A new randomized clinical trial published in the journal Menopause finds that replacing animal protein with plant-based sources produces measurable weight loss in postmenopausal women, even when total protein consumption remains the same.
The study, led by Hana Kahleova, MD, PhD, director of clinical research at the Physicians Committee for Responsible Medicine (PCRM), tracked dietary changes in 84 postmenopausal women assigned to either a low-fat vegan diet supplemented with daily soybeans or an omnivorous control group over 12 weeks. In the vegan group, animal protein intake fell by 23.3 grams per day while plant protein rose by 22.1 grams per day, with no net change in total protein. Body weight dropped significantly in line with that shift.
The data suggest that a reduction of roughly 16 grams of animal protein per day, paired with an increase of around 13 grams of plant protein, corresponded to approximately 1 kilogram of weight loss. Critically, these effects held independent of calorie intake.
A metabolic mechanism
The study points to methionine as a likely factor. The amino acid is found in higher concentrations in animal products including meat, eggs, and dairy, and lower methionine intake was significantly associated with reductions in BMI across study participants. Lower methionine has previously been linked to improved fat oxidation and metabolic efficiency, though the researchers note that this area of inquiry is still developing.
“These findings suggest that the quality of protein and its amino acid composition may influence metabolism in ways we are only beginning to understand,” said Kahleova.
Compounding benefits for menopausal health
The results carry additional weight given the metabolic and symptomatic changes common in postmenopause, including a tendency toward weight gain. In the broader trial from which this analysis draws, moderate-to-severe hot flashes fell by 88% in the vegan group. The study authors attribute part of this effect to isoflavones in soy, which are known to reduce vasomotor symptoms.
Kahleova stated, “For many women, menopause brings both metabolic and quality-of-life challenges. This approach addresses both — weight and symptoms — with a single, practical dietary change.”
“This study challenges the common assumption that protein is all the same. It’s not just how much protein you eat — it’s where that protein comes from that matters for body weight and metabolic health.”
