Iñigo San Millán, Ph.D.: Zone 2 Training and Metabolic Health (Ep. #85 Rebroadcast)

Episode Summary

Executive Summary: Peter Atiyah and Inigo Sanmilan revisit zone 2 training, explaining how exercise intensity maps to fuel use, lactate production, and mitochondrial function. They argue zone 2 is a practical proxy for metabolic health, contrasting elite endurance athletes, recreational exercisers, and people with diabetes, and explore how metformin, diabetes, and even cancer may alter these pathways.

Main Topics: Zone 2 as a marker of mitochondrial health: Zone 2 is presented as the highest sustainable intensity where fat oxidation is maximized and lactate remains near steady state, making it a useful functional test of mitochondrial capacity. Energy systems, muscle fibers, and lactate: The discussion explains aerobic vs anaerobic metabolism, slow- vs fast-twitch fiber recruitment, and why lactate is a fuel and signaling molecule rather than mere waste. Elite athletes vs diabetes in metabolic function: Sanmilan contrasts world-class cyclists with recreational athletes and people with type 2 diabetes, showing large differences in power output, lactate handling, fat oxidation, and mitochondrial efficiency. Lactate, RQ/RER, and indirect testing of metabolism: Resting respiratory quotient and exercise lactate curves are used as noninvasive proxies for mitochondrial dysfunction and metabolic flexibility, especially in insulin resistance. Exercise, glucose handling, and diabetes management: The conversation highlights how zone 2 and cool-downs improve glucose uptake, reduce insulin needs, and may help both type 1 and type 2 diabetes through non-insulin-dependent glucose transport. Metformin, lactate, and possible tradeoffs: Atiyah and Sanmilan discuss whether metformin raises lactate and blunts exercise adaptation by affecting mitochondrial function, with the idea that its effects may differ by metabolic state. Cancer metabolism and the Warburg effect: They extend the lactate discussion to cancer, where lactate may act as a signaling molecule that promotes oncogenic transcription and the acidic tumor microenvironment.

Key Arguments: Zone 2 is the intensity that best stimulates mitochondrial adaptation because it maximizes fat oxidation while keeping lactate near steady state. Lactate is not a waste product; it is a major fuel used by the brain, heart, kidney, and slow-twitch muscle fibers, and it also functions as a signaling molecule. Elite endurance athletes are not just 'more fit'; they have dramatically better mitochondrial density, lactate clearance, and metabolic flexibility than recreational athletes or people with diabetes. Resting RQ/RER near 0.9-1.0 in a fasted person is a red flag for poor fat oxidation and likely mitochondrial dysfunction. Type 2 diabetes and metabolic syndrome can be understood partly as failures of skeletal muscle mitochondrial function and fuel partitioning. Exercise increases non-insulin-dependent glucose uptake via GLUT4 translocation, which is why zone 2 and cool-downs can lower glucose even without insulin. Metformin may increase lactate and alter mitochondrial function; its benefit-risk balance may depend on the person’s baseline metabolic health and exercise state. Cancer may exploit lactate both as fuel and as a signaling molecule that upregulates oncogenic pathways, helping explain the Warburg effect.

Data Points: Resting lactate: 0.7-1.0 mmol/L - Typical resting blood lactate in a healthy individual Zone 2 lactate: 1.5-2.0 mmol/L - Approximate lactate range associated with zone 2 / fat max Recreational athlete lactate at same power: ~12 mmol/L - Example of a non-elite athlete at 300 W producing much more lactate than an elite cyclist World-class cyclist lactate at same power: ~2.5-3 mmol/L - Example of elite athlete at 300 W with much lower lactate Elite cyclist zone 2 power: ~300 W - Approximate power at which world-class cyclists reach zone 2 threshold Recreational athlete zone 2 power: ~200 W - Approximate power at which a fit recreational athlete reaches zone 2 threshold Type 2 diabetes zone 2 power: ~120 W - Approximate power at which people with type 2 diabetes reach zone 2 threshold Professional cyclist FTP/peak example: 6-6.5 W/kg - Typical Tour de France climbing performance cited for top riders Historical peak cycling performance: ~7 W/kg - Performance levels associated with the doping era discussion Tour de France carbohydrate intake: ~12 g/kg/day - Reported daily carbohydrate intake for riders during stage racing Tour de France sugar intake: ~1500 kcal/day from sugar - Approximate amount of simple carbohydrate consumed during racing Type 1 diabetes athlete insulin use: ~8-10 units/day - Example of a highly active type 1 diabetic athlete with very low insulin needs Type 1 diabetes glycogen content: ~25% higher than non-diabetics - Observed in Team Novo Nordisk cyclists in the cited study Zone 2 training frequency: 2 days/week maintains; 3-5 days/week improves - Sanmilan’s practical guidance on dose/frequency Weight loss example: ~47-50 lb - Sanmilan’s own weight loss after resuming zone 2 training Metformin dose change: 14 g/week to 3 g/week - Atiyah reduced metformin and observed lower lactate during zone 2 Cancer cell lactate exposure: 10-20 mmol/L - In vitro lactate concentrations used in breast cancer cell experiments Cancer media lactate: ~25-30 mmol/L - Lactate accumulation observed in glucose-only cancer cell media Population burden: ~50% of US adults - Referenced as having prediabetes or diabetes

Pivotal Quotes: "Lactate is the most important, if not the most important fuel for the body." — Inigo Sanmilan: He explains why lactate should be viewed as a fuel and signaling molecule, not a waste product. "Zone two is the exercise intensity where I see the biggest improvement in fat burning and the biggest improvement in lactic clearance capacity." — Inigo Sanmilan: He describes why zone 2 is central to mitochondrial adaptation and metabolic health. "The ability of humans to perform relies on the ability to convert chemical energy into mechanical energy." — Inigo Sanmilan: He contrasts mechanical output with the underlying metabolic processes that determine performance.

Implications: Zone 2 training is a practical tool for improving metabolic health, insulin sensitivity, and mitochondrial function. The episode suggests lactate and RQ can help assess disease risk, while also raising questions about metformin, diabetes care, and cancer metabolism.