The Definitive Guide to Intermittent fasting and Working Out

If you dabble in intermittent fasting, you’ve probably wondered how to best incorporate exercise into your routine. 

Which types of exercise are best in a fasted state? Can fasted training help you burn more fat? What are the risks of working out while intermittent fasting?

These are excellent questions, and answering them will require a few minutes of your time. Stick around. It’ll be time well-spent. 

What Is Intermittent Fasting?

Intermittent fasting (IF) is the practice of taking regular breaks from food. Depending on the type of intermittent fast, these breaks can last anywhere from 12 to 36 hours.

Protocols like 12/12, 16/8, and one meal a day (OMAD) entail fasting every day, while protocols like 5:2 and alternate day fasting (ADF) entail fasting on a weekly schedule. Unlike the daily protocols, the weekly protocols often allow limited calories (around 25% of normal intake) on fasting days. 

Why practice intermittent fasting? For most people, it’s about weight loss or weight management.[*] When you eat less frequently, you’re less likely to overeat. 

By keeping the hormone insulin low, IF also helps you enter fat-burning mode. This fat-burning, in turn, fuels exercise. 

Intermittent Fasting and Exercise

Back in prehistoric times, fasted exercise was a normal and necessary behavior. When our ancestors ran out of food, they had to exert themselves to find calories. 

These expeditions—Sunday morning spearfishing, spirited berry picking, etc.—were powered by a deep and bountiful energy reserve: body fat. If you do the math, a lean person with 10% body fat still has about 80,000 calories of energy lining their frame. 

And so it makes sense, in the absence of other energy sources, that fasted exercise boosts fat-burning. It’s been shown, for instance, that fat utilization goes up when low-intensity exercise (walking, light cycling, etc.) is performed in a fasted state.[*] 

But not all types of exercise follow this rule. With moderate or intense exercise, fat-burning is about equal between the fed and fasted states. 

The “why” is interesting. With harder efforts—fed or fasted—your body releases more fatty acids into circulation than it can burn.

In other words, you hit a fat-burning limit. In one study, athletes burned the same amount of fat (and performed better) while cycling at moderate intensity in a fed state.[*] 

Potential Benefits of Fasted Training

Why work out while fasting? Consider the following benefits.

#1: Fat-burning and ketosis

If fat loss is your goal, a light workout before breakfast is a good strategy. You’ll burn more fat.

And when you burn more fat, you produce more ketones. Being in ketosis is a useful metabolic state for appetite control, mental clarity, and stable energy. 

#2: Endurance

Over time, fasted aerobic training promotes better endurance capacity. In one study, four weeks of fasted cycling increased VO2 max (maximum oxygen utilization) in both men and women.[*]   

#3: Growth hormone and muscle adaptations

A two-day fast has been shown to increase human growth hormone (HGH) five-fold.[*] This is hypothetically good for strength goals, but when you look at the evidence on HGH supplementation, it’s not clear that additional growth hormone helps build muscle.[*] 

Another study found that strength training while fasted increased muscle phosphorylation after the workout, suggesting enhanced recovery.[*] Note: the participants consumed a carbohydrate and protein-rich drink directly after each session. 

Risks of Working Out Fasted

In terms of fasting risks, all the usual caveats apply. Pregnant and nursing women, underweight people, growing children, and those with a history of eating disorders should avoid fasting altogether. 

But there are other risks.

For example, it’s been shown that fasted cardio increases proteolysis (muscle protein breakdown) during exercise.[*] This cardio-induced muscle loss is blunted during the fed state. 

What about strength training? That likely depends on feeding habits. If protein isn’t consumed shortly following the workout, muscle growth and repair will be impaired. 

Another risk is fatigue. Fasted training may enhance endurance in the long run, but not so much in the short run. 

The fed state seems to work better for performance. In one study, cyclists stayed on their bikes longer when they consumed carbohydrates.[*] 

Finally, both fasting and exercise are stressors—and they’re easy to overdo.

Think of your stress capacity like a bucket. You can only fit so many stressors—work stress, personal stress, poor sleep stress, exercise stress, fasting stress, etc.—in the bucket before it overflows. 

And when it overflows, the resulting surge in cortisol (your “stress hormone”) can make it difficult to lose weight.[*] That’s why people who train hard can still struggle with fat loss. 

Tips for Working Out While Fasting

Ready to get moving? Mind these tips to optimize your fasted training. 

Favor low-intensity fasted workouts

Low-intensity means yoga, hiking, light jogging, or light cycling—about 50 to 60 percent of your max heart rate. Light fasted aerobics not only enhances fat-burning, but also reduces the risk of stress and fatigue. Save the more arduous efforts for your fed state. 

Hydrate properly with electrolytes

When you train fasted, you lose water and electrolytes at higher rates through sweat and urine.[*] This can cause weakness, cramps, and low energy due to dehydration or electrolyte deficiencies. Solve this problem by taking electrolytes (especially sodium) along with your fluids.

Get enough calories (especially protein)

If you practice intermittent fasting, you need to work extra hard to maintain muscle. Why? Because you have fewer opportunities to consume the nutrients that keep muscles around. 

Protein is the main nutrient to focus on. High-protein foods like meat, eggs, fish, and whey protein powder provide the amino acids required for muscle growth and should be consumed close to strength training 

Carbs can also help by boosting insulin, an anabolic hormone. Be aware, however, that boosting insulin suppresses fat-burning.

Intermittent Fasting and Working Out: Should You Do It?

Sure, give it a try. But go into it carefully. 

Start with easy workouts in a fasted state. These can be effective fat-burning sessions. 

If that feels good, consider upping the intensity a bit. Don’t push it, though, because this can break down muscle, make you tired, and stress you out. 

Save the hard stuff for your fed state. The exercise will still be beneficial, and you’ll still burn fat.