The Complete Guide to Fasting

The Complete Guide to Fasting: Health Benefits, Stages & How It Works

Last Updated: October 2025 | By Brooke Rieth NP & Dr. Tremblay

Hello to our awesome readers at Action Direct Care! We’re passionate about helping you achieve optimal health through evidence-based approaches. Today, we’re diving deep into fasting—a powerful metabolic tool that’s been part of human biology since our earliest ancestors.

This comprehensive guide covers everything you need to know about fasting, from the science behind it to the specific stages your body goes through. Whether you’re interested in intermittent fasting, extended fasting, or simply understanding how your metabolism works, this article will provide the clarity you need.

Table of Contents

• What Is Fasting and Why Does It Matter?
• The Science: How Fasting Works in Your Body
• The 5 Stages of Fasting Explained
• Health Benefits of Fasting
• Who Should NOT Fast
• Getting Started with Fasting
• Frequently Asked Questions

What Is Fasting and Why Does It Matter?

Fasting has been woven into human DNA since our tree-dwelling existence. It’s not a modern diet trend—it’s a fundamental metabolic state that our bodies are designed to enter regularly.

Your body operates in two primary metabolic states:

• Fed State (High Insulin): Storing food energy
• Fasted State (Low Insulin): Burning stored energy

When eating and fasting are balanced, weight remains stable. However, if you spend most of your time in the fed state (frequent meals and snacking), weight gain occurs. Fasting helps restore this metabolic balance by increasing the time your body spends burning stored energy.

How Fasting Works: The Science Behind Metabolic Switching

Dr. Jason Fung explains this elegantly in his book, The Complete Guide to Fasting. The process is remarkably straightforward:

During Eating:

• Blood sugar rises as you absorb nutrients
• Insulin levels increase to move glucose into cells
• Excess glucose converts to glycogen (stored in the liver) or fat

During Fasting:

• Insulin levels drop significantly
• Your body switches to burning stored energy
• Glycogen depletes (usually within 24 hours)
• Fat breakdown begins for sustained energy

This metabolic flexibility—the ability to switch between burning glucose and burning fat—is key to understanding fasting’s powerful health effects.

The 5 Stages of Fasting: What Happens to Your Body

Research by George Cahill mapped the precise physiological changes that occur during fasting. Understanding these stages helps you know what to expect:

Stage 1: The Feeding State (0-6 Hours)

Blood sugar and insulin levels rise as your body processes incoming food. Your cells use glucose for immediate energy, while excess gets stored as glycogen in the liver or converted to fat for long-term storage.

Stage 2: The Postabsorptive Phase (6-24 Hours After Fasting Begins)

Blood sugar and insulin levels begin declining. Your liver starts breaking down glycogen stores to release glucose and maintain energy levels. Glycogen reserves typically last 24 to 36 hours.

This is when intermittent fasting (16:8 or 18:6) operates.

Stage 3: Gluconeogenesis (24 Hours to 2 Days)

Glycogen stores are depleted. Your liver begins manufacturing new glucose from amino acids through a process called gluconeogenesis (literally “making new glucose”). In non-diabetic individuals, blood glucose levels fall but remain within the normal range.

Stage 4: Ketosis (2-3 Days After Fasting Begins)

This is where metabolic magic happens. Low insulin levels trigger lipolysis—the breakdown of body fat for energy.

Here’s what occurs:

• Triglycerides (stored fat) break down into glycerol and fatty acids
• Glycerol supports continued glucose production
• Fatty acids fuel most body tissues
• The liver produces ketone bodies for brain energy

Ketones cross the blood-brain barrier, providing an efficient fuel source. After four days of fasting, approximately 75% of brain energy comes from ketones. The two major ketones—beta-hydroxybutyrate and acetoacetate—can increase over 70-fold during fasting.

Stage 5: The Protein Conservation Phase (5+ Days)

Your body becomes remarkably efficient:

• Growth hormone levels surge to maintain muscle mass
• Energy comes almost entirely from fatty acids and ketones
• Increased norepinephrine (adrenaline) prevents metabolic slowdown
• Protein turnover continues normally but isn’t used for energy

Important: Fat is your body’s stored food energy. During fasting, your body burns fat stores—not muscle—until fat reserves are completely depleted.

Health Benefits of Fasting: Why It Works

Reduces Insulin Resistance

Fasting dramatically lowers insulin levels, which reduces insulin resistance over time. This is crucial because insulin resistance is linked to numerous chronic conditions.

Conditions Linked to Insulin Resistance:

• Heart disease and stroke
• Type 2 diabetes
• Alzheimer’s disease and cognitive decline
• High cholesterol
• Hypertension (high blood pressure)
• Abdominal obesity
• Fatty liver disease (NAFLD)
• Polycystic ovarian syndrome (PCOS)
• Gout
• Atherosclerosis (hardening of arteries)
• Gastroesophageal reflux disease (GERD)
• Obstructive sleep apnea
• Certain cancers

Metabolic Health Improvements

By cycling between fed and fasted states, you improve metabolic flexibility—your body’s ability to efficiently switch between fuel sources. This flexibility is a hallmark of metabolic health.

Weight Management

Fasting creates a natural caloric deficit while preserving muscle mass (thanks to growth hormone). Unlike traditional calorie restriction, fasting doesn’t slow your metabolic rate.

Cellular Cleanup (Autophagy)

Extended fasting triggers autophagy—your body’s cellular recycling process that removes damaged components and promotes cellular renewal.

Who Should NOT Fast: Important Contraindications

While fasting is safe for most people, certain individuals should avoid it:

Do NOT Fast If You:

• Are malnourished or underweight (BMI < 18.5)
• Have a history of eating disorders
• Are pregnant or breastfeeding
• Are a child or teenager (still growing)
• Have Type 1 diabetes (without medical supervision)

Consult Your Healthcare Provider Before Fasting If You:

• Take medications (especially diabetes or blood pressure medications)
• Have a history of gout
• Have any chronic health conditions

Periods of low food availability have always been part of human history. We’ve evolved sophisticated mechanisms to adapt to fasting—otherwise, we wouldn’t have survived as a species. However, individual health circumstances matter.

Getting Started with Fasting at Action Direct Care

Interested in incorporating fasting into your health routine? Here’s how to begin:

Start Simple

• 16:8 Method: Fast for 16 hours, eat within an 8-hour window
• 12:12 Method: Start with a 12-hour overnight fast
• 24-Hour Fast: Once or twice weekly

Listen to Your Body

Pay attention to hunger signals, energy levels, and how you feel. Fasting should feel manageable, not torturous.

Stay Hydrated

Water, black coffee, and unsweetened tea are your friends during fasting periods.

Break Your Fast Mindfully

Start with easily digestible foods—bone broth, vegetables, or lean protein.

Frequently Asked Questions About Fasting

Next Steps: Personalized Fasting Guidance

Fasting should be part of everyone’s health routine in some form. The approach that works best depends on your individual health status, goals, and lifestyle.

Want personalized guidance? Our team at Action Direct Care can help you:

• Determine which fasting protocol suits your needs
• Monitor your health markers as you begin fasting
• Adjust any medications that might be affected
• Troubleshoot challenges and optimize results

Additional Resources

For more comprehensive information, we recommend:

• The Complete Guide to Fasting by Dr. Jason Fung
• Consultation with our healthcare providers
• Schedule an appointment to discuss fasting

The Bottom Line on Fasting

Fasting is a powerful metabolic tool backed by evolutionary biology and modern science. By understanding the stages your body goes through and the profound health benefits available, you can make informed decisions about incorporating fasting into your wellness routine.

Remember: fasting isn’t about deprivation—it’s about working with your body’s natural metabolic states to optimize health, reduce insulin resistance, and potentially prevent chronic disease.

Ready to explore fasting? Contact Action Direct Care today to schedule a consultation with our experienced providers.

Written by Brooke Rieth, NP and Dr. Tremblay
Action Direct Care – Your Partners in Optimal Health
Braintree, MA

Related Articles:

• Understanding Insulin Resistance
• Metabolic Health: Why It Matters
• Weight Management Strategies That Work
• Nutritional Guidance for Optimal Health

Contact Us: Schedule your consultation at actiondirectcare.com

Medical Disclaimer: This information is for educational purposes only and should not replace professional medical advice. Always consult with your healthcare provider before beginning any fasting regimen, especially if you have existing health conditions or take medications.