Not all body fat is created equal. There is white adipose tissue, which stores excess energy, brown adipose tissue, which burns energy, and a third category known as “beige” fat. This type of fat can emerge from white fat under certain conditions and take on the energy-burning properties of brown fat, making it a compelling target for obesity research.
A new study suggests that protein consumption may influence this process with help from the gut microbiome.
The research, published in the journal, Nature, found that a low-protein diet in mice triggered a cascade of biological signals that promoted the browning process for white fat. Importantly, this transformation did not occur in mice that had been engineered to have no bacteria in their gut, underscoring that gut bacteria play an active role in the transformation process.
In normal mice, when protein intake dropped, specific microbes altered their metabolism, producing signals that ultimately reshaped their fat tissue.
“The most exciting moment for us was when we observed that a consortium of just four bacterial strains was sufficient to induce browning under a low-protein diet,” said Kenya Honda, co‑senior author of the study and adjunct professor at City of Hope, a US cancer research and treatment organization.
The scale of the fat tissue transformation particularly struck Dr. Honda and his colleagues.
The researchers observed a surge in mitochondria (the energy-producing structures within cells) alongside increased innervation by sympathetic nerves in white fat, meaning that the nerves grew denser and more elaborate networks in the fat tissue. Both of these are changes that are hallmarks of tissue shifting toward a calorie-burning state.
At the center of this transformation are two biological pathways that appear to work in parallel. The first involves bile acids, which activate a receptor known as FXR (Farnesoid X receptor). This signaling pathway acts on precursor cells in fat tissue, effectively priming them to become beige fat cells. The second pathway is driven by ammonia, a byproduct of bacterial metabolism. This ammonia travels to the liver, where it stimulates production of the hormone FGF21 (Fibroblast growth factor 21), a key regulator of energy balance.
According to Dr. Honda, neither pathway is sufficient on its own. Instead, they reinforce each other: FXR prepares fat cells for transformation, while FGF21 may enhance the neural inputs that activate them.
Don’t reduce protein intake just yet
“This is a proof-of-principle that nutrients and microbiota interact to alter browning of fat tissue,” said Jon Schertzer, a professor of biochemistry and biomedical sciences at McMaster University, who was not involved in the study. “This may increase energy expenditure at rest, which is potentially important in obesity.”
Dr. Schertzer emphasized that the study’s strength lies in its demonstration of a clear mechanism for fat browning, particularly in animal models where such pathways can be dissected in detail. The researchers did demonstrate a link to humans, as mice colonized with gut bacteria from people with high levels of brown fat showed similar browning responses when fed a low-protein diet, but Dr. Schertzer — and the study authors — caution against drawing conclusions for humans.
“It is still speculative that this mechanism would translate to humans in a way that significantly alters energy expenditure,” he said.
Also, the mice in this study consumed about one third of the protein that laboratory mice typically consume. The researchers caution that very low protein diets are not appropriate for people, particularly older adults at risk of muscle loss.
“Our goal is not to tell people to eat extreme diets,” said study first author Takeshi Tanoue of City of Hope and Keio University. “The real opportunity is to understand these pathways well enough to design therapies that safely mimic their benefits.”
Still, the study does point to a broader shift in how scientists think about the microbiome. Rather than focusing solely on which bacterial species are present, researchers are increasingly interested in what biochemical functions these microbes perform.
The findings also challenge some prevailing assumptions. At a time when high-protein diets are widely promoted for weight management, this research raises the possibility that less protein combined with the right microbial context could induce fat browning.
