6 Things to Know About Intermittent Fasting & Working Out

Woman running on a treadmill

Favored by those on the low carb diets like Atkins and keto, intermittent fasting (IF) is the method of eating only during certain times of day—or certain days of the week—and fasting for others. Whether it’s alternate day fasting, our preferred option of 16:8 (fast for 16 hours a day and eat during the remaining 8 hours), or something else entirely, fasting has been shown to help some with weight loss, among other health benefits like stabilizing blood sugar levels. 1

Because exercise has its own health perks, it’s understandable that you would want to maintain a regular workout schedule while fasting. Here are six things you should know about safely and effectively working out while fasting.

1. Yes, you can continue working out while fasting. But take it easy.

Fasted workouts have their benefits. Working out on an empty stomach could help you lose weight because your body will rely on stored fuel, in the form of glycogen and fat 2 —rather than burning your most-recent meal.

While exercising in a fasted state, however, there is a chance your body could start breaking down muscle for fuel. That’s because high-intensity activities rely on carbohydrates for fuel. Which means running sprints or doing your regular CrossFit routine while fasting, or at the end of your fast, may decrease the benefits of your workout. You may also feel less energized to work out hard if you’re new to IF.

So while fasted high-intensity weight lifting may have negative results, low-intensity cardio—which runs primarily on fat—before breaking your fast is A-Okay. The best intermittent fasting workouts for cardio include walking, jogging, yoga, cycling, and gentle Pilates.

2. The timing of your workout is up to you.

Just like your eating windows, when you work out should depend on what makes you feel best, the types of workouts you are planning, and your fitness goals. Remember, it’s best to stick to low intensity exercise while fasting, and save the moderate- to high-intensity workouts for windows after you’ve broken your fast.

Activities that require power and speed may also benefit from having fuel in your system, so schedule weightlifting and other high-intensity exercises during your feeding windows. Not only will this boost your performance, but fueling your body before and after your intermittent fasting workout will help support muscle repair and recovery. 3

If you don’t have the opportunity to exercise during your fueling window, right after your fueling window is your next best option. Most importantly, if you don’t like how you feel, don’t do it! Whether or not they’re doing IF, some people perform better on an empty stomach, and some people prefer to work out after eating a small meal. If you are pushing yourself too hard or begin to feel light-headed, take a break!

3. Refuel on protein after your workout.

Optimize your fast by eating adequate protein, high fiber carbs, and healthy fats during your fueling windows. Follow-up any higher-intensity training with protein within 30 minutes of finishing your workout. 4

If you’re doing low intensity cardio on an empty stomach, work out towards the end of your fasting period so you can refuel right after. Stick to whole, unprocessed foods that combine protein and carbs. We love scrambled eggs with veggies, or, if you are on the go and in search of a quick and convenient post-workout fueling option, try a protein bar or protein shake.

4. Hydration is everything.

It is so important to consume plenty of water and electrolytes while fasting, and especially when fasting and working out. Headaches, dizziness, low blood sugar, low blood pressure, nausea, and cramping can occur if your electrolytes aren’t balanced properly. 5 Replenish electrolytes with unsweetened coconut water, electrolyte tablets, or zero calorie electrolyte drinks. Avoid sports drinks that are high in sugar as well as caffeine and other diuretics. The primary electrolytes lost in sweat are sodium and chloride, with some potassium lost as well, so make sure you are consuming adequate sodium and potassium along with being well hydrated.

5. Don’t start intermittent fasting until you have adapted to a fat burning (keto) metabolism.

If you want to combine keto and IF, avoid beginning both at the same time. It takes a few weeks for your body to adjust to any changes or new routines, so introduce low carb living before fasting to give your body time to adapt. If you feel any mental fog, weakness, dizziness, exhaustion, burnout, injuries, nausea, or are slow to recover from your workouts, it’s time to take it back a notch. Intermittent fasting and exercising can be difficult to manage. And be warned: Extra workouts will make you feel hungrier in general, which can make fasting even more difficult—especially if the intensity is too high.

6. Most importantly, listen to your body.

If you have certain health conditions (especially those that may cause dizziness like low blood pressure or low blood sugar) working out while fasting may not be in the cards. Listen to your body, and do what feels right! If you do feel weak while working out, stop, refuel, and hydrate. Before starting IF, or before adding in exercise to your IF program, consult with your doctor or healthcare provider for guidance on what’s right for you.

Learn more about intermittent fasting, and find healthy tips, inspiration, and motivation with our online community when you start your Atkins journey today.

Evidence Based
PubMed Central, National Library of Medicine,
2022: A scoping review of intermittent fasting, chronobiology, and metabolism

Chronobiology plays a crucial role in modulating many physiologic systems in which there is nutritional synergism with meal timing. Given that intermittent fasting (IF) has grown as a flexible dietary method consisting of delayed or early eating windows, this scoping review addresses the effects of IF protocols on metabolism as they relate to clinical nutrition and the circadian system…

Evidence Based
PubMed Central, National Library of Medicine,
2018: Is exercise best served on an empty stomach?

The objective of this review paper is to evaluate the impact of undertaking aerobic exercise in the overnight-fasted v. fed-state, in the context of optimising the health benefits of regular physical activity. Conducting a single bout of aerobic exercise in the overnight-fasted v. fed-state can differentially modulate the aspects of metabolism and energy balance behaviours…

Evidence Based
PubMed Central, National Library of Medicine,
2018: Effects of fasted vs fed-state exercise on performance and post-exercise metabolism: A systematic review and meta-analysis

The effects of nutrition on exercise metabolism and performance remain an important topic among sports scientists, clinical, and athletic populations. Recently, fasted exercise has garnered interest as a beneficial stimulus which induces superior metabolic adaptations to fed exercise in key peripheral tissues…

Evidence Based
PubMed Central, National Library of Medicine,
2016: The response of muscle protein synthesis following whole‐body resistance exercise is greater following 40 g than 20 g of ingested whey protein

The currently accepted amount of protein required to achieve maximal stimulation of myofibrillar protein synthesis (MPS) following resistance exercise is 20–25 g. However, the influence of lean body mass (LBM) on the response of MPS to protein ingestion is unclear…

Further Reading
Mayo Clinic,

Hyponatremia occurs when the concentration of sodium in your blood is abnormally low. Sodium is an electrolyte, and it helps regulate the amount of water that’s in and around your cells… Call your doctor if you know you are at risk of hyponatremia and are experiencing nausea, headaches, cramping or weakness. Depending on the extent and duration of these signs and symptoms, your doctor may recommend seeking immediate medical care.

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