The Hour-by-Hour Benefits of Fasting: What Happens When You Fast

Fasting has gained momentum as a health trend, with people adopting various methods, from intermittent fasting to extended fasts lasting days. Whether it’s for weight loss, mental clarity, or overall well-being, fasting offers a range of benefits that unfold hour by hour. In Episode 2 of The Mack & G Show, we dove deep into fasting, sharing our personal journeys and breaking down the hour-by-hour benefits. Spoiler: even though we may not seem like the types to fast, we’ve both done it several times because it genuinely helps our bodies. In this blog we’ll walk through what happens to your body as you fast, starting from the moment you skip that first meal.

A Quick Disclaimer

Before diving into the details, it’s important to note: We’re not doctors. The experiences we share here are based on our personal journeys with fasting, and while it’s been beneficial for us, fasting isn’t one-size-fits-all. We highly recommend consulting with your doctor before starting any fasting regimen or diet to ensure it’s safe and appropriate for you.

Who Should Avoid Fasting?

While fasting offers numerous benefits, it’s not suitable for everyone. The following individuals should abstain from fasting:

• Children under 18: Growing bodies require consistent nutrition for proper development.
• Pregnant or breastfeeding women: Nutrient intake is critical for both mother and baby during these periods.
• Anyone who is malnourished or underweight: Fasting can worsen existing nutritional deficiencies.

The following individuals should consult a doctor before considering fasting:

• People with diabetes: Fasting can cause blood sugar levels to become dangerously high or low.
• People on medications: Certain medications may require food for proper absorption or to avoid side effects.
• People with gout: Fasting can increase uric acid levels, potentially triggering a gout flare-up.
• People with GERD: Fasting may worsen acid reflux symptoms for some individuals.

Note: Even if you’re not on this list, some people can still have adverse reactions to fasting. It’s always a good idea to listen to your body and consult a healthcare provider if you’re unsure.

0-12 Hours: Transition to Fat Burning

In the first 12 hours of fasting, your body begins to switch from using glucose (sugar) for energy to burning fat. This process, known as ketosis, can begin as early as 12 hours into your fast, depending on your body and your previous meals. During this phase, insulin levels drop, allowing stored fat to be released and used as fuel.

Many people start fasting using intermittent fasting methods, like the popular 16:8 ratio, which involves fasting for 16 hours and eating within an 8-hour window. In the early hours, your body is still using glucose stores, but it’s already beginning to adjust to fasting mode.

12-16 Hours: Transition to Fat Burning

After 12-16 hours of fasting, your body starts to run low on glycogen (the storage form of glucose). At this point, the switch to fat-burning intensifies, and the liver begins producing ketones, molecules derived from fat breakdown, to serve as energy for the brain and body. For those new to fasting, this is the point where the benefits become more noticeable: improved mental clarity, reduced hunger, and even mood stabilization.

16-24 Hours: Cellular Repair and Autophagy

As you cross the 16-hour mark and head towards 24 hours, autophagy kicks in. Autophagy is the body’s way of cleaning out damaged cells and regenerating new, healthy ones. It’s one of the key mechanisms behind fasting’s potential to promote longevity and reduce the risk of diseases like cancer.

This is when we noticed the shift in how we felt during extended fasts. While hunger can come and go, the focus and mental sharpness were undeniable. Our bodies had transitioned fully into fat-burning mode, and our cells were working on repairs. It’s like giving your body a reset button.

24-36 Hours: Heightened Autophagy and Growth Hormone Surge

By 24 hours, your body is experiencing heightened levels of autophagy, and growth hormone production surges. Human Growth Hormone (HGH) helps preserve muscle mass while promoting fat loss, making fasting an attractive option for those trying to maintain strength while shedding fat. At this stage, we felt the most significant physical benefits: energy levels soared, and any feelings of hunger had subsided.

Growth hormone increases also play a key role in muscle recovery, which is important for people like us, who are just regular guys trying to maintain a healthy lifestyle. We may not be fitness gurus, but these benefits certainly made us realize how powerful fasting can be.

36-48 Hours: Deepened Autophagy and Fat Adaptation

During this phase, your body becomes more efficient at using fat for energy—a process known as “fat adaptation.” By now, the body has switched almost entirely to burning fat, and the prolonged autophagy enhances the repair of cells and tissues. Many fasting experts, such as Dr. Jason Fung, advocate for fasts of 36 hours or more to fully experience these benefits.

For me (Johnny Mack) this is where I usually hit a bit of a rough patch. Around the 48-hour mark, I often experience a bad headache, which can be tough to push through. However, when I wake up the next day, I feel great—clear-headed, energized, and surprisingly not hungry. It’s important to note, as we did in our podcast, that everyone experiences fasting differently. Some may feel discomfort at this point, like me with headaches, while others may breeze through these hours with little trouble.

48-72 Hours: Insulin Sensitivity and Increased Fat Loss

By the time you reach 48 hours of fasting, your insulin sensitivity is greatly improved. Insulin sensitivity refers to how effectively your cells respond to insulin, which helps regulate blood sugar. Improved sensitivity can reduce your risk of developing Type 2 diabetes.

During this time, the body continues to burn fat at an accelerated rate. Both Ricky and I have done multiple 72-hour fasts, and each time, the results are noticeable—not just in terms of weight loss, but also in mental sharpness and overall well-being.

Beyond 72 Hours: Stem Cell Regeneration

Once you cross the 72-hour mark, the benefits become even more profound. Extended fasting beyond 72 hours can promote stem cell regeneration, which may help repair and regenerate tissues. This phase is often the realm of therapeutic fasting, used to support disease prevention and longevity.

We didn’t always push beyond 72 hours, but when we did, the effects were truly impressive. It’s like giving your body the ultimate reset. However, we always remind our listeners to consult a doctor before embarking on any extended fasts, as they can be physically and mentally demanding.

When Should You Break Your Fast?

While fasting can bring amazing benefits, knowing when to stop is just as important. Your body will give you signs if it’s time to end the fast for health reasons. Here are some red flags to look out for:

• Dizziness or fainting: Mild dizziness is common in early fasting stages, but if you experience extreme dizziness or fainting, it’s a sign that you should break your fast and eat something to stabilize your body.
• Severe fatigue or confusion: Feeling sluggish is normal in the beginning, but if you’re struggling with severe fatigue, brain fog, or confusion, your body may be running low on essential nutrients or electrolytes, and it’s time to break the fast.
• Heart palpitations: If your heart starts racing or skipping beats, it could be due to electrolyte imbalances, which can be dangerous if ignored during a fast. Break your fast and replenish electrolytes.
• Vomiting or nausea: If you start feeling nauseous or vomiting, it could indicate your body isn’t handling the fast well. This is a clear signal to stop fasting and seek medical attention if needed.
• Cold hands and feet: While feeling colder during a fast is normal due to lower metabolic rates, extremely cold hands or feet can indicate poor circulation, which may signal it’s time to eat.
• Intense headaches: Headaches are a common fasting side effect, often caused by dehydration or electrolyte imbalance. If drinking water and taking electrolytes doesn’t help, it’s a sign you should stop fasting.

Always listen to your body. Fasting should never feel like you’re pushing your body into a dangerous state. If any of these symptoms become severe or persistent, break your fast safely with easily digestible foods like bone broth, smoothies, or light meals, and consult a healthcare professional.

Conclusion: Why We Fast—and Why You Should Consider It Too

Whether you’re curious about fasting for health, weight loss, or mental clarity, the benefits of fasting are too compelling to ignore. From hour one to beyond 72 hours, your body goes through an incredible transformation that promotes fat loss, cell repair, and even longevity. While fasting may not be for everyone, our experiences have shown us that it can be a powerful tool for improving health.

Check out Episode 2 of The Mack & G Show for more in-depth discussion on fasting, where we dive into different fasting methods, share our personal journeys, and break down the hour-by-hour benefits. Have you tried fasting? We’d love to hear about your experiences!

Books and Products to Help With Your Fast

Whether you’re new to fasting or looking to take your practice to the next level, there are plenty of resources and tools that can help make your fasting journey smoother and more effective. Here are some recommended books and products to support you along the way:

Books

Products

Please note: We might earn a small commission if you purchase any of these books or products through the links provided. It won’t cost you anything extra, but it helps support our show and keeps us going—so thank you for your support!