On brown fat, clocks and fat burning 🔥
Brown fat burns more energy in the morning and in the cold, and it barely runs while you sleep. A new preprint finds one mitochondrial protein that responds to the body clock, cold, and dietary fat all at once.
Researchers from the University of Copenhagen, UPenn, and collaborators studied SLC25A34, a mitochondrial transporter that no one had assigned a job to. Mitochondria are the compartments in your cells that burn fuel for energy in the form of ATP, and transporters are the gates that move molecules in and out of them.
A little primer on some of the concepts of the study:
Brown fat is a heat-producing tissue. Its cells run a loop: they build fat in the cytosol, the fluid outside the mitochondria, then burn it inside the mitochondria. Building and burning at the same time looks wasteful. That waste is the point, because it releases energy as heat. In humans, brown fat activity is linked to better metabolic health.
What the study found:
• Cold exposure raised SLC25A34 about 90-fold in mouse brown fat within 24 hours, making it the most enriched tissue for this gene, ahead of the heart
• During sleep, the clock protein REV-ERBα keeps the gene switched off. Waking or cold removes that brake
• A fat-sensing protein called PPARα then switches the gene on, and it responds to the fats released when cells break down stored fat
• A ketogenic diet (93% fat) made it the second most induced gene in the entire brown fat transcriptome. A standard high-fat diet had a much smaller effect
• Silencing the gene in brown fat cells reduced respiration, fat synthesis, and mitochondrial content, and acetyl-CoA levels dropped
Acetyl-CoA is the raw material cells use to build fat, and it also helps signal genes to make more mitochondria.
The proposed mechanism: when cells make acetyl-CoA outside the mitochondria, a leftover molecule called oxaloacetate gets stranded in the cytosol. SLC25A34 appears to carry it back in, which keeps the building and burning loop moving. Without the transporter, oxaloacetate piled up outside and fell inside the mitochondria. Adding acetate, an alternative source of acetyl-CoA, largely rescued the damage.
There is also an early human signal. Across 23 clinical studies, SLC25A34 in subcutaneous fat was linked to lower BMI, waist circumference, triglycerides, and insulin resistance, and to higher HDL. The correlations were small, and no link appeared in belly fat around the organs.
This is a preprint, most of the work is in mice and cultured cells, and direct proof that the protein transports oxaloacetate is still pending. Whether the same circuit matters in human brown fat is unknown.
What this means for longevity and health optimization:
No one has tested whether you can raise SLC25A34 on purpose, so none of this is a protocol. But the findings line up with habits that already have support, and they offer a possible mechanism for why those habits work.
• Cold exposure: Cold was the strongest trigger in the study. It fits with research linking regular cold exposure to more active brown fat, though the best dose for people is still unclear
• Sleep and circadian rhythm: The clock brake that keeps this gene off at night is released around waking. Consistent sleep and wake times, and morning light, may help your brown fat switch on on schedule
• Diet composition: Very high fat intake, as in a ketogenic diet, turned the gene on in mice. That doesn't mean keto boosts brown fat in humans, and a standard high-fat diet had a much smaller effect
• Mitochondrial health: Brown fat declines with age and with obesity. Keeping it active may be one way to preserve mitochondrial capacity in a tissue that burns fat and clears glucose
The larger point is that timing, temperature, and fuel all converge on the same mitochondrial gate.