Trying to slim down? The kind of protein on your plate may make a difference

A new study suggests that eating less animal protein and more plant protein is associated with greater weight loss, though other changes in the diet may also be driving the result.

If you are trying to lose weight, protein advice can make every meal feel like a math problem. The new research indicates that total protein may not be the only factor that matters.

In a study of adults with overweight who followed two diets, participants who cut back the most on animal protein and increased plant protein tended to lose the most weight.

But they also reduced fat intake and ate more carbohydrates, so the researchers could not say protein alone caused the weight loss.

Hana Kahleova, director of clinical research at the Physicians Committee for Responsible Medicine, was the senior author. The Washington, DC-based nonprofit conducted the trial.

“Patients are constantly told to worry about getting enough protein,” Kahleova said.

Same participants, two diets

The trial took place over several months and included 62 adults with overweight in Washington. At the beginning, their average weight was about 216 pounds (98 kilograms).

Roughly half started on a low-fat vegan diet and the other half on a Mediterranean diet. They stayed on their assigned plan for 16 weeks, took a four-week break, and then switched to the other diet for another 16 weeks.

During the vegan phase, participants ate vegetables, fruit, grains and legumes such as beans.

They did not eat meat, fish, dairy or eggs, and they also avoided added fats and cooking oils.

The Mediterranean diet emphasized vegetables, fruit, legumes, fish and nuts, along with nearly four tablespoons (50 grams) of extra-virgin olive oil each day. It also favored lean white meat over red meat.

What the food logs showed

Neither diet came with a calorie cap. At four points during the study, volunteers recorded everything they ate for three days, and a registered dietitian reviewed the logs.

In total, 52 participants completed the full 36 weeks. An earlier report from the trial found that volunteers lost about 13 pounds (six kilograms) on the vegan diet, while their weight did not change consistently on the Mediterranean plan.

The new analysis revisited those records with a focus on protein.

Researchers wanted to know whether changes in both the amount and type of protein tracked with changes in body weight.

Protein levels changed in different ways

On the vegan diet, animal protein dropped from about 51 grams a day to fewer than three. Plant protein increased, from about 29 grams to 43 grams per day.

Even so, the added plant protein did not fully replace the animal protein that had been removed. Overall protein intake fell by about 33 grams a day, from roughly 80 grams to 46.

In other words, this was not a simple swap of one protein source for another. Because participants ate less protein overall, it is hard to separate the effects of quantity from the effects of source.

The Mediterranean diet produced little change in either animal or plant protein. Total protein intake remained close to 73 grams a day.

Weight changes tracked the shift

For this comparison, the researchers focused on the first 16 weeks, when about half of the volunteers were on each diet. They matched each person’s protein changes with that person’s change in weight.

People who made the largest cuts in animal protein generally lost the most weight. A similar pattern appeared among those who increased plant protein the most, as well as those who reduced total protein the most.

All three associations were moderate, and each remained significant under a stricter statistical test the researchers used to limit the chance of random findings.

“This doesn’t mean protein is the enemy,” Kahleova said. “It means the source matters.”

A closer look at amino acids

Protein is built from smaller molecules called amino acids, and different foods contain different amounts of each one. Using the food records, the team estimated how much of each amino acid the volunteers consumed.

On the vegan diet, participants ate less of every amino acid the researchers tracked. Methionine fell by more than half, from about 1.8 grams a day to 0.7 grams.

Lower intake of nearly every amino acid was associated with greater weight loss.

Only four of those links held up under the strictest statistical test: isoleucine, lysine, methionine and tyrosine.

Earlier research cited in the paper found that plant protein contains less isoleucine, lysine and methionine than animal protein.

Methionine contains sulfur. In a trial cited by the authors, people eating a vegan diet low in sulfur-containing amino acids lost about 20% more weight than a comparison group.

Other diet changes also mattered

The findings do not show what would happen if only the protein source changed and everything else stayed the same. Participants changed several parts of their diet at once.

On the vegan diet, fat made up about 17% of daily calories, down from 38%. Carbohydrates rose from 42% to 69% of calories.

Daily cholesterol intake dropped by more than 270 milligrams, and each gram of food contained fewer calories. The authors also pointed to fiber and other nutrients as possible contributors.

Earlier analyses of the same volunteers also linked weight loss to other features of the vegan diet, including a score reflecting how much of their diet came from plant-based foods.

These are correlations based on the same dietary changes, not separate proof of what caused the weight loss.

Limits of the study

Food records depend on memory and accurate reporting, so errors are possible. About 78% of the volunteers were women, and the average age was 57.

All of them also lived in one city. The results may not apply to men, younger adults or more diverse groups.

The Physicians Committee supports plant-based diets and funded the study. Three of the five authors work there, though the authors said the sponsor did not influence the results.

People did lose weight on the vegan diet, but how much of that came specifically from plant protein remains unclear.

The authors described the findings as a starting point for future research rather than proof.

They called for trials that alter only the protein source while keeping calories, fat and fiber unchanged. That would give researchers a cleaner way to test whether protein itself deserves the credit.

The study was published in the journal BMJ Nutrition, Prevention & Health.