If you’ve ever tried to lose a few pounds or just stay at a healthy weight, you’ve likely encountered a dizzying array of diets, each with passionate proponents: low carb, low fat, keto, paleo, vegan, Mediterranean, and so on. Yet most nutrition experts agree on one thing: it’s best to steer clear of processed foods. Now, there’s some solid scientific evidence to back up that advice.

Intriguingly, the weight differences on the two diets occurred even though both kinds of foods had been carefully matched from a nutritional standpoint, including calorie density, fiber, fat, sugar, and salt. For example, breakfast for the processed group might consist of a bagel with cream cheese and turkey bacon, while the unprocessed group might be offered oatmeal with bananas, walnuts, and skim milk.

The explanation for the differences appears to lie in the fact that study participants were free to eat as little or as much food as they wished at mealtimes and to snack between meals. It turns out that when folks were on the processed diet they ate significantly more—about 500 extra calories per day on average—than when they were on the unprocessed diet. And, as you probably know, more calories without more exercise usually leads to more weight!

This might not seem new to you. After all, it has been tempting for some time to suggest a connection between the rise of packaged, processed foods and America’s growing waistlines. But as plausible as it might seem that such foods may encourage overeating, perhaps because of their high salt, sugar, and fat content, correlation is not causation and controlled studies of what people actually eat are tough to do. As a result, definitive evidence directly tying ultra-processed foods to weight gain has been lacking.

Processed food drives increased consumption

The researchers enrolled 20 healthy volunteers who were given three meals a day and had access to either processed or unprocessed snacks, as well as water. Every meal was carefully matched to a counterpart meal (in the opposite diet) to ensure carbohydrate, fat, protein, sugar, fibre, and sodium levels were equal, and the total calorie count was the same in both meals. A typical breakfast in the processed diet consisted of Honey Nut Cheerios, whole milk with added fibre, a packaged blueberry muffin, and margarine.

Collage of pub food including cheese burgers, wings, nachos, fries, pizza, ribs, deep fried prawns and calamari.

In the unprocessed diet, breakfast included a parfait made with plain Greek yogurt, strawberries, bananas, walnuts, salt, and olive oil, and apple slices with freshly-squeezed lemon. The participants were told they could eat as much (or as little) as they wanted, and the researchers measured how much they consumed. At lunch, for example, one of the study’s processed meals consisted of quesadillas, refried beans, and diet lemonade. An unprocessed lunch consisted of a spinach salad with chicken breast, apple slices, bulgur, and sunflower seeds with a side of grapes. The main difference between each diet was the proportion of calories derived from processed versus unprocessed foods as defined by the NOVA diet classification system. This system categorizes food based on the nature, extent, and purpose of food processing, rather than its nutrient content.

So, it appears that a good place to start in reaching or maintaining a healthy weight is to follow the advice shared by all those otherwise conflicting diet plans: work to eliminate or at least reduce ultra-processed foods in your diet in favor of a balanced variety of unprocessed, nutrient-packed foods.