You don't ask much do you
I suggest you read Gary Taubes' "Good Calories, Bad Calories" AKA "The Diet Delusion" in the UK. I also recommend following the links John (xMenace) has in his sig for Micheal Pollan and find out about whole foods.
I'll take a stab at answering some of this. Although I suspect you may get different opinions from others here on DF, I will try to limit myself to known scientific fact. I make no apologies for keeping the science simple...
Everything that we eat can be classified as one of three types of macro-nutrients:
Fats, Proteins and Carbohydrates.
Carbohydrates are chains of sugars - some more complex than others - but nearly all break down to sugar when digested. Some break down as soon as they hit the saliva in the mouth, while - at the other extreme - there is fibre/cellulose, which the human digestive system is unable to break down.
Fats are digested into their basic building blocks; Fatty Acids, and Protein into Amino Acids.
The body uses these basic building blocks to manufacture what it needs... insulin for example, is a protein constructed from Amino Acids.
Some Fatty Acids and Amino Acids are "essential" which means that the body cannot manufacture them and can only get them from eating Fat and Protein.
So far as we know, there are no "essential" sugars that must be ingested. Glucose for example, can be made by the Liver from Amino Acids in a process called Gluconeogenesis.
While Glucose and Ketone bodies (from Fatty Acids) can be burned by most of the cells of the body as energy, the brain is only able to use Glucose.
Glucose is often referred to as the "preferred" fuel, but my understanding is more along the lines of; Glucose provides the quick-burst "on-demand" energy after which the ketones kick in for the "long-haul".
Sounds simple enough so far..? Except that they discovered that in addition to these "macro" nutrients we also need "micro" nutrients... first the Vitamins came along, then Phyto-chemicals, anti-oxidants etc... with new ones of these being found every other day. On one hand this has led to a huge industry providing supplements, but I agree with Michael Pollan that what we really need is the "whole" package of "real" food. Bear in mind also that no natural food is simply one thing... steak for example is mainly protein and fat but is also very rich in all kinds of micro-nutrients (do a search at the USDA nutrient database).
Back to Carbs...
It is Carbohydrates that mainly drive our need for insulin.
As we all know, insulin is required so that the Glucose can pass into cells to be used as energy.
Insulin also regulates the conversion of sugars into fatty acids/Triglycerides for storage in adipose/fat cells... high levels of insulin will quickly mop up any blood glucose and lock it away as fat (for use later... in theory). High levels of insulin prevent fat from being released from the fat cells to be used as energy. It can be argued that high-levels of insulin perform this role so efficiently that it can leave the body starved of ready energy and explains the phenomenon of someone eating a large high-carb meal but still being hungry immediately after. High levels of insulin over time can also lead to Insulin Resistance and ultimately Pancreatic burn-out. Hence the argument that eating carbs is largely responsible for the downward spiral that develops into Type 2 D.
In short "Carbs + Insulin = Fat"
So, if we turn that around and minimise our carbs we also minimise our need for insulin - a basal amount is still required for daily function, and remember that Protein can be made into Glucose - lower insulin levels will allow fat to come out of the fat cells to be used as energy and yes this does lead to weight loss and improved BG levels - without the spikes driven by eating carbs.
