How Your Body Uses Food When You Train
To fully understand why fasting and strength training don't mix, it's important to brush up on some biology. When you eat a pre-workout meal or snack, your body breaks down this food to fuel your workout: Your body stores carbohydrates as glycogen in its muscles, which it then uses for energy when you lift, according to the American Council on Exercise (ACE).
But when you don't have any fuel in the tank, your body fatigues quickly and you may not reap all the strength-building benefits of your training, according to Noam Tamir, CSCS, owner of TS Fitness in New York City. And that's exactly what you don't want when your goal is building strength and muscle.
Keep scrolling to learn exactly how an empty fuel tank affects your strength-training workouts and muscle gain.
1. You'll Probably Fatigue Faster
As you may already know, progressive overload is the key to building strength and muscle. Essentially, this is where you gradually increase your training volume (either your sets, reps or weight) in order to grow stronger. But consistently making your workouts more challenging demands quite a bit of energy; without the proper fuel, you may not have any extra resources to use.
"Strength training while fasted can lead to quicker fatigue," Tamir says. "Your glycogen is already depleted, so that means your intensity needs to be lower and you can't push as hard." And when you can't push as hard, lifting more weight or more reps can feel infinitely more difficult.
Not to mention, your brain may not feel as sharp as usual. Low blood sugar can cause your brain to feel fatigued, foggy or irritable, according to the Mayo Clinic. Not exactly ideal when you're in need of a distraction-free workout.
2. You May Lose Muscle
Your muscles need energy (glycogen) in order to exercise. That's why your body stores about three-quarters of your total glycogen in your muscles, so they have a large supply of energy, according to the Cleveland Clinic.
Your muscle glycogen goes down with intense exercise, and the tougher your workout, the more quickly you deplete your glycogen stores. And these stores aren't restored until you eat a meal with carbohydrates.
When you don't have enough glycogen stored in your muscles, your body needs to rely on other sources of energy, including your own fat and muscle tissue. So, you can start to break down the exact type of tissue you're trying to strengthen and build. In that case, you defeat the purpose of strength training entirely.
3. You May Not Gain Strength
Muscle is difficult to build and you need to be in a calorie surplus in order to gain strength, according to the National Academy of Sports Medicine (NASM). In other words, you need to be eating more calories than you burn.
Also, as mentioned above, you need to constantly increase the challenge of your workouts in order to gain strength. If you're not able to do that and you're in a calorie deficit, your body's ability to build muscle is hindered.
After eight weeks of intermittent fasting and consistent weight training, participants of a small October 2016 study in the Journal of Translational Medicine found no noticeable strength gain. While they did lose some body fat, participants' testosterone levels also dropped and cortisol (the stress hormone) levels were elevated.
So, if building muscle is your goal, fasting probably isn't the best way to do it.
4. You Put Yourself at Risk of Injury
This can't be said enough: Strength training with progressive overload is tough. Chances are, the last few reps of each set will feel pretty challenging, which is where proper form is especially important, according to Carolina Araujo, CPT, a New York-based strength coach.
"When you're lifting a heavier weight than before, you're at higher risk of injury," she says. "If you're already low on energy, lifting to fatigue becomes riskier because your form could break down."
Although strength training in a fasted state isn't exactly dangerous, it probably won't help you get the muscle-building results you're seeking — and may even put you at risk of injury.
If you don't love eating a huge meal before training, even a little pre-workout snack can fuel your workout, giving you enough energy to make the most of your training session.