If you’re interested in the benefits of fasting, you have lots of company. Fasting has never been more popular in the health and wellness community.
As someone who started writing about fasting in 2005, I have mixed feelings about this trend.
On the plus side, fasting does have benefits. It can be a powerful medicine to help folks lose weight, burn fat, and increase productivity.
And I’m not just talking about multiple-day fasts. Even shorter, intermittent fasts (also called time-restricted feeding) can improve your metabolic health. And for a large chunk of folks—including yours truly—a daily 14 to 16 hour fast is highly sustainable.
But as the fasts get longer, I start to get concerned. Will the person maintain muscle? Will they have insomnia? Will they develop nutrient deficiencies?
These concerns are doubled for high-octane athletes, pregnant women, underweight people, and a few other groups. Fasting just isn’t for everyone. Very few things are.
And some benefits of fasting (like autophagy) are overplayed, in my opinion. I’ll dig into that controversial topic soon. First, let’s lay a solid foundation of what fasting is and how it works.
What Fasting Means
Different people define fasting differently. Some people say a fast MUST be water only. Others say that limited calories are okay.
I lean towards the second camp. Fasting is a period of temporary calorie restriction, not necessarily a zero-calorie effort.
Limited calorie fasts like 5:2, modified alternate-day fasting, and Dr. Valter Longo’s Fasting Mimicking Diet suggest that you don’t need to eliminate calories to benefit from fasting. These programs also tend to be easier to comply with than water-only fasts.
You also have intermittent fasts like 16/8 and OMAD that compress your daily feeding window. Me? I’m a big fan of 2 meals per day, one late morning and one early evening. The rest of the time I abstain from calories.
Then there are extended fasts. An extended fast starts at around 36 hours and can span for days or weeks. There’s interesting animal research on the potential benefits of extended fasting, but it’s not something I practice or generally recommend. For me, the cons (sleep issues, stress, performance declines, muscle loss) outweigh the potential pros of stem cell proliferation and cellular rejuvenation.
For the most part, the benefits of extended fasting line up with the benefits of intermittent fasting. If intermittent fasting is like hoisting a sail to reach these benefits, extended fasting is like strapping a rocket to your boat.
But quicker doesn’t mean better. For instance, if you’re not used to running on fat for energy—from fasting or keto dieting—a 2 or 3 day fast can be excruciating, especially if you don’t have your electrolytes dialed in. With that caveat in mind, let’s talk fasting benefits.
Benefits of Fasting
I like to break the benefits of fasting into five main categories. Let’s start with the major one.
#1: Weight Loss
Weight loss is the main reason people practice fasting. It’s why intermittent fasting has become so popular.
The research is promising. For example, a recent review found that various forms of intermittent fasting are effective for weight loss, irrespective of body mass index. In other words, it’s not just obese people losing weight.
It’s not surprising that fasting promotes weight loss. When you compress your feeding window (or limit calories multiple days per week), you eat fewer calories overall. Fewer calories in equals more weight lost.
This principle of energy balance applies here. When you eat fewer calories than you expend, you’re in negative energy balance. And negative energy balance is a non-negotiable for losing weight.
What kind of weight is lost during a fast? Two main types:
- Water weight
- Fat mass
You lose water weight because fasting depletes stored glucose (glycogen), releasing a deluge of fluids. This weight is largely regained when feeding resumes, though there are ways to mitigate this.
You lose fat because fasting keeps the hormone insulin low, allowing you to access body fat for energy. This is the sustainable fat loss benefit we’re after.
What about muscle? That’s a tough one. Resistance training while fasting can forestall a significant amount of muscle loss, but folks often feel a bit lethargic while fasting, so this can be difficult to accomplish. The longer the fast, the harder it will be to maintain muscle.
#2: Blood sugar regulation
When someone has type 2 diabetes, they can’t effectively regulate their blood sugar. As a result, blood sugar stays chronically elevated and drives a host of nasty complications.
Diabetics have blood sugar regulation issues because insulin (the blood sugar boss hormone) stops working as it should. This is called insulin resistance, and it’s an unfortunate consequence of sedentary lifestyles combined with high-sugar Western diets.
The topic of insulin resistance is too hairy to unpack here, but there’s preliminary evidence that fasting can improve insulin function and lower blood sugar levels in type 2 diabetics. Critical note: This intervention shouldn’t be pursued without medical supervision.
Nutrient deprivation (aka, fasting) activates a cellular recycling program called autophagy. When your cells undergo autophagy, old damaged parts are broken down into their constituent proteins and lipids—then these building blocks are used to make new cellular machinery.
It’s like renovating your house. Same house, fresh interior.
That said, I’m not crazy about pursuing fasting to maximize autophagy. Why? For one, we can’t reliably measure autophagy in humans. There are a few biomarkers to track, but you really need a biopsy (no thanks) to measure this phenomenon in various tissues.
Another thing I don’t like: the goal of autophagy pushes lean, healthy people towards longer fasts. And then they start to lose muscle. And feel weak. And they can’t sleep. And their libido goes down the tubes.
Folks often forget that exercise and coffee ALSO stimulate autophagy. Fasting is ONE tool that is capable of driving some possible benefits of autophagy, but it’s not the only tool.
Chasing the phantom of autophagy isn’t a valid reason to suffer these consequences. Not in my book, anyway.
#4: Mental enhancement
Recall that fasting keeps insulin low. Low insulin, in turn, gets you burning fat and making ketones.
Ketones then fuel the brain with clean, efficient energy. This can lead to enhanced mental clarity.
Multiple studies have found that higher ketone levels can improve cognition, especially in older people. And speaking from personal experience, (relatively) younger guys like me also feel their sharpest in ketosis.
Eating and meal prep requires a boatload of time and effort. Going from 4 meals per day to 2 can save you hours.
What will you do with that extra time? That’s up to you.
Many find that the morning fasting window is the most productive time. With ketones elevated, folks find they are motivated to get stuff done.
What About Fasted Training?
My views on fasted training depend on what kind of exercise we’re talking about.
If we’re talking about easy yoga or a hike in the woods—sure, go for it. Light aerobic exercise while fasting is a great way to burn fat.
Fasted strength training? I’m not crazy about that one. Restricting protein intake at any level is not optimal for muscle growth or recovery.
I’m also not crazy about longer or harder efforts (like CrossFit WODs or marathons) in a fasted state. Both fasting and hard exercise are powerful stressors, and combining them can overflow your limited stress capacity. Do situations exist where fasted training may be valuable? Sure. Some even find they log personal records while fasting—but I’d be careful training this way day in, day out.
While we’re on the topic, LMNT and Ketogains co-founder Luis Villasenor wrote a fantastic article on fasted training. Check it out if you have a sec.
Fasting, Stress, and Sleep
It’s useful to think about fasting against a broader backdrop of stress in your life. I’m talking about work stress, insomnia stress, personal stress, training stress, etc.
If your stress is approaching the level of “rabbit being chased by dog”, take it easy with the fasting. Opt for shorter daily fasts, perhaps capping them at 18 hours or so. Even a 12 or 13-hour fast can be beneficial.
Also, consider easing up if you aren’t sleeping well. Fasting stimulates alertness chemicals like adrenaline and orexin-A that can make it hard to wind down and get restorative sleep.
Other Groups Who Shouldn’t Fast
Whenever I write about fasting, I have to issue a bunch of caveats. And the main caveat is that fasting isn’t for everyone.
For some groups, fasting can be harmful or dangerous. These groups include:
- Pregnant and nursing women
- Underweight people
- Those with eating disorders
The common thread here is that these populations need more nutrients, not less. Fasting generally entails less.
To be clear, I’m not talking about overnight fasting. Almost everyone can benefit from 12 or 13 hours without food. It’s good for the circadian rhythm and therefore sleep quality.
But any fast longer than that should be entered mindfully.
Final Thoughts on Fasting Benefits
The benefits of fasting are driven primarily by one thing: Eating less.
When folks eat less (and less frequently), they lose weight, burn fat, and produce ketones. It’s an effective remedy for the 24/7 food frenzy that we call modern society.
But we need to be careful with fasting. It’s a tool, not a cure-all. And if it’s overused, we won’t feel or perform our best.
The trick is to find a healthy balance between fasting and feeding. Start with an overnight fast and gradually compress your feeding window until you find a sustainable routine.
Just don’t neglect the pillars of health in the name of fasting—nutrition, sleep, exercise, stress management, and social ties are all important facets of performing your best.