CICO vs Hormones

For decades we’ve been told that people either have a “fast metabolism” or a “slow metabolism.” If you struggle with weight, fatigue, or stubborn belly fat, you’ve probably blamed a “slow metabolism” at some point.

But the truth is more nuanced.

From the perspective of low-carbohydrate and ketogenic researchers like Ken D. Berry, Shawn Baker, Anne Bosworth, and Robert Kiltz, metabolism isn’t primarily about speed. It’s about fuel preference.

In most modern metabolic dysfunction, the body becomes stuck primarily burning glucose (sugar) instead of fat. When this happens, the problem isn’t a slow metabolism—it’s a metabolically inflexible one.

Understanding this difference changes everything about how we approach fat loss, energy, and long-term health.

Metabolism Is Your Body’s Energy Engine

At its core, metabolism simply refers to all the chemical reactions your body uses to create energy. Every heartbeat, breath, and muscle contraction requires fuel.

Your body has two main fuel systems:

Glucose metabolism (burning sugar/carbohydrates)

Fat metabolism (burning stored or dietary fat)

Both systems are natural and important. Humans evolved to move between them depending on food availability.

When carbohydrates were available, the body could burn glucose.

When food was scarce, the body could burn stored fat.

This ability to switch between fuel sources is called metabolic flexibility. Historically, humans were very metabolically flexible.

Today, many people are not.

The Problem: Chronic Sugar Burning

Modern diets keep the body in a near-constant state of glucose metabolism.

Consider the typical eating pattern:

• Breakfast cereal, toast, or oatmeal
• Mid-morning snack
• Sandwich and chips for lunch
• Afternoon snack
• Pasta or rice at dinner
• Dessert

This pattern floods the body with carbohydrates throughout the day.

Every time carbohydrates are consumed, the pancreas releases insulin, a hormone that directs glucose into cells.

Insulin has an important side effect:

It suppresses fat burning. 

As long as insulin is elevated, the body prioritizes burning glucose instead of fat.

Over time, this repeated pattern trains the body to rely almost exclusively on sugar as its primary fuel source.

This is where metabolic problems begin.

As long as insulin is elevated, the body prioritizes burning glucose instead of fat.

When the Body Forgets How to Burn Fat

If the body constantly receives glucose, it has little reason to burn fat.

The metabolic machinery for fat oxidation becomes underused. This leads to a situation where the body is technically capable of burning fat—but rarely does.

The result is a person who:

• Feels hungry every few hours
• Experiences energy crashes
• Struggles with stubborn fat loss
• Feels shaky or irritable when meals are delayed

This isn’t a slow metabolism.

It’s a metabolism that has become dependent on sugar.

When glucose levels drop between meals, the body struggles to transition smoothly into fat burning.

Instead, it sends powerful hunger signals to restore glucose quickly.

Insulin Resistance and Metabolic Inflexibility

A major driver of this problem is insulin resistance.

When insulin is repeatedly elevated for years, cells begin to respond less effectively. The body compensates by producing even more insulin.

This leads to chronically elevated insulin levels—a condition sometimes referred to as hyperinsulinemia.

High insulin has several metabolic effects:

• Fat burning is suppressed
• Fat storage increases
• Hunger signals become stronger
• Blood sugar fluctuations worsen

From the outside, this looks like a “slow metabolism.”

But internally, the body is simply locked into a sugar-burning mode.

Fat Burning: The Metabolic System We Were Designed For

Human physiology is remarkably well designed to burn fat.

Fat contains more than twice the energy per gram compared to carbohydrates. Even lean individuals carry tens of thousands of calories stored as body fat.

When the body shifts toward fat metabolism, several changes occur:

• Hunger stabilizes
• Energy becomes steady
• Fat stores become accessible
• Blood sugar swings decrease

This metabolic state is what ketogenic nutrition aims to encourage.

By significantly reducing carbohydrate intake, insulin levels fall and the body is forced to rely more heavily on fat for fuel.

Over time, the metabolic machinery for fat burning becomes more efficient again.

This process is often called fat adaptation.

What Happens During Fat Adaptation

When someone transitions to a ketogenic diet, the body undergoes several adjustments.

First, liver glycogen (stored glucose) begins to decline.

As glucose availability decreases, the body increases its use of fat for fuel. The liver also begins producing ketones, which serve as an alternative energy source for the brain.

During this transition phase, some people experience temporary symptoms such as fatigue or brain fog. This is often referred to as the “keto adaptation period.”

Once adaptation occurs, many people report:

• Longer periods without hunger
• Stable mental clarity
• Improved energy levels
• Easier fat loss

This isn’t because their metabolism suddenly became “faster.”

It’s because their metabolism became more flexible.

Protein and the Role of Gluconeogenesis

One concern people sometimes raise about low-carbohydrate diets is the need for glucose.

Certain tissues, such as red blood cells, require small amounts of glucose to function.

The body solves this problem through a process called gluconeogenesis—the creation of glucose from non-carbohydrate sources, primarily amino acids and glycerol.

Importantly, gluconeogenesis is demand-driven, not supply-driven.

In other words, the body produces the glucose it needs, when it needs it.

This is one reason why adequate protein intake is critical in a ketogenic lifestyle.

Protein provides the building blocks for:

• Muscle repai
• Hormone production
• Immune function
• Glucose production when necessary

Experts such as Ken D. Berry and Shawn Baker often emphasize that protein is not the enemy in a ketogenic approach—it is foundational.

The Hormonal Side of Metabolism

Another reason the “fast vs slow metabolism” idea is misleading is that metabolism is heavily regulated by hormones.

Several key hormones influence fuel usage:

Insulin – directs the body to store energy and suppress fat burning

Glucagon – encourages fat release and energy availability

Cortisol – mobilizes energy during stress

Leptin – regulates hunger and satiety

When insulin remains chronically elevated, these hormonal signals become disrupted.

Fat remains locked in storage even when energy is needed.

This hormonal imbalance—not metabolic speed—is often the real reason fat loss becomes difficult.

Why Eating Less Isn’t Always the Answer

Traditional dieting advice often focuses on eating less and exercising more.

While calorie balance does matter, severe calorie restriction can create another problem: metabolic adaptation.

When the body perceives prolonged calorie scarcity, it may reduce energy expenditure by:

• Lowering thyroid hormone activity
• Reducing spontaneous movement
• Decreasing body temperature
• Increasing hunger signals

This is why many people regain weight after restrictive diets.

The body is simply trying to restore balance.

A ketogenic approach attempts to solve this by addressing hormonal signaling first, allowing the body to access stored fat for fuel rather than relying solely on incoming calories.

The Goal: Metabolic Flexibility

The healthiest metabolism is not defined by speed.

It’s defined by adaptability.

A metabolically flexible person can:

• Burn glucose efficiently when carbohydrates are available
• Burn fat efficiently when carbohydrates are low
• Maintain stable energy between meals
• Access stored body fat when needed

This flexibility was once the natural state of human metabolism.

Reclaiming it often requires breaking the cycle of constant carbohydrate intake and elevated insulin.

A Different Way to Think About Metabolism

Instead of asking whether your metabolism is fast or slow, a better question is:

Is my metabolism flexible?

Can your body comfortably go several hours without eating?

Can it draw on stored fat for energy?

Or does it constantly demand quick sources of sugar?

These questions reveal far more about metabolic health than calorie math alone.

Final Thoughts

Metabolism isn’t a fixed speedometer.

It’s an energy management system designed to switch between fuels depending on availability.

Modern diets and eating patterns often trap the body in a sugar-dependent state, leading to hunger, fatigue, and difficulty losing fat.

By lowering carbohydrate intake, stabilizing insulin levels, and prioritizing nutrient-dense protein and fats, many people can restore their ability to burn fat efficiently.

In other words, the goal isn’t to speed up metabolism.

The goal is to teach it how to burn the right fuel again.

When that happens, the body often does what it was designed to do all along—maintain energy, regulate hunger, and access stored fat when it’s needed most.

References

Stephen Phinney, & Jeff Volek. The Art and Science of Low Carbohydrate Living. Beyond Obesity LLC, 2011.

A foundational text explaining fat adaptation, carbohydrate restriction, and metabolic flexibility.

Benjamin Bikman. Why We Get Sick: The Hidden Epidemic at the Root of Most Chronic Disease—and How to Fight It. BenBella Books, 2020.

Explores how insulin resistance drives many metabolic diseases.

George Cahill. “Fuel Metabolism in Starvation.” Annual Review of Nutrition, 2006.

Classic research describing how the body shifts from glucose to fat and ketone metabolism.

David Ludwig & Cara Ebbeling. “The Carbohydrate–Insulin Model of Obesity.” BMJ, 2018.

Explains the hormonal role of insulin in fat storage and metabolic regulation.

Eric Westman, Stephen Phinney, & Jeff Volek. The New Atkins for a New You. Fireside, 2010.

Clinical insights into how carbohydrate restriction improves metabolic health.

Clinical perspectives and educational materials from practitioners including Ken D. Berry, Shawn Baker, Anne Bosworth, and Robert Kiltz, who emphasize insulin regulation and metabolic flexibility in ketogenic nutrition.