Study: Replacing Animal Protein with Plant Protein More Effective for Weight Loss

A randomized crossover clinical trial published in BMJ Nutrition, Prevention & Health last October found that overweight adults who replaced animal protein with plant protein within a low-fat vegan diet successfully lost weight, even though their total protein intake actually decreased. Conversely, when the same participants followed a Mediterranean diet, there was no meaningful change in body weight.

The study involved 62 participants who underwent two intervention periods of 16 weeks each, with a four-week break without a specific diet in between. During the low-fat vegan diet phase, participants avoided all animal products, added fats, and cooking oils. Their protein intake fell from 80 grams to 46 grams per day, with animal protein decreasing by 47.7 grams and plant protein increasing by 14.4 grams. The average weight loss reached 6 kilograms.

Meanwhile, the Mediterranean diet, which followed the PREDIMED protocol, did not restrict animal or plant protein intake. Participants consumed about 73 grams of protein per day, consisting of 42 grams of animal protein and 34 grams of plant protein. Despite the higher protein intake, there was no significant weight loss in this group.

The study's senior author, Hana Kahleova, MD, PhD, emphasized that the source of protein plays an important role. According to her, shifting the balance toward legumes, lentils, tofu, and whole grains—and away from meat, dairy, and eggs—provides metabolic benefits and weight loss that are not obtained merely by eating more vegetables in a Mediterranean diet.

However, the researchers themselves acknowledged that they could not determine whether the change in protein alone caused the weight loss, because changes in fat intake, carbohydrates, and calorie density also played a role. Mir Ali, MD, a bariatric surgeon at the MemorialCare Surgical Weight Loss Center at Orange Coast Medical Center, California, who was not involved in the study, judged the findings to be in line with a growing body of intervention trials comparing plant-based and Mediterranean diets. He highlighted that eliminating animal products inherently lowers energy density and fat intake while increasing fiber.

Ali also cautioned that because the dietary changes occurred simultaneously, the study is not a pure comparison between animal and plant protein. "The data come from a randomized crossover clinical trial—the gold standard in nutrition interventions—reinforced by three-day food records kept by registered dietitians. However, further studies are needed," he said.

Yoshua Quinones, an internal medicine specialist at the Medical Offices of Manhattan, added that a low-fat vegan diet can promote weight loss for several reasons, one of which is that vegetables and grains provide fewer calories than animal protein, while fiber increases satiety. Nevertheless, he stressed that these data do not show that a vegan diet in general is better than a Mediterranean diet for weight loss. "The Mediterranean diet is a better choice for certain groups of people, for example those with high triglycerides or metabolic syndrome, because the Mediterranean diet lowers triglycerides more," Quinones explained.

He also highlighted that the Mediterranean diet can be an option for older adults with sarcopenia (progressive loss of muscle mass), because keeping protein intake stable through fish, dairy, and legumes can help protect muscle better than a reduction of 33 grams of protein per day. Ali agreed, adding that the Mediterranean diet "remains an excellent and sustainable choice for individuals focused on long-term cardiovascular health and metabolic flexibility."

For people accustomed to eating meat, switching to a vegan diet for weight loss is considered realistic, at least in the short term, provided there is structured support. Quinones suggested making legumes the base of meals, keeping foods whole, limiting added oils, refined grains, fried foods, sweets, and processed vegan products, and not forgetting an exercise regimen, especially strength training. Ali added that a gradual and structured transition is usually most effective, with a "crowding out" approach—adding plant-based foods several days a week instead of focusing only on restriction.

Nevertheless, these findings raise further questions: whether the weight-loss benefits can be sustained in the long term, and how they would apply in Indonesia, whose diet is rich in animal protein such as meat, fish, and eggs. Controlled feeding trials that equalize calories and fiber while varying the amount and source of protein are needed to establish causality. Until then, the main message is simple: it is not just how much protein is consumed, but where it comes from.