betrayal
Banned
The goal of your fasting period should be to empty out your carb storage (glycogen). This "opens up the pathway" for your body to use stored fat. If you have available carbs, then your body will use that. The body stores about 400-600 grams of glycogen and uses it throughout the day.
To echo @ssolitare , sugar is your enemy.
"Burn more calories than you use" isn't exactly wrong, but it's far too simplistic. Alcohol and sugar will still be processed by the liver into a fat cell even if you are in a calorie deficit. Fat storage will still be ignored if your insulin levels are too high. There are factors at work beyond mere calories.
Bringing down your insulin makes your tissue more insulin-sensitive (duh!). That includes your fat tissue. Fat tissue that is sensitive to insulin (and therefore readily available for burning) is the normal state of your body fat storage. We've thrown our bodies out of whack by having too much sugar. Most of our hunger cravings come from sugar, too: your brain gets too addicted to the easy and readily-available energy. It starts to prefer it. When your "blood sugar is low" (not a real thing unless you're actually a diabetic) that's just your brain complaining that it doesn't have a bunch of easy-to-use energy. It's not actual hunger.
The Keto dietabusesuses this facet of your metabolism. Cutting carbs gets you out of this cycle. That's why some call it a "hack" because it seems like it's cheating. It's not cheating. We just have terrible diets in the West, so when you return to something normal it seems like a miracle.
Fun fact: "losing your water weight" refers to your glycogen storage. 1 gram of glycogen is packed with 3-4 grams of water each, so when you empty out your glycogen stores you also lose a few pounds of extra water.
Sure, if you're fat and inactive, your body will have trouble dealing with high insulin. It's all about insulin sensivity, like you said, which will get worse the fatter you are. If you stay relatively lean, eat at least a semi healthy diet and exercise regularly you will never have any problems.
Yes, insuline causes fat cells to absorb glucose and fatty acids and also supresses an enzyme called hormone-sensitive lipase, in short HSL, which also helps to break down body fat into fatty acids so it can be burned and used as energy. But just like insulin dietary fat also suppresses HSL. There are also other enzymes like acylation stimulating protein which also helps storing dietary fat as body fat, even if you have low levels of insulin. That's why it makes no difference in fat loss if you eat fat only or sugar only as long as you maintain a let's say 500 calorie deficit. The result, the final fat loss, is exactly the same. Basically we're back were we started...energy balance matters.
So many people got the concept of insulin wrong. High or low levels of insulin do not make us fat or burn body fat. The thing is, that when our body produces insulin, we receive instructions to gain fat. In response we eat more and / or decrease energy expenditure. You can have constantly extremely high insulin levels and still lose weight. You can also have abysmal low insulin levels and gain a shitload of body fat.
The marketing Keto is not a "hack" or better than any other diet, the only thing it hacks is your wallet. If you have a 1000 people and put them into two groups, one with a keto diet and the other with a high carb diet. All are trying to lose weight with 500 calories daily deficit. Whats the result? Both groups will lose the same amound of body fat and there will be no difference in body composition. That's just simple facts. Everyone can use google and check the countless number of studies, that were not commissioned by enterprises and people who promote and sell "Keto diets".
Stop falling for this marketing bullshit. If you want to lose weight, just pick a tool aka diet that works best for you and which you can most easily incorporate into your everyday life. Try to eat more healthy. Be more active. Don't buy any "magic" supplement. Maintain a caloric deficit. That's all it takes.