The heart-healthy diet is rich in fruits, vegetables, fish and whole grains, along with healthy fats like olive oil, nuts and avocados.
"I hope these rankings steer people in the direction of doing something healthful", said nutritionist Samantha Heller of NYU Langone Health in New York City.
Research has shown that people in Mediterranean countries tend to live longer and have lower instances of cancer and cardiovascular disease, compared to other populations around the world.
The Mediterranean diet taking the top spot should come as no surprise. U.S. News ranked it high on the list for being nutritionally complete, easy to follow, and providing long-term weight loss as well as heart health benefits.
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WW's new global integrated campaign, "For Every Body", which launched on December 26, is based on the insight that everyone has their own reason to get healthy and what makes the most meaningful impact along the journey is everything you gain. You can occasionally add eggs, chicken or fish. To be top-rated, a diet had to "be safe, relatively easy to follow, nutritious and effective for weight loss".
The magazine asked a panel of nutrition experts to review 41 diet plans. These types of diets can help teach lifelong positive eating habits. It's more flexible than fully vegetarian or vegan diets.
"I think that diets that don't differentiate between healthy and unhealthy fats, over time are ultimately an unhealthy approach to losing weight", Heller said.
Weight Watchers ranked in the top five for best diets overall and also obtained the No. 1 rankings for Best Commercial Diet and Best Weight Loss Diet for 2019. It's very similar to the Mediterranean plan. "We are committed to not only being the world's partner in weight management, we're also going to be the world's partner in wellness, helping people, all people, lead healthier lives".