Food source may affect weight loss more than processing level

A small study suggests that eating more animal foods may be linked to weight gain, raising the possibility that where food comes from matters as much as how it is processed.

People are often told that fresh food is healthier than packaged or heavily processed products. But a new analysis suggests the story may be more complicated when it comes to body weight.

For example, both a chicken breast and an apple count as minimally processed foods. Even so, researchers found that these two foods may be associated with weight in different ways.

In an analysis of overweight adults, higher intake of animal foods such as meat, fish, eggs, and milk was tied to weight gain, even when those foods had undergone very little processing.

The study was led by Hana Kahleova of the Physicians Committee for Responsible Medicine in Washington, DC, with collaborators from the US and Canada.

The team reviewed data from an earlier study of 62 adults.

The findings raise intriguing questions, though they do not prove that eating animal foods directly causes weight gain.

“A chicken breast is minimally processed. So is an apple,” Kahleova said. “In our study, they did not move the scale in the same direction.”

Not all processed foods act the same way

Ultra-processed foods are industrial products made with additives and little or no intact whole food. In some populations, they now provide more than half of total calories.

Previous research has suggested that high intake of these foods may contribute to weight gain and type 2 diabetes, but the health effects do not appear to be identical across all ultra-processed items.

A 2023 analysis of three large studies of American adults found that ultra-processed animal foods and sugary drinks were associated with a higher risk of type 2 diabetes.

By contrast, ultra-processed breads and cereals, along with packaged sweet and savory snacks, were linked with a lower risk.

That pattern raises an important question: does it matter whether food comes from plants or animals, even when the level of processing is the same?

Kahleova and her colleagues looked for an answer by examining weight changes in people following two different diets.

Two eating plans compared

The researchers used information from 62 overweight adults. Their average age was about 57, and most were women.

Each participant followed both diets, with the order assigned at random.

For 16 weeks, participants ate a low-fat vegan diet built mainly around vegetables, fruits, beans, and grains.

They also spent 16 weeks on a Mediterranean diet that included plant foods along with fish, nuts, beans, and olive oil.

There was a four-week break between the two diet periods.

Unlike many weight-loss programs, the study did not require people to count calories.

Participants could prepare their own meals and choose foods within the diet guidelines.

They also attended weekly nutrition classes and received guidance from dietitians.

At different points, they recorded everything they ate over three days.

The researchers then sorted those foods according to processing level and whether they came from plants or animals.

More plant foods on the vegan diet

As expected, switching to a vegan diet changed what participants ate.

Their daily intake of minimally processed animal foods fell by about 174 grams. By the end, they were eating only about 10 grams a day on average.

At the same time, they ate substantially more plant foods.

Compared with the Mediterranean diet, their daily intake of minimally processed plant foods rose by about 202 grams.

Still, the researchers noticed something else as well.

Processed foods still played a role

Going vegan did not mean giving up packaged or processed foods.

Even near the end of the vegan diet, participants were still eating about 266 grams of ultra-processed plant foods each day.

That gave the team a chance to examine more closely how different food groups related to changes in body weight.

When the researchers compared food records with weight changes over time, one result stood out.

During the second 16-week period, participants who increased their intake of minimally processed animal foods were more likely to gain weight.

The team examined 18 possible links between food intake and weight. This was the only one that remained statistically significant after the strictest test.

A few other results pointed in the same direction, but the evidence was not strong enough to support firm conclusions.

For instance, during the first diet period, eating more processed animal foods was associated with weight gain.

During the second period, people who ate more minimally processed plant foods tended to lose weight.

There was no clear connection between ultra-processed plant foods and weight changes in either period.

That does not mean all processed plant foods are beneficial. It simply means the study did not find a clear link between eating them and gaining or losing weight.

“The degree of processing still matters, but it is not the whole story,” Kahleova said.

Fiber may help explain the pattern

In the original trial, published in 2021, volunteers lost about 13 pounds, or 6 kilograms, on average on the vegan diet and showed no net change on the Mediterranean diet.

The researchers suggested fiber as one possible reason plant foods were linked to weight loss.

Apples, vegetables, beans, and whole grains contain lots of fiber and water, so each bite delivers fewer calories.

Fiber also slows the rate at which food leaves the stomach and helps trigger hormones that signal fullness.

The trial did not directly test these explanations.

Another factor may be eating speed. Whole fruits and vegetables generally take longer to chew, and slower eating may make it easier to notice when you are full.

The researchers also suggested that the body may absorb fewer calories from intact plant foods than from foods broken down by heavy processing.

These ideas are scientifically plausible, but the study did not examine them directly.

What the study cannot prove

The study was small, with 62 adults whose food diaries covered only three days at four different points.

All but one of the food-weight links also failed to meet the team’s most rigorous statistical threshold.

The research was funded by the Physicians Committee for Responsible Medicine, whose mission statement supports plant-based diets.

It cannot show that animal foods themselves caused weight gain.

People who reduced animal foods may also have eaten more fiber, taken in fewer calories, or changed other habits at the same time.

Processing level is only part of the picture

This does not mean chicken, fish, eggs, or milk automatically cause weight gain. It also does not mean every plant-based food is healthy.

The findings do suggest that classifying foods only by processing level leaves out important details.

The processing category alone says nothing about fiber content or calories per bite, and the researchers pointed to both as possible reasons plant foods may support weight loss.

An apple and a chicken breast fall into the same processing category.

In this analysis, minimally processed plant and animal foods were linked differently with weight, but only the animal-food finding passed the strictest statistical test.

The full study appeared in the journal Nutrition Research.