The Fat-Burning Bottleneck: What New Research Reveals About Why Some People Struggle to Burn Fat

TL;DR

Scientists have identified a specific protein — SLC25A45 — that acts as a gatekeeper for fat burning inside your mitochondria. Without it, your body can’t make enough carnitine to shuttle fat into the cellular furnace, and defaults to burning sugar instead. This helps explain why some people plateau on keto and why GLP-1 drugs work differently for different people.

Ever feel like your body just won’t switch to burning fat, no matter what you do? You cut carbs, you fast, you do everything right — and yet the scale barely moves.

A new study published in Science in February 2026 may have found a key reason why: a molecular bottleneck deep inside your cells that controls whether your body can actually burn fat for fuel.

The Carnitine Connection

To understand the discovery, you need to know one thing about fat burning: your body can’t just dump fat into the mitochondria (your cellular power plants) and set it on fire. Fat molecules — specifically long-chain fatty acids — need a shuttle to get through the inner mitochondrial membrane.

That shuttle is carnitine.

🔬 What Is Carnitine?

L-carnitine is an amino acid derivative found primarily in meat and dairy. It transports long-chain fatty acids into mitochondria for β-oxidation (fat burning). Your body can make it, but it also gets it from food — which is one reason animal products support fat metabolism.

Without enough carnitine, fatty acids pile up outside the mitochondria with nowhere to go. Your cells default to burning glucose instead. Even if you’re eating low-carb.

This has been known for decades. What’s new is the discovery of what controls carnitine production in the first place.

The Discovery: SLC25A45

A team led by Shingo Kajimura identified a mitochondrial transport protein called SLC25A45 that acts as the gatekeeper for carnitine biosynthesis.

Here’s how it works:

Carnitine is made from trimethyllysine (TML), a modified amino acid
TML needs to enter the mitochondria to be processed into carnitine
SLC25A45 is the transporter that carries TML into the mitochondria
Without it, TML can’t get in, carnitine doesn’t get made, and fat burning stalls

When researchers knocked out SLC25A45 in mice, the results were striking:

Carnitine levels dropped significantly
Fatty acid oxidation was impaired — cells couldn’t burn fat properly
Cells shifted to burning carbohydrates for energy instead
The mice became more susceptible to metabolic dysfunction

In other words: remove this one protein, and the entire fat-burning machinery grinds to a halt.

Why This Matters for Keto and Carnivore Dieters

This research has direct implications for anyone eating low-carb:

1. It Explains “Keto Plateaus”

If your SLC25A45 activity is low — due to genetics, nutrient status, or other factors — your body may struggle to ramp up carnitine production even when you cut carbs. You’re giving it fat to burn, but the shuttle system can’t keep up.

2. Carnitine Comes From Meat

Your body makes carnitine, but it also gets it from dietary sources. The richest sources? Red meat, organ meats, and dairy — exactly the foods emphasised in carnivore and keto diets.

This isn’t a coincidence. When you eat a meat-heavy diet, you’re supplying both the raw materials (TML from protein) and the finished product (carnitine from meat). You’re feeding the system from both ends.

3. It May Explain Why GLP-1 Drugs Work Differently for Different People

The researchers specifically note implications for GLP-1 receptor agonist drugs (like semaglutide). These drugs promote weight loss partly by shifting metabolism toward fat burning — but they rely on the carnitine shuttle being functional.

If someone’s SLC25A45 system is underperforming, GLP-1 drugs may be less effective. This could explain the wide variation in outcomes that doctors see with these medications.

The Bigger Picture: Mitochondria as Metabolic Gatekeepers

What makes this study remarkable is the finding that mitochondria themselves regulate fuel switching. It’s not just about what you eat or how much insulin you produce — there’s a checkpoint inside the cell that determines whether fat gets burned or not.

The authors describe mitochondria as a “regulatory checkpoint in fuel switching.” Your cells aren’t passive furnaces that burn whatever fuel is available. They actively control which fuel gets used, and the carnitine system is a critical part of that control.

🧬 Fuel Switching

Your body can run on two main fuels: glucose (from carbohydrates) and fatty acids (from fat). The ability to switch between them efficiently is called “metabolic flexibility.” Poor metabolic flexibility — being stuck in sugar-burning mode — is a hallmark of insulin resistance and metabolic syndrome.

What You Can Do

While you can’t (yet) directly boost your SLC25A45 levels, you can support the carnitine system:

Eat red meat and organ meats — the richest dietary sources of carnitine. Beef, lamb, and particularly heart and liver are excellent choices
Don’t fear animal fat — the combination of fat and carnitine in whole animal foods is metabolically synergistic
Stay hydrated and maintain electrolytes — metabolic processes depend on proper mineral balance, especially during keto adaptation
Be patient during adaptation — if you’re new to keto, your carnitine system may need time to upregulate. The “keto flu” period may partly reflect this bottleneck
Consider that plateaus are biological, not failures — this research suggests that fat-burning capacity has genuine biochemical limits that vary between individuals

The Bottom Line

For years, the keto and carnivore communities have emphasised that fat burning isn’t just about calories — it’s about metabolic machinery. This study from Science provides hard molecular evidence for that intuition.

Your body’s ability to burn fat depends on a specific transport protein that controls carnitine production inside your mitochondria. Without adequate carnitine, fat can’t enter the cellular furnace, and your body defaults to burning sugar.

The practical takeaway? Eat the meat. The very foods that keto and carnivore diets emphasise — red meat, organ meats, animal fats — are exactly the ones that supply and support the carnitine system your body needs to burn fat.

Science is catching up to what many people eating this way already feel: when you fuel your body with the right foods, fat burning just works.