Until recently, fat and health were as compatible as oil and water. A prerequisite for making your diet healthier was to cut fat down to no more than 30 percent of your daily calories—the less the better. More than 30 percent, many nutrition experts said, would set the stage for heart disease, obesity, cancer, and other ills.

Several major health organizations endorsed this view, and the big fat scare was on. Cookbook authors, diet programs, and the media all jumped on the low-fat bandwagon. The alternative view, held for decades by many leaders in nutrition research, was that the key to health was the type, not the amount, of fat.

The second view turned out to be correct. Fat is a major energy source for your body and also helps you absorb certain vitamins and nutrients. In the average American diet, about 35 percent of calories come from fat. People trying to reduce their cholesterol level should try to keep their fat intake between 25 percent and 35 percent of their calories.

But just as important as the amount of fat you eat is the type. Saturated fats (found mainly in meat, butter, whole milk, and cheese) and trans fats (which come mostly from the partially hydrogenated oils used in restaurant fryers, many margarines, and packaged snacks and baked goods, and in lesser amounts from dairy products and meats) are the ones to stay away from.

Saturated fats increase heart attack risk by increasing LDL cholesterol and triglycerides. Trans fats do the same, but they also pack a second punch: they actually decrease the heart-healthy HDL cholesterol levels. Good fats, on the other hand, decrease your LDL cholesterol.

That’s why indiscriminately cutting fat out of your diet isn’t a good idea—it could actually worsen your cholesterol profile. Good fats include monounsaturated fats and omega-3 and omega-6 polyunsaturated fats. The polyunsaturated fats are the healthiest, which is why so many dietary recommendations include fish.

The NCEP guidelines say that only 7 percent of your daily calories should come from saturated fat. Unfortunately for lovers of red meat, butter, ice cream, and cheese, these foods are rich in saturated fats and their intake should be carefully rationed.

It is not possible to avoid saturated fat entirely because even the healthiest oils are mixtures of saturated and unsaturated fats. And contrary to previous expert advice, holding your fat intake to the low end of the spectrum is not necessarily healthier than letting it reach 35 percent.

A low-fat diet is no guarantee of good health. In fact, a diet with only 20 percent of calories from fat can be virtually a junk-food diet if you make up for the lost fat calories with sugary foods such as soft drinks, nonfat cookies, and high-starch carbohydrates such as white bread and potatoes. An overabundance of these foods increases the risk of heart disease and diabetes.

What’s the difference between a good fat and a bad fat? All fats have a similar chemical structure: a chain of carbon atoms bonded to hydrogen atoms. What differs is the length of the chain and the shape of the carbon atoms in the bonds they form with each other and the hydrogens. Seemingly slight differences in structure translate into crucial differences in the way the body handles these fats.

Bad Fats

The two forms of unhealthy fat, saturated and trans fats, share a physical trait: they are solid at room temperature. Think of butter or the marbleized fat in a steak. But not all the foods that contain a lot of bad fats are solid. Whole milk, ice cream, and some oils also contain abundant amounts of bad fats.

Saturated Fats

The word saturated refers to the number of hydrogen atoms these fats have. In a saturated fat, the chain of carbon atoms holds as many hydrogen atoms as possible, making it literally saturated with hydrogen atoms. Each carbon atom in the chain is connected to the next by a single bond, leaving the maximum number of bonding points available to hold hydrogen.

There are about twenty-four different saturated fats. Not all of them are equally bad for your health. The saturated fat found in butter, whole milk, and other dairy products increases LDL levels the most, followed by the saturated fat in beef.

Curiously, the saturated fat called stearic acid, found in pure chocolate, is more like unsaturated fat in that it lowers LDL levels. Even some vegetable oils such as palm oil and coconut oil contain saturated fat.

Trans Fats (partially hydrogenated oils)

These fats occur naturally in meat, but their main source is packaged baked products such as cookies, cakes, breads, and crackers, as well as fast foods and some dairy products. Trans fats were artificially created in the laboratory about a hundred years ago to provide cheap alternatives to butter.

Food chemists found that they could solidify vegetable oil by heating it in the presence of hydrogen. The process, called hydrogenation, gives the carbon atoms more hydrogen atoms to hold. As a result, the structure of polyunsaturated fat (a good fat) becomes more like saturated fat.

Thus, solid vegetable fats such as shortening and margarine came into being. Today, trans fats are found not only in solid foods such as these but also in foods that contain “partially hydrogenated oil.” Even some cooking oils are partially hydrogenated to keep them fresh.

The Institute of Medicine expert panel says that trans fats have no known health benefits and that there is no safe level. In 2006, food manufacturers will be required to list the amount of trans fats a product has. Until then, the words hydrogenated or partially hydrogenated in the ingredients list are the red flags.

Keep purchases of oils and packaged foods with these words in their ingredients to a minimum, and try to find products with the lowest amount of these substances whenever possible.

Good Fats

Good fats come mainly from vegetable and fish products. They differ from bad fats by having fewer hydrogen atoms bonded to their carbon chains. They are liquid, not solid. There are two broad categories of beneficial fats: polyunsaturated and monounsaturated.

Polyunsaturated Fats

When you pour liquid cooking oil in a pan, there’s a good chance you’re using polyunsaturated fat. Corn oil is a common example. Polyunsaturated fat has two or more double carbon bonds.

There are two major types of polyunsaturated fats: omega-3 (n-3) fatty acids and omega-6 (n-6) fatty acids. (The numbers refer to the distance between the end of a carbon chain and the first double bond.)

Polyunsaturated fats are essential fats, meaning they are vital to normal body functions, but your body can’t manufacture them. Therefore, it’s important to get polyunsaturated fats from food. Polyunsaturated fats help build cell membranes, the exterior casing of each cell, and the sheaths surrounding nerves.

They’re vital to blood clotting, muscle contraction and relaxation, and inflammation. They reduce LDL cholesterol more than they lower HDL, improving your cholesterol profile. Even better, they also lower triglycerides.

Both the omega-3 fatty acids and omega-6 fatty acids offer health benefits. Research has shown that omega-3s help prevent and even treat heart disease and stroke. Evidence also suggests they have similar benefits against autoimmune diseases such as lupus, eczema, and rheumatoid arthritis.

Omega-3s come mainly from fish but also from flaxseeds, walnuts, canola oil, and unhydrogenated soybean oil. Fatty fish such as salmon, tuna, mackerel, and sardines are especially good sources of omega-3s. The Institute of Medicine has set the daily reference intake (DRI) for alphalinolenic acid, the omega-3 in vegetable oils, at 1.6 grams per day for men and 1.1 for women.

Omega-6 fatty acids also lower the risk for heart disease. High levels of linoleic acid, an omega-6, are in such vegetable oils as safflower, soybean, sunflower, walnut, and corn oils. The DRI for linoleic acid is 17 grams per day for men ages nineteen to fifty and 12 grams for women that age. For adults ages fifty-one to seventy, the DRI is 14 for men and 11 for women.

Monounsaturated Fats

When you swab your bread in olive oil at an Italian restaurant, you’re getting mostly monounsaturated fat. Unlike a polyunsaturated fat, which has two or more double bonds of carbon atoms, a monounsaturated fat has just one. The result is that it has more hydrogen atoms than a polyunsaturated fat but fewer than a saturated fat.

Although there is no DRI for monounsaturated fats, the Institute of Medicine recommends using them as much as possible along with polyunsaturated fats to replace the bad saturated fats and trans fats. Good sources of monounsaturated fats are olive oil, peanut oil, canola oil, avocados, and most nuts.