'Good Calories, Bad Calories' covers much of the research related to 'Syndrome X'/Metabolic Syndrome. Some research seems to indicate that consumption of high glycemic index foods over an extended period can cause permanent damaging metabolic changes.

I think it is slightly misleading to equate damage to the metabolic system due to historical factors with a more general claim that healthy metabolisms vary widely however. The presence of Type 2 Diabetes is clearly a very relevant data point to be aware of when advising on diet and would lead to different advice than to a 'normal' healthy individual. I think the same is likely true of other types of metabolic damage that may have occurred in the past. Of course given the current state of understanding of the biology underlying all this it's difficult to find unambiguous answers.