Teaching Your Body to Burn Fat Again
Metabolic Health Series | Issue 08
Here's a question worth sitting with: when did you last go more than four or five hours without eating and feel completely fine? No energy crash, no brain fog, no urgent craving pulling you toward the nearest snack? For many people, that experience is a distant memory — or has never happened at all.
That inability to go without food comfortably isn't just inconvenient. It's a metabolic signal. It means the body has lost — or never developed — the ability to switch cleanly between fuel sources. And recovering that ability may be one of the most important things you can do for your long-term metabolic health.
This issue is about metabolic flexibility: what it is, why so many of us have lost it, and how healthy fats, ketones, mitochondrial function, and strategic fasting can help restore it.
What Metabolic Flexibility Actually Means
A metabolically flexible body can shift between burning carbohydrates and burning fat depending on what's available and what the situation demands. After a meal rich in carbohydrates, it burns glucose efficiently. During a fast, it transitions smoothly to fat burning. During sustained exercise, it draws on whichever fuel is most appropriate for the intensity.
A metabolically inflexible body — which describes most people eating the Standard American Diet (SAD) — is essentially stuck in glucose-burning mode. It has become so accustomed to a constant supply of dietary carbohydrates that it has lost the enzymatic machinery and mitochondrial capacity to access stored fat efficiently. The result: persistent hunger, energy crashes between meals, difficulty losing body fat despite effort, and a metabolism that is highly dependent on the next meal.
Sami Inkinen — founder of Virta Health, seven-time Hawaii Ironman finisher, and a man who completed a 2,750-mile rowing crossing of the Pacific Ocean largely fat-adapted — has spoken extensively about this. He reversed his own prediabetes not through calorie restriction but by shifting his metabolism toward fat as a primary fuel. His work at Virta, which has helped more than 100,000 people reverse metabolic disease, is built on the same principle: restore the body's ability to burn fat, and metabolic health follows.
Fat Is Not the Enemy — But Type Matters Enormously
For decades, dietary fat was cast as the villain in the metabolic story. That framing has been substantially revised by the evidence — but not all fats are equivalent, and the distinctions matter.
Fats that support metabolic health:
Monounsaturated fats — found in extra virgin olive oil, avocado, and most nuts — are the most studied fats in the metabolic literature and consistently associated with reduced inflammation, improved insulin sensitivity, and lower cardiovascular risk. The Mediterranean diet's metabolic benefits rest largely on this fat category.
Omega-3 fatty acids — from fatty fish (salmon, sardines, mackerel, anchovies), walnuts, and flaxseed — are potently anti-inflammatory. EPA and DHA, the long-chain forms found in marine sources, support cell membrane function, reduce triglycerides, and have measurable effects on insulin sensitivity and inflammatory markers.
Saturated fats from whole food sources — grass-fed butter, ghee, coconut oil, eggs, and quality meat — are metabolically neutral to beneficial for most people in the context of a low-carbohydrate, whole-food diet. The research increasingly suggests that saturated fat's effect on cardiovascular risk is highly context-dependent: it appears more problematic when consumed alongside high refined carbohydrates and less so when carbohydrate intake is low.
Fats that work against metabolic health:
Industrial seed oils — soybean, corn, cottonseed, sunflower, and safflower oils — are high in linoleic acid, an omega-6 fatty acid that, in excess, promotes cellular inflammation and oxidative stress. These oils dominate the ultra-processed food supply and the fryer oils of most restaurants. They are not acutely toxic, but their chronic overconsumption — in the amounts typical of a standard Western diet — is associated with the inflammatory milieu that underlies metabolic dysfunction.
Trans fats — largely regulated out of the food supply but still present in some processed foods under labels like "partially hydrogenated oil" — remain problematic wherever they appear.
The practical shift: cook with olive oil, avocado oil, butter, or ghee. Eat fatty fish two to three times per week. Add avocado, nuts, and seeds as regular dietary staples. Reduce/eliminate restaurant fried food and ultra-processed snacks.
Ketones: More Than a Backup Fuel
When carbohydrate intake drops and fat oxidation increases, the liver produces ketone bodies — primarily beta-hydroxybutyrate (BHB), acetoacetate, and acetone. These ketones circulate as fuel for the brain, heart, and muscles. But researcher Dom D'Agostino, neuroscientist and professor at the University of South Florida, has been at the forefront of demonstrating that ketones do something far more interesting than simply stand in for glucose.
BHB is now understood to be a potent signaling molecule — one that suppresses inflammation, activates antioxidant defense pathways, and influences gene expression in ways that go well beyond its caloric contribution. D'Agostino has described it as functioning almost like a hormone: "an endogenous metabolite working as a signaling molecule, independent of metabolism, suppressing inflammation."
This has implications for who can benefit from increasing ketone production — not just those pursuing strict ketogenic diets, but anyone who can create regular windows of low insulin and fat-burning through fasting, reduced carbohydrate intake, or sustained aerobic exercise. You don't need to be in deep ketosis to benefit from the metabolic signaling that ketone production provides. Even mild elevations in BHB, achieved through an overnight fast or a carbohydrate-restricted meal, produce measurable biological effects.
Those who tend to benefit most from more sustained ketone elevation include people with significant insulin resistance, prediabetes, PCOS, non-alcoholic fatty liver disease, or elevated triglycerides — conditions where the ability to run on fat and ketones has been most compromised and where restoring it produces the most rapid metabolic improvement.
The Mitochondrial Connection
Underneath metabolic flexibility is a cellular story: the health and density of your mitochondria.
Mitochondria are the organelles responsible for energy production in every cell — they take in fuel (glucose or fat) and produce ATP, the body's energy currency. A metabolically healthy person has abundant, efficient mitochondria in their muscle and liver cells. A metabolically compromised person typically has fewer, less efficient ones — a state called mitochondrial dysfunction.
Mitochondrial dysfunction is increasingly recognized as a central feature of metabolic syndrome, not merely a consequence of it. Dysfunctional mitochondria burn fuel inefficiently, generate excess oxidative stress, and impair both insulin signaling and the ability to oxidize fat. Rhonda Patrick has documented how chronic inflammation — the kind generated by visceral fat, poor sleep, and ultra-processed diets — directly impairs mitochondrial function, creating a feedback loop in which metabolic ill health perpetuates itself at the cellular level.
The good news: mitochondria are highly responsive to training. The interventions that most powerfully stimulate mitochondrial biogenesis — the creation of new mitochondria — are:
Zone 2 aerobic exercise, which drives mitochondrial density in slow-twitch muscle fibers over time
HIIT, which creates a rapid stimulus for mitochondrial adaptation (covered in Issue 4 and Issue 6)
Fasting and carbohydrate restriction, which activate mitochondrial quality control pathways including autophagy — the cellular housekeeping process that removes damaged mitochondria and recycles their components
Cold exposure, which activates brown adipose tissue and stimulates mitochondrial uncoupling — a process that generates heat and increases metabolic rate
Building mitochondrial density and efficiency is, in a very real sense, building metabolic capacity itself.
Fasting as a Metabolic Tool
Fasting is not a new idea — it is arguably the oldest metabolic intervention available, built into human biology across millennia of food scarcity. What is relatively new is our understanding of the precise mechanisms through which it works.
When you fast, several things happen in sequence:
Liver glycogen (stored glucose) depletes over the first 12–18 hours
Insulin falls, and fat cells begin releasing stored fatty acids into circulation
The liver converts those fatty acids into ketones, which supply the brain and other tissues
Autophagy is upregulated — cellular cleanup and mitochondrial renewal accelerate
Growth hormone rises, protecting muscle mass during the fast
Inflammatory markers fall, partly through BHB's signaling effects
Even a 12-hour overnight fast — finishing dinner at 7pm and not eating until 7am — produces meaningful metabolic effects, particularly when practiced consistently. Blood glucose and insulin reach their lowest points in the final hours of this window, providing a daily period of metabolic recovery that most people eating from morning to late evening never experience.
A 16-hour fast (commonly called 16:8 intermittent fasting) extends these benefits and is where most of the clinical research on time-restricted eating has been conducted. For most metabolically healthy people, this is a sustainable daily rhythm: eat within an 8-hour window, fast for 16.
Longer fasts — 24 to 72 hours — produce more pronounced ketosis, deeper autophagy, and more significant metabolic reset effects. D'Agostino has extensively researched multi-day fasting protocols and maintains that, with adequate electrolyte management and sufficient baseline fat adaptation, extended fasts are safe and metabolically powerful for appropriate individuals. Inkinen completed his Pacific rowing crossing in a state of deep fat adaptation, demonstrating how far human metabolic capacity can extend when the machinery is properly trained.
Fasting is not appropriate for everyone — it is contraindicated in pregnancy, eating disorder history, certain medications, and some medical conditions — and longer fasts should be undertaken with appropriate supervision. But for most metabolically healthy adults, regular shorter fasting windows are one of the most accessible and evidence-supported metabolic tools available.
Putting It Together: The Metabolic Flexibility Protocol
Building metabolic flexibility is not about following a strict diet. It is about consistently creating the conditions in which the body practices fat-burning — so that it becomes the default, not a struggle.
The core practices:
Eat more healthy fat, fewer refined carbohydrates. Shift the macronutrient balance toward quality fat and protein, and away from the refined carbohydrates that keep insulin chronically elevated and fat-burning chronically suppressed.
Create a daily fasting window. A consistent 12+ hour overnight fast is the lowest-barrier way to practice metabolic fuel switching every day.
Exercise in a way that builds mitochondria. Zone 2 cardio and HIIT both stimulate mitochondrial biogenesis through complementary pathways. A weekly movement protocol that includes both (as outlined in Issue 6) is the most powerful tool for building the cellular machinery of fat burning.
Reduce eating frequency. Three meals with no snacking — or two larger meals — allows insulin to fall between meals and creates the hormonal conditions for fat oxidation. Constant eating, even of healthy food, keeps insulin elevated and metabolic flexibility suppressed.
Be patient with adaptation. Transitioning from glucose dependency to fat adaptation takes time — typically two to six weeks of consistent lower-carbohydrate eating before the body rebuilds the enzymatic machinery for efficient fat oxidation. The first two weeks often feel harder before they feel better. This is normal, not a sign of failure.
Three Things to Do This Week
Extend your overnight fast by two hours. If you usually eat breakfast at 7am, push it to 9am for five days and observe how your hunger, energy, and mental clarity feel during the extension. You are practicing fat-burning.
Replace one refined carbohydrate with a fat-rich whole food. Crackers with avocado instead of crackers alone. Eggs cooked in olive oil instead of toast. Salmon instead of a grain-based lunch. Small substitutions compound.
Check the oils in your kitchen. If soybean, canola, vegetable, or sunflower oil are present, consider replacing them with extra virgin olive oil or avocado oil for everyday cooking. This one swap meaningfully changes the inflammatory load of your diet over time.
Next issue: "How Chronic Stress Shapes Your Metabolism — and What Restores It" — how chronic stress drives blood sugar dysregulation, visceral fat accumulation, and hormonal chaos, and what the nervous system science says about actually reversing it.
This newsletter is educational and does not constitute medical advice. Work with a qualified healthcare provider for personalized guidance.
Lisa Marlene Thompson is a Functional Nutritional Therapy Practitioner, somatic facilitator, and health and lifestyle guide for anyone ready to feel more alive in their body. With advanced certifications in menopause science with Dr. Stacy Sims, somatic movement with Michaela Boehm, and sleep and brain health informed by the research of Lisa Mosconi and Matt Walker, her work sits at the intersection of cutting-edge science and deep body wisdom.
Her signature program — Strength, Sleep & Sensuality — is an invitation to come home to your body: to build real strength, reclaim restorative sleep, and rediscover a nervous system that is grounded and provides aliveness in the body that belongs to this chapter of life. She is based in Los Angeles and works with clients worldwide.
Your most vibrant chapter is still ahead.
I'd love to explore what's possible together → Reach out: lisa@lisamarlenethompson.com