Just the oil facts.

Is canola oil as good for you as olive oil? Should you eat more polyunsaturated fat or less? Is this a good time to switch from margarine back to butter?

Few subjects cause as much confusion as fats and oils. How do we know? We've started measuring our fat mail by the pound. So, at the request of our letter cartier, here's a little fat and oil guide.

Are some oils "fattier" than others? No. A tablespoon of any oil will set you back about 120 calories and 14 grams of fat. That's a quarter of your day's fat quota. It doesn't matter if it's olive, canola, corn, or coconut. Every fat or oil, by the way, gets 100 percent of its calories from fat.

Don't butter and margarine have less fat? Slightly. That's because they contain water. A tablespoon of butter or full-fat stick margarine has about 100 calories and 12 grams of fat. Tub margarines or spreads can go from almost as much fat as regular margarine to no fat at all.

Which fats contain cholesterol? Only animal fats. While most cholesterol is in the flesh, a little does get into the fat. It ranges from 11 mg per tablespoon (chicken fat) to 25 mg (butter). (A large egg has 215 mg.) Vegetable oils and margarines are cholesterol-free.

But any fat that contains a high percentage of saturated fat can raise your blood cholesterol. That means coconut oil, palm kernel oil, cocoa butter, butter, palm oil, beef tallow, and lard. While the cholesterol you eat raises your blood cholesterol level, the sat fat you eat probably raises it even more.

Where do trans fats fit into the picture? They're unsaturated fats that raise cholesterol, possibly as much as saturated fats. They're formed when manufacturers partially hydrogenate (add hydrogen to) liquid oils.

Partially hydrogenated oils are less likely to become rancid, can be re-used more times for deep-fat frying, and make baked goods tender and flaky. They're found mostly in margarines, fried fast foods, and cookies, frostings, pies, pastries, and doughnuts. If a food is fatty and is made with "partially hydrogenated" oil, it's probably high in trans.

Does the trans fat in margarine mean that I'm better off eating butter? No. Butter's got far more saturated fat. And you can now buy margarines or "spreads" with little or no trans, although their taste may take a little getting used to. In general, "diet" tubs have the least trans. Try Promise Ultra Fat Free, Nucoa Smart Beat, or Weight Watchers Extra Light Spread.

Why is olive oil considered to be so healthy? Mostly because populations that eat large amounts of it--and very little meat, poultry, cheese, or other saturated fat (the so-called Mediterranean diet)--have lower rates of heart disease. Researchers think it's because close to 75 percent of the fat in olive oil is monounsaturated and only 13 percent is saturated. The only other oil that comes close is canola (Puritan is the major brand). It's about 60 percent mono.

But canola oil has a little less saturated fat than olive oil. Doesn't that make it better? Maybe. Maybe not. What's comforting about olive oil is that people have been eating it--with no apparent damage--for thousands of years. Canola oil has only been popular for about a decade. It still looks like one of the best oils you can use, though.

A few years ago we were told to eat more polyunsaturated fats. Is that advice still good? Nope. Polys are more easily oxidized than monos once they're incorporated into the LDLs (low density lipoproteins) in your blood. And that may promote heart disease. Also, large amounts of vegetable polys cause some cancers in animals (though that may not happen in people).

What's more, no large populations have eaten enough polys for long enough to see if they pose a health risk. So go easy on the safflower, walnut, sunflower, corn (Mazola is the major brand, soybean (Wesson is the major brand), and cottonseed oils...as well as all the processed foods that contain them.

How much fat or oil should I eat? The less the better. Don't worry about eating too little. Deficiencies are virtually non-existent because most foods contain at least trace amounts. Look for "low-fat" foods (which contain no more than three grams of fat per serving) and "low-saturated-fat" foods (no more than two grams of sat fat).

One painless way to cut the fat is to forget the bottle and use a few spritzes of a vegetable oil spray like Pain when sauteeing. You'd have to keep your finger on the nozzle for 15 seconds or so to get a tablespoon of oil.

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