When it comes to losing weight, protein gets a lot of attention. But could where your protein comes from matter as much as how much you eat?
A new study suggests that people who shift toward plant-based protein may lose more weight. However, the researchers couldn't determine whether the benefits came from the protein itself or other changes in what participants ate.
Published in BMJ Nutrition, Prevention & Health, the study revisited data from an earlier clinical trial involving 62 adults with overweight. The research was conducted and funded by the Physicians Committee for Responsible Medicine, an organization that advocates for plant-based diets.
Participants followed two different diets for 16 weeks each, with a four-week break in between. One was a low-fat vegan diet featuring vegetables, fruits, whole grains and legumes, with no animal products or added oils. The other was a Mediterranean-style diet emphasizing many of the same plant foods, along with fish, nuts and olive oil.
Neither group was instructed to count calories or intentionally eat less.
During the vegan portion of the study, participants lost an average of about 13 pounds. Their weight did not change significantly during the Mediterranean portion.
Researchers wanted to know whether differences in protein intake might help explain why.
They found that during the vegan phase, participants ate about 14 grams more plant protein and nearly 48 grams less animal protein per day. Their total protein intake also dropped by about 33 grams daily.
Across both diets, people who made the biggest shifts away from animal protein and toward plant protein tended to lose the most weight.
The researchers also looked at amino acids, the building blocks of protein. They found that lower intake of several amino acids commonly found in greater amounts in animal foods was associated with more weight loss.
But there's an important distinction between finding a connection and knowing what caused it.
Participants didn't simply swap chicken for tofu or beef for beans. Their entire eating pattern changed.
The vegan diet was much lower in fat and included more carbohydrates than their previous diets. It also emphasized foods such as beans, vegetables and whole grains, which can provide fiber and help people feel full.
With so many changes happening at once, researchers couldn't determine how much of the weight loss, if any, was specifically related to switching protein sources.
The study also had limitations. Only 52 participants completed the full trial, most were middle-aged women and researchers relied on participants' food records to track what they ate. The findings also came from a new analysis of an existing trial rather than a separate experiment.
Still, the results add to evidence that plant-focused eating patterns can support weight management. They also offer a reminder that the foods providing our protein may matter for reasons beyond protein itself.
For example, replacing some meat with lentils or beans doesn't just change the type of protein on your plate. It also changes the amount of fiber, fat and other nutrients you're eating.
That doesn't mean everyone needs to give up animal products or that Mediterranean diets aren't beneficial. The study compared two specific eating plans, and its findings don't establish that one approach is best for everyone.
Instead, the research adds another piece to the ongoing conversation about protein and weight loss, suggesting that what comes with your protein may be just as important as where it comes from.
The study was funded by the Physicians Committee for Responsible Medicine, which advocates for plant-based diets. Several authors are employed by the organization, another received compensation for his work and one serves as its unpaid president.