Your body depends on fats for a host of functions. Fats form a major energy source for cells. They make up adipose tissue, which stores energy, cushions and protects vital organs, and provides insulation. Cholesterol, which is technically not a fat, is needed to make cell membranes and the critically important sheaths around nerves and is a building block from which the body makes many hormones. In order for all this to happen, fats must somehow get from your digestive system to your cells. This isn’t as simple as it sounds. Like oil and water, fats and blood don’t mix. If your intestines or liver simply dumped digested fats into your blood, they would congeal into unusable globs. Instead fat is packaged into protein- covered particles that mix easily with blood and ﬂow with it. These tiny particles, called lipoproteins (lipid plus protein), contain some cholesterol to help stabilize the particles.
Like a highway at rush hour, your bloodstream carries many sizes and shapes of fat-transporting particles. Lipoproteins are generally classiﬁed by the balance of fat and protein they contain. Those with a little fat and a lot of protein are heavier and more dense than the lighter, ﬂufﬁer, and less dense particles that are more fat than protein. The proteins do more than just shield fat from water. They also act like address labels that help the body route fat-ﬁlled particles to speciﬁc destinations.
With regard to heart disease, the most important lipoproteins are high-density lipoprotein (HDL), low- density lipoprotein (LDL), and very-low-density lipoprotein, which is composed of triglycerides.
LDL is often referred to as the bad cholesterol. When your bloodstream carries too many of these particles, they can end up inside cells that line blood vessels. Once there, LDL is attacked by highly reactive free radicals and transformed into oxidized LDL. Oxidized LDL can damage the artery lining and kick off a cascade of reactions that clog the artery and set the scene for artery-blocking blood clots.
In contrast, HDL particles sponge up excess cholesterol from the lining of blood vessels and elsewhere and carry it off to the liver for disposal. They also help the liver recycle other lipoprotein particles.
Triglycerides make up most of the fat that you eat and most of the fat that circulates in your bloodstream. Triglycerides are essential for good health, since your tissues rely on them for energy. But as is the case for cholesterol, too much triglycerides may be bad for the arteries and the heart.
When you have your cholesterol checked, the number you get back is usually your total cholesterol. This number tells you how much LDL and HDL are circulating in your blood. The ideal total cholesterol level is under 200 milligrams per deciliter (tenth of a liter) of blood. The National Cholesterol Education Program (NCEP) deﬁnes borderline high cholesterol as a total cholesterol level between 200 and 239 milligrams per deciliter and high cholesterol as 240 milligrams per deciliter or higher.
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Because total cholesterol is a mix of bad and good, it doesn’t tell the whole story. That’s why many physicians also check HDL levels along with total cholesterol and then calculate the LDL level with a simple formula. The lower the LDL the better, with anything under 130 milligrams per deciliter considered healthy. For healthy people, levels between 130 and 159 milligrams per deciliter are borderline high, and 160 milligrams per.
The human body can build most of the diﬀerent fats it needs from any other fat in the diet, or from carbohydrates, for that matter. A few, though, can’t be made from scratch. These so-called essential fats, which are all polyunsaturated fats, must come directly from food.
• Saturated fat. The term saturated means that the carbon atoms in a chain hold as many hydrogen atoms as they can. This happens only when each carbon atom is connected to its carbon neighbors by single bonds. Saturated fats look like straight chains.
About two dozen diﬀerent saturated fats exist in nature. They are abundant in meat and animal fat, dairy products, and in a few vegetable oils like palm and coconut oil. At room temperature, saturated fats are solid rather than liquid, something you see every time you let the drippings from cooked bacon or hamburger congeal in a pan.
When it comes to their eﬀects on cholesterol and the artery-clogging process known as atherosclerosis, saturated fats come in gradations of bad. The saturated fats in butter and other dairy products most strongly increase LDL (bad)
cholesterol. Those in beef fat aren’t quite as powerful at boosting LDL, and those in chocolate and cocoa butter have an even smaller impact.
• Monounsaturated fat. The Greek preﬁx mono, meaning “one,” hints at the structure of these fats. At one point along the carbon backbone, two carbons are connected by a double bond. This seemingly small change leads to several key diﬀerences. It reduces the number of hydrogen atoms the carbon chain can hold by two. It changes the shape of the molecule from a straight chain to a bent stick. And it makes the fat a liquid at room temperature. Basically, monounsaturated fats are oils. Olive oil, peanut oil, and canola oil are all high in monounsaturated fats. Avocados and most nuts are also excellent sources.
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• Polyunsaturated fat. Two or more double bonds make a polyunsaturated fat. These hold even fewer hydrogen atoms than a monounsaturated fat with the same number of carbon atoms and look like a stick with a double bend. Polyunsaturated fats can be subdivided into the omega-3 or omega-6 groups, with the number referring to how far the ﬁrst double bond is from the end of the carbon chain. Each type plays diﬀerent roles in the body. Polyunsaturated fats are also liquid at room temperature. Our bodies don’t make polyunsaturated fats, so we need to get these essential fats from plant oils like corn and soybean oil, seeds, whole grains, and fatty ﬁsh such as salmon and tuna.
• Trans fats. More than one hundred years ago, food chemists discovered that they could solidify a polyunsaturated vegetable oil by heating it in the presence of hydrogen gas and ﬁnely ground particles of nickel metal. During the process, called partial hydrogenation, hydrogen latches on to some—but not all—of the double-bonded carbons, changing them into single bonds. At the same time, some of the remaining double bonds twist into a new straightened shape, which gives the fat new chemical and physical properties.
Why did anyone bother ﬁguring this out? It’s easier to ship and store solidiﬁed vegetable oils than liquid oils, and partially hydrogenated vegetable oil can be used in place of butter or lard in baking. And a lesser degree of hydrogenation yields a still-liquid oil that doesn’t become rancid as quickly as unprocessed vegetable oils. Without this process we wouldn’t have had margarine or vegetable shortenings such as Crisco. We also would have less heart disease and at least thirty thousand fewer deaths from it each year.
Not long ago, an FDA advisory panel said that trans fats are even more harmful than saturated fats. The Institute of Medicine went a step further, concluding that the safest amount of trans fats for humans is zero.