Fasting has seen quite the resurgence in the health and wellness community over the past several years and, to be honest, I have mixed feelings about this. You see, a lot of people sensationalize fasting for overhyped benefits like autophagy when there are more impactful benefits at stake.
Intermittent fasting should be done with measurable, perceivable goals — goals that prioritize feeling, performing, and looking your best in a sustainable way. To that end, fasting can be a powerful medicine to help folks burn fat, lose weight, and increase productivity. And shorter, intermittent fasts can even improve your metabolic health.
It can be highly sustainable and beneficial, for instance, to fast between 12 and 18 hours per day. But as people extend their fasts to span days in the name of minor, marginal benefits, I begin to worry. While specific populations may benefit from careful extended fasts, many others will likely shed muscle mass and end up fatigued and nutrient-deficient. These concerns are doubled for athletes and a few other populations.
Fasting just isn’t for everyone. But is fasting for you? Stick around for 5 minutes and you’ll be in a good position to decide for yourself. I’ll cover how fasting works, a few ways to go about it, and its potential benefits in this article.
Types of Fasting (Intermittent, Extended, & Limited-Calorie)
There are multiple ideas around what constitutes a fast. Some people draw a hard line, abstaining from ALL calories during a fast. Meanwhile, others consume a limited amount of calories during their fasting period.
Intermittent fasting entails consuming zero calories for a set number of hours per day. Here are a few popular methods:
- 12/12: Consume zero calories for 12 hours, and eat within the remaining 12 hour period. This is a great place for beginners to start experimenting.
- 16/8: Consume zero calories for 16 hours, and eat within the remaining 8 hour period. Work up to this if it’s comfortable and beneficial for you.
- OMAD: One-meal-a-day. While some people prefer this method for its simplicity, it can be difficult (and even uncomfortable) to get all of the nutrients you need in a single meal.
Extended fasts also entail consuming zero calories, but they can span anywhere from 36 hours to several days or weeks. In my opinion, the cons (sleep issues, stress, performance declines, and muscle loss) outweigh the potential cardiometabolic benefits. In short, I would only practice extended fasts under the recommendation and supervision of a trusted medical professional.
I loosely follow the 16/8 approach to intermittent fasting. I eat one meal in the late morning and one in the early evening. The rest of the time I stick to water and electrolytes. But if intermittent fasting isn’t your jam, it may be worth exploring limited-calorie fasts. They tend to be easier to stick to compared to the zero-calorie regimens above. Some examples include:
- 5:2 diet: Five days per week, eat normally. Two consecutive days per week, limit eating to about 25% of your typical calorie intake or less.
- Alternate-day fasting: Alternate between days of eating normally and limiting eating to about 25% of your typical calorie intake or less.
- Fasting Mimicking Diet (FMD): Each month, choose 5 consecutive days to limit eating to about 800 calories per day. Specifically, eat foods that are low in carbohydrates and sugar, and high in unsaturated fats. Repeat for at least 3 months.
Both limited-calorie and zero-calorie fasts can be beneficial. The important thing is to experiment and find whatever makes the most sustainable impact toward your goals. Quicker results are not better results unless they stick around for the long haul. With that caveat in mind, let’s talk about the benefits of fasting.
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
While the phrase “intermittent fasting” may evoke images of corny tabloids and clickbait videos touting transformation photos, the scientific research behind fasting and weight loss is actually quite promising. That goes for intermittent fasting, the 5:2 diet, alternate-day fasting, and the fasting-mimicking diet.
It’s not surprising that fasting promotes weight loss. When you compress your feeding window (or limit calories for a few days), you eat fewer calories overall. If you eat fewer calories than you expend in total, the principle of energy balance dictates that you lose weight. Additionally, fasting depletes your body’s preferred form of stored energy (glycogen). Without access to glycogen, your body must adapt to access and break down stored fat for energy.
It’s important to note, however, that not all weight loss during fasting is body fat. There are two types of weight you should intentionally preserve while fasting: water weight and muscle mass.
In short, you lose a lot of water and sodium during a fast due to its diuretic and natriuretic effects. It’s important to replenish water and electrolytes to keep your body running optimally. To maintain muscle, I recommend resistance training, which has been shown to forestall muscle losses while fasting. The longer the fast, the harder it will be to maintain muscle, so be sure to keep your fasting period within reasonable bounds.
#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 trouble regulating blood sugar because their body has built up a tolerance to insulin, the hormone in charge of lowering blood sugar. This insulin resistance is an unfortunate consequence of sedentary lifestyles combined with high-sugar 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.
#3: Autophagy
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, the building blocks are used to make new cellular machinery. It’s like recycling a can—melt down the materials, and you can create something brand new.
In animals, autophagy has been linked to longevity. And when autophagy is impaired, multiple organ systems (including the heart) suffer. While this is super interesting, I don’t believe people should pursue fasting simply for the sake of autophagy.
For one, we can’t reliably measure autophagy in humans. Second, pursuing autophagy often distracts people from the other effects of fasting, like weight loss. Why should a lean, healthy person extend their fast in the name of autophagy, just to lose muscle mass, hamper their sleep quality, and hinder their overall performance? They shouldn’t!
There are better ways, folks. Exercise and coffee also stimulate autophagy. Fasting is one tool, 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 minimizes blood glucose, keeping insulin low and depleting your body’s glycogen (stored glucose). With no fuel in the tank, your body kicks on the backup generator: It breaks down body fat to create an alternative fuel source called ketones.
Ketones are clean, efficient energy. Multiple studies have found that higher ketone levels can improve cognition, especially in older people. Speaking from personal experience, I certainly feel my sharpest in ketosis. It’s brought me a new level of mental clarity for many decades now.
#5: Productivity
This one’s super simple. Shopping, cooking, and cleaning can take a boatload of time and effort. Therefore, eating one or two less meals per day can save you a lot of time. Many people I’ve spoken with prefer to fast through the morning to keep their day on track, but this is mostly personal preference. Do what suits your routine.
What About Fasted Training?
My views on fasted training depend on what kind of exercise we’re talking about. If we’re talking about a relaxing yoga flow or a stroll through the woods—sure, go for it. Light aerobic exercise while fasting is a great way to burn fat.
For strength training, however, I’d advise you to wait until you’ve eaten. Restricting protein intake (on any diet) is not optimal for muscle growth or recovery. Similarly, I recommend avoiding longer or more intense efforts (like CrossFit WODs, marathons, or HIIT training) in a fasted state. Both fasting and hard exercise can be powerful stressors, and combining them may overflow your capacity for stress.
Fasting, Stress, and Sleep
It’s useful to think about fasting against a broader backdrop of stress in your life. I’m talking about stress due to work, insomnia, personal matters, training, and anything else.
Fasting can stimulate alertness chemicals like adrenaline and orexin-A that can make it hard to wind down and get restorative sleep. So if you’re not sleeping well, or if your stress is approaching the level of a rabbit being chased by a dog, take it easy.
Shorter, 12-hour fasts can even be good for the circadian rhythm and therefore sleep quality. The key is to experiment with both when and how long you’re fasting, and be mindful to not push yourself beyond your means.
Other Groups Who Shouldn’t Fast
Fasting is not for everyone. While some people simply don’t enjoy or feel any benefit from it, there are other groups that should downright avoid fasting. These groups include pregnant and nursing women, children, those who are underweight, and people with eating disorders. The common thread here is that these populations need more nutrients, not less.
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 are oftentimes better able to burn fat, produce ketones, and lose weight. It’s an effective principle to follow if you’re trying to detach from the 24/7 food frenzy available to modern society.
But we need to be careful with fasting. It’s a tool that must be used with a particular goal in mind. And if it’s overused, we won’t feel or perform our best. 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.
