Humans have been practicing intermittent fasting for millennia. Our ancestors didn’t put a label on it, but they were known to miss breakfast, lunch, and dinner from time to time.
Even today, many people practice intermittent fasting without knowing it. If you’ve ever gone twelve hours between dinner and breakfast, congratulations: You’re in the club.
What Is Intermittent Fasting?
Intermittent fasting (IF) is the practice of taking regular breaks from calories. Depending on the fasting protocol, these breaks generally span from 12 to 36 hours. Another term for IF is Time-Restricted Eating (TRE), which has been shown to reduce body weight, improve glucose tolerance, reverse fatty liver, lower cholesterol, and decrease blood pressure in studies.[*]
You can take these caloric breaks on a daily or weekly basis. Maybe you fast for 16 hours every day (the 16/8 protocol), or maybe you fast for 24 hours every other day (alternate day fasting). We’ll explore these regimens in more detail soon.
A “break” from calories doesn’t necessarily mean a 100% break from calories. While the daily regimens usually entail zero-calorie fasts, the weekly regimens often allow limited calories (up to 25%) on fasting days. These limited-calorie fasts still have health benefits, and they’re easier for many folks to stick with.[*][*]
Intermittent fasting works by keeping the hormone insulin low, which in turn promotes the burning of body fat and the production of ketones. This metabolic state (called ketosis) has many potential health benefits that we’ll review momentarily.
From an evolutionary perspective, the ability to burn body fat was essential. It kept our ancestors powered up while they searched for food and shelter.
Benefits of Intermittent Fasting
Why practice intermittent fasting? Consider these potential benefits.
1. Weight Loss
Mostly: people tend to eat less food on IF protocols. It’s a natural consequence of compressing the feeding window.
To be clear, people needn’t intentionally restrict calories for this to happen. It happens naturally.
When people were forced to eat the same number of calories on IF vs. 3-meals-per day, the weight loss benefit vanished.[*] But the IF folks still lost comparatively more fat, probably due to the metabolic benefits of ketosis and improvements in insulin resistance.
2. Cognitive Function
Animal studies suggest that fasting leads to neurogenesis (the generation of new brain cells) and an increase in Brain-Derived Neurotrophic Factor (BDNF). BDNF, by the way, is critical for memory formation.[*]
3. Reduced Inflammation
Many scientists believe chronic inflammation (a low-grade unnecessary immune response) underlies most chronic diseases. The good news is that intermittent fasting shows promise for reducing inflammatory biomarkers like CRP, which means improvements in common inflammatory ailments like fatigue and joint pain.[*]
Fasting activates a cellular recycling program called autophagy. During autophagy, your cells clean out old, damaged parts and replace them with new ones.
Autophagy has been linked to longevity in animals, but we still don’t know the optimal protocol for humans.[*] It’s notoriously hard to measure, but adopting an 8-hour eating window is probably a good start.
5. Heart health
Heart disease is the number one killer globally. Relevant here: fasting may improve markers of cardiovascular health.
In one study, eight weeks of alternate-day fasting decreased total cholesterol, LDL cholesterol, and triglycerides in obese men and women.[*] Other research has found similar results.[*] Intermittent fasting may also raise sub-optimal HDL (good cholesterol) levels, which is key for protection against heart attacks.[*]
Fasting shows promise for fighting cancer. Consider the following:
- By keeping blood glucose low, fasting may starve cancer cells of their favorite fuel: glucose.[*]
- Fasting keeps insulin low, and high insulin levels (i.e. insulin resistance) are linked to higher rates of cancer.[*]
- A 24-hour fast reduced the toxic side effects of chemotherapy in women with HER-2 negative breast cancer.[*]
Stay tuned. More research on this front is coming.
7. Insulin function
Insulin resistance describes the inability of the hormone insulin to regulate blood glucose levels. It’s at the heart of type 2 diabetes.
Fasting may help. A 2014 study showed that intermittent fasting reduced visceral fat mass, fasting insulin, and insulin resistance in people with type 2 diabetes[*]
8. Sleep enhancement
By fasting overnight, you send sleep signals to your body.[*] This helps promote the release of your sleep hormone, melatonin, among other benefits. An overnight fast also minimizes snacking opportunities. If weight loss is your goal, that’s a good thing.
Different Fasting Schedules
Let’s go through the main IF protocols now. Then you can decide which regimen makes the most sense for you.
12/12 is a simple overnight fast and a good starting point for beginners. Just stop eating after dinner and resume eating the next morning. For example, if your last bite occurs at 8 PM, your next bite will occur at 8 AM the next morning.
16:8 (Lean Gains)
The Lean Gains protocol entails a 16-hour fast followed by an 8-hour feed. The simplest way to practice? Just skip breakfast.
For example, you might skip breakfast, eat a big meal at noon, a snack at 4 PM, and conclude dinner at 8 PM. Rinse and repeat.
People like OMAD (one meal a day) because it saves time, money, and may accelerate fat loss goals. But since it requires nearly 24 hours of fasting, it’s not ideal for beginners.
To practice, simply consume all your daily calories in one meal. (No need for conscious restriction). You might, for instance, consume your one daily meal at 6 PM so you can join family or friends for dinner.
The Warrior Diet is a form of OMAD that extends your “one meal” feeding window to four hours. So four hours of feeding, twenty hours of fasting. This is also sometimes stylized as a 20:4 IF diet.
5:2 is a weekly fasting protocol that entails five days of normal eating and two fasting days per week. (Fasting days are 0-25% of normal calories). It’s been shown to be effective for weight loss, among other metabolic benefits.[*]
To practice 5:2, select two non-consecutive days per week (say, Tuesday and Friday) to fast. The other five days, eat normally.
Alternate-day fasting (ADF) is the least beginner-friendly protocol on this list. People typically practice ADF for significant weight loss, to manage a condition like type 2 diabetes, or (less commonly) to promote autophagy.
To practice ADF, simply consume 0-25% of your normal calories every other day. So Monday you eat 500 calories, Tuesday you eat normally, Wednesday you eat 500 calories, and so on. The timing of the 500 calories isn’t especially important, but try to eat them while the sun is shining for circadian rhythm enhancement.
Who Should Avoid Fasting?
The following groups should avoid fasting altogether:
- Pregnant and nursing women
- Growing children
- Underweight people
- Those with a history of disordered eating
- Those undergoing chemotherapy
These groups need more nutrients, not less.
People with diabetes should also be careful with IF because combining fasting with blood sugar medications may cause a low blood sugar state called hypoglycemia. Medical supervision is strongly encouraged. Always consult your doctor before starting any new diet or exercise regimen, especially if you have a serious medical condition.
How long should I fast for?
If you’re new to fasting, start with an overnight, 12-hour fast. Work your way up from there, but don’t force it. Listen to your body.
Can I drink liquids during a fast?
Yes, non-caloric liquids like water, tea, coffee, and lemon water are fine.
Can I take supplements while fasting?
Yes, you can take supplements when fasting. Electrolytes like sodium and potassium are especially important for hydration and energy. Understand, however, that the fat-soluble vitamins A, D, and K are best absorbed with food.
Can I work out while fasting?
You can, but you may need to reduce the intensity. Fasting tends to improve fat burning, but not performance.
How can I manage hunger while fasting?
First, understand that hunger ebbs and flows. It doesn’t keep rising ad infinitum.
Second, keep hunger triggers (sights and smells of food) out of your environment. Finally, consume non-caloric beverages to curb hunger pangs.
Can I Combine Keto With Fasting?
The Keto diet and intermittent fasting work well together. Both keep insulin low and get you running on fat for energy.
Going Keto can make fasting easier. Hunger hormones are suppressed in a state of ketosis.[*]
Excited to combine Keto and fasting? Try them separately first. Go Keto for about a month, then ramp up your fasting regimen. This will ease any shock from drastic dietary changes.
How To Get Started With Intermittent Fasting
Getting started with intermittent fasting is simple. Just start with the minimum protocol (12/12) and work your way up from there.
Take it slow. Go one hour at a time, and back off if things get hairy.
Feeling hungry is normal. Feeling weak, tired, or shaky is not.
All the while, monitor your sleep, energy levels, mood, appetite, body weight, and bloodwork. These should be improving. If not, you may do better on a conventional eating schedule.
Remember, intermittent fasting isn’t one-size-fits-all. Find the size that fits you, and you’ll be glad you did.