When you eat carbohydrates (carbs), your body releases insulin into your blood stream. The insulin tells your cells what to do. It tells most cells to absorb the sugar out of the blood and tells your fat cells not to release any fat to burn (because there are carbs already in the blood to burn).
Correct, but keep in mind, even when one consumes protein and or fat, insulin is also released into the blood stream, just not as high amounts as carbs and specifically simple carbs.
One of insulin's jobs is to take sugar out of the blood the sooner the better, because sugar in the blood damages the body. The reason diabetics have such bad health problems (blindness, circulation problems, etc.) is because of the damage sugar does when the person can't make enough insulin.
The other issue is not just not manufacturing insulin, but insulin insensitivity. Some people make plenty of insulin, but they have lost sensitivity to it.
When you eat something with a lot of sugar in it, the body kind of panics and usually overdoes it. Too much insulin is released to mop up the sugar. It takes all the sugar out of your blood. Then your blood sugar is too low, so you crave more sweets.
This depends on the person and their particular insulin sensitivity.
So insulin saved the day, keeping that sugar from damaging your body, but in the process, it makes your body store fat and prevents you from burning fat.
This is where exercise is crucial. People who have a high level of fitness and exercise frequently and at a high enough intensity, will store that sugar as glycogen (fuel for exercise) in the liver and in the muscle tissue. Those who are sedentary, have no need to store the same levels of glycogen and the sugar gets stored as fat.
A low carb diet, limits carbs, so you don't put very much insulin into your system, so your body burns any fat you eat as fuel, and if you need more, your fat cells freely release fat into your blood stream to be burned as fuel and you lose weight.
If one is sedentary, low carbs becomes even more important.
But when you eat a large amount of carbs a hundred grams, two hundred grams, or more (most Americans get about 300 grams a day) your body releases insulin in huge amounts, which makes it almost impossible to burn fat. And makes it really easy to gain weight. (Some people don't gain weight, of course, even though they eat lots of carbs, they are the lucky ones who can eat whatever they want and not gain weight.) That is the basic process. This is why people lose weight so easily and quickly when they cut carbs, because carbs increase insulin, which leads to storing fat and preventing fat from being burned, and creates cravings.
Again, this depends on each person and most importantly the physical demands they place on their body. For an endurance athlete, 300 grams of carbs is next to nothing, as some consume over 500 grams, because of the need they have for glycogen storage and the insulin sensitivity, that exercise helps to promote.
Bottom line, it is a good idea to limit the release of insulin whenever you can, but this is especially true for sedentary people. Those who are very active, not so much.