How much of that greasy burger with French fries did the last bike ride burn off? Can I still squeeze that bowl of chips for TV into my calorie budget for the day? Many people think that questions like these can be accurately answered with calorie counting. It might not be as easy as that. Let’s look at how hard it is to estimate how many calories you burn in a day.

We already know from the previous article that tracking calorie intake with any sort of accuracy is nearly impossible. To make matters worse, tracking calories expended might be just as hard if not harder.

Calorie-burn estimates are very imprecise

According to an online calorie expenditure calculator, you just burned 700 kcal with that hour-long bike ride. You put in your weight, age, average speed, and everything. That should be pretty accurate, right? Wrong. Unless you’re willing to spend a lot of money to go into a hermetically sealed isolation chamber in a lab to have direct calorimetry done, you’re out of luck. This method only has a 3,3 % margin of error but is rarely used. The vast majority of calorie estimates you’ll find are done via indirect calorimetry methods measuring gas exchange. Those have a margin of error of up to 45 %! Also consumer fitness trackers are off by 9 – 23 % for aerobic exercises and by 30 % for total calorie expenditure.

Everyone burns calories differently

How many calories you burn while resting or exercising depends on a lot of factors. For example a single variation in the FTO gene causes people who have it to burn 160 kcal less per day. In cold environments, people burn up to 400 kcal more per day if they have a lot of brown fat tissue (it fat has more mitochondria and is used to heat up the body). Hormones are also a substantial factor, the basal metabolic rate can increase by 10 % during a women’s menstrual cycle.

How much you eat will affect how much you burn off

In theory, if you eat 1000 kcal more than you need every day for 8 weeks, you should gain about 7,4 kg (16 pounds) of total body weight. In reality, your metabolism increases as a reaction to a higher calorie intake and you gain much less. According to studies, you would gain only something between 0,4 to 4,2 kg (0,8 to 9,3 pounds). The variance is quite substantial. According to estimates you also burn more energy digesting protein (20 – 30 %), than carbs (5 – 10 %) or fat (0 – 3 %).

All in all, it’s clear that measuring expended calories accurately is nearly impossible, whether you are sitting down or riding your bike. Just like with counting calories in, there’s a high margin of error. It has been estimated that for both calories in and calories out the average error is at least 25 %. With the average 2000 kcal daily calorie budget, you might easily be off by 500 kcal either way, and that’s just not acceptable. So, what should you look for in food if not calories? We will take a look at that next time.

Next up in Calorie Counting Doesn’t Work series

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