As an avid faster and fitness buff, I’ve experimented extensively with mixing intermittent fasting and exercise. When done strategically, I’ve found the combination can be safe and supercharge your results.
But getting it right hinges on understanding your body, fine-tuning your workouts, and—above all—exercising common sense.
In this epic deep dive, I’ll explore the nitty-gritty science, cutting-edge research, and hard-won lessons behind training in a fasted state. I’ll also distill it all into easily digestible guidance so you can confidently incorporate fasting and fitness into your routine.
Let’s get after it!
Fasting and Fitness Fundamentals
Before jumping headlong into fasted workouts, let’s quickly level-set on what intermittent fasting actually entails.
Intermittent Fasting 101
At its core, intermittent fasting simply means alternating intervals of eating and not eating. This differs from conventional diets that focus primarily on what you eat.
Instead, IF centers around when and how often you should refrain from food.
There are several popular IF approaches, including:
- 16:8 fasting: Confine eating to an 8-hour window, like 12–8pm daily
- Alternate day fasting: Fast every other day
- 5:2 diet: Eat normally 5 days per week, limit calories 2 non-consecutive days
- 24-hour fasts: Once or twice weekly 24-hour fasts…basically skipping a day eating
The supposed benefits of these intermittent fasts range from weight loss and heart heath to anti-aging. But what does the research actually indicate?
The Science-Backed Benefits
A growing pool of studies suggest several science-based perks:
- Triggers weight and body fat loss: By restricting eating windows, IF makes lowering overall calories easier. In one study, 16:8 fasting produced 3–8% weight loss over 3-12 months [1].
- Improves markers of heart health: IF may reduce blood pressure, LDL cholesterol, inflammation and other cardiovascular risk factors [2].
- Regulates blood sugar: IF supports glycemic control, improving insulin sensitivity by over 20% in some trials [3].
- Supports healthy longevity: Studies link IF with enhanced cell repair, gene expression, and longevity [4].
For fitness folk fixated on body composition like myself, the fat scorching and muscle preserving potential of intermittent fasting is highly enticing.
But is it actually safe to continue exercising through fasting periods? What about us gym rats striving for PRs…can we still crush workouts without food?
Let’s explore those critical questions next.
Is Exercising While Fasted Safe?
I’ll cut straight to the chase on this one:
For most people, exercising while fasting is generally safe.
Despite prevalent myths, fasting does not completely drain your body of fuel stores. Various metabolic sources like glycogen and fat remain available for energy.
However, your risk level depends largely on two key factors:
- Duration of your fast: Fuel availability and demand differs vastly for shorter versus prolonged fasts.
- Degree of metabolic fat adaptation: This determines your capacity to tap into fat for energy when glucose is low.
With that overview, let’s examine exercise safety considerations for shorter and longer term fasts:
During Short-Term Fasts
For time-restricted fasting windows under 24 hours, exercising is generally secure with proper precautions:
| Duration | Typical Approach | Exercise Guidance |
| 12-16 hours | Overnight fast | – Low/moderate intensity
– Listen to body |
| 16-20 hours | 16:8 or 20:4 fasting | – Light/medium intensity
– Monitor energy levels |
In these shorter fasts, your body retains reasonable glycogen stores for quick energy even if not fully fat adapted. You also avoid extreme energy deficits.
As such, lower and moderate pace exercise is perfectly safe here as long as you listen carefully to your body. Pay attention to signs of low energy or fatigue and cut back intensity if needed.
Hydration is also critical during short fasts. With limited carbs and lower liver glycogen, dehydration can manifest quickly and exacerbate issues. Consistently sip water before, during and after workouts.
Longer Fasting Periods
Once fasts extend beyond 24 hours and liver glycogen nears depletion, exercise safety depends far more on your metabolic conditioning:
- Fat adapted: Body readily breaks down fat for ketones to power tissues and workouts [5].
- Not fat adapted: Struggles to tap fat for energy, risks hypoglycemia and muscle loss.
| Duration | Typical Approach | Exercise Guidance |
| 24-36 hours | Alternate day fasts |
– Low intensity only – Fat adapted strongly advised |
| >36 hours | Prolonged fasts | – Avoid exercise
– Break fast if training |
When fasting beyond 24 hours, intense workouts can quickly drain glucose reserves risking hypoglycemia. Hitting the gym in a severely glycogen-depleted state also augments muscle loss.
As such, only low intensity activity is advised for longer fasts. And for fasts beyond 36 hours, avoiding exercise altogether or breaking the fast pre-workout is strongly encouraged.
Bottom line? If you’re fat adapted, exercising while fasting is generally safe…if not adapted, extreme caution is warranted for longer fasts.
Now that we’ve established exercise safety protocols based on fast duration and metabolic conditioning, let’s highlight some unique benefits that might inspire you to experiment with fasted workouts.
3 Science-Backed Benefits of Training Fasted
Once deemed safe for your situation, certain forms of exercising while fasted can uniquely enhance fitness and accelerate results.
For example, research shows select benefits like:
1. Burns More Fat
During fasting periods with depleted glycogen reserves, your body heavily relies on fat breakdown to meet energy needs.
For example, in one study individuals burned 80-90% more triglycerides cycling when fasted vs fed [6]. This shows the depth of fat stores your body can penetrate when pushed into a carb-depleted state.
Several mechanisms provoke accelerated lipolysis and fat oxidation:
| Driver | Impact |
| Depleted glycogen | Forces increased fat burning for energy |
| Enhanced adipose enzymatic activity | Boosts fat breakdown |
| Rising catecholamines | Accelerates fat release and burning |
This research clearly demonstrates fasting’s potent fat torching effects. So if your goal is dropping fat, incorporating some fasted training can skyrocket results.
2. Enhances Metabolic Flexibility
Bouncing back and forth between fuel sources is a hallmark of metabolic health. This trait, called metabolic flexibility, confers benefits like:
- Fat burning efficiency: Readily taps fat when glucose is low.
- Muscle building capacity: Changes fuel sources to accommodate growth.
- Resilience: Adapts nimbly to fuel availability.
Fasted training powerfully enhances flexibility by forcing increased oxidation of fat to fulfill energy needs when carbs are unavailable [7].
Doing so trains your metabolic machinery and muscles to become supremely adaptable at burning fat, building muscle, and thriving through varying dietary phases.
3. Stimulates Anabolic Hormones
Certain workouts in a fasted state can also strategically spike key hormones that:
- Burn fat: Like growth hormone and norepinephrine [8].
- Preserve muscle: Such as testosterone [9].
- Support adaptation: Through raised cortisol [10].
| Hormone | Role | Impact |
| Growth hormone | Fat metabolism | Elevated up to 5X fasting [11] |
| Catecholamines | Fight-or-flight | Surge during fasting |
| Cortisol | Energy release | Rises to meet demand |
Strategically timed fasting and training can ignite this cocktail of hormones that torch fat, fuel performance, protect muscle, and drive fitness to the next level.
Tips for Exercising Effectively While Fasted
Convinced of the potential benefits and ready to experiment with fasted workouts?
Here are my top tips for training efficiently while fasting:
1. Mind Your Workout Intensity
Without quick carbs, your max workout capacity may be blunted during fasting periods.
As such, avoid long or extremely challenging sessions that rapidly drain reserves. Opt for lighter weights, moderate pace cardio, gentler classes, etc. instead.
Also, build intensity gradually and remain keenly aware of energy lulls, lightheadedness, or other warning signs indicating it’s time to back off intensity.
2. Listen to Your Body
Speaking of warning signs…you absolutely must tune into feedback from your body here.
Pay close attention to any symptoms of low energy, fatigue, nausea, mental fog, cramping, etc. Stop immediately if concerned. Your health and safety is #1.
3. Stay Extremely Hydrated
Maintaining optimal hydration is critical when training fasted:
- Sip frequently during workouts and the full day
- Sports drinks may provide electrolytes if training hard
- Consume sodium post-workout to retain fluid
Poor hydration exacerbates issues when glycogen is low so diligently drink up!
4. Time Nutrient Intake
To optimize results, carefully time your nutrient intake:
- Pre-workout: 10g BCAAs to reduce muscle breakdown
- Post-workout: 20-40g whey protein to ignite recovery & growth
- Next meal: Consume carbs + protein within 60-90 minutes after training to restore glycogen
This nutrient timing allows you to reap the benefits of fasted workouts while supporting muscle repair and recovery.
5. Supplement Smartly
The following supplements can help counteract risks like fatigue, muscle loss, low blood sugar, cramps, and hyponatremia:
- Caffeine: Fights fatigue and enhances lipolysis
- Sodium: Prevents cramping and hyponatremia
- Magnesium & potassium: Alleviates muscle fatigue and strain
- Citrulline malate: Reduces fatigue and aids hydration
- Beta alanine: Enhances power, strength, and endurance
Talk to your doctor before supplementing.
Avoid Fasted Training If…
While I believe most healthy adults can safely experiment with fasted workouts, they may be ill-advised for certain groups, including:
1. Uncontrolled Diabetes
If you have uncontrolled diabetes or blood sugar regulation issues, exercising without quick fuel availability poses safety risks related to the potential for hypoglycemia.
Be sure to carefully monitor blood glucose levels with medical guidance before attempting fasting plus fitness.
2. Endurance Athletes
Extreme endurance athletes requiring maximum muscle glycogen may struggle with sessions exceeding 60+ minutes due to fuel depletion.
Carbing up pre-workout is likely prudent here even if experimenting with post-workout fasted periods.
3. Very Low Caloric Intake
If you are already restricting calories substantially, throwing fasting on top can put you at greater risk for side effects like hypoglycemia, nutrient deficiencies, unwanted muscle loss, and fatigue.
Fuel appropriately for your activity level before adding fasting.
4. Poor Metabolic Conditioning
As emphasized earlier, if you are not fat adapted, longer fasts quickly exhaust glucose reserves raising safety issues—especially when mixing in high intensity or lengthy workouts.
Enhance metabolic flexibility first before combining longer fasts and challenging training.
5. Ignoring Warning Signs
Above all, paying attention to signs from your body is mandatory here. Lightheadedness, severe fatigue, cramping and other symptoms while training fasted demand an immediate stop.
Attempting to push through despite signals of distress can have serious health and safety repercussions.
So if you struggle with any of the above risk factors, I’d advise extra caution with fasting and exercise. Speak to your doctor if unsure!
In Closing
I realize this was an epic deep dive on fasted workouts…but I trust it’s now clear that strategically incorporating fasting and fitness can be safe and hugely rewarding.
The keys are minding the duration of your fast, listening carefully to your body, fueling appropriately around workouts, considering your metabolic flexibility, and adjusting intensity levels wisely.
If done with care, intermittent fasting and exercise can ignite fat burning, amplify anabolic hormones, enhance metabolic conditioning, and escalate your fitness to entirely new levels!
Just remember—progress slowly, remain hyperaware of warning signs, stay incredibly well hydrated and fueled, and check with your doc before diving in.
Now go enjoy experimenting with those fasted gains! Just please be smart about it.
Let me know if you have any other questions. I’m always happy to nerd out on this stuff!