Rebuilding Metabolism After Menopause: Dr. Iñigo San Millán on Lactate & Zone 2
Dr. Vonda Wright sits down with cellular-metabolism researcher Dr. Iñigo San Millán to unpack lactate as both the fuel and the language of your mitochondria. They explain why zone 2 training paired with short sprint intervals rebuilds metabolic flexibility, and why it is never too late — even after menopause — to reverse metabolic decline.
Overview
In this episode, orthopedic surgeon Dr. Vonda Wright interviews Dr. Iñigo San Millán, a leading researcher in cellular metabolism and coach to world-class athletes, about how midlife bodies produce and use energy. He explains that lactate is not a waste product but the preferred fuel of most cells and the clearest window into mitochondrial health, measured through a simple finger-prick test at rising exercise intensities.
A robust metabolism keeps lactate near 2 mmol in a state he calls metabolic equilibrium, while dysfunctional mitochondria let it climb even at rest. San Millán describes zone 2 training — sustainable, mostly low-intensity work — as the best stimulus for building mitochondria, complemented by roughly 20% high-intensity sprint intervals. Heart rate serves as a practical surrogate for lactate, and as fitness improves the same zone 2 demands more power, which is why periodic retesting matters.
The conversation turns to the brain, where lactate fuels neurons through the astrocyte-neuron shuttle and may bear on Alzheimer's, estrogen loss and cognitive decline in menopause. San Millán frames skeletal muscle as the largest and most active organ, the first to show type 2 diabetes and one that essentially never develops cancer, making muscle health central to longevity. He closes with an inside-out hypothesis of metabolic decline and a hopeful message: with the right guidance, pre-diabetes and metabolic dysfunction are often reversible in three to six months.
Key quotes
4If we didn't produce lactate, the muscles would not be able to function.
Sometimes people are so metabolically dysfunctional that they are above zone 2 sitting in a chair.
Why the largest organ in the body does not see cancer is because it's extremely well protected.
It's never, never, ever too late to exercise and to reverse the conditions.
Key ideas
8Lactate is the language of your mitochondria
At rest no non-invasive test reveals mitochondrial status, so you stress the system and read lactate. It is the best proxy for how well your mitochondria and metabolism are working.
Fuel switching makes lactate mandatory
As intensity rises the body shifts from fat to glucose, and burning carbohydrates always produces lactate. Without it, glycolysis and muscle contraction would come to a stop.
Metabolic equilibrium sits near 2 mmol
Robust mitochondria produce and clear lactate at the same rate, holding blood levels around 2 mmol. When they are overwhelmed, lactate spills into the blood and climbs to 3-6 mmol.
Some people are above zone 2 at rest
With 70% of Americans inactive, many show resting lactate of 2-3 mmol — roughly like a glucose of 200. It signals significant metabolic dysfunction.
Polarized training: mostly zone 2, some sprints
Elite athletes spend the vast majority of training in low zones because high intensity is not sustainable. The same 80/20 balance works for midlife bodies and metabolic disease.
The astrocyte-neuron lactate shuttle
Astrocytes break down glycogen into lactate that feeds neurons for energy. Lactate raises BDNF and neurogenesis, and its dysfunction is linked to Alzheimer's.
Muscle is the largest, most active organ
Skeletal muscle oxidizes about 80% of post-meal glucose and is the first organ to show type 2 diabetes. It is also the one tissue that essentially never develops cancer.
Pre-diabetes is reversible through muscle plasticity
Caught early, pre-diabetes often reverses in three to six months of strength and cardio work. Muscle's plasticity lets even lifelong sedentary people reset their metabolism.
Practical takeaways
6- 1
Split training 80/20 26:00
Spend roughly 80% of your time in zone 2 and about 20% in all-out intervals to build both efficiency and top-end capacity.
- 2
Use heart rate as your lactate proxy 22:00
Find the heart rate where your lactate stays near 2 mmol and train there most of the time instead of chasing a finger-prick every day.
- 3
Retest as you get fitter 32:00
As mitochondria improve, your old zone 2 becomes too easy — retest so you keep training at the right intensity and avoid plateauing.
- 4
Add short sprints twice a week 40:00
Do brief all-out efforts, such as 30 seconds on with full recovery repeated a few times, to protect VO2 max and glycolytic power.
- 5
Follow FACE for whole-body training 52:00
Flexibility, Aerobic conditioning, Carry a load twice a week, and Equilibrium and balance keep muscle and mitochondria strong for life.
- 6
Catch pre-diabetes early and move 55:30
If your fasting glucose is creeping up, don't settle for 'watch your diet' — build muscle and add cardio to reverse the trend.
Topics & chapters
15Cold open and welcome
Teaser clips on muscle, lactate and zone 2 set up the episode, followed by Dr. Wright thanking her audience.
Meet Dr. Iñigo San Millán
Dr. Wright introduces her guest, a cellular-metabolism researcher and elite-athlete coach whose lab sits in her Lake Nona building.
What a lactate threshold test measures
Why you stress mitochondria like a cardiac stress test and read lactate as the surrogate for metabolic status.
How muscles switch fuels and make lactate
The shift from fat to glucose with rising intensity, and why burning carbohydrates always produces lactate.
Lactate as a buffer and preferred fuel
Lactate mops up protons and becomes the fastest, preferred fuel for most cells, including the brain.
Metabolic equilibrium and robust mitochondria
How healthy mitochondria hold lactate near 2 mmol, and what it means when it climbs into the blood.
Heart rate as a surrogate for lactate
Using heart rate to stay in the flexible zone, and how the sedentary can sit above zone 2 at rest.
Why zone 2 trains mitochondria best
Lessons from decades with elite athletes on why sustainable low-intensity work builds the most mitochondria.
Retraining, and why zone 2 gets harder
Learning to slow down, how a fit rider can hold zone 2 at 300 watts, and why you must retest.
Dr. Wright's own progression
Her personal story of testing at Lake Nona, gaining weight through menopause and rebuilding flexibility.
The role of sprint intervals and VO2 max
Why short all-out efforts preserve glycolytic capacity, and how lactate discriminates better than VO2 max.
Lactate as brain fuel
Endorphins, blood flow and lactate flooding the brain, plus the astrocyte-neuron shuttle and BDNF.
Estrogen, menopause and brain energetics
How estrogen loss may shift brain metabolism, and the metabolic theory of Alzheimer's as type 3 diabetes.
Muscle: the largest organ and FACE
Skeletal muscle as the first organ to see diabetes, its resistance to cancer, and the FACE framework.
Inside-out hypothesis, reversal and hope
How decline starts inside the muscle, why pre-diabetes reverses in months, and a critique of quick-fix longevity clinics.