Here’s an interesting story explaining one reason why:

Consider bread…when consumed, the carbohydrate it contains eventually becomes glucose in your bloodstream. Your body absorbs the glucose with the help of insulin. Because white bread (as well as white rice, crackers, and other refined grain products) are highly processed and lack fiber, they can cause a spike in blood glucose, which stresses the body, and in the long-term can send you on the road towards diabetes, heart disease and even cancer. The interesting thing is, fat can decrease the size of the glucose spike…which is a good thing. But don’t go buttering your bread with the excuse that it will make the bread healthier, because the type of fat plays a crucial role.

For example, if you add butter to bread, you will experience a smaller spike in glucose (compared to bread sans butter). Even though this sounds like a good thing, there’s a catch…your body will still release a large amount of insulin, despite the smaller glucose spike. This is a problem. Reason being…in the long-term, your body will become insensitive to insulin (because insulin is getting released for no reason, the body simply stops paying attention to it). Insulin insensitivity eventually leads to diabetes and heart disease.

However, on the other hand, if you dip your bread in olive oil, you will get a smaller spike in glucose, alongside an appropriately attenuated amount of insulin. Thus, when fed olive oil, the body somehow adjusts the amount of insulin it releases, something it is unable to do when fed butter.This is just one of the reasons why the “olive oil people” of the Mediterranean tend to experience less heart disease than the “butter people” just North of them (or at least prior to the globalization of the food supply this was the case).

Olive trees near Agios Nikolaus, Greece

I learned about this fact from: Rasmussen et al. Differential effects of saturated and monounsaturated fat on blood glucose and insulin responses in subjects with non-insulin-dependent diabetes mellitus. AJCN, 1996;63:249-253.