Fasting Fundamentals – The Science Behind Intermittent Fasting and Weight Loss

intermittent fasting

Intermittent fasting has exploded in popularity over the last decade as a dietary strategy for weight loss, disease prevention, and overall health. But is intermittent fasting just another fad diet or is there real scientific evidence to support its growing hype? In this blog post, we’ll dig into the biological mechanisms and research that underlies how and why intermittent fasting can be an effective tool for weight loss and boosting metabolism.

What Is Intermittent Fasting

A Brief History of Fasting

While intermittent fasting may seem like a modern diet trend, fasting practices have been used for health purposes for thousands of years. Fasting for religious and spiritual reasons has been a part of human culture since ancient times. Greek philosopher Hippocrates recommended fasting for healing and believed it slowed the aging process. Ancient Greek doctors prescribed fasts as treatment for various maladies.

In more recent centuries, fasting clinics and sanitariums gained popularity in the 19th and early 20th centuries. Patients checked into these facilities to undertake medically-supervised fasting regimens, claiming benefits for diabetes, epilepsy, high blood pressure, and obesity. With the rise of modern pharmaceuticals, these therapeutic fasting centers declined but have seen resurgence with the recent intermittent fasting movement.

Intermittent fasting began drawing mainstream scientific attention in the 1980s and 1990s, spearheaded by researchers examining the physiological effects of fasting in animals. Over the last 20 years, research on fasting has accelerated rapidly with numerous human clinical studies demonstrating powerful health benefits, including for weight loss.

So while fasting may be on-trend right now, it actually has centuries’ worth of tradition and therapeutic use behind it. Today’s intermittent fasting methods leverage old ancient wisdom with new scientific breakthroughs to unlock optimal health.

Varieties of Intermittent Fasting

Intermittent fasting is a broad term encompassing various meal timing schedules that cycle between fasting and eating. It’s contrasted with the normal pattern of eating three meals per day plus snacks. There are several specific intermittent fasting protocols:

  • 16:8 method – This popular approach involves fasting for 16 hours per day and consolidating eating into an 8-hour window, like 11am to 7pm. It’s relatively simple to follow and allows flexibility in aligning the fasting/eating windows to your schedule and needs.
  • Eat Stop Eat – This plan from fitness expert Brad Pilon entails a 24-hour fast once or twice per week, eating normally on the other days. It’s essentially skipping 1-2 meals every week.
  • 5:2 diet – Also called “Fast Diet”, this technique lets you eat normally for 5 days while restricting calories to 500-600 for 2 non-consecutive fasting days per week.
  • Alternate day fasting – As the name implies, alternate day fasting consists of eating regularly one day alternating with a fast day of no food or a single small meal (500 calories or less).
  • Periodic fasting – This involves fasting for extended periods of 2-5+ days, done a few times per year. It mimics fasting done for religious, spiritual or cleansing purposes.

The common thread among intermittent fasting strategies is the cycling of extended fasting periods with periods of normal eating. This contrasts the typical Western pattern of eating throughout the waking day.


Metabolic Switch from Glucose to Ketones

So what happens in your body when you stop eating for many hours straight? Quite a lot, actually. On a cellular level, fasting kickstarts a dramatic metabolic switch from burning glucose to burning fat as your primary fuel source.

When you eat food, particularly carbohydrates, your body breaks this down into glucose molecules. Glucose circulates in your bloodstream to provide energizing fuel for cells throughout the body and brain. Any excess glucose gets stored in the liver and muscles as a molecule called glycogen.

During fasting, as glucose reserves get used up, the body starts tapping into glycogen for energy. Glycogen binds to water molecules, so as glycogen stores decline, you begin releasing bound water weight.

Once glycogen starts running out, which happens 12-36 hours into a fast for most people, the body makes another crucial metabolic shift. This is when your metabolism switches its main fuel source from glucose derived from food to ketones derived from your fat stores. The ketones are produced in the liver from free fatty acids released from adipose tissue. Your brain can use these circulating ketones as an alternative fuel source in the absence of glucose.

This transition to “ketosis” is the secret sauce that enables your body to tap into fat storage. Without a constant supply of food-derived glucose, your body adapts by burning fat for fuel. Metabolically, fasting primes your body to readily jettison stored fat to keep energy levels high. This is why intermittent fasting can be such a powerful weight loss tool.

Hormonal Impact on Weight Loss

In addition to facilitating the metabolic switch to fat burning, fasting triggers a cascade of hormonal changes that further enable weight loss and improve health. Key hormones impacted by fasting include:

  • Insulin – The anabolic hormone insulin is dramatically reduced during fasting periods. Lower insulin levels enable your body to access stored fat for energy rather than locking it away. Improved insulin sensitivity can continue even after resuming normal eating.
  • Growth hormone – Levels of growth hormone skyrocket during fasting, increasing up to 5-fold. Growth hormone powerfully mobilizes fat from storage and facilitates its breakdown into free fatty acids for energy.
  • Norepinephrine – Often called the “stress hormone”, norepinephrine keeps your metabolism revved and energy levels high during periods of fasting to facilitate fat burning.
  • Cortisol – This catabolic hormone also rises during fasting and promotes the breakdown of stored fat to be burned for fuel. However, chronic elevated cortisol can be detrimental, so intermittent fasting helps strike a balance.
  • Ghrelin – Known as the “hunger hormone”, ghrelin increases during fasting. However, this effect normally lessens after the first day. Ghrelin also boosts growth hormone.
  • Leptin – Leptin suppresses appetite by signaling satiety. Levels drop during a fast, which sparks hunger initially. But leptin surges back after refeeding, promoting satisfaction.

In summary, fasting induces hormonal changes designed to provide energy from stored body fat when food isn’t coming in. Your body ramps up fat burning when fasting both through the shift to ketosis and these amplification effects from fasting-induced hormones.

fasting clock for weight loss

Autophagy – The Cellular Rejuvenation Process

Fasting triggers a truly remarkable process in cells throughout your body called autophagy. Autophagy is like the cellular waste management and recycling system, removing damaged cell components and turning them into energy.

Why does this matter? Autophagy is your body’s natural mechanism for replacing dysfunctional or aged cells with new healthy cells. This keeps cells and tissues functioning optimally. As a housekeeping process, autophagy occurs at low levels at all times. But fasting significantly ramps up autophagic activity.

Emerging research suggests enhanced autophagy provides anti-aging effects in the body. It reduces oxidative stress on tissues that drives disease progression and aging. By spurring autophagy, fasting allows your body to accelerate this cellular “cleanup” process.

Like the metabolic shift to fat burning, ramped up autophagy may also promote weight loss. Damaged cells build up toxic byproducts that can impede proper organ function. Clearing out this cellular “garbage” improves organ function, enabling more efficient fat metabolism.

Autophagy also clears out misfolded proteins that clump together in neurons and contribute to neurodegenerative disorders. Enhancing autophagy through activities like fasting may help prevent diseases like Alzheimer’s and Parkinson’s.

Benefits Beyond Weight Loss

The profound metabolic, hormonal and cellular effects induced by fasting deliver powerful benefits extending far beyond simply weight loss and calorie restriction. Here is a high level summary of the plethora of researched health benefits:

Brain Health

  • Enhanced cognitive function
  • Reduced risk of Alzheimer’s and Parkinson’s
  • Anti-depressant effects
  • Protection against strokes and ischemia

Heart Health

  • Decreased blood pressure
  • Improved cholesterol profile
  • Reduced inflammation
  • Lower cardiovascular disease risk

Diabetes Prevention

  • Increased insulin sensitivity
  • Decreased blood sugar levels
  • Reversal of Type 2 diabetes

Cancer Prevention

  • Slowed tumor growth
  • Blocking cancer cell glucose intake
  • Increased resistance to oxidative stress


  • Increased lifespan
  • Gene expression changes similar to calorie restriction
  • Activation of sirtuin anti-aging proteins

Gut Health

  • Increased bowel motility
  • Promoted growth of beneficial gut bacteria
  • Reduced inflammation and intestinal permeability

The depth and breadth of benefits from intermittent fasting highlight that it is far more than just a weight loss tool. Fasting triggers a biological cleansing process with whole-body effects encouraging optimal health.

Getting Started with Intermittent Fasting

Intermittent fasting is safe, simple and sustainable for most people when approached gradually. Here are some tips for getting started:

  • Start conservatively, like 12-14 hours fasting, and progress slowly to allow adaptation.
  • Consume water, unsweetened teas and black coffee during fasting periods to support satiety and hydration.
  • Focus your meal timing on when it best fits your schedule and not forcing yourself to eat when not hungry.
  • Rest well during fasting periods and don’t undertake overly strenuous workouts when getting started.
  • Adhere to a nutritious whole food diet during eating periods. Fasting complements healthy eating.
  • Listen to your body. Hunger comes in waves and will stabilize as your body adapts. Avoid torturing yourself.
  • Track fasting periods and progress to stay motivated, but don’t become obsessed with apps and timers.

Be patient through initial adaptation difficulties like hunger, weakness or irritability. These typically resolve within 1-2 weeks of regular intermittent fasting. Pay attention to any concerning symptoms and talk with your doctor, especially if pregnant, breastfeeding, diabetic, etc.

fit woman result of weight loss benefits of intermittent fasting

Putting it All Together

When you consider the multifactorial biological effects – ketosis, fat burning hormones, autophagy, anti-aging pathways, cellular cleansing, etc. – it makes evolutionary sense that intermittent fasting provides such profound health benefits. Our ancestors evolved surviving routine cycles of famine and feasting. Fasting flips the metabolic switch back to our factory setting.

While research continues to reveal new mechanisms and impact on disease pathways, the proven weight loss and health benefits speak for themselves. If you are looking to lose weight sustainably and improve metabolic health, intermittent fasting provides a scientifically-validated approach with minimal downsides when applied judiciously.

Implementing sensible intermittent fasting enables you to work with your body’s natural functioning rather than constantly fighting against hunger and cravings. Fasting offers both an ancestral secret and modern biohack for taking health into your own hands.


We hope this deep dive on the science of intermittent fasting provided helpful insights as you consider different dietary strategies and tools. Thank you for reading! Don’t forget to grab our free nutrition guide with fasting tips and delicious recipes to level up your health. Here’s to feeling and doing your absolute best!

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