What we eat matters. Researchers are still searching for the 'best' diet.

Part 3 of a six-part USA TODAY series examining America’s obesity epidemic.

Campaigns to cut fat and then sugar from America's diet didn't make a dent in the obesity epidemic. Nor did keto or paleo,  Atkins, exercise plans or a TV reality show.

Kevin Hall knows, because he has studied all of them at the National Institutes of Health, where he has worked for 19 years. Hall considers it his mission to help people live healthier lives through food.

He has seen study participants lose weight on virtually every type of diet, regardless of the mix of fats, carbs, sugars or protein. 

So a few years ago, when he decided to test the latest Diet Enemy No. 1, ultra-processed food, he expected to see much of the same.

He didn't.

In his study, two groups of 10 people each were fed either mainly chef-prepared whole foods for two weeks or mainly ultra-processed foods – the snack items, cereals and pre-made meals that make up about half of the typical American diet. Then they switched.

Both were offered the same number of calories and balance of fats, sugars, carbs, protein and salt. Both reported feeling equally full and equally satisfied with their food options. 

But when they followed the mostly whole-food diet for two weeks, the volunteers lost about 2 pounds each. On the highly processed meal plan, the same people unintentionally consumed about 500 calories more a day on average, gaining both fat and 2 pounds each in just two weeks.

It was the first time he'd seen weight gain in one of his studies and Hall, whose research takes place at the National Institutes of Health Clinical Center in Bethesda, Maryland, couldn't help but conclude that ultra-processed food plays a significant role in the obesity epidemic.

"By changing people's food environment, you can change how much they're eating by a huge amount without them choosing," Hall said.

Highly processed foods are fast and filling. They are designed to taste good and be hard to stop eating. They can sit for months on store shelves. And they are cheap.

(Click here to read the rest.)

(Photo: Kevin Hall)