The concept of CICO(calories in - calories out) reduces the human body down to a bomb calorimeter. That isn't all that useful because the body is a complicated organic machine that has evolved over millions of years. Bodies have hormones and to simply restrict calories doesn't necessarily mean that weight loss will happen.
When you reduce caloric intake, your body responds by downregulating the amount of energy your body burns - it literally understands that you're not eating enough food and then tries to hold on to what it does have - and this means you get tired easier and you burn less energy. Plus, you are constantly dealing with hunger - your body wants more food and is trying to return to it's set weight. Eventually, the hunger gets the best of a long term CICO dieter and they cheat and binge and then leave the diet. Because the body's hormones just dealt with a long-term fast - they are locked into a pattern where they hold onto calories they do get - so on the off-CICO binge, the body regains even more weight and settles at a higher body mass - perhaps to have a better shot at surviving the next starvation period. Most people do continuous bouts of CICO and end up fatter and sicker than ever.
Is there a better way? I think so. The key is to control hunger. How do you control hunger? Be satiated! Try a r/keto or r/zerocarb diet where you eat as much as you want when you're hungry - and stop when you're full. You're generally eating just fat and protein and restricting carbohydrates (sugars, grains, starches). This means you retrain your hormonal system to expect plenty of energy - but the meals you eat actually satiate you. It also means your body enters nutritional ketosis and the liver creates ketone bodies by breaking up fat into smaller molecules that can be 'eaten' by cells that don't have the full fledged mitochrondria that are needed to burn normal fat. In this way, your liver is creating a complementary fuel source of glucose (gluconeogenesis) and ketones (ketogenesis) while relying on dietary fat or body fat for the rest of it's fuel needs- and your hunger nearly completely goes away. Because you're never hungry - you don't feel the need to binge. Another issue is that when you eat carbohydrates, you break them down quickly into blood sugar and then your pancreas must dump a great deal of insulin into the bloodstream to usher the blood sugar into cells. Why does it do this? High blood sugar glycates the arteries and cells and is lethal over a long term - the body wants homeostasis of around 83 mmol in the blood - no more than 5 grams of glucose at any one time. When you eat the typical diet of 250-350 grams of carbs a day - you're riding a glucose and insulin wave. When your pancreas responds to a big deal of sugar - it may push too much sugar into cells - leading to an episode of hypoglycemia. This is why, after a high carb meal, you feel dizzy or faint or tired or hungry and then feel like you need another meal. But when you stop this whole rollercoaster phenomenon, your hunger normalizes around eating protein and fat. The main issue is that our bodies never evolved to deal with the copious amounts of carbohydrates that are now the recommended mainstay of the human diet. If you don't understand the science of these hormonal systems and treat the body as simply a flame - you're doing yourself a huge disservice. Maybe your CICO diet will last a couple of months - but you will be hungry and you'll be at the whim of this carb rollercoaster, and eventually your body will force you to eat more.
CICO works for a short time and it seems to make intuitive sense - which is why it has lasted so long as the primary way for weight loss - but in reality, it has been the brainchild of Big Food companies who want to push the idea that a steak and a bottle of soda are nutritionally the same as long as they have the same number of calories. They want you to think that everything in moderation, including their junk food, is a healthy path forward - but they ignore the hormonal systems at play. Plus, they try to say that carbs are better for you simply because each gram has half the calories of a gram of fat - but your body is much smarter than CICO - it knows how much macronutrients you eat and responds accordingly. Plus, the calories that your body spends, the CO, is 80-90% based on your metabolism and how your body normally works. You cannot easily work off more calories - that means that if you restrict the CI, your CO will also be reduced and you will get weaker.
I constantly see people at r/keto who are posting success stories say they tried an r/CICO diet and it wasn't sustainable - but because it's the most common advice - they think it's the only way to lose weight. In actuality, understanding how hormones work allows you to not be hungry - and over just a few days your body will enter a mode where it is completely reliant on burning fat for energy - which just so happens to be the fuel source that your body stores. The body stores only about 2,000 calories of glycogen - a sugar molecule similar to starch, but stores 40,000+ calories of fat. By not eating carbs, you're not dumping insulin into your bloodstream - which means the fat cells can release their fat calories into the lipid distribution system to be turned into ketones or be used by muscle cells as Free Fatty Acids. Insulin is a storage hormone that tells the body to stop burning fat so it can focus on burning the highly toxic sugar.
I just wanted to type this up for those of you struggling with CICO. I'm a moderator at r/ketoscience and have absorbed books, podcasts, videos, and lectures on these topics. The other amazing thing is that once in nutritional ketosis - you have a renewed amount of energy because your body isn't dealing with waves of toxic glucose. This means brain fog goes away, arthritis reduces, gut issues resolve, and many chronic diseases beyond obesity are helped beyond belief. If you're trying to lose weight with CICO - you may be losing weight but how good do you really feel? Once you get a couple weeks into keto - it is common to hear of the incredible energy that burning fat allows you to have, instead of having to cycle between a hodgepodge of glucose and fat.