So Which Is Better For Weight Loss: Cardio or Weights?

By Dominique Michelle Astorino

Updated on 12/2/2017 at 5:10 PM

POPSUGAR Photography | Benjamin Stone
POPSUGAR Photography | Benjamin Stone

It's been a hotly debated topic for longer than some of us have been alive: is a cardio or strength-training workout better for weight loss?

Let's look at exactly what happens in your body when you do a cardio workout vs. a strength-training workout, all the way down to the smallest level: cells! We'll explain how it all affects weight loss. Nicole Aurigemma, M.S. physiology at Women's Health and Exercise Lab at Penn State, gave us a little science lesson to explain which workout is better.

Long-Term Cardio Training (Running, Cycling, Etc.)

This is what goes down in your cells when you're sweating it out at a cycling class or on a half marathon. Prepare for a refresher in "seventh grade biology," as Nicole put it.

  • More energy and more energized muscles. This happens for a few reasons. Nicole told us that with cardio, you'll experience an "increased number of mitochondria in skeletal muscle cells" (mitochondria is the "powerhouse," or energy center of the cell). More mitochondria equals more ATP. ATP (adenosine triphosphate) is energy that supports your body's functions. More energy for your muscles is a good thing. You'll also experience changes in actual muscle fiber type. "Skeletal muscle can be found in three 'flavors,'" she said. "Fast twitch, slow twitch, and intermediate (mix of fast and slow)." More long-term cardio creates more slow twitch fibers. "These fibers are slower to fatigue and generally have more mitochondria than fast twitch." She said an example of this would be that a runner who trains at longer distances "will have primarily slow twitch fibers in their leg muscles." And as mentioned, more mitochondria means more energy.
  • Increased endurance. "More blood vessels means more oxygen delivered to mitochondria; that means more energy produced." Cardio creates an increase in capillaries ("very small blood vessels that transport blood and oxygen to the working skeletal muscle"); this also sends more oxygen to the aforementioned cellular powerhouses mitochondria. Why is this important? In Nicole's words, "we can continue attending (and kicking ass) at Spin class."
  • More fat burn. Increased mitochondrial content also means that "the muscle is now more efficient at using fat stores as energy — this is called fatty acid oxidation." In addition to having more endurance, more readily available oxygen, and stronger muscles, you'll also be burning more fat, which Nicole said could lead to weight loss.

Long-Term Resistance Training (Lifting)

Nicole made a note that "many women shy away from weightlifting because they do not want to look masculine" and said that "this is pretty misguided; unless you are lifting seriously heavy weights on a regular basis (and also have abnormally high testosterone levels), you will not bulk up like a dude." Wise words from a hip scientist.

Here's what happens when you lift.

  • Larger individual muscle fibers. After you lift, your body starts building bigger muscles for hours. "In response to resistance exercise, the cell mechanism for making more muscle — a process is called protein synthesis — is ramped up significantly for four hours postexercise and is maintained up to 24 hours," she said.
  • More muscle means more weight loss. "In reality, having more muscle is actually beneficial to weight loss," said Nicole. "Muscle is more metabolically active that fat." What does metabolically active mean? "Simply put, muscle uses more energy and thus burns more calories than fat." Makes sense.
  • Better bones and disease prevention. "Added bonus: resistance training also increases bone density, which prevents the onset of osteoporosis, and increases glucose handling, which aids in the prevention of type II diabetes."
  • Fast twitch muscles equals more power. Looking for a boost in the gym? Changes in muscle fiber type also happen with resistance and strength training. As Nicole puts it, the "muscle transitions to having more fast twitch fibers. This fiber type has quicker enzymatic function, meaning that power is produced faster." Power sounds good to us.
  • More energy. Sound familiar? Cardio isn't the only exercise that gives you more of those energy factories in your cells. And as we just learned, more mitochondria equals more energy.
  • Boosted metabolism. "Because you are increasing muscle size and content with weight training, you'll have muscle that will be more metabolically active than fat,'" she said. "This means that muscle is using more of the body's stored fuel sources such as fat and glycogen (this is a form of carbohydrates that is stored in the muscle and liver)." Using up fat and glycogen means you'll have "a slightly higher resting metabolic rate, meaning that your metabolism is increasing with training."
  • Fat and weight loss. Nicole continued on the last point to say, "Ultimately, this can lead to the decreases in adipose tissue (fat), which can also contribute to weight loss."

Wait, but Which Is Better?

That's the exciting part: they're both great for weight loss. Nicole told us that "it depends on your final goal." Trying to burn fat and lean out? "If you don't care about muscle definition, cardio is the way to go." But "if your main goal is to look and feel stronger, resistance exercise is key, and the weight will come off if you stick to a plan."

She said it's important to keep in mind that because you'll be gaining muscle mass through resistance and weight training, you might initially maintain or even gain a few pounds. Give it time. She told us, "if you are resistance training three times a week and allowing your body appropriate rest days, you'll start seeing awesome changes within your physique."

How does she know? From personal experience. Not only is Nicole a physiologist, but she's also an athlete. "I was pretty fit in college; I ran on the cross country and track team. But after college I gained about 20 pounds, because I wasn't running at the same intensity. This past Winter I started lifting regularly three times a week and I'm almost back to my college weight, but my body looks better." You go, girl.