As a fitness professional, I’m often asked about exercise routines and their effectiveness. What’s so disheartening for me is that so many people are operating under false assumptions – most of them promoted by years of popular media. Case in point: a friend recently sent me her latest exercise routine, which involves lots of burpees, kettlebell swings, box jumps, thrusters, and high speed intervals on the treadmill. She said to me, “This is one of my fat-burning sessions. I want to lose weight, so I’m doing this instead of strength training.” While these kinds of workouts do burn lots of calories, and can improve cardiovascular fitness and endurance – they won’t necessarily make you lose fat.
If someone wants to lose fat, conventional wisdom, fueled by a million fitness article headlines, promotes the same types of workouts as my friend described – moving frantically from one exhausting high intensity exercise to the next until they can’t take another step. We’re told that metabolisms will be stoked, muffin tops will be melted, and pounds will drop off at a rapid rate. This has to work, right? Unfortunately, not so much. Your body has other plans.
Your Body Fights Back
In 2012, a team of researchers in Denmark ran a very simple experiment. They had a group of overweight young men run or bike six days per week for 13 weeks. Half of them exercised for 30 minutes a day, burning around 300 calories per workout. The other half exercised for 60 minutes each day (twice as long), burning about 600 calories per workout. At the end of 13 weeks, the researchers measured the amount of body fat lost by each participant.
What was the result? While you’d expect the biggest calorie-burners would have lost the most fat, in fact the amount of fat lost by both groups was virtually identical.
How is that possible? Several interesting factors come into play here, both mental and physical.
High intensity exercise makes you hungry (duh). So what very often happens is that you end up quickly replacing the calories you worked so hard to burn – and then some. The harder you work, the hungrier you get, and the more you eat – making that caloric deficit disappear. There’s also an emotional phenomenon called “moral licensing” where being “good” (exercising hard) gives you permission to be “bad” (eating sugary junk).
Workouts that burn a lot of calories are physically and mentally taxing. They leave you exhausted, and often sore. Once you’re finished one of these workouts, you may find you move much less than you otherwise would have – because you simply don’t have the energy!
So instead of cooking a healthy meal at home, you run through the drive-through. Instead of doing chores, you put them off. Instead of taking a walk after dinner, you binge the latest Netflix hit.
The Body’s Pilot Light
The body has a particular physiological process with a complicated name: non-exercise activity thermogenesis, or NeAT for short. It represents the calories your body burns during physical activities other than sleeping, eating, or structured exercise—things like typing, cooking, gardening, housework, or even just fidgeting.
It’s surprising how much NeAT contributes to daily calorie expenditure – the difference between two people could be as much as 2,000 calories per day.
The approximate number of calories burned at complete rest (sleeping) is what’s commonly known as the Resting Metabolic Rate (RMR). For an average person, this is about one calorie per kilogram of body weight per hour. Then come the calories burned through NeAT. So if you’re sitting at a desk, you’re burning about 5 percent more calories, or an extra 10 to 20 per hour. Stand up and walk around, and you’re burning about 10 percent more. Even the simplest physical activities (like fidgeting) can increase your energy expenditure by 20 to 40 percent above your RMR.
The Body’s Energy Budget
A growing body of research shows that if you burn lots of calories via exercise, your body adjusts by spending less energy elsewhere, independent of NeAT or your energy (food) intake.
The human body, for reasons not yet fully understood, appears to put a cap on the number of calories it will burn from physical activity.
Researcher Herman Pontzer, PhD, explains this phenomenon of “constrained energy expenditure” this way :
If we push our bodies hard enough, we can increase our energy expenditure, at least in the short term. But our bodies are complex, dynamic machines, shaped over millions of years of evolution in environments where resources were usually limited; our bodies adapt to our daily routines and find ways to keep overall energy expenditure in check.
In effect, your body budgets for the cost of additional activity by cutting back on the calories it would ordinarily use for running the basic metabolic functions that keep you alive.
What This Means for You
Diet and exercise are different tools with different benefits. When it comes to losing fat, the food you eat (or don’t eat) is just as important as how you exercise. The fact is, human metabolism is too complex to let you to manipulate any one aspect of it without affecting other aspects.
Understanding this, it’s no surprise that “fat-blasting workouts” don’t live up to the hype. They may burn a large number of calories – but they also cause your body to fight back by ramping up your appetite, and dialing down your activity level and metabolism, making the fat loss even more difficult.
Focus instead on increasing strength, endurance, and muscle mass, all of which will contribute to a longer, healthier life and, over time, will help you get – and stay – leaner.
 Rosenkilde M, Auerbach P et al. Body fat loss and compensatory mechanisms in response to different doses of aerobic exercise – a randomized controlled trial in overweight sedentary males. American Journal of Physiology, 2012 Sep 15;303(6):R571-9. www.ncbi.nlm.nih.gov/pubmed/22855277
 Pontzer, Herman. Debunking the Hunter-Gatherer Workout. New York Times, 24 August 2012. www.nytimes.com/2012/08/26/opinion/sunday/debunking-the-hunter-gatherer-workout.html