Intermittent Fasting For Bodybuilding

Intermittent fasting (IF) means eating inside a set window each day, or on set days, and fasting the rest of the time. For bodybuilders, its main value is practical: a shorter eating window makes it easier for many people to eat less without constant hunger, which helps when cutting and when staying lean year-round.

It is not a fat-burning switch. When calories and protein are matched, IF produces about the same fat loss as a normal eating pattern; what it changes is how easy the diet is to stick to. Used carelessly, it can also make it harder to hit your protein target and train hard.

For muscle gain, IF needs more care. If you are a beginner who mainly wants to add size, more meals and more evenly spread protein will usually serve you better. This guide covers the main protocols (16/8, 20/4, OMAD and alternate-day fasting), how long to fast for your goal, what to eat, when to train and whether you can build muscle while fasting.

How Long Should I Fast?

This is the most common question bodybuilders have about intermittent fasting. The answer depends on your goal and your starting point more than anything.

A 36-hour fast, for example, makes little sense if your goal is to gain as much size as possible. You would go a day and a half without protein or training fuel: muscle protein synthesis stays at its low fasted baseline while breakdown continues, cortisol tends to rise, and you miss the feedings that drive growth. Long fasts are sometimes used for weight loss in people with obesity, ideally with medical guidance, but they are not a muscle-building tool.

At the other end, eating from 8 am to midnight and only fasting the 8 hours you sleep is unlikely to get you to around 8% body fat. A longer fasting window, combined with other appetite-control habits, can make a deep cut less of a fight with hunger, stress and fatigue.

Here is how the main protocols compare for a bodybuilder:

Alternate-Day Fasting

Alternate-day fasting (ADF) means alternating “fast days” with normal eating days. On a fast day you eat nothing or, in most research versions, about 25% of your energy needs (often around 500 kcal); on the next day you eat freely. It is not the same as eating normally one day and doing 16/8 the next.

ADF has mostly been studied for weight loss in people with overweight or obesity. In a one-year trial of 100 adults with obesity, ADF produced similar weight loss to ordinary daily calorie restriction, but more people dropped out of the ADF group (Trepanowski et al., 2017, JAMA Internal Medicine).

For bodybuilders, true ADF is a poor match: every second day you would train on little or no food and miss most of your protein feedings.

Training-Day / Rest-Day Fasting for a Lean Bulk

A better fit for a lean bulk, and one of the cases where intermittent fasting and building muscle can go hand in hand, is to fast on rest days only:

• Training days: eat normally with only the usual overnight fast (around 8 hours), so you get the carbs, calories and recovery in.
• Rest days: follow your fasting plan, such as 16/8.

This keeps your pre- and post-workout nutrition strong on the days you train, while the shorter eating window on rest days makes it harder to overshoot your calorie surplus. Your protein still needs to reach your daily target on rest days.

OMAD (One Meal A Day) for Bodybuilding

OMAD can work as a short cutting strategy, especially after a bulk that pushed body fat to around 16-18% or higher.

Eating one large meal a day makes it easy to cut calories hard. To control hunger during the day, sip on water, sparkling water, plain tea or black coffee.

The weak spot is protein. Eating 150-200 g in one sitting is hard for most people. A simple fix is a large protein shake as “dessert” straight after your main meal, which helps you meet your daily protein needs without breaking the one-meal structure.

A large single dose of protein is not wasted. In one study, 100 g of protein after a full-body workout produced a larger and longer-lasting muscle-building response than 25 g (Trommelen et al., 2023, Cell Reports Medicine). Even so, one meal a day gives you one protein-driven rise in muscle protein synthesis instead of three or four, so OMAD is better as a short cutting tool than a long-term plan.

20/4 Fasting

This is 20 hours fasting and 4 hours eating.

Like OMAD, it can make an effective cutting diet, especially if you gained more fat than planned in your off-season.

The difference is that you get two main protein feedings a few hours apart, and your calories don’t have to be quite as low. That generally makes it kinder to muscle retention than OMAD, although results depend on how much fat you gained while bulking, your training, your sleep and your total protein.

But what if your body fat sits in the 10-15% range, as it does for many bodybuilders? If your goal is fat loss, you will probably want something like the following.

16/8 Fasting For Bodybuilding

This is the most popular version of IF, and the easiest to follow: 16 hours fasting, 8 hours eating. Many people eat from 12 pm to 8 pm, but any window works; the best one is the one that suits your schedule and your training time.

Starting is simple. Push breakfast back by about two hours in the first week, then by another two hours in the second week, and you are there.

Black coffee in the morning helps many people get through the fast. Caffeine blunts hunger for some people and causes a small, short-lived rise in energy expenditure, but it is a helper, not a fat-loss tool on its own.

16/8 also has the best research in lifters of any IF protocol. In an eight-week trial, 34 resistance-trained men ate either within an 8-hour window (meals at 1 pm, 4 pm and 8 pm) or on a normal schedule, with matched calories and macros and the same training. The 16/8 group lost fat mass while keeping fat-free mass and maximal strength (Moro et al., 2016, Journal of Translational Medicine). A similar eight-week trial in active women doing resistance training found the same gains in fat-free mass and strength with an 8-hour window as with a normal schedule, with protein intake kept high (Tinsley et al., 2019, American Journal of Clinical Nutrition).

Without lifting and enough protein, the picture is different. In a 12-week trial of 16/8 eating in adults with overweight or obesity, who were given no training or protein targets, weight loss was small and no better than a normal meal schedule, and the 16/8 group lost a significant amount of lean mass (Lowe et al., 2020, JAMA Internal Medicine). The eating window does not protect muscle on its own; training and protein do.

So why does 16/8 suit bodybuilding? The main benefits are:

• Appetite control. Many people find they are less hungry on a fixed schedule, so eating less takes less willpower.
• Lower insulin for much of the day. Insulin stays low during the fast. Fat loss still comes down to your overall calorie balance, not insulin alone.
• Flexibility. There are no rules on macros or food types like there are with keto or paleo.
• Room for enough protein feedings. An 8-hour window still fits three or four protein meals a few hours apart, which matches the research on protein distribution better than the one or two meals of OMAD or 20/4.

On that last point: when 80 g of protein was spread over 12 hours after training, 20 g every 3 hours stimulated more muscle protein synthesis than the same amount given as two 40 g or eight 10 g servings (Areta et al., 2013, Journal of Physiology). A review of the evidence suggests about 0.4-0.55 g of protein per kg of body weight per meal, across at least four meals, to reach 1.6-2.2 g/kg a day (Schoenfeld & Aragon, 2018, Journal of the International Society of Sports Nutrition).

All of this makes 16/8 a sustainable route to low body fat for many lifters. It won’t make you leaner than the same calorie deficit eaten any other way, but if it makes the deficit easier to stick to, it will get you there.

What Foods Should You Eat?

There are no strict rules on food choices or macros with IF, but a few general pointers help.

If your eating window is full of junk food, you will struggle to lose fat over the long term, and your health and energy will suffer too. Build most meals around lean protein, vegetables, fruit, whole-food carbs and some healthy fats.

Breaking The Fast

There is no special trick to breaking a fast: your first meal is simply your first meal. Many lifters prefer a protein-rich, lower-carb first meal (around 12 pm on a standard 16/8), such as fish, eggs, red meat or Greek yogurt with some vegetables. Protein first helps you feel full, which makes a cutting diet easier overall.

The common claim that a high-carb first meal “spikes your insulin too high and crashes your energy” is overstated. In healthy people, insulin does its job and blood sugar returns to normal after a carb-rich meal. Some people do feel sleepy after a large, carb-heavy meal, so if that is you, keep the first meal lighter on carbs and save more of them for around training. If you train soon after breaking the fast, carbs in that first meal are a good idea.

The Evening Feast

Many lifters put most of their carbs in the last meal of the day. If you train in the afternoon or evening, it doubles as your post-workout meal, and the carbs help replenish glycogen and support recovery. Some people also find a carb-rich evening meal helps them sleep.

So alongside your chicken, turkey, steak or fish, have rice or potatoes. The amounts need to match your size, activity level and fat-loss goal, which is where a structured meal plan comes in, but the above covers the basics.

There is also a practical upside that gets less attention:

The evening feast is what makes IF sustainable for many people, because it is enjoyable.

A lot of dieters cut out social meals with friends or their partner, which makes dieting painful and low body fat harder to maintain.

With a big evening meal, you can eat a large share of your daily calories (for some people around 70%) at dinner, whether you are alone or with friends and family.

What About Intermittent Fasting And Working Out?

Can you work out while fasting? Yes, but whether you should depends on where you are in your cut.

Early in a cut, especially after 6-12 months of eating in a surplus, many people train fasted without problems: glycogen stores are well stocked and energy is high. Fasted training gives no extra fat loss, though. In a four-week study, young women doing the same cardio either fasted or fed, on the same calorie deficit, lost similar amounts of fat (Schoenfeld et al., 2014, Journal of the International Society of Sports Nutrition).

Once you have been in a calorie deficit for a few weeks, or you are below about 12-14% body fat, most lifters train better with carbs beforehand. Keeping intensity high is what tells your body to hold on to its muscle.

So the suggestion for bodybuilders for the majority of a cut is:

Train during your eating window, rather than your fasting window.

For most people this means training later in the day, but on 16/8 you could equally eat from 8 am to 4 pm and train in between. It’s up to you.

Either way, get your pre-workout meal in so you can train hard enough to keep your hard-earned muscle. A typical 16/8 training day with a 12 pm to 8 pm window could look like this:

For most lifters, this is a good balance of muscle retention and fat loss on intermittent fasting.

Does Creatine Break A Fast?

One simple rule answers this and almost every similar question: if it has calories, it breaks the fast.

Plain creatine monohydrate has no calories, so it does not break your fast (flavored creatine products with added sugar are a different story). It is also one of the best-researched supplements for strength and lean mass, with 3-5 g a day the usual maintenance intake (Kreider et al., 2017, Journal of the International Society of Sports Nutrition), so it is worth taking every day if you care about your muscle mass.

Research on timing is mixed and any differences are small; taking it consistently matters most. If you like to take it in your morning coffee, see the truth about creatine in coffee, and for the different forms, our guide to the types of creatine.

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Does Bulletproof Coffee Break A Fast?

Yes. Bulletproof coffee is coffee blended with butter, MCT oil or both, which adds fat calories, often a few hundred, so it does break your fast.

Those fat calories are better saved for cooking proper meals later in the day, such as steak, fish or chicken with vegetables, where they come with protein and fiber. If you want something in the morning, black coffee or plain tea will not break the fast.

Can You Build Muscle While Fasting?

Yes, but it depends on several factors.

A calorie surplus makes muscle gain much easier, especially for trained lifters. Beginners, people returning after a break and those with more body fat can gain some muscle at maintenance or even in a small deficit; the further you are into your training career, the more you need the extra calories.

Your starting point matters too. If you have only built 3-5 pounds of muscle so far, you may keep gaining some size on a tight eating window such as OMAD, for a while. Our guide to muscle gain per month shows what is realistic at each stage, and you can check where you stand with beginner, intermediate or advanced lifter standards.

But what if you have gained 30 pounds or more of muscle over your bodybuilding career? Then the calorie surplus is your muscle-gaining lifeline, and over the mid to long term a training-day / rest-day split, or a normal eating schedule, will serve you better than OMAD or 20/4.

Protein matters as well. Across studies, extra protein stopped adding fat-free mass at around 1.6 g per kg of body weight a day, with some lifters benefiting up to about 2.2 g/kg (Morton et al., 2018, British Journal of Sports Medicine). Spreading that over several meals is easier with a wider eating window. If you struggle to eat enough, calorie-dense whole foods help; see our list of high-calorie foods for bulking.

If fat loss is your main goal, though, intermittent fasting is one of the most practical tools you can use.