What Happens To Your Body While Intermittent Fasting

May 07, 2021

What Happens To Your Body While Intermittent Fasting

In this article:

  • Intermittent fasting in a nutshell
  • The stages of fasting
  • Making the most out of your fast
  • Track your fasting

Intermittent Fasting in a Nutshell

The health benefits of fasting make it well worth it - in short term periods, of course. First and foremost, intermittent fasting is not a diet, it's an eating schedule. Instead of dictating what you can't eat, intermittent fasting helps us to regulate when we eat in order to maximize our body's natural ability to burn fat and turnover new cells. Despite our modern eating schedules, our bodies are actually able to go fairly long periods of time without food. Just think back to when we had to rely on hunting and gathering to get our next meal. One missed shot and dinner is canceled.

There are multiple types of fasting schedules. The most popular are:

  • The 16:8: Sixteen hours of fasting followed by an eight-hour eating window
  • The 5:2: Five days of normal eating and two days of calorie restriction
  • Alternate day fasting: One 24 hour day with calorie restriction, followed by a day with normal, healthy, eating patterns

There are many benefits to this simple lifestyle change. Essentially, during a well-fed state your body is in growth mode. Your cells want to grow and divide and so, that's exactly what they'll do. During this stage however, they have no concern for eliminating harmful, cellular waste or recycling themselves through autophagy. Growth is good but without cleanup and repair, it can be too much of a good thing. Intermittent fasting is basically a short-term version of starvation. This physical stress tells our bodies to go into preservation mode and begin autophagy.

Without digging too deeply into the different stages of fasting (we'll do that below), autophagy is essentially what happens when we fast and is also what brings on the health benefits. During a fast, our bodies do not have access to glucose for energy. When insulin levels are low, the body starts to use other sources for energy such as lactate, amino acids, and fat. Meanwhile our bodies are conserving energy, and our metabolic rate becomes more efficient. The benefits of intermittent fasting include:

  • Better regulation of blood sugar levels:During a fast, insulin sensitivity is heightened. This is a useful benefit for those who have type 2 diabetes and those at risk for it.
  • Decreased levels of inflammation
  • Weight loss
  • Improved digestion
  • Better mental clarity and energy
  • Alzheimer’s prevention
  • Increased metabolism
  • Muscle growth
  • Lowered blood pressure
  • Reduced risk of heart disease

The Stages of Fasting

So what constitutes a fast? If I skip lunch and eat a late dinner, am I reaping all the benefits listed above? Achieving a fast isn't a finite goal. There is a certain time threshold that we need to surpass in order to start taking advantage of all its benefits. But remember, more is not always more. To a certain point, prolonged fasts can benefit our bodies, but if we go too long with little or no calorie intake the benefits are reversed. The process of fasting has five different stages, each with its own unique effects on the body.

Stage 1 (8-12 hours): We technically begin fasting about eight hours after our last meal. During this phase, glucose levels drop and your body will begin sending signals to feed it again. You may feel fatigued, have cravings, and hear lots of rumbling coming from your stomach. This is when emotional mastery over hunger really becomes important. If you can make it a bit longer and get over this hurdle, usually after about 12 hours your body enters partial ketosis, the beginning of the fat burning "journey". Around this time, your glycogen stores become depleted and your blood sugar levels stabilize, so the body begins breaking down fat cells for energy rather than carbohydrates.

Stage 2 (13-18 hours): Ketosis and Fat Burning: Once you hit the 16 or 18-hour marker, your body should be in full ketosis and your fat stores will become your body's primary source of energy. At this stage, the liver begins using fat cells to create ketones or ketone bodies and releases them into the bloodstream. These ketone bodies help to fuel your brain and reduce inflammation throughout the body, giving you a little extra kick of energy. They also help to reduce your appetite by reducing ghrelin, the hormone that makes you want to eat a lot of food. What’s more, ketone bodies support insulin level normalization. Not only will your stomach quiet down, but you’ll likely have more mental clarity, alertness, and a better mood due to an increased production of brain-derived neurotrophic factor (BDNF). If fat loss is your goal, stage 2 of fasting should be your goal.

Stage 3: (19-24 hours): Autophagy: When you start approaching the 24 hour mark, you enter a phase of cellular recycling called “autophagy”. Autophagy is like spring cleaning for your cells; they begin "self-eating" and removing whatever damaged, cellular material and proteins may be lying around. This is extremely important for your mental health. When cellular waste builds up, your risk for certain neurodegenerative diseases like Alzheimer’s and cancer increases. Autophagy happens less and less as we age, so triggering it through fasting is a clever anti-aging "hack" you can implement throughout your week.

Stage 4: (36-48 hours) Growth Hormone Rises: If you've made it this far, you can probably drop the "intermittent" off of "intermittent fasting". During this phase, your human growth hormone (HGH) levels begin to rise, providing many different benefits in the short and long run. Growth hormone is extremely important for athletes as it supports muscle growth, injury recovery, strength, and performance. Low levels of HGH have been linked to obesity and chronic diseases. Interestingly, one study found that human growth hormone secretion went up by 400% after a 48 hour fast.

Stage 5: (72+ hours): Immune System Function: After a whopping 3 days without food, your immune function improves drastically. One study found that a 72 hour fast triggered a near complete overhaul of the immune system via immune cell regeneration. This same study found that fasting helped chemotherapy patients to strengthen their immune system during a time when it is extremely vulnerable.

Making the Most Out Of Your Fast

Fasting isn't always easy to get used to. That's why when you fast, you should do these five things to get the most benefit out of every minute that has passed without food!

  1. Stay hydrated: This is the golden rule of fasting. Not only is going without water harmful to your health, but it will make your energy levels plummet and the minutes will drag by. Try these drinks to stay hydrated and shake up your "drink menu" on fasting days.
  2. Rest and relax: Plan your workout schedule around fasting, don't try to do too much at once. You can easily become dehydrated, lightheaded, and exhausted after a fasted workout if you're not used to it. Don't ask too much of your body during fasting days, keep it light. Try yoga and walking to stay active so you won’t crash or need to break your fast.
  3. Eat high-volume food: If you're following an intermittent fasting plan that allows food consumption on fasting days (like the 5:2 method) aim to eat high-volume, low-calorie, nutrient-dense foods, or foods that "take up a lot of space" and help you to feel full. High-volume, low-cal foods include leafy greens, broccoli, melon, apples, oatmeal, air-popped popcorn, brown rice, beans, eggs, and fish.
  4. Increase the taste: Every calorie counts when you're fasting, so enjoy them! Eating healthy yet flavorful foods you love during the eating periods of your fast can help give you the mental push you need to go on. Try eating the foods mentioned above and go heavy on the spices! Healthy, filling food doesn't have to be bland.
  5. Sprinkle in some supplements: Supplements can help ensure you're feeding your body nutritionally without adding in extra calories or throwing off your fast. If you have specific dietary needs, meet with a doctor or registered dietitian before you begin your fast to see what supplements you should add during and after fasting days. Electrolytes including potassium, sodium, magnesium and zinc are extremely important during a fast. Protein supplements can also be taken to help you feel full throughout the day without adding excessive calories.

Track Your Fasting

The best way to track your fasting experience is to use a fasting app on your mobile device. Here, you'll be able to track what you ate, how long you've gone without eating, how much water you drank, and how you’ve felt. Tracking this information helps with your next fast and allows you to keep tabs on your progress. Here are some of our favorites apps:

  1. Window: It has everything you need to get started! With helpful reminders and suggestions, weight tracking tools and more, this is a must-have app for IF newbies.
  2. Fastic: Fastic keeps you on track and alerts you of what's going on with your body all throughout your fast.
  3. Zero: Fasting & Health Tracker: Zero gives you personalized insights into how to maximize your fast and helps you to "troubleshoot" when you're feeling sluggish, or struggling to make it to the finish line.
  4. BodyFast Intermittent Fasting: BodyFast gives you a new personalized plan every week to help you progress and grow stronger and healthier with every fast!
  5. Fastient: Mobile, desktop - it does it all! Fastient lets you access your personal data and compiles it into easy to read graphs that you can view on the go or at your desk.

Fasting can support many different health goals, but that doesn't mean it's right for everyone. Jumping right into a fast unprepared can be dangerous and extremely difficult. It's always best to work your way up from a partial fast (like 16:8) to get to know your body's triggers, understand how it affects your energy levels, and know when it's time to call it quits. For this reason, fasting can be dangerous for people who have struggled with eating disorders.

Want more information on getting started with fasting? Check out the Amandean Blog where our pros weigh in on how to Retain your energy during a fast, the benefits of autophagy, and answers to the highly debated topic "Can supplements break a fast?"

Summary Points

  • During a fast, our bodies do not have access to glucose for energy
  • When insulin levels are low, the body starts to use other sources for energy such as lactate, amino acids, and fat
  • Usually after about 12 hours your body enters partial ketosis, the beginning of the fat burning "journey"
  • Once you hit the 16 or 18-hour marker, your body should be in full ketosis and your fat stores will become your body's primary source of energy
  • When you start approaching the 24 hour mark, you enter a phase of cellular recycling called “autophagy”
  • The best way to track your fasting experience is to use a fasting app on your mobile device




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