The Skinny on Fats, Both Good and Bad

In our discussion of olive oil last month, we erroneously said that the monounsaturated fat in olive oil was even more unsaturated than polyunsaturated fats, such as corn, safflower and sesame oil. A number of readers noted that we should have written that although monounsaturated fats are more saturated than polyunsaturated fats, they are as effective as polyunsaturated fats in lowering blood cholesterol.

For anybody who’s still confused, here’s a short course on fats.

Cholesterol: A chemical compound manufactured in the body. It is used to build cell membranes and brain and nerve tissues. Cholesterol also helps the body make steroid hormones and bile acids. The body produces enough for its needs, so you don’t actually have to consume any extra.

There are primarily two types of cholesterol produced in the body--LDL (bad) and HDL (good). (See lipoproteins, below.) The relationship between the two is very complex and is crucially important in preventing heart disease.

The important numbers to remember are that total cholesterol should be 200 or below, LDL should be under 130, and HDL should be over 35.

Dietary cholesterol: Cholesterol is found in animal products that are part of the human diet. Egg yolks, liver, meat, some shellfish and whole-milk dairy products are all sources of dietary cholesterol.

There is no such thing as good or bad dietary cholesterol, only good and bad blood cholesterol.

Fatty acid: A molecule composed mostly of carbon and hydrogen atoms. Fatty acids are the building blocks of fats.

Fat: A chemical compound containing one or more fatty acids. Fat is one of the three main constituents of food (the others are protein and carbohydrate). It is also the principal form in which energy is stored in the body.

It is not healthy to remove all fat from the diet since fatty acids are important for growth of children, for healthy skin, for regulating cholesterol metabolism and in the production of certain essential hormones.

Dietary fat also carries certain so-called fat-soluble vitamins, like A, D, E and K. As those of us with some extra body fat know, fat deposits also serve as insulation and to cushion--not only our derriere, but also our body’s internal organs.

Hydrogenated fat: A fat that has been chemically altered by the addition of hydrogen atoms (see trans-fatty acid). The purpose of hydrogenation is to make the fatty acids more stable and to convert liquid fats to a solid or semi-solid form. Margarine is an example of a hydrogenated fat.

Lipid: A chemical compound characterized by the fact that it is insoluble in water. Both fat and cholesterol are members of the lipid family.

Lipoprotein: A chemical compound made of fat and protein. Lipoproteins that have more fat than protein are called low-density lipoproteins (LDLs). Lipoproteins that have more protein than fat are called high density lipoproteins (HDLs). Lipoproteins are found in the blood, where their main function is to carry cholesterol.

LDLs drop off fat into the arteries; HDL acts like a little vacuum cleaner and picks up excess fat. Thus, the designations “good” and “bad” cholesterol.

Monounsaturated fatty acid: A fatty acid that is missing one pair of hydrogen atoms in the middle of the molecule. The gap is called an “unsaturation.” Monounsaturated fatty acids are found mostly in plant foods and seafoods.

Monounsaturated fat: A fat made of monounsaturated fatty acids. Olive oil and canola oil are monounsaturated fats. Mono-unsaturated fats tend to lower levels of LDL cholesterol in the blood.

Polyunsaturated fatty acid: A fatty acid that is missing more than one pair of hydrogen atoms. Polyunsaturated fatty acids are mostly found in plant foods and seafoods.

Polyunsaturated fat: A fat made of polyunsaturated fatty acids. Safflower oil and corn oil are polyunsaturated fats. Polyunsaturated fats tend to lower levels of both HDL cholesterol and LDL cholesterol in the blood.

Saturated fatty acid: A fatty acid that has the maximum possible number of hydrogen atoms attached to every carbon atom. It is said to be “saturated” with hydrogen atoms. Saturated fatty acids are mostly found in animal products, such as meat and whole milk.

Saturated fat: A fat made of saturated fatty acids. Butter and lard are saturated fats. Saturated fats tend to raise levels of LDL cholesterol (“bad” cholesterol) in the blood, more even than eating dietary cholesterol. Elevated levels of LDL cholesterol are associated with heart disease.

Trans-fatty acid: A polyunsaturated fatty acid in which some of the missing hydrogen atoms have been put back in a chemical process called hydrogenation. Trans-fatty acids are the building blocks of hydrogenated fats.

Trans fats: These occur naturally in meat and dairy products, and any food made with hydrogenated oils. These are now seen as another important influence on elevated LDL cholesterol, much like saturated fats.

In general, trans fats make up less than 5% of our diet, compared to 12% for saturated fats and 34% for total fat.

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The guidelines for fat consumption haven’t changed very much in the last few years, but here’s a summary, just in case you’ve forgotten:

* Try to limit your total fat intake to no more than 30% of your overall calories, and of this, no more than 10% should come from saturated fat. Put another way, if you’re eating 2,000 calories a day, you should have no more than 60 grams of fat per day. If you can reduce your fat intake even further, you’re way ahead of the game.

* Try to limit your cholesterol intake to no more than 300 mg per day.

* Make sure you have your blood cholesterol levels checked regularly, and try to keep the total cholesterol below 200 mg per deciliter (mg/dl). The HDL (“good” cholesterol level) should be 60 mg/dl or more. You can try to boost your “good” cholesterol by losing weight, not smoking, exercising and, if you’re a menopausal woman, taking hormone replacement therapy.

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Dr. Sheldon Margen is a professor of public health at UC Berkeley; Dale A. Ogar is managing editor of the UC Berkeley Wellness Letter. Send questions to Dale Ogar, School of Public Health, UC Berkeley, Berkeley, CA 94720-7360, or e-mail to daogar@uclink4.berkeley.edu.