Do You Really Need to Exercise to Lose Weight?

When most people decide to lose weight, they go on a diet. It's just automatic.

Exercising, however, tends to be a totally different story. After all, exercise takes time out of our already over-packed schedules, gym memberships don't come cheap and, let's face it, exercise isn't the most exciting thing you can do with the spare time you do have.

So we tell ourselves -- and anyone who tries to force us to exercise -- "diet makes a bigger dent when it comes to weight loss, anyway."

And you wouldn't be completely wrong.

[See: The 38 Best Diets Overall.]

Exercise's Weight-Loss Weakness

Weight loss boils down to calories in versus calories out. To lose weight, you have to maintain a caloric deficit, meaning that you're consuming fewer calories than you're burning each day.

And, when it comes to achieving that caloric deficit, minimizing calories in proves to be easier than maximizing calories out. "While it may take an hour to burn 500 calories by exercising, it takes just a few minutes to eat that all back," says registered dietitian and certified strength and conditioning specialist Alissa Rumsey, spokeswoman for the Academy of Nutrition and Dietetics.

What's more, it may actually take more than an hour to burn those 500 calories. While a systematic review from Montclair State University found that the actual act of exercise burns fewer calories than we've always thought, a 2016 Current Biology study determined that after a certain point, exercising for longer periods does not necessarily burn more calories.

In the study, people who worked out moderately burned about 200 more calories per day than sedentary folks. However, those who exercised even more did not burn as many calories per day as would be expected given their activity levels. Researchers concluded that physical activity wasn't all that important to weight loss.

[See: 13 Fun Sports That Burn Calories.]

Metabolic Rate -- or Why Exercise Still Matters for Weight Loss

There is, however, a huge difference between body weight and body composition. Body weight is the number on your bathroom scale. Body composition is the percentage of that number that is fat, versus the percentage that is made up of lean tissues like muscle. And, even though we all obsess over the scale, what really determines health -- or even physique for that matter -- is body composition, Rumsey explains.

It's important to realize that the Current Biology study controlled for both. It didn't investigate how exercise affects body composition. And, to be clear, when it comes to body composition, exercise rules supreme.

That's because, apart from burning calories in the gym, exercise (or more specifically, the right exercise) has the potential to greatly increase your resting (or basal) metabolic rate, according to research published in Proceedings of the Nutrition Society. Resting metabolic rate is the number of calories that you burn sitting, breathing and letting your heart beat. The number of calories you burn simply by living. The study's authors note that resting metabolic rate is the single largest determiner of how many calories you burn each day. So when it comes to weight loss and improving body composition, resting metabolic rate matters.

The authors even shrug off the fact that the direct effects of exercise on energy expenditure, meaning the number of calories you can burn during an exercise session, are just a sweaty drop in the bucket of overall caloric expenditure. They basically say, "Sure, you can only burn so many calories during 30 minutes of exercise. But exercise will still help you burn more calories the other 23 hours and 30 minutes of the day."

Exercise can increase resting metabolic rate in two ways. First, it creates what's termed excess post-exercise oxygen consumption, or EPOC, an increased caloric burn that continues even hours after you've left the gym as your body works to return to a homeostasis environment. EPOC is largely determined by the intensity of your exercise. Work closer to your max capacity, and the harder your body will have to work afterward to lower your core body temperature, repair muscle fibers, clear your body of metabolic waste and get you ready for your next exercise session. Second of all, exercise, especially strength training exercise, increases your body's levels of lean muscle, which, apart from uncontrollable factors like your age, sex and genetics, makes the biggest impact on your resting metabolic rate.

So, to connect the dots, exercise increases muscle, which increases your resting metabolic rate, which increases your daily energy expenditure, which helps you achieve a caloric deficit, which results in weight loss -- even on days that you don't exercise.

Exercise's ability to increase resting metabolic rate becomes even more impressive when you consider what weight loss does to your metabolism. According to the International Journal of Obesity, losing 10 percent of your body weight results in your resting metabolic rate slowing by 20 to 25 percent. Called "metabolic adaptation" or "adaptive thermogenesis," it's a byproduct of your body trying not to lose weight, which it interprets as starvation, even when it's achieved in a healthy, balanced way. (Of course, this metabolic adaptation is even worse in people who excessively cut calories, but that's a whole other story!) It also partially explains why, four to five years after successfully losing weight, about two-thirds of people actually weigh more than they did to begin with, according to research from the University of California -- Los Angeles.

[See: How to Weigh Yourself the Right Way.]

"In order to lose weight and keep it off in the healthiest possible way, you need to maintain lean body mass and lose only your excess fat. If you simply cut calories, but you don't maintain a healthy level of exercise, you will lose not only body fat, but you'll lose muscle mass as well. When you are losing both fat and lean body mass, or muscle, you are essentially starving yourself. Starvation results in hormone imbalances, muscle wasting, loss of essential minerals and a long list of physical problems," explains Brian Quebbemann, a metabolic and bariatric surgeon and founder of the N.E.W. Program weight-loss system in California. "Achieving a healthy balance between body fat and muscle requires a healthy balance between exercise and calories consumed. By focusing only on cutting calories, you are only looking at half the equation."

So yeah, you really need to exercise to lose weight. But not because exercise actually burns that many calories.

K. Aleisha Fetters, MS, CSCS, is a freelance Health + Wellness reporter at U.S. News. You can follow her on Twitter, connect with her on LinkedIn, find her on Facebook or the Web, or email her at kafetters@gmail.com.