In my Grandma's day, there was no such thing as bad fat.

All fat was "good" simply because it tasted good. My Grandma fried her eggs in bacon grease, added bacon grease to her cakes and pancakes, made her pie crusts from lard, and served butter with her homemade bread. My grandmother was able to thrive on all that saturated fat—but not my grandfather. He suffered from angina and died from heart failure at a relatively young age.

My grandfather wasn't alone. Population studies from the first half of the 20th century showed that Americans in general had a much higher risk of cardiovascular disease than people from other countries, especially Japan, Italy and Greece. Was all that saturated fat to blame? The Japanese were eating very little fat of any kind, while the people of the Mediterranean were swimming in olive oil, an oil that is very low in saturated fat but high in monounsaturated oils.

So, in the 1960s, word came from on high that we should cut back on the butter, cream, eggs and red meat. But, interestingly, the experts did not advise us to switch to an ultra low-fat diet like the Japanese, nor to use monounsaturated oils like the Greeks or Italians. Instead, we were advised to replace saturated fat with polyunsaturated oils, primarily corn oil and safflower. Never mind the fact that no people in the history of this planet had ever eaten large amounts of this type of oil. It was deemed "the right thing to do." Why? First of all, the United States had far more corn fields than olive groves, so it seemed reasonable to use the type of oil that we had in abundance. But just as important, according to the best medical data at the time, corn oil and safflower oil seemed to lower cholesterol levels better than monounsaturated oils.

Today, we know that's not true. In the 1960s, researchers did not differentiate between "good" HDL cholesterol and "bad" LDL cholesterol. Instead, they lumped both types together and focused on lowering the sum of the two. Polyunsaturated oils seemed to do this better than monounsaturated oils. We now know they achieve this feat by lowering both our bad and our good cholesterol, in effect throwing out the baby with the bathwater. Monounsaturated oils leave our HDL intact.

In hindsight, it's not surprising, then, that our death rate from cardiovascular disease remained high in the 1970s and 80s even though we were eating far less butter, eggs, bacon grease, and red meat: We had been told to replace saturated fat with the wrong kind of oil.