First, in brief:
Intermittent fasting can work sometimes with exercise, depending on what you do and how.
You should avoid intermittent fasting if you work out in the morning.
The more training you’re doing, the less likely it’s a good idea.
Most of those who practice intermittent fasting and train effectively only strength train as their only meaningful, intense exercise. Generally, their only aerobic training is whatever walking they do during the day, or very brief high intensity interval training… if they do any cardio at all.
If you don’t do much exercise at all, then yes intermittent fasting is a good idea. And you should probably get some exercise, but intermittent fasting is a good habit.
A General Overview of Intermittent Fasting:
Instead of traditional fasting, where you may go a day or more without eating… intermittent fasting is about eating all your day’s meals in a short window of time and not eating the rest of the day.
Even if you eat a similar number of calories, the long break from eating gives your body an extended metabolic break, which can help reduce inflammation and better promote healing and recovery. This is actually more of the benefit of intermittent fasting than the potential fat burning improvements that can occur during the fast.
There’s no calorie restriction on how much you eat during the food window. But, obviously, it’s going to be harder to overeat in a single 8 hour window than it would be if you ate meals throughout the day.
Still, it is possible to outeat the fast during the 8 hour window and still maintain or gain weight. The fast doesn’t cause you to lose weight in itself. While it’s obviously more difficult in a shorter window of time, you can still overeat. That said, intermittent fasting can help with food portion and weight control.
The most common intermittent fasting approach is one devised by Martin Berkhan called Leangains, where you eat all your meals in an 8 hour window and go 16 hours without food between those windows.
Other similar approaches shorten the day’s food window to 6 or 4 hours (the latter is the backbone of Ori Hofmekler’s Warrior Diet). But the 16/8 approach is most common.
The easiest and most common approach to 16/8 Intermittent Fasting is to simply skip breakfast. Your first meal is at lunch (often around 12pm), and you eat normally until dinner (often no later than 8pm). These meals of course are bigger meals, since you must fit all your nutrition into the 8 hour window.
It’s possible to move the food window back, eat breakfast, eat your last meal and start the fast earlier in the day. But hunger pangs can obviously be an issue during the longer fasting window, and this can make it hard to get to sleep or stay asleep. Going to bed shortly after your last meal makes sleep easier.
Also, placing the food window between lunch and dinner has some scientific support. NIH research finds people are more insulin sensitive during the daytime than later in the day, indicating your body best regulates its weight if you do all your eating later in the day.
The flip side is that, if you exercise a lot or are otherwise very physically active, you will have a portion of the day where you are not eating, and this may have an effect on your ability to conduct that physical activity… not to mention potential damage your muscle mass through muscle catabolization… during the fasting period.
When at rest during the fasting period, your body can rely on burning fat and a bit of stored glycogen for fuel. But if you exercise vigorously and start burning calories quickly during the fasting period, your body may eat away at your own muscle to fuel your workout and to “rob Peter to pay Paul” by catabolizing muscle in one part of your body to spur muscle recovery in another location.
So, to explain my original points:
You should avoid intermittent fasting if you work out in the morning. In Leangains, Berkhan notes that if you choose to strength train during the fasting window, you should either do so right before your feeding window starts so you can re-fuel… or take branch chain amino acid supplements (BCAAs) around your workout to minimize muscle catabolization (amino acids are the natural derivative of protein and key the recovery processes connected with protein).
However, endurance training requires either an immediate influx or replenishment of carbohydrates to maintain performance or restore glycogen stores. Obviously, during a fasting window, you’re not taking in any nutrition.
With shorter, easier workouts, this may be fine if your food window begins right after the workout. But if you’re doing harder or longer workouts, or you don’t plan to eat for some time following the workout, working out while intermittent fasting could create more problems than results.
Those who successfully endurance train and intermittent fast often do their workouts later in the day, during the food window. But not everyone can consistently practice this, plus the practice of intermittent fasting can preclude morning workouts and races, which often is the most practical time of day to run.
The more training you’re doing, the less likely intermittent fasting is a good idea. While you can eat as much as you’d like during an 8 hour feeding window, you probably have your limits in how much healthy food you can reasonably eat in that span of time.
Plus, bear in mind the effect of large meals on your energy levels, as digestion commands bloodflow and generates a stress response. A big dinner is probably no problem, because you typically are in a relaxed state at that time of day. However, a big lunch could crash your energy levels during the middle of a workday where you need your energy to remain high.
A typical weekend warrior or strength training bro might not require more than 2500 calories in a day. They can start the food window with a typical, reasonable lunch, and probably pound the bulk of their calories after work, before bed.
Meanwhile, a busy high volume endurance athlete could easily require 3000+ calories each day. If intermittent fasting there’s not much time during the food window to get all the needed calories in. A shortfall of calories (while good in principle for weight loss) can have dramatically negative effects on an endurance athlete’s energy levels and hormonal profile, beyond the long term effects of serious training.
Plus, consider that a rather large healthy meal (e.g. whole foods, no processed food, a minimum of sugar and additives) may not consist of more than 1500 calories, if that. A typical healthy lunch probably contains no more than 800 calories.
Go into a food logging app or website, and try to enter 3000 calories worth of healthy food into a day over lunch, dinner, and a midday snack. You will likely find this very challenging, and even more challenging to actually execute.
Even if it’s possible, your meal planning and execution, not to mention the timing and execution of your training, have to be on point for intermittent fasting to work with endurance training. Again, your workouts probably need to occur right before or during the food window. All your key workouts probably need to take place later in the day.
Midweek speedwork and tempo runs later in the day might not be a problem… but what about long runs? Also, what is where you live is very hot during summer, like the Las Vegas desert, making endurance workouts not just impractical but unsafe?
Most of those who practice and swear by intermittent fasting, and can train effectively, only strength train as their only meaningful, intense exercise. Generally, their only aerobic training is whatever walking they do during the day, or very brief high intensity interval training… if they do any cardio at all.
This is why you see a lot of writers online swear by intermittent fasting. Conventional broscience traditionally only practices weightlifting and light cardio as their only physical activity, and that’s the background of a lot of these writers. In that lifestyle, intermittent fasting is very practical and easy to execute.
For a more serious athlete or an endurance athlete, their calorie requirements are more substantial, their training more demanding, and so intermittent fasting doesn’t fit them as well during training.
However, you can periodically apply intermittent fasting if you are an athlete. You can intermittent fast not just during an offseason or training break, but also during off days. Given intermittent fasting’s metabolic benefits, it can help provide a metabolic break from the consistent inflammation of consuming all that food in addition to the break from training. It can help the body heal.
I personally tend to intermittent fast during my less active periods, and eat more traditionally throughout the day when seriously endurance training or if I need to work out during the morning.
If you don’t do much exercise at all, then yes intermittent fasting is a good idea. And you should probably get some exercise, but intermittent fasting is a good habit in your situation.
The more you exercise, the less likely you should intermittent fast.