In recent years, here’s what the scientific, medical and nutrition communities have said about dietary fats: Fats are bad for you; fats are good for you. Fats cause cancer; fats don’t cause cancer. Fats cause weight gain; fats don’t cause weight gain.

Get the picture?

Don’t worry, we’re here to help. Here’s a guide to help clear up some of the confusion about fat and its effects on the human body.

First of all, you need to know all types of fat pose two problems:

– Fat causes health issues when it lingers in the bloodstream.

– All fats are high-calorie foods. That’s why some researchers say any fats are bad when you get too much, and good when you get just enough.

The relationship between humans and fat began with ancient human ancestors. Fat was a rare commodity from the animals they hunted, so the human physiology learned to hold on to dietary fats.

But as time marched on, the human body found itself ill-equipped to handle a state of perpetual abundance with most food flavored with fat. That has resulted in obesity and clogged blood vessels.

People eat four basic types of fats. Each category has lots of versions, but these are the names you hear: Saturated fats, from red meat and poultry; monounsaturated fats, fish oil and olive oil; polyunsaturated fats, vegetables and grains; and trans fats, which are manufactured fats.

Saturated fat

This is the bad fat blamed for clogging arteries, causing heart attacks and stroke, and occasionally associated with cancer, especially colon and breast cancer.

Source: Red meat, poultry, whole dairy products such as milk and yogurt, lard and butter.

Benefit: It’s a powerful source of fuel. Some saturated fat in the diet suppresses appetite for hours longer than the other fats. That’s why the Atkins diet worked for some people.

Caution: Foods fried in saturated fat have vastly superior flavor. Be careful at restaurants. Those really great-tasting rolls could be made with lard or butter, and you won’t know.

Danger: Eat like this for a few years and it will probably clog your blood vessels. Also the more saturated fat you eat, the higher your bad cholesterol. And it seems to lower your good cholesterol.

Hint: The leanest meat will deliver all the saturated fat you need in your diet. If you’re a food addict, or you like great gourmet meals, you need to exercise a lot more to work off this fat.

Monounsaturated and omega-3 fats

These are considered good fats because they move through the bloodstream without clogging up the works and do some cleaning in the process.

Source: Monounsaturated fatty acids are found mostly in some plants, such as olives. Olive oil is the most popular source of monounsaturated fat. Omega-3 fats come from eating fish oil or canola oil and can be purchased in pill and liquid forms.

Benefit: They appear to reduce bad cholesterol and raise good cholesterol. Olive oil is valued because it replaces the use of butter and margarine. Omega-3 doesn’t allow sticky fats and platelets to accumulate in narrow blood vessels and helps move cholesterol back to the liver for processing.

Caution: Olive oil and fish oil have the same calorie content as any other fat: 9 calories per gram. Olive oil is 13 percent saturated fat.

Danger: Too much omega-3 fat in the blood can slow blood clotting a tiny bit. It takes a lot to be a health risk alone, but be careful with fish-oil pills if you’re taking a prescription blood thinner or aspirin.

Hint: You could get all the omega-3 fats you need by eating 6 to 8 ounces of fatty fish three times a week. When cooking with olive oil, use just enough to lubricate the pan. Don’t immerse your food.

Trans fat

These are manufactured fats. Manufacturers change natural fat to trans fats in order to prolong the shelf life of bread, milk substitutes and desserts. A tiny amount exists in nature, but the problem dosage comes from food produced in factories.

Source: Manufactured food, especially baked goods. Imitation dairy products, hard margarines and some hard cheese. Trans fats make shortening harder.

Benefits: None.

Caution: Read food labels; the government now requires that trans fats be listed.

Danger: Trans fats are associated with raising bad cholesterol and can float around the system getting in the way of nutrients. Also, the residue can be solid at body temperature, so it’s like wax floating around in your bloodstream.

Polyunsaturated fat

This generally is oil extracted from vegetables – soy mostly, but also corn and safflower oil. The Food and Drug Administration says polyunsaturated fatty acids tend to lower levels of good and bad cholesterol.

Source: Wesson and Mazola oils, margarine and some dairy substitutes that use plant oil, such as coffee creamers.

Benefit: The only real benefit is that it’s not saturated fat.

Caution: It’s still oil: 9 calories per gram. Many people deep-fat-fry with vegetable oil, including fast-food restaurants.

Dr. James Shoemaker, a researcher with St. Louis University School of Medicine, says the confidence in vegetable oil is undeserved. Vegetable oil is a very new substance to the human diet, he said.

Humans didn’t start eating vegetable oil until the second half of the 20th century. Then, when saturated fat got a bad rap in the early 1970s, vegetable oil’s popularity took off. Even fast-food restaurants stopped frying in lard and started using vegetable oil.

While that reduced the amount of saturated fat, it didn’t reduce the calories. One piece of fried chicken, for example, can deliver up to 300 additional calories because the breading has been bathed in oil.

Danger: Some early studies showed a possible association with cancer.

Hint: Read food labels. Polunsaturated fat is added to a lot of prepared foods and used to fry a lot of french fries, meat and many other nibble foods. At home, don’t fry. If you must use this oil, brush it on the pan and saute at a low heat; don’t submerge your food in any fat.

The bottom line

So, how much fat can you eat? The U.S. Department of Agriculture recommends that less than 10 percent of your daily calories come from saturated fat.

You should eat as little trans fat as possible; most fat in your diet should be polyunsaturated or monounsaturated. Keep your total fat intake at 20 percent to 35 percent of daily calories.

SOURCES: Dr. Samuel Klein, Danforth Professor of Medicine and Nutritional Science at Washington University; Jennifer Ebelhar, a registered dietitian with St. Louis University; Dr. James Shoemaker, professor, biochemist and dietitian with St. Louis University; the U.S. Food and Drug Administration; the U.S. Department of Agriculture; the U.S. Department of Health and Human Services