If you want to start an argument among psychiatrists, ask a simple question:

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What fuel does the brain actually run on?

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Glucose? Ketones? Both?

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One thing is clear: the brain is an energy-intensive organ, consuming roughly 20% of the body’s metabolic budget. When energy metabolism becomes unstable, mood, cognition, and emotional regulation suffer.

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Which brings us to a surprisingly powerful metabolic intervention: walking after meals.

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A fitness influencer recently blessed my feed with “three hacks to make fat loss faster through walking.” His advice: get your steps up, take walks after meals, and when that stops working, crank up the incline on your treadmill.

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He’s not wrong, exactly. He’s just not interesting.

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The framing is pure calories-in-calories-out: walk more, burn more, lose fat. It’s the metabolic equivalent of explaining that cars move because the wheels turn. Technically accurate. Utterly missing the point.

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Let me tell you what’s actually happening when you take a walk after dinner—and why it matters far more than the 47 calories your Apple Watch says you burned.

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The Hadza Paradox

Herman Pontzer is an evolutionary anthropologist at Duke who had a reasonable hypothesis: hunter-gatherers who walk 10-14 kilometers per day must burn dramatically more calories than sedentary Westerners.

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He was wrong.

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Using doubly labeled water—the gold standard for measuring energy expenditure—Pontzer found that Hadza hunter-gatherers in Tanzania burn approximately the same number of calories per day as American office workers. Hadza men: ~2,600 kcal/day. Hadza women: ~1,900 kcal/day. Statistically indistinguishable from adults in the U.S. or Europe after controlling for body size.

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Let that sink in. People who spend their lives scratching a living out of the earth with hand-made tools—who move more in a morning than most Americans move in a week—burn the same energy as their sedentary Western counterparts.

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Pontzer’s “Constrained Energy Expenditure” model explains why: total energy expenditure increases with physical activity at low levels, then plateaus. The body adapts. It down-regulates other metabolic processes to keep total expenditure within a narrow, evolutionarily-conserved range.

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Frankly, this stretches my understanding of physics. We’re talking about 10-14 kilometers of daily walking—somewhere in the neighborhood of 15,000-20,000 steps. What processes could possibly be down-regulated enough to account for that difference? The implication is remarkable: the range in which the human body can burn fuel to accomplish the same work is extraordinarily broad. And when there’s excess energy coming in, the body even knows how to waste it—mitochondrial uncoupling proteins can essentially disconnect the turbines and just produce heat, burning fuel without generating ATP.

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We’re not simple calorimeters. We’re adaptive metabolic systems with millions of years of evolutionary pressure to maintain energy homeostasis. The body has more tricks than your fitness tracker knows about.

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So when our fitness influencer says “push harder on the incline to burn more calories”—he’s fighting millions of years of metabolic conservation. The body will find a way to compensate. It always does.

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Exercise prevents you from getting sick. Diet is your tool for weight management. These are Pontzer’s words, not mine.

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But here’s where it gets interesting: if walking isn’t primarily about burning calories, why does it work so well for metabolic health?

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It’s the Glucose, Not the Calories

When you eat, blood glucose rises. This is true no matter what you eat—even if you consume zero carbohydrates. The very act of eating protein induces an insulin response, which in turn triggers compensatory glycogen release and even gluconeogenesis. Your body is determined to maintain glucose homeostasis, and it will manufacture glucose from protein if it has to. Eating is a metabolic event, period.

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Insulin rises in response, signaling cells to take up that glucose. In a metabolically healthy person, this happens efficiently—glucose clears, insulin drops, and you return to a state where lipolysis (fat breakdown) can occur.

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In someone with insulin resistance, this process is sluggish. Glucose stays elevated longer. Insulin stays elevated longer. And as long as insulin is elevated, lipolysis is suppressed. You’re locked out of your own fat stores.

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Here’s what a post-meal walk actually does:

It bypasses insulin entirely.

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Muscle contraction activates GLUT4 transporters through an insulin-independent pathway mediated by AMPK. You’re literally pulling glucose out of circulation without requiring an insulin spike to do it. This isn’t just “using fuel”—it’s a fundamentally different metabolic route.

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A 10-minute walk immediately after eating reduces peak blood glucose significantly—from ~182 mg/dL to ~164 mg/dL in one study. That blunted glucose excursion means a blunted insulin response. And a blunted insulin response means you return to a lipolytic state faster.

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The calories burned during that walk? Trivial.

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The reduction in insulin exposure? That’s the game.

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The Math Your Treadmill Won’t Tell You

Speaking of calories: those readouts on exercise equipment are, to use the correct meaning of the word, gross estimates. Not “ew, gross”—gross as in rough, approximate, unrefined.

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If you think you can walk on a treadmill until the display reads “500” and conclude you’ve burned a seventh of a pound of fat, you’re relying on tragically flawed math. The actual number depends on your body composition, metabolic rate, fitness level, what you ate, your hormonal state, and roughly a dozen other variables the machine knows nothing about.

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But more importantly: even if the number were accurate, it wouldn’t matter much. Because the Hadza taught us that energy expenditure is constrained. You can’t out-walk your metabolism’s determination to conserve energy.

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This brings me to something that frustrates me about the way people talk about wanting to make their bodies more “efficient.”

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In most aspects of life, efficiency is the opposite of what you want.

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You don’t want your body to be a Prius—relatively weak power output with maximized fuel storage and minimal use. That’s a body optimized for scarcity, hoarding every calorie against an uncertain future.

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What you actually want is a supercharged V-8: maximum power output, minimal storage, burning fuel freely because there’s plenty more where that came from. A body that builds muscle instead of fat. A metabolism that runs hot.

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And this will happen if you “tell” your body the right things. (Remember: listening to your body works, as long as you learn the language.)

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If your body understands that fuel availability is practically infinite—that famine is not coming—and that the number one thing needed for survival is power output, it will do what any self-respecting engineer would do: build you a muscle car.

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When insulin is low, the fat spigot is wide open, and you’re feeding the machine with plenty of high-quality protein and fat, that is exactly the message you’re sending. You’re signaling abundance and demand. The body responds by building capacity, not storage.

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What you can do is change the metabolic environment. And that’s where the post-meal walk earns its keep—not by burning calories, but by bending the glucose and insulin curves.

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The Snowball Effect (Or: Why Consistency Beats Intensity)

Here’s the part nobody talks about.

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When you walk after a meal and successfully blunt your glucose/insulin response, you create a window for lipolysis. Fat gets mobilized. Ketones get produced. And your mitochondria—the power plants in every cell—get exposed to a different fuel mix.

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Mitochondria are adaptive. They upregulate the enzymatic machinery for whatever substrate is abundant. Feed them glucose all day, every day, and they’ll optimize for glucose oxidation. Expose them regularly to fatty acids and ketones, and they’ll build the infrastructure to burn fat preferentially.

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Here’s the kicker: mitochondria turn over every 4-6 weeks. You’re not stuck with the metabolic machinery you have today. You’re building new mitochondria constantly—and they’re being shaped by the metabolic environment you create.

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So imagine you walk after meals consistently. Each walk:

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• Pulls glucose down via insulin-independent GLUT4 translocation
• Blunts the insulin response
• Opens a window for lipolysis
• Exposes mitochondria to elevated fatty acids and ketones

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Do this regularly, and you’re voting—with every walk—for what kind of metabolism you want next month.

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Over time:

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• More fat/ketone exposure → mitochondria that prefer fat oxidation
• Fat-adapted mitochondria → less glucose demand → lower baseline insulin
• Lower insulin → less insulin resistance → even easier access to fat stores
• Easier lipolysis → more ketone exposure → next generation of mitochondria even more fat-adapted

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This is metabolic compounding. The first walk is the hardest. The 100th walk is running on different hardware.

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The Caveat: Hyperinsulinemia Changes the Rules

Now, a dose of reality for those of us who aren’t metabolically pristine.

If you’re significantly insulin resistant, you likely have chronic hyperinsulinemia—elevated baseline insulin that doesn’t drop easily. In this state, you can walk for hours and primarily just deplete glycogen. Insulin stays high because it can’t drop (or won’t, because your pancreas is overcompensating for resistant tissues). Without that insulin drop, you never fully access lipolysis. You’re draining the battery without switching to the generator.

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This isn’t futile—glycogen depletion is still useful, and the GLUT4/AMPK pathway still works. But you won’t get the clean switch to fat oxidation that a metabolically flexible person gets.

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There is a phenomenon—sometimes called “the flip”—where prolonged exercise eventually forces the issue. Deplete enough glycogen, get hypoglycemic enough that your body has no choice, and insulin will finally fall. Ketones will finally rise. But we’re talking hours, not minutes.

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I experienced this firsthand on a 20-mile hike in Colorado a few years back. At the time, I was in a moderately insulin-resistant phase—fasting glucose running 90-100 mg/dL, ketones minimal at 0.5-0.7 mmol/L. For most of that hike, I felt suboptimal. Running on sugar and glycogen, never quite clicking into a higher gear.

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But at mile 20, I checked my blood. Ketones: over 4.0 mmol/L.

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That was the flip. Hours of glycogen depletion finally forced insulin down, and my liver started producing ketones at a rate I’d never seen. I’d essentially walked my way into therapeutic ketosis—the hard way.

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The lesson: if you’re insulin resistant, the post-meal walk still helps. It’s still bending the curves in the right direction. But the full metabolic cascade takes longer to unlock. Patience and consistency matter more than intensity.

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Metabolic Psychiatry (Or: Why This Matters Beyond the Mirror)

Here’s where my day job as a psychiatrist intersects with metabolic physiology.

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Glucose variability—the spikes and crashes that come from poor metabolic control—doesn’t just affect your waistline. It affects your brain. Mood, cognition, energy, emotional stability: all of these are downstream of metabolic function.

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The brain is an energy hog. It’s roughly 2% of your body weight but consumes about 20% of your metabolic budget. And while the old dogma held that the brain runs exclusively on glucose, we now know it runs beautifully on ketones—arguably better on ketones, which provide more ATP per unit oxygen consumed than glucose does.

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When you’re metabolically flexible—when you can access fat and produce ketones readily—your brain has a steady, high-quality fuel source. When you’re locked into glucose dependency with wild swings in blood sugar, your brain is along for the ride. Every spike and crash registers. The afternoon slump. The brain fog. The irritability that comes out of nowhere. The anxiety that spikes two hours after a high-carb meal.

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This isn’t just “feeling off.” Chronic glucose variability drives inflammation. And inflammation in the periphery becomes neuroinflammation in the brain. The same metabolic dysfunction that promotes insulin resistance also promotes the inflammatory signaling that underlies depression, anxiety, and cognitive decline.

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But here’s the flip side: ketones aren’t just fuel. Beta-hydroxybutyrate—the primary ketone body—is a signaling molecule. It inhibits NLRP3 inflammasome activation, one of the key drivers of inflammatory cascades. It’s essentially an endogenous anti-inflammatory that you produce for free every time you access your fat stores.

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So when you take that post-meal walk and successfully blunt your glucose response, open a window for lipolysis, and produce some ketones, you’re not just managing your weight. You’re:

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• Reducing glucose variability → more stable mood and cognition
• Lowering systemic inflammation → less neuroinflammation
• Producing beta-hydroxybutyrate → active anti-inflammatory signaling
• Providing your brain with premium fuel → clearer thinking, more sustained energy

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The subjective experience of this is hard to overstate. You don’t just feel “better”—you feel steady. The emotional rollercoaster flattens out. The energy peaks and troughs smooth into something more like a plateau. You stop white-knuckling your way through the afternoon.

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Mental health is physical health. The artificial separation between the two has always been a failure of imagination. Your brain is an organ. It runs on metabolism. Fix the metabolism, and the brain comes along for the ride.

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The Punchline

The fitness influencer wants you to think about walking as a calorie furnace. Stoke it hotter, burn more fuel, lose more fat.

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The metabolic reality is different. Walking—especially post-meal walking—is a glucose disposal intervention. An insulin-management strategy. A mitochondrial training program.

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The boots aren’t made for burning calories.

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They’re made for building mitochondria.

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Walk today, and you’re not just handling this meal’s glucose. You’re casting a vote for what kind of metabolism you want next month. And the month after that.

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That’s not a hack. That’s a compounding investment in your metabolic future.

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Nancy Sinatra knew what she was talking about.
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This article was originally published on Dr. Boz's Substack