Vegan diets have been growing in popularity in recent years. However, experts investigated the impacts of an all-plant based diet in mice and found that it had a worrying effect on their liver.
Researchers at the University of Geneva, Switzerland, saw that when mice were switched to an exclusively plant-based diet, within only a few days their liver function changed and they experienced a significant decrease in motivation to do physical activity.
The study was published online on September 4 and will be in the journal Molecular Metabolism in November.
"Being on a vegan diet is possible but it is a daily challenge of getting sufficient proteins, enough iron and calcium, and supplementation of vitamin B12," study author Pierre Maechler, a professor of cell physiology and metabolism at the University of Geneva Medical School, told Newsweek.
"Based on our study, one should also pay attention to the qualitative aspect of dietary proteins by getting enough BCAA [three essential amino acids including leucine, isoleucine and valine] from specific food, such as tofu, nuts, or lentils," he said.
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How an All-Plant Based Diet Impacted the Liver
The researchers set out to explore what impact switching from a diet of both animal and plant proteins to an all plant-based diet has on the body.
They compared two groups of mice, one on a lactovegetarian diet (which contained both plant-based and animal proteins) and one on a vegan diet.
Maechler said that in just two days, the mice on the vegan diet had less efficient glucose production. The mice had a "reduced capacity in maintaining blood glucose levels in the fasting state, normally contributed by the liver at the expense of amino acids," he said.
The impact of this was a "marked reduction in spontaneous physical activity," he said, adding that normally the mice enjoyed running on a dedicated device.
This was "fully and rapidly reversible once back on animal based protein diet," he said. The animal-based diet also did not contain meat, only casein left over during the process of cheese production (whey protein), he added.
The researchers also found that the mice experienced impaired regulation of ammonia as those on the all plant-based diet had a reduction in the production of the enzyme glutamate dehydrogenase (GDH)—which "acts as a bridge between protein metabolism and sugar metabolism," the authors said. This reduction of GDH led to a build-up of ammonia in their bodies.
He also said that the long term impacts of the all plant-based diet in mice was "mostly limited supply of vitamin B12 and iron, plus the challenge of getting enough of dietary proteins." The overall impact of this was fatigue, he explained.
Should Vegans Be Concerned?
While the findings are notable, experts have said they are not a cause for concern. Dr. Neal Barnard, a professor of medicine at the George Washington University School of Medicine, told Newsweek that this study does not "relate to human nutrition," as the enzyme being studied (GDH) is "entirely different between species."
Barnard also said that many studies have found positive associations between all plant-based diets and human health—such as better blood sugar control and liver health.
He said his 2006 research showed an entirely plant-based diet was "three times more effective at improving blood glucose, a finding that has been repeatedly confirmed since then."
"We have also shown that a plant-based diet improves liver health—removing about one-third of liver fat in 16 weeks, both in healthy people and people with type 2 diabetes," he said.
He said that because mice, like other rodents, have "different metabolic needs and also do not eat the same diets that humans eat, the results from mice are often unreliable predictors for human health."
What Needs To Happen Next?
Maechler said that the next step needs to be conducting the same research in humans, particularly as few studies currently look at the direct impact of changing diet type on the human body.
"Most of the studies in humans look at long term adaptation to vegan diet or comparing groups of people already used to such diets," he said, adding that those existing studies have already indicated "the potential issue of lacking vitamin B12 and low iron availability."
He added that a "short term adaptation survey" may improve understanding of the "impact of changing protein source on the basis of the qualitative aspect (less BCAA) before other potential deficiencies would manifest."
Reference
Yan Zhou, Angela M. Ramos-Lobo, Tingting Fu, Hector Gallart-Ayala, Julijana Ivanisevic, Pierre Maechler, (2026) Transition to dietary plant-based proteins requires functional liver GDH for proper control of ammonia and energy homeostasis. Molecular Metabolism. https://doi.org/10.1016/j.molmet.2026.102435
Contact Newsweek editors on this story: Kara Dolman and Paul Donnelley.