Recently it seems that oils have been typecast as the bad guys of the diet story, seductive villains that lure us with a moment's pleasure into a lifetime of flab as well as a host of ills. Yet science tells us that there are good oils - not just less-bad-for-you oils, but oils that positively boost health in many surprising ways. Trying to figure out "who is who" in this slippery family has been difficult for the public; over the years, scientists have shifted their views frequently. Polyunsaturated oils such as safflower and sunflower apparently wore the white hats in the fat realm for some time, only to be discredited later as possibly leading to cancer. More recently, olive oil has been the favorite, but the oil story is not that simple. To find out which oils are best, and how much of them we need, let's take a look at the basic types of plant oils and how they act in the body.

Optimal functioning of our bodies depends on fats. Among their many activities, they aid in tissue repair, metabolism, and hormone synthesis. In addition, they can affect our immune systems in various ways. Many people don't realize that our bodies manufacture most of the fatty substances we need, but not all. Certain fatty acids must be consumed - in small quantities - in our diets. There are two families of these essential fatty acids: omega-6 - found in many seeds and legumes, such as corn, soybeans and peanuts - and omega-3 - found in fish, walnuts, flax seeds and leafy vegetables.

When our bodies don't get enough EFAs (essential fatty acids), our red blood cells cannot carry as much oxygen to our tissues, and our white blood cells (important in the functioning of our immune systems) become less active. Although both omega-6s and omega-3s are helpful in many ways, they compete for absorption within the body. This was not so important in the past, when oil consumption in general was lower than it is today. But modern times have seen the rapidly increasing use of vegetable oils such as corn and safflower, high in omega-6s. Experts state that in olden times the dietary ratio of omega-3s to omega-6s was about 1:1; the ratio has now soared to 1:10. With our modern eating habits, we are unlikely to have a relative deficiency of omega-6s -- yet imbalances in omega-6 to omega-3 ratios have been associated with a higher incidence of some cancers. All this implies that most people need to increase their intake of omega-3s.

Since intake of omega-3s has become more important in today's society, let's take a closer look at this class of fatty acids. Omega-3s are apparently good guys in the world of fats. Not surprisingly, omega-3 supplements are popular -- especially among seniors and baby boomers. In addition to the sources listed earlier they are found in pumpkin seeds and wild greens such as purslane leaves. Omega-3 is also available in moderate amounts in canola oil and tofu.

Many studies have demonstrated the positive effects of diets containing adequate amounts of these fatty acids. Population studies as well as lab experiments indicate that diets rich in omega-3s may help guard against heart disease. Even when dietary cholesterol was high, omega-3-rich diets prevented those ever-important blood cholesterol levels from rising. And mice fed such a diet have enhanced immune cell production when challenged with viruses.

Certain substances that contribute to inflammation are derived from arachidonic acid, which is found in animal fat, but which can be produced from omega-6s as well. Oils high in omega-3s, on the other hand, tamp down inflammatory responses in the body. This trait is particularly helpful for people with arthritis, resulting in a reduction of joint tenderness and morning stiffness. Other inflammatory diseases - such as inflammatory bowel disease - appear to respond as well.

Recently, a number of fascinating studies hint that omega-3s may have a positive effect on mental health. In 1995, the American Journal of Clinical Nutrition reported that these fatty acids appear to reduce the incidence of depression, while a British study found that a dietary increase in omega-3s reduced 20 symptoms of schizophrenia in psychiatric patients.

A word of caution: Omega-3-rich oils are highly polyunsaturated. This means that unless they are very, very fresh, they - like other polyunsaturates (safflower and sunflower oils, for example) - may deteriorate in such a way as to cause harmful effects in our bodies. The bottom line: refrigerate and consume any flax seed oil or seeds within a few weeks.

It is partly because of the freshness issue that monounsaturated oils, such as olive, have become so popular. Their chemical structure is such that they remain stable - that is, they do not oxidize - as readily as do polyunsaturates. As with other monounsaturates, intake of olive oil reduces levels of the harmful form of cholesterol in the blood, compared to more saturated oils.

Trans fatty acids are some of the outlaws of the oil group. They don't appear naturally in the plant kingdom; rather, they are formed when oils are hydrogenated in order to harden them, such as in the production of margarine sticks. The process takes oils that are naturally liquid at room temperature, and treats them so they become solid - like animal fats.

According to the American Journal of Clinical Nutrition, trans fatty acids raise cholesterol levels just as much as do saturated fats. But that's not the full extent of their damage. Studies at Harvard indicate that trans fatty acids may be responsible for 30,000 deaths from heart disease each year, and that people who ingest the highest amounts of various sources of trans fatty acids - including margarine - have twice the risk of heart attack as those who consume the least. The Dutch government is so concerned about the risks that it has banned the use of margarine containing appreciable amounts of hydrogenated oils.

Look at food labels; baked goods, crackers, and margarines are likely sources for hydrogenated oils. The key phrases to look for (and avoid) are "hydrogenated oil" and "partially hydrogenated oil." If manufacturers were forced to acknowledge the cholesterol-raising effects of trans fatty acids, many boxes of baked goods would have to remove their boasts that they have "no cholesterol," and sticks of margarine would have to confess that most of them are just as bad for the heart as is the much-reviled butter they attempt to replace. If you need a buttery spread on your morning toast, health-conscious tub margarines - such as Smart Balance and Spectrum Spread - are usually the best bet; they are softer simply because they have not been hydrogenated. Avoid deep-fried fast foods, including french fries. Despite claims to be cholesterol-free, which technically they are, most fast foods are fried in vegetable shortening, the cholesterol-raising hydrogenated form of oil.

Just how much fat will suffice to stay fit? Experts disagree. Laurie Meyer, R.D., a spokesperson for the American Dietetic Association, suggests it's 14 to 20 grams daily. Dean Ornish, M.D., well-known champion of the low-fat diet, believes that we need only 4 to 6 grams each day. To get some sense of the quantities being discussed, one tablespoon of oil contains about 10 grams. In any case, we need only a very small amount. Considering that virtually all foods contain some fat - a cup of cooked tomatoes and a cup of cooked squash contain one gram each - we hardly need to add oils to our diets.

But since nothing quite replaces that special something that a little oil does for our taste buds, it's wise to carefully choose which oil to use. A small amount of fresh flax seed oil is great when used uncooked on salads and popcorn, while canola and olive are best used for cooking. Switching allegiance to the good guys of the oil world is easy, health-enhancing, and tasty.

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Nettleton, J. Omega-3 fatty acids: comparison of plant and seafood sources in human nutrition. Journal of the American Dietetic Association, 91(3), 1991.

Goodnight, S and Cairns, J. Therapeutic use of n-3 fatty acids for vascular disease and thrombosis. Chest, 108(4), 1995.

Weinberg, L. The truth about trans fatty acids: scientists can't agree on risk. Environmental Nutrition 19(2), 1996.