There are many vast oversimplifications in this, but is is a very complex system, and making it understandable to someone unfamiliar requires such.
The cast of players:
Hilde: a human
Insulin: a hormone, helps sugar get into cells, also passes messages around the body, produced in the pancreas.
Hungry Hormones: (Ghrelin) stimulates hunger, produced by the stomach and pancreas.
Full Hormones: (Leptin) stops hunger. Released by the pancreas.
Receptors for hormones: all hormones have receptors, they are usually produced by the individual cells. An important fact is that high levels of a hormone cause the cell to make less of the receptor, and can create a similar effect as having a low level of the hormone.
Muscle Cell: a metabolically active cell, it needs energy to do its job. It likes doing its job, if it does not have enough energy to do it, it will ask for more. It thinks in the short term, does not save much for the future.
Fat Cell: A cell with Hilde's long term survival in mind. It will take up energy in times of plenty, and keep it around for her to use when the going gets tough. It likes to think of the future, will sometimes make the other cells skimp on the present to save enough, especially if it thinks that hard times are likely because they have happened before.
The Story
Hilde eats carbs. They are changed to sugar in her digestive system, and absorbed into the blood stream.
Her stomach feeling full, releases Full Hormones.
The pancreas notices the sugar, and does it's job, releases insulin and Full Hormones.
Hilde is no longer hungry, she stops eating.
The insulin binds to the insulin receptors on various cells, the combination of insulin-insulin receptor transports the sugar into the cell. The cell gets energy and does whatever it's job is better.
The level of sugar in the blood goes down, the pancreas stops producing as much insulin, low blood sugar + low insulin causes a release of Hungry Hormones, Hilde gets hungry , eats again.
This goes on and on. Then something goes wrong:
Hilde eats carbs. They are changed to sugar in her digestive system, and absorbed into the blood stream.
Her body overreacts, produced too much insulin, quickly the blood sugar drops too low, she gets hungry again (a deep-down, can't ignore hunger, craving for what her body is low on, carbs, a craving in the lizard brain, one that sends signals of deprivation, her body thinks it is being starved, the part that sees and responds to the low blood sugar does not see that this all started from a bit of an over reaction, it just see that there is not enough fuel to run the body, and acts to correct this life-threatening condition), she eats more carbs.
This cascade of overreaction of the hormonal systems continues, resulting in an excess of insulin in the blood, and widely varying blood sugar levels.
Now, lets see what her individual cells see.
First, Muscle Cell (or other cells with a metabolic job, a non-fat cell). The part of it that is making the insulin receptors sees that there is an excess of insulin. Muscle Cell takes action to protect itself, it lowers the number of insulin receptors on it's surface. When there is low blood sugar, this lowered amount of receptors means that not enough energy can get into the cell. This low amount of energy, called starvation, causes the Muscle Cell to ask for more, it sends out signals to make more Hungry Hormones, to ask Hilde to eat more to send more energy its way. These two parts of the cell are doing their jobs, they have very overbearing bosses, so even when they meet at the water-cooler and decide they can correct each others problems, they go back to their desk and their bosses make them do what they were doing before, cutting down on insulin receptors and sending out signals that they are starving.
This leads to high blood sugar at the same time as high insulin. This is called Type 2 diabetes.
Now, Fat Cell, it sees the increase in blood sugar. It interprets this as the body having extra energy around. It was made for just this case, it steps up to action and fulfills its destiny. It uses the insulin (it loves it so much it is less likely to down regulate the receptors) to take in the extra sugar, and converts it to the most efficient (in terms of weight per unit energy) energy storage material it can, fat. During times of low blood sugar, it interprets this as starvation, a vindication for it, proof that it is needed, so it makes itself more efficient at making more energy to store. It gets a sense that it is very important, and starts taking energy at lower and lower levels of excess energy in the system.
This lowers the overall amour of available energy for Muscle Cell. Who gets hungry and asks for more. Also of note, if Hilde was starving herself, Fat Cell would see this, and increase its ability to grow in the future. It has done this every time she put herself on a diet in the past.
The initial over response to sugar, leading to an increase in insulin, and a subsequent decrease in insulin receptors what started this whole cascade. Individual people have vastly different likelyhoods that this will happen. It is when there are too many carbs for the individuals tolerance for a time longer then the individuals tolerance that it starts. Every body is different, some people can eat lots of sugar and never have it effect them, others can't.
The diabetes is causing the Fat Cell to grow and multiply. It is responding to the imbalance in insulin and sugar, and even protecting the body (in the short term) from even higher blood sugar. Once this all starts, the cycle behaves like a positive feed back loop, both types of cells are doing what they need to do in the moment to stay alive and perform their function. The overall loop gets worse and worse for Hilde, but the cells are all being somewhat self centered. Of course if all cells in the body sacrificed themselves for the good of the whole, the whole would not live. In each instant they are doing the best they can. They are in a type of crisis mode.
Now, how does Hilde stop this and get her system back to normal?
As long as there are too few insulin receptors on the muscle cell, it will continue to starve. One way she can increase the number is by exercise. Exercise directly increases the number of insulin receptors. Another way is to lower the amount of insulin in the blood stream. This means that she needs to stop stimulating the pancreas to produce it. This means she needs to lower her blood sugar concentration.
Again, a couple of ways to do this. Exercise immediately lowers blood sugar. And diet. Eating fewer carbs will slow down the cycle. They turn immediately into blood sugar. Protein will also slowly turn into blood sugar, it will take a bit of energy to do this, but if your system is calling for MORE SUGAR, I AM STARVING NOW, it will use high protein to make it, but not at the same levels as carbs. Fat bypasses this system, providing energy outside of the insulin/sugar system. Fat will keep you alive while you keep your blood sugar at a low enough level to stop stimulating your pancreas, lowering the amount of insulin in the blood, causing the cells to make more insulin receptors, allowing them to again use sugar as an energy source.
I have not said anything about glycogen stores in the liver or muscles, a simple way of looking at them is a sponge for extra blood sugar, a buffer against fluctuations. Sugar gets released when cells call for it. In order to lower blood sugar enough to lower the production on insulin, you need to stop dietary intake long enough to lower the glycogen stores.
This is why the Atkins diet works. Induction goes on long enough to use up some of the glycogen stores and lower the blood sugar long enough to lower the insulin levels. This causes a up regulation of insulin receptors, and allows the person to slowly increase the amount of carbs until they find their individual balance point, where they are getting good energy from the carbs, but not letting their body overreact to them.
This is also why the Atkins diet does not work. If your issues with weight are not caused by this system, you can loose some weight on Atkins, but it will not be the same dramatic, feel better, reset the metabolism diet that it is for some.
People are individuals, do what works for you! And make it about health, not weight. If you are diabetic, improving your metabolism by cutting back on carbs and exercising will cause you to lose a little bit of fat, the 10% or so that studies show helps diabetes. But it is the slowing of the diabetes that causes the weight loss, not the weight loss that slows the diabetes.
If all this had gone on for a long time (long being defined by Hilde's pancreas' individual tolerance) her pancreas would get tired, and start producing less insulin. Raising blood sugar more, stimulating the pancreas more, exhausting it more. Even if the number of receptors starts to increase in response to the lowered amount of insulin, it is not enough. The high blood sugar is damaging the body is many other ways, and that an immediate medical issue. That can impact immediate survival, immediate circulation, immediate kidney function. It is more important to get the sugar down now then it is to effect a long term cure, because if you don't, you will not be alive for the long term. This is why insulin is given by doctors, to keep the person alive for one more day.
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