Women's Health July 17, 2026 0 Comments

Fasted Workouts: The Secret Hack That Could Skyrocket or Sabotage Your Gains—Which Side Are You On?

Ever find yourself racing out the door for that 6 a.m. workout, sans breakfast, feeling like you’ve totally got this fasted workout thing down? But here’s the kicker—just because you crushed that session on an empty stomach doesn’t mean it’s the golden ticket to fitness nirvana. Fasting, technically going 8 to 12 hours or more without eating, has sparked a fiery debate among fitness buffs and scientists alike. Remember when the science seemed to back fasted workouts as the secret sauce for muscle gains, turbocharged energy bursts, and weight slashing? Well, recent research is flipping that script and urging us to rethink our pre-sweat snack habits. So, before you swear off breakfast forever, let’s unravel what’s actually happening in your body when you hit the gym fasting—and whether it’s magic, myth, or somewhere in-between, according to the experts who know their stuff. LEARN MORE

8 min read

Picture this: You wake up at the crack of dawn with just enough time to make that 6 a.m. workout class, if you leave home…now. So, you speed to the studio without having breakfast, and feel fine during the workout. Since you didn’t notice any issues, does that mean it’s totally okay to do fasted workouts moving forward?

First, a reminder: “Fasting is defined as a period of time when you have not eaten for 8 and 12 hours,” or longer, says Nadia Agha, PhD, Assistant Teaching Professor of Kinesiology at Rice University. Fasted workouts are quite controversial: The science used to suggest that they’re more effective than fed training when it comes to growing muscle, muscular oxidative capacity (which you need for high bursts of energy in a workout), and weight loss. Some endurance athletes even swear by fasted workouts.

Meet the experts: Nadia Agha, PhD, is an Assistant Teaching Professor of Kinesiology at Rice University. Grace Horan, ACSM-EP, RRCA Coach, is an exercise physiologist at the Hospital for Special Surgery in New York City. Michael Fredericson, MD, is the Director and Professor of Physical Medicine and Rehabilitation at Stanford University, and team physician for Stanford Track & Field and Swimming.

However, more recent research shows that skipping a meal before a sweat sesh may not be as beneficial as previously thought. Ahead, learn what the new science says about fasted workouts, and whether or not they’re good for anyone, according to experts.

Your Body on a Fasted Workout

Let’s go back to high school science class for a sec: There are three energy pathways—fueled by carbohydrates, fats, and proteins—that your body taps into when you exercise (or do anything else), according to the Cleveland Clinic. While all three work together to sustain your workout, one pathway will provide the most effective source of energy, depending on the kind of exercise you’re doing.

For instance, anaerobic exercise (like HIIT, heavy resistance circuits, or sprint intervals) uses carbs to produce spikes of effort. Whereas aerobic exercise, like running or biking, relies on both carbs (primarily stored as glycogen) and fats (fatty acids) to keep you going. And exercises involving explosive power, like box jumps or power cleans, rely on molecules called creatine phosphate that the body stores in limited amounts, but naturally replenishes with rest (and can be nourished with lean proteins) says Grace Horan, ACSM-EP, RRCA Coach, an exercise physiologist at Hospital for Special Surgery in New York City.

When you’re doing a fasted workout, your body doesn’t have its usual energy sources to pull from—namely, glycogen. Instead, it resorts to alternative sources like stored fat “to meet energy demands of daily functional and sport-specific activity,” says Horan. While this might seem positive for anyone with weight loss goals, there are a number of other factors to consider before skipping a pre-workout meal. (More on that below!)

What the Evidence Actually Shows About Fasted Workouts

According to a substantial amount of research, when muscle glycogen (energy) is limited, “athletes may see decreases in how much work they can do during a session, or [experience] earlier fatigue,” says Horan. “It may reduce training quality for some individuals and it delays the availability of amino acids needed to support muscle protein synthesis,” or how muscles break down protein for energy and repair. Since fasting can cause you to generate less energy needed for a workout, it may take longer for you to build muscle than it would with consistent fueled workouts, she adds.

However, some research tells a different story: A small 2023 study in Nutrients tracked 11 female CrossFitters’ progress through a range of exercises (like the standing long jump and a handgrip strength test) to gauge how being fasted for 24 hours impacted their power, overall strength, and speed. After completing the tests in both fasted and fed states, the researchers found little difference between the participants’ performances, and suggested that fasting doesn’t impair workout performance.

Another small 2025 study in Nature Communications had 13 participants (seven men and six women) fast for seven days as they exercised and worked their jobs. During this time, they underwent testing for their resting metabolic rates, completed muscle biopsies and DXA scans, and completed strength and endurance testing. The participants maintained muscle strength, but their high-endurance capacity decreased.

While the data from these smaller studies is compelling, most experts believe you’re better off strength training in a fed state. “Consuming protein and carbohydrates before exercise can help support training performance, allowing an individual to potentially lift heavier loads, which is an important driver of muscle growth,” Horan adds.

Data is murky on how fasting impacts other anaerobic workouts.

One small 2025 study in the Journal of Bodywork and Movement Therapies had 14 trained men perform various workouts (an anaerobic sprint test and leg and bench press repetitions to failure) in fed and 14-hour fasted states. Their anaerobic performance increased by 10 percent during the fasted state, and they were able to complete more leg presses to exhaustion.

However, more research is needed to assess how this approach may affect women. Horan says that because anaerobic workouts require “readily available carbohydrates,” athletes are better off fueling ahead of their workout.

Fasting may be harmful for long endurance workouts.

“Fasting during [endurance] exercise lasting longer than two hours can negatively impact your perceived exertion and overall performance,” says Agha. When you lose touch with your rate of perceived exertion (how much effort you think you’re putting in), you may push harder than intended. Research backs up this notion: The above study in Nature Communications found that participants’ VO₂ max (how much oxygen your body can transform into energy while working out) suffered during a high-intensity exercise test after fasting for six days.

Additionally, “consistently underfueling can contribute to Relative Energy Deficiency in Sport (RED-S), which can disrupt menstrual function and impact bone health, recovery, and performance,” Horan adds. If left untreated, athletes may experience heightened risk of menstrual irregularities, impaired cardiovascular health, and premature labor and pre-term delivery (for those diagnosed prior to conception). RED-S can also impact how well you recover and build muscle.

So, it’s imperative to fuel before long endurance sessions, like training for a marathon or triathlon. A 2024 study in Frontiers in Physiology looked at marathon performance research over the past 15 years and highlighted how important carbs, protein, and electrolytes are for maintaining energy, skeletal muscle mass, and hydration, respectively, when training for a marathon.

That said, fasting may support shorter, low-intensity aerobic workouts.

Fasting before a brief recovery run or chill walk or bike ride may help some athletes. “During these sessions, exercising in a fasted state may increase the proportion of fat being used as fuel during the workout itself and may stimulate some metabolic adaptations related to fat oxidation,” says Horan. (Meaning, when implemented strategically as part of a cohesive training program, your body can become more efficient at switching from using glycogen to fat for fuel.)

However, she notes that those metabolic changes don’t necessarily lead to meaningful results over time: “Long-term outcomes related to fitness, body composition, and endurance performance are often better when the nutrition intake matches the training load.”

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Fasted workouts might accelerate weight loss.

Since your body uses fat for energy when it doesn’t have glucose, “fasted workouts could be an advantage if you’re trying to lose weight,” says Michael Fredericson, MD, Director and Professor of Physical Medicine and Rehabilitation at Stanford University, and team physician for Stanford Track & Field and Swimming.

Some studies have looked at intermittent fasting—or only eating during certain hours of the day—in the context of fasted workouts. A 2025 study in Nutrition had participants diagnosed as overweight or obese participate in moderate exercise and either follow an intermittent fasting plan or a calorie-restricted diet. (It’s unclear how the study defines “moderate exercise,” but according to the CDC, it’s any movement that raises your heart rate and causes you to breathe harder than normal and break a sweat.)

After four weeks, participants in the intermittent fasting group exhibited a “statistically significant” amount of weight loss compared to the other group. During the four-week maintenance period, both groups experienced weight loss and improvements in body composition. At the six- and 12-week check-ins, the same group showed “superior” weight maintenance and less weight regain. However, research on how non-overweight populations might react to this approach is still lacking.

Additionally, a 2021 meta-analysis of studies in Physiology and Behavior looked at the impact of intermittent fasting (such as time-restricted feeding, alternate day fasting, and periodic fasting) and resistance training on the body. It concluded that pairing the two methods could result in lower body mass, body mass index (BMI), and body fat percentage without losing muscle. Still, the research wasn’t clear on what type of intermittent fasting provided the highest results in the long term.

Who, if Anyone, Can Benefit From Training Fasted

It may be easier to lose weight with fasted workouts than fed ones, at least in the short term. The key to losing weight is to remain in a calorie deficit (or consuming less calories than you burn), so it can be helpful to pair intermittent fasting with moderate exercise, including resistance training, says Agha.

However, fasted workouts may not be a long-term solution for those with weight loss goals “since body composition changes are driven more by total energy balance, daily nutrition, and consistency over time,” Horan says.

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Overall, your fitness performance or gains likely won’t improve in the long term with fasted workouts. For folks with other fitness goals, like “high-intensity sessions, long endurance efforts, or strength training, where fueling plays a critical role in performance and adaptation,” fasted workouts are not the move, says Horan.

You should always make sure your body has enough energy for workouts and to just get through the day, adds Fredericson.

If you want to try a fasted workout, connect with your healthcare provider to see what they think in the context of your overall wellness and fitness status. If you get the go-ahead, be sure to monitor how you feel during the workouts. Say you’re dizzy, lightheaded, or not performing as you should—in that case, “fasted workouts may not be ideal for you,” says Agha.

How to Fuel for Your Workouts

Ideally, eat at least one hour before exercise, according to the Mayo Clinic. Here are a few optionsHoran suggests trying, based on your workout:

Before HIIT, CrossFit, sprint workouts, and early morning gym sessions: A banana or applesauce pouch will provide quick-digesting carbohydrates without causing gastrointestinal discomfort.
Before moderate-intensity strength training or mixed training sessions: Greek yogurt with fruit combines carbohydrates and protein before exercise.
Before tempo runs or interval cycling: Toast with honey or jam will supply carbohydrates for higher-intensity efforts.
Before long runs, rides, and endurance training lasting over 90 minutes: Oatmeal with fruit provides a larger carbohydrate load for sustained energy release.

Oh, and fueling doesn’t stop when your workout is over. “Following training sessions, athletes should prioritize carbohydrates to replenish glycogen stores, and protein to support muscle repair and recovery,”says Horan, ideally within one to two hours after exercise. What you eat depends on the intensity, duration and type of workout. When in doubt, you can always bet on chocolate milk or a protein shake with a banana, she suggests. Or, go for a more robust meal like salmon, veggies and rice, or a chicken burrito bowl with rice and beans.

Ultimately, you know what works best for your body. But, to optimally support your fitness goals, grab a piece of fruit or some toast (at least) on your way to your next workout class.

Liz Doupnik is a freelance culture and lifestyle journalist based in Nashville, TN who writes about books, fashion, and wellness. With nearly 20 years of experience, Liz cut her teeth working as a full-time editor at WWD, StyleCaster, and Seventeen, and has led editorial teams at SHAPE and Nike. A lifelong athlete, Liz has also worked as a certified personal trainer and obtained her Master Training certification from Focus Personal Training Institute. As a freelancer, her work has appeared in W, Marie Claire, Dwell, Teen Vogue, Well & Good, The Zoe Report, and many others. When she’s not busy at work, you can find her logging a long run on one of Nashville’s many trails, perusing the shelves at her favorite local bookstore, or searching for her future rescue pup.