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The Truth About Fasting and Exercise
There are a lot of reasons a person would consider exercising on an empty stomach. Many people simply like to exercise first thing in the morning. Then we have trending diet plans like Intermittent Fasting that make it difficult to schedule exercise around food consumption. Others incorporate fasting into their lifestyle for faith-based reasons. All the while, our exercise routines continue according to or despite our nutritional timing.
So, what happens when you exercise on an empty stomach? Is it good or bad for weight loss? Does it help you burn more fat or does fasting negatively impact your workout?
Let’s tackle these questions one by one. Here’s what you need to know if you’re considering working out while fasting:
What Happens to Your Body When You Fast?
When we say “fasting” we typically mean you’ve gone 8 to 12 hours without food. This would be like waking up in the morning after a good night’s sleep and deciding to work out before you have your first meal. If you participate in Intermittent or Prolonged Fasting, you may be going without (or with very little) food for 12, 16, 48, or even 72 hours.1 Here’s what’s happening in your body when you fast:
By 6-8 Hours – Your body is still burning through its glycogen supply.2 This is the most readily available form of energy.
By 12 Hours – You enter the metabolic state called ketosis in which your body begins to break down fat.1
By 18 Hours – You’re now in “fat-burning mode.” Your body is generating more ketones which tell your body to reduce inflammation and to make repairs to damaged DNA.1
By 24 Hours – Your body starts a process called autophagy. This means your cells work harder to recycle old components and to break down misfolded (damaged) proteins. Misfolded proteins are connected to Alzheimer’s and other diseases.1
If you’re interested in learning what happens once you hit 48, 54, and 72 hours of fasting, check out this article on the 5 Stages of Intermittent Fasting. For the purpose of this article, we don’t need to delve that far.
What Happens if You Exercise While Fasting?
Now that we know what’s happening on a biological level while fasting, let’s take a look at what happens when exercise is added to the equation.
As you exercise, your body starts by using glycogen for energy. You typically have enough stored up to last you about 24 hours.3 If you manage to deplete your glycogen stores (say you’re an endurance athlete running a triathlon and you haven’t been replenishing your energy as you go), you’ll hit exhaustion. Your body still has plenty of energy stored as fat, why doesn’t it use it when glycogen levels are low?
The body is not adapted to this. However, it can be trained! Following a low-carb diet or exercising while fasting can teach your body to draw energy from your fat-stores.3 Before this all starts to sound too good to be true, let’s define what we mean by “drawing energy from your fat-stores.”
A terrific example by Dr. Jason Fung in his article on Fasting and Exercise, starts to illustrate this concept pretty well: Imagine that your glycogen supply is like energy stored in the refrigerator. It is ready to use but the supply is limited. Your fat is like energy stored in the freezer; it takes a greater process to make it usable but you can store a larger supply.
Over time, exercising while fasting increases the production of fat-metabolizing proteins.3 Our muscles essentially become more efficient at breaking down fat in order to use the energy. It’s all part of how the body adapts to its circumstances.
What Other Studies Say
So, what does other research have to say about this? An article from the Strength and Conditioning Journal reviewed the results of multiple studies on the subject.
The review found that endurance trained individuals who performed moderate to high-intensity cardio (while fasting) break down significantly more fat than the body can actually use.4 So, yes, the body does break down that fat, but what isn’t immediately used goes back to its original form.4 The ultimate conclusion was that the net effect of exercising while fasting was negligible and may even have negative effects on muscle strength and growth.4
Another study examined body composition changes between a group that fasted before exercise and a group that did not. Their findings showed no difference between the two groups! They both lost significant amounts of weight and fat mass but fasting seemed to have no implication on the results.5
So, the evidence seems to lean in two directions. The body is clearly responding to the need for energy but there are split conclusions on whether this response is beneficial. Until more research is done, fasting before exercise may come down to a matter of preference. If you’re still seeking some answers and still interested in giving it a try for yourself, let’s find out a little bit more.
Will Fasting Before Exercise Cause You to Eat More?
A study on exercising in the fasted state hypothesized that exercising on an empty stomach would increase calorie intake throughout the day. They were surprised to find that not only did their participants eat less, they were also more motivated to work out.6 These results are promising if you’re worried about your hunger causing you to overeat on days you both fast and exercise.
Does Fasting Decrease Your Ability to Effectively Work Out?
Well, it depends what you’re doing. If you’re working out on an empty stomach, keep in mind that your body has been running on its glycogen stores while you fasted. It will pull from those same stores when you exercise, at least until you run out. If you’re doing steady cardio, you’re likely to be just fine.
High-intensity exercises, however, rely on glucose for muscle contraction.7 If your energy stores are low because you’ve been fasting, your body might break down your lean muscle to get you through your workout.7 If you recall, this was the same concern that came up in the article from the Strength and Conditioning Journal.
Clearly, this subject merits further research before any definitive conclusions can be drawn. Evidence can be found to back both sides of the argument. Some athletes swear by it, and there is science to prove that something is in fact happening to release and utilize your fat stores, but is it all enough to make a difference and are there significant adverse effects?
With the research at your fingertips, it remains up to you to decide whether you want to fast or feast before your workout. Which do you prefer? Let us know in the comments below!
For more thought provoking posts, check out What Happens to Your Body When You Stop Working Out, or, read this post to debunk some big Muscle Building and Fat Burning Myths. To access our monthly blog post highlights, subscribe to our newsletter, today!
- Jarreau, Paige. “The 5 Stages of Intermittent Fasting – LIFE Apps: LIVE and LEARN.” LIFE Apps | LIVE and LEARN, 22 May 2019, https://lifeapps.io/fasting/the-5-stages-of-intermittent-fasting/.
- “4 BIG Health Benefits of 12 Hour Intermittent Fasting.” Clean Cuisine, 5 Dec. 2019, https://cleancuisine.com/12-hour-intermittent-fasting/.
- Fung, Jason. “Fasting and Exercise.” Diet Doctor, 14 Sept. 2018, https://www.dietdoctor.com/fasting-and-exercise.
- Schoenfeld, Brad. “Does Cardio After an Overnight Fast Maximize Fat Loss? : Strength & Conditioning Journal.” LWW, Strength and Conditioning Journal, 2011, https://journals.lww.com/nsca-scj/Fulltext/2011/02000/Does_Cardio_After_an_Overnight_Fast_Maximize_Fat.3.
- Schoenfeld, Brad Jon, et al. “Body Composition Changes Associated with Fasted versus Non-Fasted Aerobic Exercise.” Journal of the International Society of Sports Nutrition, BioMed Central, 18 Nov. 2014, https://jissn.biomedcentral.com/articles/10.1186/s12970-014-0054-7.
- Bachman, Jessica L, et al. “Exercising in the Fasted State Reduced 24-Hour Energy Intake in Active Male Adults.” Journal of Nutrition and Metabolism, Hindawi Publishing Corporation, 2016, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5050386/
- Niedziocha, Laura. “What Happens If I Workout Without Eating?” LIVESTRONG.COM, Leaf Group, 24 Mar. 2019, https://www.livestrong.com/article/501975-what-happens-if-i-workout-without-eating/.