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Let’s Talk Fasting

When I bring up the idea of fasting to my patients, some of them look at me like I’m crazy.

“Fast? Like for a few hours? Or skip a meal or two?”

For some people, the idea of fasting feels almost barbaric—mainly because they’ve never skipped a meal in their entire life. I tend to get the biggest reaction when I mention a 3-day fast… or anything longer.

Fasting—especially in America—can feel like a completely foreign concept. Our society is built around food. Three meals per day is the norm: breakfast shortly after waking, lunch around noon, and dinner somewhere between 5–8 p.m. (or later depending on work and social life). And in between those meals? Snacks.

Walk into Costco or any supermarket and you’ll find aisles upon aisles of processed snacks—chips, cookies, crackers, trail mixes loaded with sugar, and more. At places like Costco Wholesale, you can even buy these in bulk, making constant eating even more convenient.

Now, don’t get me wrong—I’m not saying snacking is inherently bad, and I’m not saying three meals a day is wrong. The point is this: many of our lives revolve around food. We structure our schedules, our energy, and even our mood around when we eat.

So let’s step back and ask a simple question:

What is food, actually?

From a physiological and biological standpoint, food is any substance consumed by an organism to provide energy, support growth, and maintain life. It supplies macronutrients (carbohydrates, fats, proteins) and micronutrients (vitamins and minerals) that fuel cellular processes.

In other words, food is fuel and building material.

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What Happens When We Don’t Eat?

Without food for an extended period, the body doesn’t just “shut down.” It adapts.

Your body begins shifting through different energy systems:

  • 0–4 hours after eating:

    You’re in a “fed state.” Your body is using glucose from your recent meal for energy.

  • 4–16 hours:

    Insulin levels drop. The body begins using stored glycogen (carbohydrates stored in the liver and muscles).

  • 16–24 hours:

    Glycogen stores start running low. The body begins increasing fat breakdown.

  • 24–48 hours:

    Fat becomes a primary fuel source. The liver starts producing ketones.

  • 48–72 hours:

    Ketone production increases significantly. The brain begins relying more on ketones for energy.

Despite what many people think, your body doesn’t immediately start “eating muscle.” It’s actually designed to preserve muscle for as long as possible by shifting toward fat utilization.

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So… Why Fast?

Fasting is, in many ways, a metabolic reset.

It gives your digestive system a break and forces your body to become more metabolically flexible—meaning it can efficiently switch between using glucose and fat for energy.

Once glycogen stores are depleted and the body enters ketosis, some interesting things happen:

  • Ketones become a primary energy source
  • Many people report improved mental clarity and focus
  • Blood sugar and insulin levels stabilize
  • Inflammation markers may decrease

Some research suggests ketones may provide a more efficient or “cleaner” fuel for the brain under certain conditions, particularly during periods of low glucose availability.

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Autophagy: The Real Reset

Around the 24–48 hour mark (varies by individual), the body begins increasing a process called autophagy.

Autophagy literally means “self-eating,” but that’s a bit misleading. It’s better understood as cellular cleanup and recycling.

During autophagy, the body:

  • Breaks down damaged or dysfunctional cellular components
  • Removes waste and misfolded proteins
  • Recycles materials to build healthier cells

Think of it like a cellular “spring cleaning.”

This process is regulated in part by pathways like mTOR signaling pathway, which is suppressed during fasting and allows repair mechanisms to ramp up.

Potential benefits of autophagy include:

  • Improved cellular efficiency
  • Reduced accumulation of damaged proteins
  • Support for metabolic health
  • Possible roles in longevity and disease prevention

There is also growing research exploring the relationship between autophagy and conditions like cancer. Some studies suggest autophagy may help the body remove damaged cells before they become problematic, though this relationship is complex and not fully understood. It’s not accurate to say fasting “cures” cancer—but it may play a supportive role in overall cellular health.

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Who Should NOT Fast (or Should Be Careful)?

Fasting is not for everyone.

Some groups need to be cautious or avoid it altogether:

1. Women of Childbearing Age

Female physiology is more sensitive to energy availability due to hormonal balance.

  • During the luteal phase (second half of the menstrual cycle), the body has higher energy demands. Fasting during this time may increase stress hormones and disrupt hormonal balance.
  • During the follicular phase (first half of the cycle), shorter fasts (12–16 hours) are generally better tolerated.

Longer fasts (24+ hours) should be approached cautiously and not done frequently without proper guidance.

2. Pregnant or Breastfeeding Women

Energy and nutrient demands are higher—fasting is not appropriate.

3. Individuals with a History of Eating Disorders

Fasting can trigger unhealthy patterns.

4. People with Certain Medical Conditions

This includes:

  • Type 1 Diabetes
  • Advanced Type 2 Diabetes (especially if medicated)
  • Chronic kidney disease
  • Underweight individuals

5. Highly Active Individuals or Those Under High Stress

Fasting can add additional stress if recovery, sleep, and nutrition aren’t dialed in.

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Final Thought

Fasting isn’t new. It’s been part of the human experience since the beginning.

Before modern food systems, humans regularly went periods without eating—hunting, gathering, and moving when resources ran out. Even historically and spiritually, fasting has played a role in discipline, clarity, and reflection.

We live in a time of constant access. Food is always available. And while that’s a blessing, it also means we’ve lost touch with the body’s natural ability to function without it.

Fasting, when used appropriately, isn’t about deprivation.

It’s about giving the body space to do what it was designed to do.