Do fitness trackers really work? Probably, but it's not as simple as you think.

As some of you may know, I’m not only a tech lover — I’m also a big fitness enthusiast. Therefore, the idea of a fitness tracker — which combines tech and fitness into a single product — seems right up my alley. Getting all your biometrics right on your wrist and seeing your progress charted over time is like something from a superhero movie.

There are practical, less nerdy benefits too. A fitness tracker should help you lose weight, improve your fitness, and adopt healthier habits around sleeping, eating, exercising, and moving.

Do fitness trackers really work, though? What is your fitness tracker actually measuring anyway? Just how accurate is it really? As much as I love this tech, I can see some serious inherent problems.

Don’t miss: The best fitness trackers 

The concept

To find out whether fitness trackers really work, first, we need to know precisely what they do. Fitness trackers come in all shapes and sizes and include various sensors and features depending on the brand. Almost universally, they rely on a heart rate monitor to calculate rough calorie expenditure and motion sensors to detect movement.

The premise is simple: the harder your heart is working, the more you’re exerting yourself, the more calories you burn.

Now, if you measure the calories coming in via a diet app like MyFitnessPal, you can see the difference and attempt to maintain a “deficit.” If you consume less than you use, your body burns fat, and you lose weight. If you consume more than you use, your body has a surplus of calories — which it stores as fat — you gain weight.

It’s simple math, but it has loads of problems.

The problem with heart rate monitors

The heart rate monitors used in most trackers work via a technology called pulse oximetry. Infrared light penetrates the skin and looks for subtle changes in the color of the blood. These colors represent the oxygenation of the blood, which increases with each heartbeat.

This technology is far from perfect. Many other things can impact blood oxygenation or otherwise change the appearance of the blood vessels. When you’re lifting weights, contractions alter the amount of blood reaching the limbs, so do changes like vasodilation. This is why chest-worn heart rate straps are more reliable than wrist-worn devices.

Also read: The best heart rate monitors

The best fitness trackers use complex algorithms analyzing movement to ascertain what activities you engage in and alter the heart rate interpretation accordingly. These are best described as guesstimates.

Those aren’t the only problems. Heart rate alone is not an ideal indicator of calorie burn. The assumption is that when the heart pumps faster, you are creating a demand for oxygen and energy and thus probably engaging in an activity that is “costly” from an energy perspective.

In reality, many things can increase your heart rate. Fitter individuals actually have lower resting heart rates because the strength of each heartbeat is greater — allowing their hearts to beat less frequently. Blood pressure, air pressure, ambient temperature, mood, and more can impact your heart rate.

Good fitness trackers don’t rely on the heart rate alone. They measure heart rate and movement combinations compared against patterns derived from huge data sets using machine learning. Regardless, it is still ultimately an inaccurate picture.

Finally, it’s also problematic that some smartwatches only measure your heart rate once every few minutes or so, unless you are engaged in some kind of workout. This is intended to conserve energy while allowing for an average resting and maximum heart rate. The amount of calories you burn during a gym session actually only has a small impact on your total daily calorie expenditure. Far more important are your levels of activity throughout the day. Your smartwatch may remind you to get up and walk around now and again, but it might also miss your quick impromptu dance routine in the kitchen.

It’s possible the real secret to impressive weight loss is to be more energetic and active throughout the day — and your fitness tracker has no way of effectively tracking this. Whether fitness trackers actually work depends on if you’re a runner or a weightlifter or whether you’re looking to lose weight throughout the day or just during your workouts.

Do fitness trackers really work?

What’s even more concerning is what is ultimately done with all this information.

The central hypothesis driving the smartwatch industry is that losing weight is a simple matter of calories in versus calories out. This view is held by many fitness experts, bodybuilders, and nutritionists. But not all.

Another crowd maintains that counting calories is actually ineffective for a large number of the population. According to this perspective, the answer has more to do with metabolism, hormone balance, insulin sensitivity, gut bacteria, nutrient density, etc. These people push intermittent fasting, ketogenic diets, and the paleo lifestyle.

Allow me to be so bold as to present my take. Burning more calories than you consume will result in weight loss. However, we have no idea how many calories we’re actually burning a lot of the time.

Smartwatches and the calorie-counting crowd rely on a number called an active metabolic rate (AMR) to do their math. This is the number of calories that you burn during a normal day of activity. In turn, this is calculated as your basal metabolic rate (BMR) — the number of calories required for breathing and thinking — plus any activity on top.

To get your BMR and your AMR, enter your height, your weight, and sex. This is necessary because taller people require more energy to go about their usual business — that’s why bigger folks tend to eat more. Likewise, men and women burn slightly different amounts of calories — with women usually requiring a little less food daily.

There’s a glaring omission here: muscle mass. Muscle is metabolically active; it requires energy to maintain and to use. Someone who is 165lbs and very muscular will burn more calories than someone who is 165lbs with no muscle.

There are ways around this. You can do a similar BMR calculation using your lean body mass, which is a much more useful figure for learning calories burned. Very few fitness trackers allow you to enter this information. None that I know of ask for a body fat percentage by default. Most users would likewise understand why that might be a problem.

However, the problem runs much deeper than that. Individual differences in calories burned don’t just come down to body weight and muscle mass. Two people roughly the same height and weight probably won’t burn the same number of calories.

Hormone balance plays a big role here. If you have a higher level of testosterone, you will burn more calories and build more muscle. Likewise, thyroid hormones, cortisol levels, insulin, and more all play a role.

Related: The best running watches you can buy

Don’t believe that hormones can make that much difference to your physique? Look at someone who uses steroids. They would have the same numbers as anyone else on a fitness tracker, but they’d look a whole lot different.

Now I’m not suggesting you use steroids, but while you might not have the sky-high testosterone levels of someone who uses them, a slightly higher or lower number could still have a minor impact on your weight loss.

Likewise, many women who start using oral contraceptives will find they either gain or lose weight as a result.

Similarly, many health conditions like hypothyroidism can drastically alter the way your body uses energy from food. You might not have hypothyroidism, but if you struggle to lose weight despite what the numbers are saying, you might still have a slower metabolism than average. It’s better to think about these things as spectrums rather than binaries in many cases.

Many other things might influence your actual BMR, too, such as your gut bacteria. Our guts harbor a lot of friendly bacteria that we are increasingly learning plays a very vital role in our health and wellbeing. These critters help us produce hormones and neurotransmitters, aid with our digestion, and fend off bad bacteria. Several case studies have shown us that fecal transplants (yep, it’s what it sounds like!) can result in rapid weight loss or weight gain thanks to the loss or addition of the bacteria.

If it were possible to count calories going in and out precisely, this would lead to more accurate weight gain or weight loss predictions. However, our idea of calculating calorie burn is probably pretty primitive, and the resulting number might end up being more misleading than helpful. This all casts some doubt when asking if fitness trackers work.

Should I throw out my fitness tracker?

If all this sounds very negative, forgive me. I’m actually a big fan of fitness trackers.

For most people, the calorie calculations are accurate enough to be useful. Calorie restriction is a great place to start, and it’s only when you find it doesn’t work that you need to start considering other options to improve your metabolism and general health and fitness.

As Tim Ferriss points out, sometimes just having a single metric to track can be incredibly valuable — even if that metric isn’t completely accurate or doesn’t paint a complete picture. Do fitness trackers work? If it’s been working for you, then yes! I know plenty of people personally who have lost weight using them.

That’s really where fitness trackers come into their own. They are highly useful devices for simply letting us track our general activity levels and identify areas where we could be doing better. Trying not to break the chain is a seriously motivating reason to keep going to the gym or keep your calorie intake low (just like Seinfeld).

Of course, fitness trackers also do much more than count calories and steps — they’re useful for training in heart rate zones (see my post on seven-minute workouts) and they provide sleep tracking, meditation modes, and more. I’m really excited for the future of these devices and how they might advise us on our chronotype or look at our heart rate variability to estimate recovery. Devices like the Oura Ring and Withings ScanWatch show promise in this regard.

Even by simply making us more aware of more aspects of our health, any fitness tracker can help improve many areas of our lives. That which is measured, improves.

Just understand the limits of these devices. Don’t take their results as law. There’s far more going on in our bodies than they can measure or than even we fully understand. It’s not as simple as asking whether or not fitness trackers work. Nothing about the body is simple!