What's the best fat loss workout? Back in the day, it used to be that if you wanted to shed fat, you went out on a long run. For some people, it still is. The reason goes back to the 60s, when the concept of aerobics—exercise that uses free oxygen—first emerged as a way to improve cardiovascular health and, yes, lose weight. Of course, Aerobics by Kenneth Cooper, the book that jump-started the scientific community’s fascination with cardio, dissed strength training in the process.

Cardio was for slimming down and overall health. Strength training was for professional weight lifters and bodybuilders trying to get extremely strong or bulk up. The problem was, not a whole lot of research actually backed up the claim that aerobics—low- to moderate-intensity, steady-state cardio—was all that helpful for fat loss, explains Hayden Steele, CPT, CSCS, an Oklahoma City–based personal trainer and strength coach. He says that a bias toward aerobics led to researchers neglecting strength training as a form of exercise for decades.

For example, in one 1987 study published in the British Journal of Clinical Practice, when sedentary women followed a 10-week jogging program, they reaped “no alteration in body weight or body composition.” As Steele says, researchers had more questions than answers. It wasn’t until the mid-90s before research looking at strength training and more intense forms of cardio started to become academically accepted.

Fat Loss Workout:  Muscling Up

In 2007, a groundbreaking study examined the effects of resistance exercise on fatty tissues. The results? “Significant,” Steele says. “This was the first piece of exercise-based research to suggest resistance exercise has a direct impact on fat metabolism, the body’s ability to break down and utilize fat stores for energy, a physiological process known as lipolysis.”

This fat-burning benefit had long been the calling card of steady-state cardio, says kinesiologist Ryan Campbell, a training specialist at Anytime Fitness of Southern Wisconsin. “In the not-so-distant past, there was the scientific concept that working at an aerobic level [low intensity] versus an anaerobic level [high intensity] allows the human body to use fat more efficiently,” he explains. In fact, many cardio machines have built-in programs that keep exercisers in the “fat-burning zone.” The idea is that, when working between 60 and 70 percent of your maximum heart rate, a pretty low intensity that can be maintained for long durations, the body uses fat, rather than carbohydrates, for energy. However, the truth is that, while low-intensity cardio burns a greater percentage of calories from fat compared to high-intensity work like heavy lifting, it burns fewer total calories per minute—including those from fat, Campbell explains.

Exercise physiologist Abbie E. Smith-Ryan, PhD, CSCS, director of the Applied Physiology Laboratory at the University of North Carolina–Chapel Hill adds that high-intensity strength training also has the advantage of burning calories and fat after you finish your workout. That happens through a process called excess post-exercise oxygen consumption (EPOC), in which the body recovers and gets back to baselines. Smith-Ryan explains that high-intensity strength training depletes the body’s stores of ATP (energy) along with the anaerobic energy substrates glycogen and creatine phosphate.

The impact of EPOC on total calories burned depends on your exact fat loss workout, but one Colorado State University study suggest a metabolic boost of about 4.2 following an hour and 40 minutes of strength training. Research suggests EPOC may last anywhere from 12 hours to a few days after a tough anaerobic workout, with one Journal of Sports Science review noting that intensity is the primary driver of EPOC. “Higher intensity exercise like strength training, although hard as hell, provides a greater stimulus and burns more calories and fat over a shorter period of time spent exercising,” Smith-Ryan says.

Even more importantly, at least as far as a good fat loss workout, is that strength training also builds muscle. According to researchers, that explains why, in one 2014 study in the journal Obesity, when Harvard scientists followed 10,500 men for 12 years, they found that those who performed 20 minutes of strength training per day gained less abdominal fat than those who spent the same amount of time doing in cardio exercise.

“Muscle contributes approximately 22 percent of your body’s resting metabolism,” explains Steele. “Muscle demands fuel, and the more muscle you have, the more calories you burn.” Estimates suggest that each pound of muscle burns approximately six calories per day at rest.

But we aren’t advocating for a sedentary lifestyle, now are we? “By improving strength potential, muscle growth can directly improve speed, power, and athletic performance,” Steele explains. Translation: muscle gained helps you perform more work at a given exercise intensity, complete new feats of strength and power, burn even more calories and fat, and make continued progress toward your body composition goals.

That’s in contrast with steady-state cardio, which your body is actually phenomenal at adapting to. “Based on the current literature, cardio sessions can often lead to burning fewer calories and fat for a given exercise, which is counterproductive to weight-loss goals,” Smith-Ryan explains.

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Still, Not All Cardio (or Strength Training) Is the Same

Despite all of the back-and-forth between cardio and strength training at to which is the best fat loss workout, it’s important to realize that both are pretty huge catchall terms.

After all, all exercise, when you really get down to it, is cardiovascular, Campbell explains. (If the heart isn’t beating, the body isn’t moving!) And high-intensity  cardio routines like Tabatas are actually primarily anaerobic in nature, so while they might not build as much muscle as high-intensity strength workouts, they will still burn a significant number of calories both during and after your workouts through EPOC, Smith-Ryan says.

Meanwhile, curling five-pound dumbbells is radically different that performing heavy deadlifts, even if they both qualify as strength training, Campbell adds. One 2017 study suggests that total workout volume (weight times reps times sets) is a primary determiner of strength training’s effects on body fat. Choosing multi-joint movements that allow you to move more weight per rep is one simple way to increase volume, Smith-Ryan explains. Performing more exercises, reps, and sets is another way.

Basically, intensity matters. For an optimal fat loss workout she recommends including both heavy strength workouts (that emphasize building muscle) and fast-paced circuit-style resistance workouts (that jack your heart rate up). Your low-intensity cardio days—yes, like those slow and steady jogs—can help your body recover from high-intensity strength training so you can give each heavy lift your best effort, suggests one study in the American Journal of Physical Therapy & Rehabilitation. “A well-planned and progressed training program should blend both low-intensity and high-intensity training to prevent injury, overtraining, and psychological burnout,” Steele says.

As far as current science can tell, every form of exercise has its place, even in fat loss. And for maximum results, try our new 20% Tougher series of workouts, exclusively available on the Spartan FIT app.