Why fitness apps use calories and how they can help you lose weight

Why fitness apps use calories to help you lose weight


There's no barcode on this doughnut so it doesn't count
There's no barcode on this doughnut so it doesn't count
Photo: Graham Bower/Cult of Mac

Most fitness apps seem obsessed with calories. Go for a run, and your Apple Watch tells you how many calories you burned. Scan a barcode and MyFitnessPal tells you how many calories are in the food you are about to eat.

So what exactly are calories, and does counting them really help you achieve your fitness goals?

Calories, joules or Nike Fuel — it’s all just energy

Kilocalories, or “calories” as they are more commonly known, are a unit for measuring energy. Not the spiritual kind of energy that yoga practitioners talk about, but the technical kind. One kilocalorie is approximately the amount of energy required to heat a kilogram of water by 1 degree Celsius. Scientists describe this kind of energy as “the ability of a system to perform work.” In fitness terms, the “system” in question is your body, and “work” is all the things that your body does.

There are other units used by apps for measuring energy, such as the joule (a standard unit equivalent to roughly 0.25 calories) and Nike Fuel, a propriety unit used by Nike apps to “measures your movement.” Two to three units of Nike Fuel are roughly equivalent to 1 kilocalorie, although Nike has never provided an official conversion rate. Proprietary units of measurement like this are not great, because they lock you in to a single platform. (Even though Apple’s Health app still supports Nike Fuel, hopefully people will stop using this measurement now that Nike’s FuelBand has been scrapped.)

Kilocalories, more commonly known as just “calories,” are what most of us are familiar with, because they are used in food labeling and most fitness apps.

How does MyFitnessPal know how much you should eat?

Energy enters your body via the food you eat. Most of this energy is used simply to keep your body alive. A beating heart, breathing lungs and even a thinking brain all require energy. The bare minimum energy you need just to stay alive is known as your “basal metabolic rate,” or BMR.

You can think of your BMR like the fuel that an automobile uses to keep the engine idling when the car is not actually going anywhere.

BMR varies from one person to another depending on a wide variety of factors. Apps like MyFitnessPal make an estimate of your BMR in order to tell you how many calories you can eat per day. They use a formula to do this, based on your age, gender, weight and height. It provides an average guide, but it is not 100 percent accurate for every individual. Factors like health, fitness, body fat and your unique genetic makeup all influence your actual BMR.

If you’re interested, you can use the BMR calculator on the MyFitnessPal website to get an estimate of your BMR.

What Apple means by “Active Energy”

In addition to your BMR, your body also needs energy to fuel your daily activity — things like going to work and exercising. The amount of energy consumed in this way is what Apple Watch and the Health app in iOS 9 refer to as “Active Energy.” In earlier versions of iOS, this was known as “Active Calories”.

When you combine your BMR and your Active Energy, you get your total daily energy expenditure, or TDEE:

BMR + Active Energy = TDEE

How calories affect your weight

When the total calories you eat are less than your TDEE, this is known as a “calorie deficit,” and it will result in weight loss. As a guide, a deficit of 500 to 1,000 calories per day should result in a loss of 1 to 2 pounds per week.

To achieve a calorie deficit, you normally need to increase your TDEE by doing more exercise and reduce the calories you are eating. If you don’t want to give up your junk food, it might seem tempting to focus just on the exercise part. But it is hard to burn sufficient calories through exercise alone. Changes in diet are usually required as well.

Not everyone wants to lose weight. Some people may be advised by their doctors to gain weight. And professional athletes such as bodybuilders often want to gain weight by packing on more muscle. To achieve this, the same principles apply, but in reverse. You need to be in calorie surplus — eating more calories than your TDEE.

Calories in: Why Einstein was wrong about doughnuts

Ask Siri how many calories there are in food and you may get a surprising answer. Take that doughnut I’m eating in the photo above. It weighs 30 grams (about 1 ounce). When I ask Siri “how much is 30 grams in calories?” I get this surprising result: 6.444 x 1014. That’s a scientific way of saying 644,400,000,000 kilocalories. That’s more than enough calories to fuel my next run. In fact, it’s the kind of energy released by a large nuclear bomb.

Can there really be that many calories in my humble Strawberry Frosted Sprinkles Donut? Not according to Dunkin’ Donuts, which says the doughnut only contains 290 calories.

The reason for this difference is because Siri is using Einstein’s famous equation E=mc2 to answer the question. From a physicist’s perspective, the matter comprising my doughnut is the equivalent of that much energy, but there is no way that my humble digestive system would get that much out of it. I would literally have to vaporize the entire doughnut out of existence to release that much energy.

Instead, when I eat the doughnut, my body uses various chemical reactions, known as “metabolic pathways,” to access the “macronutrients” (fats, carbohydrates and proteins) stored in the doughnut and ultimately convert them into adenosine triphosphate or “ATP” — the chemical used by the body for transferring energy.

The amount of energy our body can extract from food in this way is known as the bioavailability of the nutrients. The most common estimates for how many calories we can get from macronutrients are 4 calories per gram from protein or carbohydrate and 9 calories per gram from fat. But this can vary depending upon factors such as how the food is prepared and how well your digestive system is functioning.

So the amount of energy I’m actually going to get out of that doughnut can vary — the calorie estimate on the label is only a rough guide.

You Apple Watch can't assess the efficiency of your running style
Your Apple Watch can’t assess the efficiency of your running style.
Photo: Graham Bower/Cult of Mac

Calories out: Are you a gas-guzzling SUV?

Fitness gadgets like Apple Watch tell you how many calories you have burned from exercise. Unfortunately, this estimate can also be quite inaccurate.

To calculate how much energy you are burning, fitness apps use a system called Metabolic Equivalent of Task values, or METs. These values are based upon a statistical analysis of how much energy different types of exercise burn. While they take into account your body weight, together with the nature and duration of the exercise, they don’t factor in the fact that we all perform exercises differently and our energy efficiency varies as a result.

Take different running styles, for example. The amount that you move your arms as you run will affect your energy efficiency. So too will your unique biomechanics — the shape and size of your limbs and joints and how well suited they are to the exercise you are performing. MET values can’t take any of this into account.

That is a bit like estimating how much fuel your car will use on a journey based upon the average fuel efficiency of all cars on the road. Some of us are gas-guzzling SUVs while others are more like hybrids. As the Environmental Protection Agency always likes to say: Your mileage may vary.

If calories are inaccurate, why do fitness apps use them?

The precise calorie estimates that fitness apps present us with provide a reassuring sense of accuracy. But the truth is, this is just an illusion. As we’ve seen, many estimates are involved, and inaccuracy can occur at every stage in the process. Estimating your basal metabolic rate, the bioavailability of energy in food and the calories burned from exercise are all prone to significant errors. But that does not mean we can afford to ignore calories altogether.

Calories may offer only an imperfect view of what is going on, but that view is better than not measuring anything at all. While it does not make much sense to attempt to precisely balance your energy in and out to the exact calorie in MyFitnessPal, it is important to monitor your calories if you want to lose or gain weight. Using these apps helps raise your awareness of the choices you are making that affect your health. And by monitoring calories in and out as you make changes in your diet and workout regime, you will gradually discover the right balance for you.


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