How the Body Clock and Cold Team Up to Burn Fat

Summary:

Metabolic researchers led by the University of Copenhagen have discovered that a little-known mitochondrial transporter, SLC25A34, acts as a master energetic switch in brown fat. The protein integrates signals from the circadian body clock, environmental cold exposure, and dietary fuel availability to control both the synthesis and burning of fat. Silencing the transporter blunted thermogenesis in brown fat, while human clinical data linked higher levels of the protein to lower body fat and improved metabolic health.

Key Facts:

• The Triple-Input Master Switch: SLC25A34 is controlled by three distinct physiological levers: the circadian repressor REV-ERBα keeps it silenced during sleep; cold exposure overrides this schedule to induce a 90-fold surge; and dietary or stored fatty acids activate it via PPARα.
• The “Build-to-Burn” Paradox: Both fasting and insulin stimulate SLC25A34 expression because active brown adipose tissue builds new lipid molecules specifically to burn them for thermogenesis, a substrate cycle fueled by SLC25A34 shuttling oxaloacetate into mitochondria.
• Human Metabolic Relevance: Silencing SLC25A34 in human brown fat cells significantly reduced fuel consumption, and an analysis of 24 clinical cohorts revealed that individuals with higher expression of the protein in subcutaneous fat were leaner and metabolically healthier.

Source: University of Copenhagen / NNF Center for Basic Metabolic Research

Brown adipose tissue (BAT) functions as an internal furnace, consuming glucose and lipids to generate heat through non-shivering thermogenesis. Under typical conditions, this energetic output adheres strictly to a circadian timetable, dipping during sleep when energy conservation is prioritized and ramping up prior to waking to prepare the body for daily activity.

Yet the biological environment is rarely predictable. A sudden drop in ambient temperature or a skipped meal introduces urgent thermal and energetic demands that run counter to the pre-programmed clock. How brown adipocytes balance an inflexible 24-hour schedule with the immediate need to improvise has remained an enduring biological puzzle.

Now, an international team led by researchers at the Novo Nordisk Foundation Center for Basic Metabolic Research (CBMR) at the University of Copenhagen has unmasked the molecular junction linking these disparate systems.

The study reveals that SLC25A34, a previously uncharacterized transporter embedded in the inner mitochondrial membrane of brown fat cells, serves as a central switch that synchronizes internal circadian rhythms, external thermal cues, and nutritional intake.

“We usually think of the body clock, the response to temperature, and the response to food as separate systems. A mitochondrial transporter that is tuned by the time of day, the temperature, and what we eat raises the possibility of therapies that shift when and how the body burns fuel. That would be a different kind of lever from today’s obesity and diabetes treatments,” said corresponding author Zach Gerhart-Hines, Ph.D., Associate Professor at CBMR.

Uncovering an Overlooked Thermogenic Engine

The investigators initiated their search by mining large-scale functional datasets to identify mouse brown fat proteins that respond simultaneously to circadian pacing and acute cold stress.

Out of thousands of candidates, only two met every screening benchmark: uncoupling protein 1 (UCP1), the canonical engine of non-shivering thermogenesis, and SLC25A34, a member of the mitochondrial solute carrier family whose physiological function had never been resolved.

In mice housed at warm, thermoneutral temperatures, SLC25A34 was barely detectable in brown fat compared to virtually every other organ. However, following 24 hours of cold exposure, the transporter experienced an astonishing 90-fold increase, elevating brown fat to the tissue with the highest concentration of SLC25A34 in the entire body.

Investigating the genetic architecture of the Slc25a34 locus in knockout models revealed a tripartite regulatory mechanism:

• Circadian Control: The clock repressor protein REV-ERBα represses Slc25a34 transcription during sleep, lifting the block just ahead of awakening.
• Thermal Override: Cold temperatures rapidly lift the REV-ERBα brake regardless of the hour, overriding the circadian clock whenever emergency heat generation is required.
• Nutritional Activation: Lipids mobilized from intracellular lipid droplets or absorbed from the diet bind to the nuclear receptor PPARα, driving direct transcription of the Slc25a34 gene.

Resolving the “Build-to-Burn” Lipid Cycling Enigma

The researchers noticed an apparent biochemical contradiction: both fasting (which drives lipid catabolism) and insulin signaling (which promotes lipid storage) provoked marked increases in SLC25A34 levels.

Rather than working at cross-purposes, this dual regulation reflects a fundamental property of brown fat biology. To generate sustained heat while clearing circulating glucose and lipids from the bloodstream, active brown adipocytes simultaneously synthesize new fatty acid chains only to funnel them directly into mitochondrial oxidation.

SLC25A34 acts as a metabolic linchpin in this futile cycle by transporting oxaloacetate across the mitochondrial membrane. When the researchers depleted SLC25A34, brown fat cells exhibited blunted fuel consumption, and knockout mice suffered a marked deficit in their capacity to burn lipids for heat.

“Many of these mitochondrial transporters still have no known function. This one turned out to be needed both for building fat and for burning it. And we are only scratching the surface: SLC25A34 is also highly expressed in the heart and is implicated in brain and liver metabolism, but what it does in those organs remains a mystery,” said first author Iuliia Karavaeva, Ph.D., of CBMR.

Clinical Potential for Obesity and Metabolic Disorders

To assess human relevance, the researchers silenced SLC25A34 in human brown adipocytes harvested from donor tissue, which resulted in an immediate reduction in cellular fuel consumption in three out of four donor lines.

Furthermore, across 24 human clinical cohorts, higher expression of SLC25A34 in subcutaneous white adipose tissue consistently correlated with lower body mass index (BMI), reduced adiposity, and enhanced systemic metabolic fitness.

While the authors emphasize that this clinical correlation does not establish direct causality, the convergence of molecular, animal, and clinical findings establishes SLC25A34 as an attractive candidate for metabolic pharmacology. Developing targeted agonists capable of activating this mitochondrial transporter could offer a novel avenue for treating obesity and type 2 diabetes by retraining fat cells to burn fuel on demand.

Funding: The research was supported by, among others, the Independent Research Fund Denmark through a Sapere Aude Starting Grant, the European Research Council through a Starting Grant (aCROBAT), and the Novo Nordisk Foundation, including a Bioscience PhD fellowship and a postdoctoral fellowship to Iuliia Karavaeva and support for the Center for Adipocyte Signaling (ADIPOSIGN).

Questions & Answers

Editorial Notes:

• This article was edited by a Neuroscience News editor.
• Journal paper reviewed in full.
• Additional context added by our staff.

About this neuroscience Research:

• Media Contact: Peter Andrew Stanners
• Source: University of Copenhagen
• Image Credit: Image credited to Neuroscience News
• Original Research is Open Access: Science (Oct 1, 2026). “Mitochondrial carrier SLC25A34 links clock, diet, and temperature control of adipocyte lipid cycling.” Authors: Iuliia Karavaeva, Astrid Linde Basse, Samuel A. J. Trammell, Mohammed Faiz Hussain, Lasse Kruse Markussen, Jesper F. Havelund, Marie Sophie Isidor, Adam Chatoff, Andrea Andress Huacachino, Hannah J. Richter, Meghna Birla, Sabina Chubanava, Yann Deleye, Rini Arianti, David Tandio, Sarah E. Utzon, Zafir Kaiser, Yachen Shen, Ditte Neess, Hannes Embring, Dilip Menon, Olivia J. Conway, Frederike Sass, Fabian Finger, Lidia Argemi-Muntadas, Tao Ma, Elahu G. Sustarsic, Cecilie Kynding Kristensen, Rebecca L. McIntyre, Genesee J. Martinez, Anna Sofie Husted, Matthew J. Emmett, Zachary A. Kipp, Mikkel Frost, Mark P. Jedrychowski, Michel van Weeghel, Homa Majd, Ekaterina Zhuravleva, Robert W. McGarrah, Kaja Plucińska, Mohit K. Midha, Andreas Prokesch, Paul Cohen, James G. Granneman, Patrick Seale, Riekelt H. Houtkooper, Jacob B. Hansen, Steven P. Gygi, Thue W. Schwartz, Matthew P. Gillum, Terry D. Hinds, Jr., Raymond E. Soccio, Phillip J. White, Edmund R. S. Kunji, Thomas Moritz, Jonas T. Treebak, Endre Kristóf, Susanne Mandrup, Brice Emanuelli, Nathaniel W. Snyder, Daniel J. Fazakerley, Lawrence Kazak, Nils J. Færgeman, Mitchell A. Lazar, and Zachary Gerhart-Hines.
• DOI: 10.1126/science.adz4797

Abstract

Mitochondrial carrier SLC25A34 links clock, diet, and temperature control of adipocyte lipid cycling

INTRODUCTION

Brown and beige thermogenic adipocytes (fat cells) have uniquely high energy expenditure capacity. This activity is critical for defending body temperature in small mammals and is linked to cardiometabolic health in humans. Energy expenditure from thermogenic fat cycles daily on a rhythm set by the circadian clock, rising after waking and dropping during sleep.

At any point, however, this activity can be suddenly induced by exposure to cold temperature or by ingestion of a calorie-dense, high-fat meal. How thermogenic fat can seamlessly achieve both rhythmic continuity and acute responsiveness is a fundamental question of energy homeostasis that has remained unknown.

RATIONALE

We set out to understand how diverse cues from the body’s clock, environmental temperature, and diet are molecularly integrated in adipocytes to facilitate both anticipatory and adaptive regulation. By combining unbiased analysis of DNA-binding patterns, gene expression, and protein levels in thermogenic fat from mice that were subject to different circadian, environmental, and dietary perturbations, we identified the orphan mitochondrial metabolite transporter SLC25A34 as the convergence point of all three physiological paradigms.

RESULTS

Similar to fat’s thermogenic activity, the expression of Slc25a34 mRNA exhibited a daily rhythm, which was mediated by the nuclear receptors REV-ERBα and REV-ERBβ. REV-ERBs are transcriptional repressors in the circadian clock that recruit histone deacetylase 3 to the Slc25a34 promoter during sleep to shut off transcription. When animals wake and REV-ERB circadian repression is lifted, the peroxisome proliferator–activated receptors α and γ (PPARα and PPARγ) are able to bind the Slc25a34 promoter and restore expression.

However, at any time, when animals are suddenly confronted with an unanticipated need for adipocyte energy expenditure such as exposure to cold temperatures or eating a lipid-rich meal, REV-ERB repression is rapidly overridden and PPARs are activated by lipolytic signals to boost SLC25A34 levels. When the energy demand is met, the REV-ERB–mediated rhythmicity of Slc25a34 is reestablished.

Our biochemical and metabolic data support a functional model in which SLC25A34 transports oxaloacetate into mitochondria. In doing so, SLC25A34 helps to maintain high tricarboxylic acid (TCA) cycle activity, which is required for powering energy expenditure in thermogenic adipocytes. The need for SLC25A34 during times of high energetic demand arises because thermogenic adipocytes not only oxidize fatty acids in the mitochondria but also simultaneously synthesize lipids in the cytosol, a process collectively known as lipid cycling.

Lipid synthesis requires continuous production of acetyl–coenzyme A (acetyl-CoA) in the cytosol, and the two major pathways responsible for this acetyl-CoA pool also generate oxaloacetate as a by-product, which then needs to be transported back into the mitochondria. Increased cytosolic acetyl-CoA additionally promotes the transcription of genes linked to mitochondrial respiration. Thus, SLC25A34 supports both sides of this self-reinforcing cycle in which lipids are continually broken down and resynthesized.

CONCLUSION

These findings identify SLC25A34 as a molecular node that integrates circadian, temperature, and dietary cues and reveal how thermogenic fat activity can sustain a daily rhythm while remaining acutely responsive to sudden energy demands. This mechanism may offer a new entry point for therapeutically enhancing energy expenditure in metabolic disease.