The real myth is that you can track calories in and calories out.
How do you measure how many calories your body absorbs from the food you eat?
How do you measure your calories out, particularly your resting metabolic rate?
Here's a hint -- the body changes its resting metabolic rate in response to calories in! Probably! Unless you eat junk food, and then maybe it doesn't!
In practise you don't need to track the out because it doesn't change much per day. It's not easy to burn 200 calories but it's easy to consume them. That's why it's common to hear that weight loss happens in the kitchen.
It does require work to understand your particular body, and discipline to do it consistently. Which is again, why it is not a good technique for most people. Most people don’t have the time, motivation, or really the need to do what this woman (in the video) is doing.
Apps like Macrofactor do it for you. You just track what you eat and your weight.
One secret about using CICO is that you do not need to know the gut absorption rate! You just need to track the calories before absorption, the numbers printed on the box. Easy these days with a smartphone app. The measurement does not need to be perfect at all. It only has to be decent on average. You can have variable accuracy of 25% on any given food and still get well under 10% on average for a week. It doesn’t matter if there’s a bias either, if you track your weight. The reason absorption and bias don’t matter is because you’re doing science with a black-box (your body). Measure the known input, monitor the output. Do it for a long time and you’ll find the maintenance input. Track input above that and verify mass gain, track below and verify loss. The maintenance value you come up with will “absorb” and bias, and gut calorie conversion rate, and other inaccuracies.
Measuring output is easy with a heart rate monitor. Changing metabolism we already discussed, but important to know that the range of elasticity is like around 15%, and in practice for any give person may be lower. It’s not some massive factor you can’t predict, it’s within a narrow enough range for the majority of people, statistically, that you don’t need to worry too much. And it will be accounted for if you track calories and weight over a long enough time frame.
The real “myth” of calories in = calories out is that it’s possible to do it by feel. It takes a lot of work and math, like spreadsheet-level, to actually track calories in and out. So even though it is physically true as a concept, it’s not a good weight management strategy for most people.
Sugary snacks don’t make you feel very full for their caloric content. So unless you’re literally writing down the caloric information on the wrapper and comparing it to your total daily activity, it’s easy to overeat.