T2, Fat Burning & Brown Fat: How It Works in the Mitochondria and What the Studies Show

Does increasing fat burning automatically mean losing more body fat? And what would we need to measure to know whether an intervention preserves muscle?

In this Friday solocast, I'm revisiting my conversation with Dr. Amie Hornaman to unpack the research on T2, a lesser-known thyroid hormone derivative, and connect the mechanisms to the body composition questions we care about.

We go inside the mitochondria to explore how fatty acids enter, how our cells regulate fuel use, and how mitochondrial uncoupling releases energy as heat. I also revisit the brown fat research and potential connections with ketogenic metabolism.

Along the way, I clarify several findings from the interview—including what "50% less fat mass" meant in the rat study, why lower body weight does not establish fat-only loss, and why changes in muscle-cell metabolism do not prove muscle preservation.

The T2 research is intriguing, but human evidence remains very limited. This recap explores what the findings suggest, what they cannot establish, and the practical steps we can take now to support muscle and metabolic health.

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In this episode, I discuss:

What T2 is and how it relates to T3 and T4. What the 2005 rat study actually found about fat accumulation. The carnitine shuttle and how fatty acids reach the mitochondria. How AMPK, ACC, and malonyl-CoA help regulate fat oxidation. Mitochondrial uncoupling, ATP production, and heat generation. Brown fat thermogenesis and why an animal's thyroid status matters. Possible connections with keto—and why shared pathways do not establish added benefits. The difference between burning dietary fat and losing stored body fat. The two-participant human T2 report and Amie's uncontrolled 14-woman project. What stronger human research would need to measure. Thyroid safety questions and the importance of appropriate prescribed treatment. Lean mass versus skeletal muscle during weight loss. Practical ways to prioritize protein, resistance training, and consistency. Questions to bring to your clinician if you have persistent symptoms. Research discussed

Lanni and colleagues, 2005: T2, fat accumulation, and liver fatty acid oxidation in high-fat-fed rats.

Read the study Lombardi and colleagues, 2009: T2, skeletal muscle fa