Intermittent fasting is more than just a fleeting health fad. According to Harvard Health Publishing, intermittent fasting (typically, fitting all of your meals into a 8- to 10-hour daytime period and fasting at night) serves as both a potentially effective weight loss tactic and may even help prevent diabetes.
But what about exercise while fasting? As it turns out, not only can you exercise while fasting, but some evidence suggests that you should — the more important question lies in how to balance the two.
Exercise during intermittent fasting is often beneficial, but safety is absolutely essential.
What Happens to Your Body?
Though the research is not conclusive, a 2016 study published in the Journal of Nutrition and Metabolism found that fasting before morning exercise actually increases fat oxidation, potentially encouraging the potential for weight loss during exercise. During physical activity, your body uses carbohydrates as fuel; if you've been fasting for a while, your body naturally turns to other energy sources when carbs aren't available.
As registered dietitian Chelsea Amengual of Virtual Health Partners told Healthline, "[While fasting] your stored carbohydrates — known as glycogen — are most likely depleted, so you'll be burning more fat to fuel your workout."
So, basically, the fasting body first depletes sugar-heavy glycogen stores. When those stores are depleted the body moves on to burning fat for energy. Over time, your body "learns" to rely on that process, encouraging itself to burn fat more efficiently. Four studies assessed by the journal Cureus in 2018 indicated that intermittent fasting is an efficient weight-loss method, regardless of body mass index.
Read more: How Intermittent Fasting Can Get You Lean
Intermittent Fasting Workout Schedule Tips
Just like fasting itself, maximizing your intermittent fasting workout routine ultimately boils down to your body's specific needs, but it's also informed by your particular workout goals and the food you consume when you're not fasting. The daytime period during which you eat is known as the fueling window; a key question is whether to workout before, during or after that window. There's no "right" answer here — consider these factors as you plan your intermittent fasting workout schedule:
- Exercise during the fueling window typically serves as a reliable "all around" option for both performance and recovery; it's also beneficial if your routine focuses on post-workout nutrition (such as protein recovery).
- Some people find that they perform well on an empty stomach; if this is you, exercise before the window is an option.
- If you prefer to exercise after fueling up with food, the after window may be your best bet, especially if you're tight on time during the fueling window.
- Lower carb days are better suited to cardio or high-intensity interval training workouts.
- Because strength training requires protein recovery, it may be best scheduled in the "before" or "during" window.
Use Your Meals Wisely
Alongside regular exercise, diet is always one-half of staying in shape, but because intermittent fasting relies so heavily on that crucial fueling window, what you eat and when you eat become essential elements of the fasting lifestyle. Time your meal intake close to moderate- and high-intensity workouts to ensure that your body's glycogen stores are up to the task; reserve high-intensity workouts for after eating.
When strength training, for instance, your diet during the fueling window should incorporate more carbohydrates and protein. Likewise, strength training should be followed up with carb and protein intake (about 20 to 30 grams of quality protein) about 30 minutes after working out. As a general rule, schedule high-protein "feast" meals about once every four hours when strength training regularly.
Try coconut water for low-calorie hydration that offers electrolytes without all the sugar of most sports drinks. If you're prone to low blood sugar, ensure that you take in your meals or snacks about an hour or two before working out.
Read more: 13 DOs and DON'TS of Intermittent Fasting
Consider the Potential Drawbacks
Although the increased fat oxidation that may occur during exercise while fasting can potentially give your weight loss goals a boost, there's a flip side of the coin: It could also hinder muscle gains, as your body will instead start to break down muscle proteins as an energy source. As Ph.D., RD and certified sports dietetics specialist Kelly Pritchett told CNN, "When glycogen is in short supply, your body also reverts to breaking down protein — your muscles' building blocks — for fuel." That could lead to difficultly building muscle, or even cause muscle loss.
It's possible that your body simply will not respond well to working out while fasting. As Amengual tells Healthline, it's totally feasible that "you're more susceptible to hitting the wall [during fasting], which means you'll have less energy and not be able to work as hard or perform as well."
Similarly, it's possible that the long-term combination of intermittent fasting and regular exercise could slow down your overall metabolism, making it more difficult to lose weight in the future. This is because your body, during fasting, adapts to the lesser number of calories it receives by burning fewer calories per day.
Read more: What Does Fasting Do to Your System?
Safe Exercise While Fasting
Safety is absolutely paramount to any intermittent fasting workout routine. Above all else, pay attention to your body and look for signs of dehydration or low blood sugar. Safety guidelines to keep in mind as you exercise while fasting are:
- Stay hydrated; don't just drink water while you fast, drink more water than usual while you're fasting.
- Stop working out if you feel weak, dizzy or short of breath.
- If dizziness or weakness occurs, have a carbohydrate-electrolyte sports drink.
- If you're fasting for 24 hours or more, stay on the safe side by only practicing low-intensity exercise, such as yoga or walking.
- On the cardio front, in general, stick to lower-intensity exercise — think elliptical rather than sprints — while fasting.
- Always check with your doctor before embarking on a new fitness regimen or dietary program.
- Harvard Health Publishing: Intermittent Fasting: Surprising Update
- Healthline: How to Exercise Safely During Intermittent Fasting
- NCBI: Journal of Nutrition and Metabolism: Exercising in the Fasted State Reduced 24-Hour Energy Intake in Active Male Adults
- CNN: Intermittent Fasting: Should You Exercise on Empty?
- Intensive Dietary Management: Fasting and Exercise
- NCBI: Cureus: Intermittent Fasting: The Choice for a Healthier Lifestyle