Most Replayed Moment: Calories In, Calories Out Is A Myth! Why Most Diets Fail - Dr. Jason Fung

What if weight loss has been framed backwards?

Imagine being told that eating 500 fewer calories should automatically translate into a predictable pound of fat lost weekly. It sounds tidy. It sounds scientific. But the tidy equation collapses once you factor in how your body responds — hormonally — to what, when, and how often you eat.

Calories are energy. Food is instruction.

That sentence stuck with me. Calories measure energy, yes, but food also carries hormonal commands. Insulin is the headline here: when insulin is high, the body stores energy; when insulin falls, the body releases energy from fat. That simple distinction reframes common weight stories. It explains why two people eating the same number of calories can have opposite outcomes.

There’s a blunt metaphor that I kept returning to while listening: body fat is like money in a bank. If insulin is high, the bank vault is closed. You can't withdraw. Lower calories won’t help if the vault stays shut. That image made the physiology feel immediate — and a little unfair.

Metabolic adaptation: why "eat less" can backfire

Cutting calories often triggers the exact opposite of what dieters expect. The basal metabolic rate doesn’t sit politely; it adapts. Eat less, and your body tends to burn fewer calories. The result is exhaustion, cold intolerance, hunger, and — frequently — a rebound to higher weight once normal eating resumes. That yo-yo pattern is not moral failure. It is biological adaptation.

What I found quietly provocative was the claim that intermittent fasting can break that cycle. Fasting lowers insulin and elevates other hormones that activate the body's ability to access stored fat. In some studies, short-term fasting even raises metabolic rate — the opposite of the feared "starvation mode." That flips a long-standing assumption on its head.

Timing matters almost as much as what you eat

There’s an overlooked cultural shift since the late 1970s. Where three meals ruled, five to six eating occasions became normalized by the early 2000s. That constant nibbling keeps insulin chronically elevated. The consequence? Your body rarely gets a window to mobilize fat stores.

Breakfast myths crumble under that light. The English term itself — to "break fast" — implies a daily fasting interval is natural. Skipping breakfast, or delaying the first meal, isn’t inherently harmful. When implemented thoughtfully, it creates hormonal space for fat release without perpetual insulin spikes from frequent snacking.

Exercise is vital — but it's no magic bullet for weight loss

Expecting exercise to cancel out poor dietary patterns is wishful thinking. Physical activity offers sweeping health benefits: strength, resilience, mood, longevity. Yet in practical weight terms, typical exercise burns relatively few calories compared to daily metabolic needs. And there’s another catch: exercise can increase appetite later, partly offsetting the energy spent.

I appreciated the humility here. The speaker wasn’t dismissing movement but urging clarity. Weight loss strategies should center what people actually control day-to-day: food choices and meal timing, not the fantasy that a treadmill will erase a lifetime of hormonal signaling.

Concrete contrasts: two 1,500 calorie approaches

• Low-carb, infrequent eating (fasting windows) — insulin drops, body accesses stored fat, metabolism remains active.
• Low-fat, constant snacking — insulin stays high, body stores energy, metabolism downshifts and stalls weight loss.

Both approaches might total the same number on a calorie tracker. Yet their hormonal environments are radically different. That felt like the central, somewhat infuriating, insight: numbers alone hide important biological context.

Why some short-term diets look spectacular — then fail

I couldn’t help but think about the friend who achieved apparent success eating pizza and then crashed back. Rapid losses can mask metabolic damage. When normal eating resumes, a body that has downshifted its caloric burn will regain weight often beyond the starting point. The narrative of personal failure misses the physiological explanation.

There’s moral freight here. Diet culture interprets weight regain as sloth or dishonesty. But these patterns are predictable reactions to hormonal signals and persistent snacking. Understanding that changes the tone from blame to pragmatism.

A different lens for everyday choices

Reframing food as instruction invites small experiments rather than sweeping guilt. Try extending the overnight fast by a couple of hours. Remove one snack. Notice energy, hunger, and mood. These are practical toggles that alter insulin patterns without dramatic deprivation.

The most surprising part? Fasting isn’t some fringe fad here; it’s a long-standing physiological state used in medicine. The defensiveness around it feels cultural more than clinical. That dissonance intrigued me — and made the science more compelling.

Final thought

I left this conversation less convinced that calories are irrelevant and more convinced they’re incomplete. Biological context — hormones, timing, and the body’s adaptive thermostat — matter in stubborn, measurable ways. If we accept that food talks to our hormones, then weight becomes less of an arithmetic problem and more of a conversation with our own physiology.