Metabolic flexibility is one of those ideas that sits quietly underneath almost everything we talk about when it comes to health, yet very few of us have ever had it explained clearly. In this masterclass I have brought together some of the sharpest minds in metabolism, exercise physiology, and diabetes research to unpack what metabolic flexibility actually is, why your mitochondria sit at the centre of it, and what it means for your long-term health. You will hear from Dr Iñigo San Millán, Dr Alan Flanagan, Dr Kevin Hall, Dr Robert Eckel, Professor Roy Taylor, and Dr Christos Mantzoros.
The reason this matters is that metabolic dysfunction rarely announces itself. Most of us picture metabolic health as a line you cross the day a doctor diagnoses type 2 diabetes, but the science tells a different story. The decay begins years, sometimes decades, earlier, at the level of the cell. This conversation is about learning to see that earlier, and understanding that the tools to change course, how we move, what we eat, and how we think about fat in the body, are largely in our hands.
What We Cover
- Why Dr San Millán calls lactate the best non-invasive window we have into your mitochondrial function, and why it is a fuel source rather than a waste product
- The story of an 81-year-old whose metabolism tested like someone in their thirties, and what his history reveals about reversing decades of damage
- What “metabolic flexibility” really means, and why medicine spent so long overlooking cellular metabolism
- Whether a calorie really is a calorie, and what Dr Kevin Hall’s tightly controlled chamber studies actually found
- Why Professor Roy Taylor argues type 2 diabetes is simpler and more reversible than we were taught, and the twin cycle behind it
- The personal fat threshold, and why a normal BMI does not always mean you have no weight to lose
- Whether sugar or saturated fat does more to drive fat in the liver
- Why Zone 2 training earns its reputation, and how much of it you actually need
My hope is that this masterclass gives you a clearer mental model of what is happening beneath the surface, and a few practical levers you can start pulling today. Metabolic health is not fixed, and it is rarely too late to move in a better direction.
To learn more from the experts featured in this masterclass, you can follow Dr Iñigo San Millán on Instagram and Twitter, explore Professor Roy Taylor’s work on type 2 diabetes remission at Newcastle University, read more from Dr Kevin Hall at kevinhallphd.com or on LinkedIn, and find Dr Alan Flanagan on Instagram and through Alinea Nutrition. The masterclass also features insights from Dr Robert Eckel and Dr Christos Mantzoros.
- The 81-Year-Old Who Reversed 30 Years of Damage (00:00)
- What Metabolic Health Actually Means Now (01:57)
- Why Mitochondria Decide Your Disease Risk (06:01)
- What Is Metabolic Flexibility? (12:47)
- Hepatic Fat Oxidation and the Sex Difference (15:20)
- Is a Calorie Really a Calorie? (18:29)
- You Can Be Sick Before You’re Diagnosed (28:41)
- Why Lactate Is the Biomarker to Watch (29:20)
- Lactate Is Not a Waste Product (31:03)
- Is Lactic Acid Burn a Myth? (36:42)
- Why Athletes Clear Lactate and You Don’t (40:34)
- When High Lactate Predicts Death (43:01)
- The Study That Found Damage in Healthy People (47:13)
- The Lactate Test That Beats Blood Sugar (54:31)
- Training Zones 1 to 6 Explained by Fuel (58:50)
- Why Zone 2 Builds Mitochondria Best (1:09:18)
- How to Know You’re in Zone 2 Without a Lab (1:16:50)
- Is 150 Minutes a Week Enough? (1:23:39)
- Why You Still Need High Intensity Training (1:28:10)
- What Obesity Research Got Wrong (1:31:44)
- The Twin Cycle Hypothesis of Type 2 Diabetes (1:35:04)
- Your Personal Fat Threshold Explained (1:42:26)
- Why Some People Store Fat Safely (1:45:45)
- Why BMI Fails as a Health Measure (1:51:29)
- Sugar or Saturated Fat: What Drives Liver Fat? (1:55:07)
- 15kg Weight Loss and Diabetes Remission (2:01:05)
- Does Weight Loss Really Destroy Muscle? (2:04:39)
- Why Exercise Alone Doesn’t Drop Weight (2:07:05)
- 3 Tests That Reveal Your Metabolic Health (2:11:14)
- What Blue Zones Get Right About Movement (2:17:40)
Resources and links:
- Zone 2 Training: The Science Behind Peak Metabolic Health | Dr Iñigo San Millán | The Proof Podcast
- NAFLD Explained: Risks, Symptoms & Solutions | Dr Alan Flanagan, PhD | The Proof Podcast EP#281
- Ultra-Processed Food, Energy Balance & the Carbohydrate-Insulin Model | Dr Kevin Hall
- Lactate & Mitochondrial Function | Dr Iñigo San Millán | The Proof Podcast EP#385
- #1 Obesity Expert: Your Brain Has a ‘Fat Stat’ That Sabotages Weight Loss | Dr Robert Eckel | EP#403
- Can You Reverse Type 2 Diabetes? | Professor Roy Taylor | The Proof Podcast EP#287
- Fatty Liver Disease, Ectopic Fat & Leptin | Professor Christos Mantzoros | The Proof Podcast EP#322
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More about Dr Iñigo San Millán
Dr Iñigo San Millán is a globally recognised expert in physiology, metabolism, and performance science, with nearly three decades of experience optimising the health and performance of elite athletes, including Tour de France winners and Olympic medallists. As Director of Performance at Athletic Club de Bilbao and a longtime architect of Team UAE Emirates’ dominance in world cycling, including the development of the performance program behind Tadej Pogačar, he has played a key role in shaping modern endurance sports science through a range of groundbreaking scientific and methodological innovations.
A true pioneer, Dr San Millán was the first to measure substrate utilisation (fat and carbohydrate oxidation) in competitive athletes over 20 years ago, leading to groundbreaking changes in sports nutrition. He introduced the concept of consuming 80–100 g/h of carbohydrates in endurance events long before it became standard practice and later advanced it to 125 g/h with Pogačar in his first pro season. He also coined the now-ubiquitous term “Zone 2” training nearly 30 years ago, defining training zones based on metabolic parameters like lactate and substrate utilisation, now a cornerstone of endurance training worldwide.
Dr San Millán developed the first metabolomics platform used in professional sports and created a novel methodology to indirectly measure mitochondrial function and metabolic flexibility, a tool now applied not only in elite performance but also in clinical settings. His innovations span beyond athletics and into metabolic health, where he is a global authority. His work bridges the gap between high-performance sport and medicine, helping patients with type 2 diabetes, cancer, post-ICU complications, and those focused on longevity optimise their metabolic function.
As the co-author of the Lactagenesis Hypothesis, Dr San Millán has helped reframe the understanding of the Warburg Effect, proposing lactate as a master regulator in cancer biology. His research demonstrates lactate’s role in gene expression, opening new avenues for diagnostics and therapeutics in oncology.
An adjunct professor and published scientist, Dr San Millán continues to advise top global organisations and drive innovation at the intersection of performance, health, and medicine.
More about Professor Roy Taylor
Roy Taylor qualified in medicine at the University of Edinburgh, and is Professor of Medicine at Newcastle University. He founded the Newcastle Magnetic Resonance Centre in 2006 to develop innovative research techniques. Using these new techniques, in 2011 he showed that type 2 diabetes was a simple, reversible condition of excess fat within liver and pancreas. Subsequently he has clarified what causes type 2 diabetes and how it works. This has led to practical application in the NHS with the national ‘NHS Type 2 Diabetes Path to Remission’ programme now well underway.
Between 1986 and 2000, Professor Taylor developed the system now used throughout the United Kingdom for screening for diabetic eye disease, with major reduction in blindness due to diabetes across the UK.
He has published books in lay language explaining type 2 diabetes (eg ‘Life Without Diabetes’) plus training books on retinal screening. He has been invited to deliver named lectures including the 2012 Banting Lecture, the 2015 Harry Keen Lecture (Diabetes UK), the 2016 Samuel Gee Lecture (Royal College of Physicians of London) and Sir Robert W Philip Lecture of The Royal College of Physicians of Edinburgh 2021). He received the 2023 Robert Turner Award for Research Impact. Professor Taylor was made MBE in the King’s 2023 New Year Honours.
More about Dr Kevin Hall
Kevin Hall, PhD is an internationally recognised expert in human nutrition, metabolism, obesity, and neuroscience. For more than two decades, Dr Hall was the Integrative Physiology Section Chief at the National Institutes of Health where he made groundbreaking discoveries about how our environment shapes dietary behaviours, and how nutrition and physical activity affect human metabolism and body weight regulation. His transdisciplinary research has combined tightly controlled metabolic ward trials, advanced neuroimaging, and mathematical modelling to generate mechanistic insights into the drivers of obesity and diet-related chronic diseases. Dr Hall has published more than 150 scientific articles and is a sought-after public speaker, having delivered more than 250 presentations around the globe. Dr Hall has twice received the Director’s Award from the US National Institutes of Health and has also received the E.V. McCullum Award from the American Society for Nutrition, the Lilly Scientific Achievement Award from The Obesity Society, and the Guyton Award for Excellence in Integrative Physiology from the American Physiological Society. His work is regularly featured in major media outlets including The New York Times, The Washington Post, The Economist, CNN, BBC, and network television. Dr Hall has also been featured in several podcasts and documentaries about food, nutrition, metabolism, and obesity. He co-authored a book with health journalist Julia Belluz, Food Intelligence: The science of how food both nourishes and harms us.
More about Dr Robert Eckel
Robert H. Eckel, MD, is the former Charles A. Boettcher Endowed Chair in Atherosclerosis in the Department of Medicine at University of Colorado Anschutz Medical Campus (UCAMC) in Aurora, CO. Dr Eckel is a distinguished alumnus of the University of Cincinnati College of Medicine in Cincinnati, OH. He performed a Medical Residency at the University of Wisconsin Hospitals in Madison, WI, and a Senior Fellowship in Metabolism and Endocrinology at the University of Washington School of Medicine in Seattle, WA. For nearly 30 years Dr Eckel served as the associate and then program director of the Adult General Clinical Research Center, and subsequently as the interim vice chancellor for research. Dr Eckel is past president of the American Heart Association; American Diabetes Association (Medicine and Science); the Obesity Society, and the Association of Patient Oriented Research. He was a former member of the Extramural Advisory Council of NIDDK, been a contributor to various professional guidelines, and has received numerous awards. Dr Eckel has published over 400 articles/editorials in peer-reviewed journals and his translational research has used populations, human subjects, genetically modified mice, and molecular techniques to pursue physiologic and pathophysiological insights into cardiometabolic disorders such as obesity, metabolic syndrome, diabetes, cardiovascular disease, and more recently neurodegenerative diseases.
More about Dr Christos Mantzoros
Christos S. Mantzoros, MD, DSc, PhD h.c. mult., is a Professor of Medicine at Harvard Medical School and an Adjunct Professor at Chobanian and Avedisian School of Medicine. He has also served as a Professor of Environmental Health at the Harvard School of Public Health. He currently serves as the Chief of Endocrinology, Diabetes and Metabolism at the Boston VA Healthcare System and the Director of the Human Nutrition Unit of the Division of Endocrinology Diabetes and Metabolism at Beth Israel Deaconess Medical Center. Dr Mantzoros has been the editor of two books (one on “Diabetes and Obesity” and one on “Nutrition and Metabolism” with more than two editions each). His research has resulted in more than 800 publications in Medline, including more than 150 publications under the collaborative Look Ahead Research Group, more than 200 chapters and reviews, with more than 72,000 citations with a Hirsch Index H=121.
Dr Mantzoros is also the director for the VA Boston Healthcare System fellowship program with Boston University Medical Center (BUMC), which offers one dedicated fellowship position through BUMC, but also allows any fellow at BUMC or Harvard affiliated fellowship programs the opportunity to rotate through the VA Boston Healthcare System. He has served as a Board Member or an advisor or head of the Scientific Advisor Board of non for profit foundations as well as governmental and non-governmental entities. He is the Editor-in-Chief of the journal Metabolism and is on the editorial board of several scientific journals worldwide. For his research discoveries and public health service Dr Mantzoros has received four honorary PhDs, the Alexander Technological Institute of Thessaloniki has named their nutrition laboratories after his name and several Universities worldwide have awarded Dr Mantzoros honorary Professorships and visiting Professorships.
He has mentored more than 160 young scientists who currently hold key positions in clinical medicine, academia, industry as well as governmental agencies and nongovernmental organizations. Dr Mantzoros has been elected a member of ASCI and a Fellow of the American College of Physicians and the American Association of Clinical Endocrinology. He has received numerous awards, including from the American Association of Clinical Endocrinology, the American Diabetes Association, the Obesity Society, the American Society for Nutrition, the Humboldt Foundation of Germany, the American Federation of Medical Research, and the American Physiological Society, among others.
More about Dr Alan Flanagan
Dr Alan Flanagan, PhD, holds an MSc and PhD in nutrition from the University of Surrey, and is a former practising Lawyer (Barrister) from Dublin, Ireland. Alan is the founder of Alinea Nutrition, an online education hub dedicated to providing impartial, science-based nutrition analysis. Alan is also Research Communication Officer for Sigma Nutrition, creating educational resources for nutrition science. Originally a lawyer by background in Dublin, Ireland, Alan spent a decade practising as a barrister before moving into science.
