The type of protein, rather than the amount, could impact weight loss.
In general, a high protein diet can support weight loss by increasing satiety, which may lead to reduced food intake. However, a diet high in animal protein may increase the risk of obesity.
New research published in October in BMJ Nutrition, Prevention & Health explored whether plant protein could be more beneficial for weight loss than animal protein.
Secondary analysis of the trial found that adults with overweight who replaced animal protein with plant protein as part of a low fat vegan diet lost weight, even though their total protein intake decreased.
When the same participants followed the Mediterranean diet, they did not experience any significant changes in protein intake or body weight.
Senior study author Hana Kahleova, MD, PhD, explained in the press release that “the [protein] source matters. Shifting the balance toward beans, lentils, tofu, and whole grains—and away from meat, dairy, and eggs—gave our participants a metabolic and a weight benefit they didn’t get by simply eating more vegetables on a Mediterranean-style diet.”
When following the low fat vegan diet for 16 weeks, participants experienced an average weight loss of around 13 pounds (lbs), or 6 kilograms (kg).
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For the crossover trial, 62 adults followed either the vegan diet or the Mediterranean diet for 16 weeks. After four weeks without following either specific diet, they then switched to the other diet for another 16 weeks.
The vegan diet comprised:
It had a macronutrient composition target of:
Participants avoided animal products, added fats, and cooking oils.
The vegan diet involved consuming a total of 46 grams (g) of protein per day, a reduction from 80 g at the start of the trial.
The total protein intake was reduced by 33.3 g per day on the vegan diet. Animal protein intake decreased by 47.7 g while plant protein rose by 14.4 g. None of these three measures changed significantly on the Mediterranean diet.
The researchers determined that replacing animal protein with plant protein in the low fat vegan diet was associated with weight loss in adults with overweight.
However, they also highlight that they cannot confirm whether changes in protein alone led to weight loss, as changes in dietary fat, carbohydrates, and calorie density may also have played a role.
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Medical News Today spoke with Mir Ali, MD, bariatric surgeon, bariatric medicine specialist and medical director of MemorialCare Surgical Weight Loss Center at Orange Coast Medical Center in Fountain Valley, CA, about these findings.
“While the magnitude of weight loss is notable, the findings align with a growing body of intervention trials comparing plant-based and Mediterranean frameworks. This shift is expected, given that eliminating animal products inherently reduces energy density and fat intake while increasing fiber,” said Ali, who was not involved in the study.
“The data originate from a randomized clinical crossover trial – the gold standard in nutritional intervention -strengthened by objective tracking via three-day food records evaluated by registered dietitians. However, because dietary shifts occurred concurrently, this is not a true comparison of animal versus plant protein, and further studies are needed,” he added.
Yoshua Quinones, board certified internist with Medical Offices of Manhattan and contributor to LabFinder.com, who was also not involved in the study, explained that “a low fat vegan diet can promote weight loss for many reasons. One reason is that veggies and grains provide fewer calories than animal protein. Fiber increases satiety, making you feel full.”
Quinones also told MNTthat “[t]he weight loss itself is moderately reliable, at least over the short term in motivated adults with overweight. The idea that the protein shift caused it is hypothesis-generating at best. Testing that would need controlled feeding trials that match calories and fiber while varying protein amount and source.”
The Mediterranean diet led to no significant weight changes.
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The diet involved making no changes to the amount of animal or plant protein participants were consuming. It was based on the PREDIMED protocol, which involved consuming:
The Mediterranean diet also allowed lean white meat.
At the beginning of the diet, participants consumed 73 g of total protein, of which 42 g were animal protein. Total protein remained at around 73 g, with animal protein at around 39 g and plant protein providing 34 g.
While protein intake was higher during the Mediterranean diet — 73 g per day versus 46 g per day on the vegan diet — the vegan diet led to weight loss, while the Mediterranean diet did not.
MNTasked Quinones whether a vegan diet could be considered better for weight loss than the Mediterranean diet.
“These data don’t show that a vegan diet is generally better for weight loss than a Mediterranean diet,” answered Quinones.
“The Mediterranean diet is a better option for a different group of people. One of them is if you have high triglycerides or metabolic syndrome, as the Mediterranean diet lowered triglycerides more.”
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He added, “Another scenario is in the elderly with sarcopenia (progressive loss of skeletal muscle mass, strength, and physical function). Keeping protein intake steady through fish, dairy, and legumes may help protect muscle better than a 33 g/day drop in protein.”
Ali agreed that the Mediterranean diet could be an option for older adults at risk of sarcopenia, adding that the eating plan “remains an exceptional, sustainable option for individuals focusing on long-term cardiovascular health and metabolic flexibility.”
A reduction in four amino acids — isoleucine, lysine, methionine, and tyrosine — was associated with greater weight loss, according to the research.
Certain amino acids are associated with weight gain, and animal protein contains more essential amino acids, compared with plant protein.
However, while research findings indicate a potential link between weight loss and the amount of amino acids from protein, the researchers note that they cannot confirm that reduced amino acids could directly lead to increased weight loss.
“The correlation between reductions in specific amino acids — particularly branched-chain amino acids (BCAAs) like isoleucine and sulfur amino acids like methionine — and greater weight loss aligns with emerging research linking high BCAA and sulfur amino acid availability to impaired metabolic flexibility and weight gain,” said Ali.
“While mechanistically fascinating, the general public should avoid micromanaging individual amino acids, as it is impractical. Instead, focusing on whole food sources, shifting from heavy animal proteins toward whole plant proteins like beans, lentils, tofu, and whole grains, naturally optimizes these amino acid profiles without requiring complex tracking.”
MNTwanted to know how realistic it could be for a person to switch to a vegan diet for weight loss if they usually consume meat, and whether there are any specific steps they should take.
“A person who consumes meat can realistically switch to a vegan diet for weight loss, at least in the short term, if they get structured support,” explained Quinones.
“I advise making legumes the base as it has the best predictor of weight loss. Keep it whole food, limiting added oils and refined grains, fried foods, sweets, and processed vegan products, as they can increase the cardiometabolic risk. Don’t forget to add an exercise regimen, especially strength training.”
Ali advised that a “gradual, structured transition is often most effective.” He also recommended a “crowding out” approach, which involves “adding plant-based meals a few days a week rather than focusing solely on restriction.”
He added, “Replace familiar foods with plant-based alternatives (e.g., lentil or bean-based chilis, tofu scrambles, plant milks). […] Ensure proper micronutrient awareness (particularly vitamin B12) and adequate caloric volume to prevent energy deficits and burnout.”
