What Happens After 17 Hours of Fasting? A Science-Based Look

After roughly 17 hours without food, most people will have depleted their glycogen stores and started burning fat for fuel. This metabolically demanding state, called ketosis, signals your body to begin accessing its fat stores.

But what exactly happens, both physically and mentally, when you fast for 17 hours?

As a passionate gaming creator focused on tech, gear and the latest releases, I admittedly don’t have a medical background. But as an advocate for healthy gaming lifestyles, I get asked about trends like intermittent fasting a lot!

So in this post, I’ll walk through some of the key changes, based on the current science. My goal is to provide an objective overview so you can make informed decisions about whether longer fasts are right for you.

Overview: Metabolic Changes in the Body

Once liver glycogen is depleted (around 12-16 hours for most people), several metabolic shifts begin:

• Blood sugar and insulin plummet, triggering appetite signals
• Growth hormone is released to preserve muscle
• The body turns to fat reserves and ketone production ramps up

This shift to ketosis provides an alternative energy source for the brain and muscles when carbohydrates are unavailable. Blood ketones can reach higher levels during extended fasting periods.

Fat burning and ketone levels over time fasting (The Conversation)

These changes signal the body to start tapping its fat stores. But the duration likely needs to exceed ~48 hours before meaningful fat loss starts occurring in most people.

17 Hours Fasted: Expected Physical Effects

Here are some of the key physical effects that may be experienced after ~17 hours without food:

• Mild dehydration – Fast water loss can occur early on, increasing thirst
• Fatigue and physical weakness – As glucose drops, strength/endurance may decline
• Halitosis (bad breath) – Ketone bodies can cause a distinct acetone smell
• Constipation – Common due to fluid loss and electrolyte shifts
• Postural hypotension – Some experience low blood pressure on standing
• Difficulty sleeping – Hunger signals and cortisol changes can disrupt sleep

These effects can vary based on the individual – based on factors like activity level, starting weight, metabolism etc. They tend to peak between days 2-3 of fasting in research studies.

The Mental Side: How We Feel After 17 Hours Fasted

The physical effects overlap with some distinct mental changes around this 17 hour mark:

• Increased hunger and food preoccupation – Appetite hormones like ghrelin surge
• Irritability and impaired mood – Cortisol and adrenaline may be elevated
• Lower energy and motivation – Partly from low blood sugar levels
• Mild cognitive impairment – Particularly with complex tasks requiring focus/alertness

However, some find that ketosis provides a mild mental boost and clarity once past the initial transition period. Individual experiences can vary quite a bit.

Key Takeaways: Outlook After 17 Hour Fast

While fasting for 17 hours may provide some unique benefits, longer fasts aren‘t necessarily better. Here are a few key points:

• Most people will have depleted liver glycogen around this stage
• Hunger, weakness and other adverse effects tend to increase past 12-16 hours
• Blood sugar stability, hydration and sleep are crucial during extended fasts
• Muscle loss may occur with fasts exceeding 48 hours if protein intake is inadequate during feeding periods

The upside is that longer fasts may help those looking to burn fat or enter ketosis. But ease into it slowly paying attention to any negative or distress signals. And consult your doctor before attempting prolonged fasts over 24 hours.

The bottom line? Whether just curious or aiming for fat loss, balance and sustainability should be the goals with intermittent fasting – not extreme durations or rigid targets.

Over to you – what fasting durations have you tried? Or do you have any questions on the expected effects of a 17 hour fast? Let me know in the comments!