So I have been obsessively reading lately. This post may be too much for some, but at least it helps me work out my thoughts and hopefully it is stimulating for others.
Obviously most people here think carbohydrates play a crucial role in metabolic syndrome. However, there are varied opinions on the causes of metabolic syndrome and the best way to deal with it. Stephan Guyenet has leveled a pretty tough criticism of the CIHO (carbohydrate-insulin hypothesis of obesity) and makes some relatively convincing arguments for Food Reward Theory and the problem of the hyper-palatability of modern food. Others, I think like Dr. Kurt Harris (Archevore blog) argue that the neoloithic agents of disease may have a role in hurting our metabolisms. He used to argue more for low-carbs and was a believer in the CIHO, but has been someone convinced by Stephan and also his own self-experiments. Robert Lustig focuses on the causal role of fructose rather convincingly. People in the primal/paleo community are becoming a little more pro-some-carbs due to athletic demands for example. But also, looking at the Kitavans for example (who ate a traditional high carb diet) but had no symptoms of metabolic syndrome. Taubes indicts most all carbs, though says refined carbs are probably particularly bad. He also lays some heavy arguments against the calories in-calories out theory. A lot of people here have probably enthusiastically read him and been influenced him. However, many within the community now emphasize that calories count despite Taubes' theory. So if you are still with me - here is my theory.
Calories in and calories out are each variables that matter of course. We can affect their #s through our independent actions. However, we can be acted upon by biological mechanisms that heavily push us to ingest more or less calories or expend more or less calories. These mechanisms act via hormones - that we can manipulate to some extent towards our ends. So while Taubes is right that there are major problems with the standard interpretation and application of the first law of thermodynamics, he does not go far enough to acknowledge that, while hormones can affect our calorie intake for example and that the two sides of the equation are not in fact independent variables, we can affect our weight by limiting our calories - even if that is an oversimplification and does not address many other important factors. Lyle McDonald has a good article on his site about how the 1st law of thermodynamics holds very true, but that many factors don't usually get accounted for in calculations (digestive issues, the fact that as you lose weight, your BMR decreases, and a host of other nitty gritty issues).
I currently believe that fructose IS the biggest CAUSE of metabolic syndrome. See Lustig's bitter truth youtube video. I also believe that Food Reward and the hyperpalatibility of modern food plays a key role through the addiction of fructose, removal of fiber, addition of salt and other factors. This causes us to overeat which is a problem in itself, depending on the foods included will probably lead to excessive insulin stimulation, and if it includes fructose, further exacerbates the damage and insulin resistance caused by excessive fructose consumption. Along with an inactive lifestyle and probably some other things, we become more or less insulin resistant. This causes us to become leptin resistant. As a result, we are not able to turn our hunger off, and continue to overeat - even more than otherwise because the food is hyperpalatible - causing the problem to spiral out of control.
So low-carb diets work, IMO, not because carbohydrates were the problem PER SE, but for one because many of our metabolisms are so damaged through other mechanisms, that we MUST limit carbohydrates as a therapeutic action. In a normal undamanged metabolism, limitation of natural carbohydrates would never lead to metabolic syndrome. Our strict focus on carbohydrates is the price some of us must pay. As Mark says, it is hard to eat enough fruits and vegetables to cause a problem. But for some us, we have to pay pretty close attention. Secondly, they work through increasing satiation, thus limiting actual caloric intake. So the first reason relates to insulin levels, the second to caloric intake and satiation. As we lose weight, ingest less fructose, and move around more, we improve insulin sensitivity and correct SOMEWHAT the problems of our damaged metabolisms. We also maintain a lower caloric level. And furthermore, that lower caloric intake is SUSTAINABLE due to its satiating effects and nutritional density.