Fasted Weight Training: Research, Pros & Cons

The other day, I came across an article in Men’s Health on the subject of intermittent fasting and exercise in the morning, which cautions against lifting weights on an empty stomach.

Here’s a snippet:

Lifting weights, sprinting, doing CrossFit WODS, and other high-intensity activities all depend on carbs for fuel. If you perform any of these activities during (or worse, at the end of) your fast, your performance will suffer. Instead of getting stronger and faster, you may well get weaker and slower. If you’re a big guy with a lot of weight to lose, no big deal. Go ahead and lift on an empty stomach. You might lose a little bit of muscle, but you’ll burn fat, too—and that’s your main goal. But if you’re a slimmer guy with less muscle mass to spare, schedule your lifting workouts during your feeding window.”

Fasted lifting, it concludes, is a big mistake.

In truth, fasted weight training is not the catabolic, performance-sapping menace that many claim. A bout of strength training, even when it’s done during a fasted period, will still lead to muscle being gained.

Here’s why:

For one, your body has the ability to store plenty of carbohydrate, which can be used to fuel high-intensity exercise, such as high-intensity interval training (HIIT) or resistance training.

Depending on how much muscle you have, your body can hold upwards of 400 grams of glycogen, which is the name given to carbs stored in your muscles and liver. It’s not like your glycogen stores run out simply because you’re fasting.

You also need to consider when in the day you’re training.

Training on an empty stomach first thing in the morning isn’t a problem, although it can take a few weeks to get used to. When I made the switch from evening to morning training, I felt weaker and my performance did take a hit.

But over time, I got used to it.

However, the longer the fasting period goes on, the more likely it is that your performance will suffer. If you leave it until the afternoon or evening before you train, you won’t be able to lift as much weight or do as many reps.

Over time, this dip in strength means that the growth stimulus delivered by a given workout is going to be weaker than it otherwise would be. As a result, muscle will be gained more slowly compared to doing the same training session with a few meals inside you.

What about the claim that lifting weights during periods of fasting might mean losing a bit of muscle?

While training outside of your eating window won’t automatically lead to the loss of muscle, it does increase the potential for muscle to be lost, depending on what time of day you train and what your overall diet looks like. 

If you’re fasting all day, for example, lifting weights in the evening, then eating one big meal at night, the risk of muscle loss is certainly increased. But that’s because your short eating window means going too long without giving your muscles the nutrients they need to grow, rather than because of fasted training per se.

Bottom line? As long as the right dietary boxes are being ticked, muscle will still be gained whether weight training is done in a fasted or non-fasted state.

If you want to know more about the subject of fasted weight training, muscle growth and weight loss, I answer six of the most popular questions about it below.

Is It OK To Weight Train On An Empty Stomach?

Yes, it is OK to lift weights on an empty stomach. You can do just about anything in a fasted state, and that includes lifting weights.

However, the quality of your workouts may end up being compromised to some degree, depending on:

• The type of exercise you’re doing
• The duration of the fast (i.e. alternate-day fasting vs time-restricted feeding)
• What you’ve eaten the day before

As far as the type of exercise you’re doing is concerned, I’m going to assume you’re following a training routine designed to increase the size of your muscles.

That is, you’re lifting heavy(ish) weights, doing multiple sets of compound lifts, and pushing yourself hard in each set.

If so, fasted training first thing in the morning is unlikely to pose a problem, although it may take you a few weeks to get used to it.

But, if you leave it until the afternoon or evening before you train, chances are you won’t be able to lift as much weight or do as many reps.

In one study, leaving several hours between waking up and lifting weights had the effect of reducing repetition strength (the number of reps you’re able to do with a given weight) by 15% in the squat, and 6% in the bench press [1].

Over time, this dip in strength will mean that muscle gains happen more slowly compared to doing the same workout with a few meals inside you.

I can usually make it through until the early afternoon without eating anything. After that, I start to get hungry, tired and light headed.

For me, the idea of fasted weight training in the afternoon isn’t a pleasant one. I’ve trained on an empty stomach in the morning without a problem, but not in the afternoon and certainly not in the evening.

The amount of food you’ve eaten the day before is also going to make a difference.

A big day of eating prior to a fasted workout will leave you with plenty of energy to get through that workout. But if you’ve barely eaten anything the day before, chances are you’re going to find it difficult to give your all in the gym.

There’s also a degree of variability from person to person.

Some people love fasted training, and feel like they do better without eating anything before a morning workout. Even just a pre-workout snack leaves them feeling like they’re going to throw up the entire time.

Others find that they get shaky and dizzy when they try fasted training. Their best workouts come after they’ve eaten several times throughout the day.

All of which means you’ll need to experiment. It’ll take time for your body to adapt to fasted weight training, so give yourself a few weeks to get used to it before deciding whether or not it’s right for you.

RELATED: What I Learned from 20 Years of Intermittent Fasting

Can You Build Muscle Training Fasted?

Yes, you can build muscle training fasted. Whether you’ll build as much muscle as you would have done had your training been done after a meal is open to debate, for the reasons I mentioned earlier. But resistance training, even in a fasted state, will still stimulate muscle gains.

To maximize the muscle-building response to a bout of heavy lifting, especially if it’s done on an empty stomach, you want to get some post-workout protein inside you [2].

Training first thing in the morning in a fasted state, then waiting until the afternoon before eating any protein, will almost certainly slow the rate at which muscle is gained.

That post-workout protein can be in the form of regular food or even just a protein shake. Some whey protein mixed with water will do the job just fine.

Will Fasted Weight Training or Cardio Help With Fat Loss?

Fasted weight training can certainly help with fat loss. But it isn’t going to help you lose body fat any faster than the same workout done in a non-fasted state. In fact, the benefits of fasted workouts, from a fat loss point of view at least, are minimal.

Studies do show that fasted weight training does increase fat burning compared to the same workout done after a meal [3]. Because you haven’t eaten anything, blood sugar and insulin levels are low, which tends to favour the use of fat for fuel.

However, your body will adjust the rate of carb and fat oxidation over time. That’s why we need to look at how fasted training affects body composition changes over a period of weeks and months.

Any short term change in insulin, blood sugar levels and fat oxidation during a workout itself doesn’t tell the whole story.

There are only a handful of studies to look at the long-term effects of fasted exercise on body composition changes. All show much the same thing. Whether you train in a fasted or a fed state, it won’t make much difference to the amount of fat you lose.

RELATED: Fasted Cardio: Are There Benefits to Working Out on an Empty Stomach?

In short, I don’t think you’re going to see a huge benefit in terms of fat loss from fasted weight training (or fasted cardio for that matter).

Truth is, exercise by itself isn’t particularly effective way to create the calorie deficit required for weight loss.

It can certainly help… a little. But if you’re wondering if fasted weight training or aerobic exercise will help with fat loss, you’re focusing on the wrong thing.

As far as getting lean is concerned, the food you eat (or don’t eat) is a lot more important than what you do in the gym. Think of your workouts as a way to gain (or even just retain) muscle, and your diet as a way to put you in a calorie deficit, which is necessary for getting rid of body fat.

As long as your diet is set up properly, the decision to do cardio or strength training in a fasted or a fed state can be based largely on personal preference. When it comes to weight loss, there’s no great advantage or disadvantage to one or the other.

Will Fasted Weight Training Lead to Muscle Loss?

No, fasted weight training won’t lead to the loss of lean muscle mass. It certainly increases the potential for muscle to be lost, depending on when that fasted weight training is being done and what your overall diet looks like.

But strength training on an empty stomach, in and of itself, won’t cause muscle to be lost rather than gained.

There was an interesting study done on a group of guys during Ramadan, which involves a month of no eating or drinking during daylight hours [4]. One group of men lifted weights in a fasted state between 4-6pm, while the non-fasted participants did so between 9 and 10pm, having eaten something beforehand.

Interestingly enough, no muscle was lost. Lean body mass was maintained in both the fasted and non-fasted participants.

Other than signs of mild dehydration (not entirely unexpected if you haven’t had anything to drink all day), there were no adverse effects of training in a fasted state. Markers of renal function, immunity and inflammation also remained in the normal range.

I don’t think that either protocol was optimal for building muscle. And Ramadan only lasts for a month, so the study doesn’t tell us what would have happened over longer periods of time. However, it does show that lifting weights in a fasted state doesn’t make the loss of muscle inevitable.

If you’re doing some version of intermittent fasting that involves fasting all day, training in the evening and then eating one big meal at night, the risk of muscle loss is certainly increased. But that’s because your muscles have been starved of nutrients throughout the day, not because of fasted weight training per se.

But if you’re doing fasted weight training first thing in the morning, and following that up with multiple protein-rich meals during the day, there’s far less potential for muscle to be lost.

RELATED: Does Intermittent Fasting Make You Lose Muscle or Fat?

Can You Do Weight Training in the Morning While Intermittent Fasting?

You can do weight training in the morning while intermittent fasting. However, if you want to build muscle as fast as humanly possible, most research points to the afternoon and evening as being the best time of day to train.

Back in 2009, a team of researchers based at Finland’s University of Jyväskylä ran a very simple experiment [5].

They rounded up a group of young men, and got them to train in the morning or evening for a total of ten weeks.

The morning group trained between 7am and 9am, while the evening group did their workouts between and 5pm and 7pm. Both groups followed exactly the same training routine, which involved lifting weights 2-3 times a week.

Although the difference in muscle growth didn’t reach statistical significance, subjects who trained in the evening saw their muscles grow more quickly than the group who trained in the morning.

In fact, the evening group increased the size of their thigh muscles, on average, 30% more than their counterparts in the morning group.

A follow-up study, this time lasting six months, shows much the same results [6]. Men who trained in the morning saw their quads – specifically, their outer thigh – grow by an average of 12%. But the ones who hit the gym later in the day saw their thighs grow 50% more quickly.

Some research shows much greater variability from person to person in the strength of the growth signals sent to muscles with a morning versus an evening workout [7].

That is, lifting weights later in the day seems to produce a much more consistent rise in the various growth signals sent to muscle fibers. Training in the morning, on the other hand, leads to a more pronounced increase in some subjects, but also a decrease in others.

Again, there is a degree of individual variability. Some folks may do just fine lifting weights in the morning. Others will see better results by training in the late afternoon or early evening.

The key is to find a training time that works for you.

You’ll probably find that you’re weaker in the morning than you are in the evening, and that it takes a little longer to warm up.

However, your body can adapt to training at different times of day. Even though you may not feel as strong lifting weights in the morning, your body will get used to it, and the difference in strength between your morning and evening workouts will become smaller over time.

Some folks like to train fasted, not because of any specific fat-burning benefits, but because they don’t like having food in their stomach during a workout.

I remember when I made the switch from evening to morning training, and it was a shock to the system. Everything felt so much harder. But gradually, over time, I got used to it.

Ultimately, the best time of day to train is the time of day that works for you and fits your schedule. Timing is a lot less important than simply making it to the gym in the first place.

Getting your workouts in on a consistent basis is more important than most other things when it comes to getting in shape.

Is It Better to Workout Fasted?

Some say that intermittent fasting and fasted exercise has a beneficial effect on the body’s hormones, insulin and growth hormone in particular.

Fasted training is more than just OK, it’s actually the best way to improve both your health and your body composition.

On the flip side, others claim that high-intensity workouts shouldn’t be done following fasting periods. Your metabolism will slow down, your performance will suffer, the levels of various stress hormones, such as cortisol, will rise and you run the risk of losing rather than gaining muscle.

However, contrary to what the name suggests, human growth hormone (HGH) doesn’t have much to do with muscle growth in adults. Even growth hormone injections don’t do much for muscle protein synthesis, a prerequisite for building muscle.

Studies do show that early time-restricted feeding, which means eating most of your calories earlier in the day, improves whole-body insulin sensitivity independently of its effects on weight loss [8, 9].

This gives it a potential role to play in improving cardiometabolic health in people with type II diabetes or pre-diabetes.

However, insulin sensitivity will tend to improve with calorie restriction and weight loss, irrespective of whether that weight loss is achieved with fasted or non-fasted exercise.

There is a case to be made for carbohydrate periodization, where your low-intensity workouts (such as an easy bike ride or brisk walk on an incline treadmill) are done with a low carbohydrate availability, while more intense workouts are done with a higher carbohydrate availability [10, 11].

The idea is that doing some of your training in a fasted state with low glycogen stores can improve performance when you eventually compete with glycogen stores fully topped up.

However, that’s more of a strategy for improving performance in endurance athletes, rather than losing fat mass more quickly.