A reader sent me a copy of his exercise routine, which involved lots of burpees, kettlebell swings, box jumps, thrusters, and skater jumps.

“This is one of my sessions for burning fat,” he wrote. “I’ve been doing it instead of lifting weights, because I want to lose weight. Is this effective?”

If one of your clients wants to lose fat, conventional wisdom has it that you should put them through the type of workout my reader described, running frantically from one heart-pounding exercise to the next until they’re left on their knees, exhausted, in a puddle of their own sweat.

Metabolisms will be revved, bellies will be melted, and pounds will be lost. A million fitness-article headlines couldn’t be wrong. Or could they?

Not entirely. These workouts may well burn lots of calories. Done consistently, they’ll also offer long-term benefits, like increasing aerobic fitness and work capacity. But they’re not necessarily going to make your clients any leaner.

Let me explain why.

How the body fights back

Back in 2012, a team of researchers in Denmark ran a very simple experiment. They recruited a group of overweight young men to run or cycle six days a week for 13 weeks. Half of them exercised for 30 minutes a day, burning around 300 calories in each workout. The others exercised for twice as long, burning roughly 600 calories each time.

You might expect, not unreasonably, that those who burned the most calories would lose the most fat. But you’d be wrong. In fact, the fat loss was virtually identical. Men in the 600-calorie-per-workout group ended the study no leaner than those who did half as much exercise.

How is that possible?

The first thing to consider is the effect a workout has on hunger. If it stimulates your appetite, you end up replacing the calories you worked so hard to burn, if not more. Research has shown that some of us are compensators (we eat more following exercise) while others are non-compensators (we don’t eat more, or may even eat less).

You’ve undoubtedly seen this in your clients. The harder they work with you, the hungrier they get, and the more they eat, thus reducing (if not negating) the calorie deficit created by your program.

That’s just one way exercise is linked to more food consumption. There’s also a phenomenon known as moral licensing, where being “good” gives you permission to be “bad.”

Here’s an example from my own experience:

A while back, I went mountain biking through the Sierra Nevada, a mountain range in southern Spain. I cycled up to eight hours a day for six days. Some of the climbs were so steep and narrow that I had to pick the bike up, put it across my shoulders, and walk. It was also extremely hot, in the neighborhood of 95 degrees (35 Celsius).

With all the calories I was burning, you’d think I would have returned home several pounds lighter. But I didn’t, for a simple reason: I ate massive amounts of food at the end of each day, in part because I felt I deserved it. I told myself I could eat whatever I wanted after all those hours on the bike in the hot sun.

But increased appetite isn’t the only way your body can compensate for exercise. It can also downregulate the amount of movement you do between workouts.

A NEAT trick

You’ve probably heard of non-exercise activity thermogenesis, or NEAT for short. First described by Dr. James Levine of the Mayo Clinic in the early 2000s, it refers to the calories you burn during physical activities other than sleeping, eating, or structured exercise—things like typing, cooking, gardening, housework, or even just shifting around in your chair.

It may sound trivial, but you’d be surprised at how much NEAT contributes to our daily calorie expenditure. The difference between two people could be as much as 2,000 calories a day.

At rest, on average, most of us will burn about a calorie per kilogram of body weight per hour. That’s your resting metabolic rate, or RMR. If you’re sitting at a desk, staring at a computer screen, you’re burning about 5 percent more calories, or an extra 10 to 20 per hour, according to this study. Get up and walk around and you’re burning about 10 percent more. Even the most innocuous physical activities, like fidgeting, can increase your energy expenditure by 20 to 40 percent above your RMR.

Here’s how it relates to your clients:

Workouts that burn a lot of calories are physically and mentally taxing. They’ll often leave your clients tired, exhausted, and sore—which, of course, is exactly what many clients say they want, and why they pay you. You’re just giving them what they asked for.

But there are consequences to an apocalyptic “go hard or go home” workout. Once they leave the gym, they’ll move much less than they otherwise would have. They simply don’t have the energy.

So instead of cooking a meal from scratch, they’ll get takeout. Instead of doing household chores, they’ll put them off, or pay someone else to do them. Instead of taking a walk after dinner, they’ll binge Stranger Things on Netflix.

It’s another form of compensation, only instead of eating more between workouts, they burn fewer calories. Either way, their energy balance stays about the same despite their workouts with you.

Turning down the metabolic dial

More interesting still, 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 intake.

Say you have a new client who’s moderately active. Training with you increases her overall activity level, but it doesn’t increase her daily calorie expenditure. The human body, for reasons we don’t yet understand, appears to put a cap on the number of calories it will burn from physical activity.

Hunter College researcher Herman Pontzer, PhD, explained the phenomenon of constrained energy expenditure in this article:

f 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.

Pontzer believes that your body budgets for the cost of additional activity by cutting back on the calories it would ordinarily use on the moment-to-moment metabolic tasks that keep you alive.

What this means for you and your clients

Diet and exercise are different tools with different strengths. When it comes to losing fat, the food you eat (or don’t eat) is a lot more important than what you do in the gym. But human metabolism is too complex to allow you to manipulate any aspect of it without affecting other aspects.

Once you understand that, it’s not really a surprise that the workouts we describe as “fat-blasting” don’t work as advertised. Yes, they may burn a large number of calories. But they also cause your client’s body to fight back by adjusting the dials on her appetite, activity levels, and metabolism, making the quest to lose fat increasingly difficult.

Don’t think of a workout as a way to burn fat, and try to avoid promising that benefit to your clients, unless you’re confident you can help them manage all aspects of the energy-balance puzzle. The amount of fat a given workout burns is not the only or even the most important way to judge its effectiveness.

Focus instead on helping them increase strength, endurance, and muscle mass, all of which will contribute to a longer, healthier life, and all of which, over time, will help them get leaner.