Every Saturday, a fleet of cars and trucks pulls into a windswept parking lot just off the Mediterranean. Under flapping white awnings, women slit open eggplants the size of a large man’s thumb and stuff them with a mix of chopped garlic, red peppers and walnuts. This is Souk el Tayeb, the farmers’ market that has helped make Beirut a hot destination for globe-trotting foodies. But if you want to see how the new generation of Lebanese really wants to eat, you have to go somewhere else. You have to go to Roadster Diner.

Roadster is a chain of 1950s-Americana restaurants. Its original motto, “There goes my heart,” evokes both Elvis and his artery-clogging diet. The Roadster in my Beirut neighborhood had a life-size statue of a grinning black man with huge white teeth singing into a microphone. Unlike the strenuously authentic Lebanese restaurants beloved by tourists and visiting food writers, Roadster’s nine retail franchises across Lebanon are always packed with locals.

In Europe and the United States, the so-called Mediterranean diet — rich in olive oil, whole grains, fish, fruits and vegetables and wine — is a multibillion-dollar global brand, encompassing everything from hummus to package trips to Italy, where “enogastronomic tourism” rakes in as much as five billion euros a year. Studies at Harvard and elsewhere correlate the Mediterranean diet with lower rates of heart disease, diabetes and depression. In America, health gurus like Mehmet Oz exhort followers to “eat like a Greek.” But according to data from the United Nations Food and Agriculture Organization, Mediterranean people have some of the worst diets in Europe, and the Greeks are the fattest: about 75 percent of the Greek population is overweight. So if the Mediterranean diet is not what people in the Mediterranean eat, then what is it?

Before there was a Mediterranean diet, there was WWII and the food shortages that went along with it. When the fighting was over, Haqvin Mamrol, a researcher in Sweden, showed that mortality from coronary disease declined in Northern European countries during the war. This was, he believed, the result of wartime restrictions on milk, butter, eggs and meat. At about the same time, a Minnesota scientist named Ancel Keys, who had been studying the effects of starvation on a group of volunteer subjects, moved on to study the diets of Midwestern businessmen. He found that these well-fed Americans were more prone to heart disease than were men in war-deprived Northern Europe. Keys postulated that saturated fats led to high levels of cholesterol and from there to cardiovascular disease. To prove it, he initiated a long-term study in seven countries, including Italy and Greece. He concluded that we should cut down drastically on saturated fat and turn to vegetable oils instead. Keys and his wife wrote two best sellers that changed the way Americans ate.

The Mediterranean diet was always a composite. Spaniards love pork; Egyptians, as a rule, do not. In some regions, people made pesto with lard, not olive oil. “There is no such thing called the Mediterranean diet; there are Mediterranean diets,” says Rami Zurayk, an agriculture professor at the American University in Beirut. “They share some commonalities — there is a lot of fruits and vegetables, there is a lot of fresh produce in them, they are eaten in small dishes, there is less meat in them. These are common characteristics, but there are many different Mediterranean diets.”

The healthy versions of these diets do have one other thing in common: they are what the Italians called “cucina povera,” the “food of the poor.” In Ancel Keys’s day, Mediterraneans ate lentils instead of meat because they had no choice. “A lot of it is to do with poverty, not geography,” says Sami Zubaida, a leading scholar on food and culture.

Diet is mostly about desire. The diet that Keys and his colleagues invented bore little resemblance to what Mediterraneans actually wanted to eat. “I’m not sure whether the prestige of such a diet would be high enough for many of the people in the Mediterranean to follow it,” says Josef Schmidhuber, a senior economist at the United Nations Food and Agriculture Organization. “Because they aspire to a Western diet, which they conflate with prestige and wealth.”

Last year, Unesco added the Mediterranean diet to its list of the world’s great intangible cultural treasures in need of safeguarding. Today, more than half the populations of Italy, Portugal and Spain are overweight. In Eastern Mediterranean countries like Lebanon, obesity is growing — especially among the young, an increasing number of whom are happy to trade their eggplants for French fries and milkshakes at Roadster.