Low-fat, low-carb, low-sugar, high-protein, Mediterranean — with so many diets out there and new, sometimes-conflicting research coming out all the time on the best ones for weight loss, how can you possibly know which one will be the most effective in helping you shed pounds?

And for doctors and scientists who spend their time designing and carrying out studies, fad diets make for an added challenge.

But instead of trying to figure out which diet is best overall, doctors and scientists should focus on determining which diet is best for a certain person, said Christopher Gardner, a research professor of medicine at the Stanford University School of Medicine. Gardner gave a talk on the best diets for weight loss on Nov. 14 at the American Heart Association's (AHA) Scientific Sessions annual meeting in New Orleans. 

Gardner started his talk with a brief history of how major medical organizations have approached weight loss. In 1988, for example, the National Institutes of Health concluded that the best diet for weight loss was a low-fat diet, Gardner said. More recently, in 2013, the AHA, along with the American College of Cardiology and The Obesity Society, released recommendations that included 15 different diets, ranging from vegetarian-style diets to high-protein diets.

But there's a catch to this approach of recommending so many diets, Gardner said: All of the diets work if "a reduction in energy intake is achieved," he said. In other words, you have to consume fewer calories.

Some recommended diets explicitly call for people to cut their calorie intake, while others do not. The evidence shows that diets are effective for weight loss if they do not have a "formal prescribed energy restriction" but they do have a "realized energy deficit" — in other words, when a person isn't told to eat fewer calories but ends up doing so anyway because he or she is following the diet, Gardner said.

"This is the key," Gardner said. "What has thwarted us forever is satiety issues," he said. (Satiety is a feeling of fullness.)

If you're not told you need to eat fewer calories but you still end up eating fewer calories, it's usually because you're not hungry, Gardner said. "Different diets are working for different people because they're satiated," he said. 

Eat your nutrients

With that in mind, what should people look for when considering a diet for weight loss?

The government outlines the amounts of macronutrients people should get each day in order to keep chronic diseases at bay and stay healthy, Gardner said. These recommendations are called the "acceptable macronutrient distribution range."

The recommendations say that for adults, carbohydrates should make up 45 to 65 percent of a person's daily calories, protein should make up 10 to 35 percent and fat should make up 20 to 35 percent.

If a person stays within these ranges, he or she will get the right amount of nutrients to stay healthy, Gardner said.

A lot of diets fall within these ranges, Gardner said. For example, the diet recommended by the government's ChooseMyPlate program is about 55 percent carbohydrates, 15 percent protein and 30 percent fat. And the diet that Americans eat, on average, is about 45 percent carbohydrates, 20 percent protein and 35 percent fat.

Gardner pointed out that other diets, including some diets that are considered trendy, don't fall within these ranges. The Atkins diet, for example, is about 15 percent carbohydrates, 35 percent protein and 50 percent fat. And some versions of the paleo diet call for about 30 percent carbohydrates, 30 percent protein and 40 percent fat.

This means that, in the course of getting your calories in these distributions of macronutrients, you'd fail to get all of your adequate nutrients, Gardner said. If a diet is going to work for weight loss, it also has to meet your nutrient requirements, he said.

What diets have in common

Although popular diets can often seem at odds with one another (take, for example, the fat-rich Atkins diet compared with the very-low-fat Ornish diet), there are three things that all diets appear to have in common, Gardner said.

All of the popular diets include vegetables and suggest avoiding both added sugars and refined grains, Gardner said. These three points, taken together, account for the majority of Americans' problems with diets, he said.

Originally published on Live Science.

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