advised to use monounsaturated fat (e.g. olive oil, canola oil) as their main source of fat and to limit their intake of red meat, a major source of saturated fat," Bloomfield added by email.

Polyunsaturated and monounsaturated fats are the "good" kind that can promote healthy cholesterol levels, especially when used in place of saturated fats and trans fats, the "bad" actors in red meat.

A Mediterranean diet typically includes lots of fruits, vegetables, whole grains, legumes and olive oil and lean protein like chicken or fish.

While some previous research has linked a Mediterranean diet to weight loss and a reduced risk of heart disease and some cancers, scientists haven't conclusively proven that the diet itself is responsible, rather than other lifestyle choices made by people who eat this way.

For the current study, Bloomfield and colleagues analyzed results from 56 previously published studies.

Few trials randomly assigned participants to eat different diets to help determine if the Mediterranean diet in particular might cause specific health benefits, the authors note in the Annals of Internal Medicine.

One trial randomly assigned 7,447 older adults at risk for cardiovascular disease to follow one of three diets: a Mediterranean diet supplemented with one liter (about 34 fluid ounces) of extra virgin olive oil a week, a Mediterranean diet supplemented with 30 grams (1 ounce) of nuts a day, or a low-fat diet.

After almost five years, participants in the two Mediterranean diet groups had a combined 29 percent reduction in major cardiovascular events compared with the control group on the low-fat diet.

People in the Mediterranean diet groups were also about 30 percent less likely to develop diabetes; other studies also saw a lower diabetes risk with this diet.

In this study, a Mediterranean diet was also associated with a 57 lower risk of breast cancer; other studies didn't find a risk reduction.

This study didn't find a difference in death from all causes, based on which diet people were assigned to follow.

These findings have been published before, and this study combined with other observational research does "convincingly prove the benefits of this diet for cardiovascular disease and diabetes," Dr. Dariush Mozaffarian, a nutrition researcher at Tufts University in Boston who wasn't involved in the current analysis, said by email.

A second, smaller trial of 429 older adults in Hong Kong randomly assigned to stick to their normal eating habits or to switch to a Mediterranean diet also failed to find a benefit when it comes to death from all causes. After 33 months, 13 percent of people on the Mediterranean diet died, as did 11 percent of people who didn't change their diets.

None of the studies in the analysis did a good job of assessing how strictly people followed a Mediterranean diet over time, the authors also found.

But observational data in several studies did suggest cancer incidence and mortality were lower in people who stuck most closely to the Mediterranean diet than in people with the lowest adherence.

The take-home message is that the type of fat, total calories and quality of food we consume can all impact our overall health, disease risk and weight, said Samantha Heller, a registered dietician and exercise physiologist affiliated with New York University Langone Medical Center in New York City.

"So no, we can't sit down and eat a stick of butter wrapped in a slice of bread," Heller, who wasn't involved in the analysis, said by email.

"Eating a diet rich in healthy unsaturated plant fats helps boost immunity and reduce systemic inflammation," Heller added. "Inflammation is one of the cornerstones to many of the diseases that plague those who consume a typical Western diet."