Zone 2 training: impact on longevity and mitochondrial function, how to dose frequency and duration, and more | Iñigo San-Millán, Ph.D. (#201 rebroadcast)

Episode Summary

Executive Summary: Peter Attia and Inigo San Millan revisit zone 2 training through the lens of elite cycling, especially Tadej Pogacar, to explain how lactate, fat oxidation, mitochondrial function, and training structure interact. They argue that zone 2 is best understood as the intensity that maximizes mitochondrial oxidative capacity and fat use, that lactate thresholds vary by fitness and disease state, and that exercise prescription should rely more on metabolic testing and perceived exertion than generic heart-rate formulas. They also discuss long COVID, metformin, NAD supplements, and cancer metabolism as examples of why mitochondrial health matters.

Main Topics: Pogacar as a physiologic outlier: San Millan explains how early testing, low lactate at high power, rapid recovery, and metabolomics data revealed Pogacar’s exceptional endurance physiology and trainability. Defining zone 2: Zone 2 is framed as the exercise intensity that most strongly stresses mitochondrial oxidative capacity, maximizes fat oxidation, and supports lactate clearance with minimal glycolytic spillover. How to measure zone 2: The discussion compares indirect calorimetry, lactate testing, heart rate, power, and perceived exertion as ways to identify the correct training zone, with emphasis on practical surrogates when lab testing is unavailable. Lactate, fat oxidation, and mitochondrial function: They detail how lactate is produced, cleared, and reused as fuel, why blood lactate is not identical across populations, and how elite athletes differ from sedentary or metabolically unhealthy individuals. Training prescription and adaptation: San Millan outlines frequency, duration, and intensity principles for zone 2, arguing that frequent moderate sessions build mitochondrial capacity best, while high-intensity work remains necessary but secondary. Clinical and disease applications: The conversation extends zone 2 physiology to metabolic syndrome, type 2 diabetes, metformin use, long COVID, ICU catabolism, and cancer, highlighting mitochondrial dysfunction as a common thread. Longevity and exercise as medicine: They conclude that exercise is the most powerful longevity intervention, with compounding benefits over time, and that mitochondrial health is central to healthspan and performance.

Key Arguments: Pogacar’s low lactate at high wattage, rapid recovery, and strong metabolomic profile indicate exceptional mitochondrial efficiency and trainability. Zone 2 is best defined physiologically, not just by heart rate: it is the intensity where fat oxidation and mitochondrial oxidative metabolism are maximized. Blood lactate is a context-dependent marker; the same lactate value can mean very different metabolic stress in elite athletes versus sedentary or metabolically impaired people. Indirect calorimetry is a strong way to estimate fat oxidation, but diet, especially ketosis or low carbohydrate intake, can distort gas-exchange-based estimates. For most people, conversational pace, heart rate, and perceived exertion are practical surrogates for zone 2 when lactate testing is unavailable. Zone 2 adaptations are dose-dependent: frequent sessions, usually 3-5 days per week, are more effective than infrequent long sessions. High-intensity training is still important for maintaining glycolytic capacity and VO2 max, but it should complement, not replace, zone 2 work. Metformin may elevate lactate and complicate interpretation of metabolic testing; its long-term effects on mitochondrial function and longevity remain uncertain. NAD-boosting supplements may not improve longevity and could theoretically support tumor metabolism; more research is needed. Long COVID may involve a mitochondrial phenotype resembling metabolic syndrome or type 2 diabetes, suggesting exercise and metabolic testing could help characterize and potentially treat it.

Data Points: Pogacar age when first tested: 19 - San Millan began working with Pogacar when he was a 19-year-old first-year pro. Tour de France stage attack distance: 35 kilometers to the finish - Used as an example of Pogacar’s confidence and race strategy in the Alps. Tour duration: about 100 hours - Attia notes the Tour is roughly a 100-hour race over about 4.5-5 hours per day. Elite short-climb power in modern era: about 6.3 W/kg - San Millan says current riders are far below the 7+ W/kg of the EPO era on short climbs. Elite longer-climb power in modern era: about 5.5-5.8 W/kg - San Millan contrasts current climbing outputs with the doping era. Metabolic syndrome resting lactate: about 1.5-3.0 mmol/L - Discussed as a common elevated resting lactate range in metabolically unhealthy patients. Healthy moderately active resting lactate: about 1.0 mmol/L - Shown in the comparative lactate graph for moderately active individuals. Elite athlete resting lactate: about 0.5 mmol/L - Shown in the comparative lactate graph for professional endurance athletes. Metabolic syndrome zone 2 threshold: about 1.0-1.3 W/kg - Estimated from the graph as the approximate zone 2 range for a person with metabolic syndrome. Moderately trained zone 2 threshold: about 2.0-2.2 W/kg - Estimated from the graph as the approximate zone 2 range for a moderately trained individual. Professional endurance athlete zone 2 threshold: about 4.0 W/kg - Estimated from the graph as the approximate zone 2 range for a pro endurance athlete. Patient aspiration target: 3.0 W/kg - Attia says his clinic sets 3 W/kg as an elite aspiration for patients. Fat oxidation in elite athlete: about 1.5 g/min - Attia describes a highly trained ketogenic cyclist with very high fat oxidation. Fat oxidation in Attia’s own test: 1.3 g/min - He reports his own maximum fat oxidation during a ketogenic period. Carbohydrate oxidation in elite athletes: up to 6.5 g/min - San Millan cites elite athletes’ maximal carbohydrate oxidation. Carbohydrate oxidation in recreational/metabolic syndrome individuals: about 4 g/min - Used to contrast elite versus less trained metabolic capacity. Heart rate surrogate for zone 2: 70-80% of max HR - San Millan and Attia agree this is a useful starting range, though broad. Conversation test for zone 2: able to talk but with strain - Used as a practical non-lab surrogate for zone 2 intensity. Recommended zone 2 frequency: 3-5 days per week - San Millan says frequent sessions are ideal for adaptation. Recommended zone 2 duration: 1-1.5 hours per session - Typical target duration for meaningful mitochondrial adaptation. High-intensity protocol example: 4 x 4 minutes - Attia describes a common VO2 max interval session with 4-minute work and recovery blocks. VO2 max elite benchmark for age: 52-53 ml/kg/min - Attia says this would place him in the elite range for his age group. Attia’s current VO2 max: about 51-52 ml/kg/min - He reports current VO2 max after years of training. Attia’s former VO2 max: 76.7 ml/kg/min - He cites his VO2 max from his competitive cycling years. Long COVID sample size: 50 patients - San Millan describes the cohort studied with National Jewish Hospital. Long COVID cohort composition: 35 females, 15 males - The long COVID study sample was female-predominant. NAD mouse pilot study: 4 mice per group - Pilot study comparing nicotinamide riboside versus water in tumor-bearing mice. Tumor growth increase with NAD precursor: about 15% - Observed in the mouse pilot study over 23 days.

Pivotal Quotes: "Zone two is the exercise intensity at the one you are stressing the mitochondria and oxidative capacity to the most." — Inigo San Millan: Core definition of zone 2 early in the physiology discussion. "The majority of the sessions are in the lower intensity. They're not intervals, intervals." — Inigo San Millan: Explaining how elite athletes actually train, despite common assumptions. "If you can exercise whatever the exercise you do and maintain a conversation like you and I are doing, you're way too easy." — Inigo San Millan: Practical conversational test for identifying zone 2 intensity.

Implications: Listeners should think of zone 2 as a metabolic target, not a generic heart-rate zone. For athletes and patients alike, frequent sustainable aerobic work plus some high-intensity training is the best path to mitochondrial health, performance, and longevity.