Protein gets a great deal of attention during and after menopause. It can help support muscle mass as the body changes with age, but quantity is not the only question worth considering. A new study suggests that where protein comes from may also deserve attention.
In a secondary analysis of a 12-week randomized clinical trial, postmenopausal women assigned to follow a low-fat vegan diet with daily soybeans maintained their overall protein intake while replacing much of their animal protein with plant protein. The dietary shift was associated with weight loss. However, the study cannot prove that protein source alone caused the change because participants modified their broader eating patterns at the same time.
The analysis was published in the journal Menopause and funded by the Physicians Committee for Responsible Medicine, a nonprofit organization that promotes plant-based diets. Several authors received compensation from the organization for their work on the study.
“This study challenges the common assumption that protein is all the same,” said lead author Hana Kahleova, M.D., Ph.D., director of clinical research at the Physicians Committee for Responsible Medicine. “It’s not just how much protein you eat — it’s where that protein comes from that matters for body weight and metabolic health.”
The analysis drew on an earlier trial involving 84 postmenopausal women who experienced at least two moderate-to-severe hot flashes per day. Half were assigned to follow a low-fat vegan diet that included one-half cup of cooked soybeans daily. The other half continued their usual diets. Participants were asked to keep their exercise habits and medications consistent.
The original intervention changed more than protein source. The women assigned to the vegan diet avoided animal products, limited added oils and ate soybeans daily. Those changes likely affected multiple aspects of their diets, including fiber, fat intake, food choices and energy density.
In the new analysis, total protein intake remained relatively stable in both groups. Among women assigned to the vegan diet, animal protein decreased by 23.3 grams per day while plant protein increased by 22.1 grams per day.
The researchers found that a decrease of approximately 16 grams of animal protein and an increase of approximately 13 grams of plant protein per day were associated with a 1-kilogram, or 2.2-pound, reduction in body weight. The association remained after researchers adjusted for reported calorie intake.
That does not mean calories are irrelevant. Food records are imperfect, and a diet higher in fiber and lower in energy density may affect fullness and eating patterns in ways that are difficult to capture precisely. The study also does not tell us whether someone would see the same results by making a single swap while keeping the rest of their diet unchanged.
Instead, the results offer a more practical message: It is possible to incorporate more plant-based foods without sacrificing protein intake. Foods such as beans, soy foods and whole grains can contribute meaningful amounts of protein while also adding nutrients and fiber.
The study also explored whether methionine may help explain the results. Methionine is an essential amino acid found in many foods, with higher concentrations in some animal products. Researchers found that reductions in methionine intake were associated with decreases in body mass index.
That finding is preliminary. The study does not establish that reducing methionine causes weight loss, and readers do not need to track individual amino acids or treat methionine as something to avoid. Protein remains essential, particularly as people age.
The original trial also examined hot flashes. Women following the low-fat vegan diet with daily soybeans experienced a substantial reduction in moderate-to-severe hot flashes. But the new analysis does not show which part of the dietary intervention mattered most. Soy isoflavones, changes in body weight, lower fat intake, higher fiber intake or a combination of factors may have played a role.
The findings do not mean postmenopausal women need to become vegan. They also do not settle the question of whether plant protein is inherently better than animal protein for weight management.
The study was small, lasted only 12 weeks and involved volunteers willing to make a significant dietary change. Larger and longer trials are needed to determine whether more modest shifts toward plant protein produce similar results and whether those changes are sustainable over time.
Still, the analysis adds useful nuance to the protein conversation. For women thinking about how to build meals after menopause, the goal does not have to be choosing one protein category and rejecting another. It may be as simple as making more room for beans, soy foods and other plant-based options while continuing to meet overall protein needs.
The Physicians Committee for Responsible Medicine funded the study. Authors Haley Brennan, Macy Sutton, Richard Holubkov and Hana Kahleova received compensation from the organization for their work on the study. Author Neal D. Barnard serves without compensation as president of the Physicians Committee for Responsible Medicine and Barnard Medical Center. He has received royalties and honoraria for books, articles and lectures related to nutrition and health.