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To make things more complicated, researchers discovered that cholesterol travels around the body in two major forms: low-density lipoprotein (LDL), the kind that does most of the damage, and high-density lipoprotein (HDL), which actually seems to keep arteries clean. Beyond that, another class of fats, known as triglycerides, also circulates in the blood, doing more or less the same kind of damage as LDL.
Doctors then began recommending foods and activities that drive down LDL and triglycerides (eat less meat, cream and butter--one recommendation that has never changed--add olive oil and fish to the diet) and at the same time push up HDL (get more exercise and lose weight).
Then came the news that taking benign foods like vegetable and peanut oils and hydrogenating them--a process that stiffens them to make stick margarine, peanut butter and solid shortening--transforms them into substances known as trans-fatty acids, which can drive LDL and triglyceride levels through the roof. Trans-fatty acids are not technically fats, which means, astonishingly, that a food labeled FAT FREE may be bursting with stuff that can give you heart disease. The fact that stick margarine is bad doesn't mean butter is suddenly good. Says Dr. Walter Willett, head of nutrition at the Harvard School of Public Health: "I think the healthiest solution is none of the above." The recommendation to stay away from meats and animal fats, thus, hasn't been overthrown. It's simply been extended slightly to encompass a substance once thought to be relatively harmless.
How about staying away from fat entirely? Bad idea. The body needs fats, in part because vitamins like A and D must be dissolved in fat to enter the body or even move about within the body. What about the super-low-fat diets, advocated by people like Dr. Dean Ornish, director of the Preventive Medicine Research Institute in Sausalito, Calif. (see box)? They seem to work but perhaps not because they're low fat; the key may be the types of foods--beans, grains, vegetables, instead of meats and cheeses--that Ornish recommends. Indeed, Mediterranean men, who get more than 30% of their calories from fat (some three times what Ornish suggests) but who eat little saturated fat, have a very low rate of heart disease.
HDL, LDL and triglycerides aren't quite the whole story either. Over the past few years, researchers have identified yet another form of fat that could rightly be labeled Bad Cholesterol II. Called lipoprotein (a), or Lp(a), it behaves like LDL in the body. But because Lp(a) levels have more to do with your genes than your diet, they can't easily be controlled. At best, doctors think they can use Lp(a) screening to find people who should be working extra hard to reduce their other heart-attack risks.
So fat, which started out as a good thing to eat and then became a bad thing, now turns out to be a collection of very different things, some good, some bad, some absolutely neutral. It's a pattern that has been repeated for a variety of cardiovascular risk factors. It's not pretty; the tortuous progress of scientific discovery rarely is.
SALT