Jennifer and her friends were eating lunch in the middle school cafeteria. Jennifer loaded her tray with french fries, a cheeseburger and chocolate pudding. There wasn't a vegetable or a piece of fruit in sight.

"You're eating a lot of cholesterol," her friend Sally said.

"So what?" Jennifer asked. "I'm not fat. I can eat another whole tray of this food and I won't even gain an ounce."

"Maybe not," Sally said. "But cholesterol's bad for you. It clogs up your veins."

"Yuck," Jennifer said. She looked at her french fries and cheeseburger. Suddenly, they didn't look quite so delicious.

What is cholesterol, anyway? And is it really as bad for you as people say?

Cholesterol is a fat. It is waxy, white and has no taste or smell. Your body manufactures it in the liver. You need cholesterol for several very important jobs: It is part of your cell walls. It helps keep your nervous system working. It aids digestion and helps produce hormones.

When cholesterol moves through the bloodstream, it combines with protein. It becomes a substance called a lipoprotein. There are two main kinds of lipoproteins: LDLs, or low-density lipoproteins, and HDLs, or high-density lipoproteins.

Too much LDL in the bloodstream can cause fat deposits to stick to the insides of blood vessels, blocking blood flow and leading to heart disease. When people talk about how bad cholesterol is, they're really talking about LDLs. HDLs, on the other hand, keep cholesterol moving through the blood and return it safely to the liver, which gets rid of it.

Medical studies over the years have shown that lowering the level of cholesterol in the blood reduces the risk of heart disease. Careful eating habits, combined with a healthy exercise program, can lower the level of bad LDLs and raise the level of good HDLs in the blood.

There has been a lot of controversy, or argument, in the medical community about cholesterol. But the consensus, or general feeling, is that lowering blood cholesterol levels is a good idea. United States government guidelines now say that the desirable cholesterol level for grown-ups is 200 or below. (Doctors use a blood test to analyze cholesterol levels.)

The American Heart Association reports that about 5 percent of kids ages 5 to 18 have cholesterol levels above 200. Many experts think that kids' cholesterol levels should be no more than 170.

Because following a low-fat diet can help reduce cholesterol levels, the Heart Association recommends that everyone follow a low-fat diet after age 2. However, other experts -- including the American Academy of Pediatrics, a large group of doctors who specialize in children's health -- think that a very restricted diet may not provide kids with all the nutrients they need for growth.

If your parents are wondering which diet would be best for you, they should ask your pediatrician or family physician for advice.

There should be some fat in a healthy diet. Fat supplies fatty acids that the body needs for normal growth. It gives energy and helps protect the body from getting too cold. It provides protective padding for vital organs like the kidneys. And fat forms an essential part of the structure of the cells that make up the body. So you wouldn't want to completely eliminate fat.

There are three different kinds of fat found in food. There's saturated fat, polyunsaturated fat and monounsaturated fat.

Saturated fat is found in animal products. It's hard at room temperature. It is found in some oils, such as coconut and palm oil. These oils are used in many processed foods. Saturated fat in large amounts is not good for you.

Polyunsaturated fat is found in vegetable oils like safflower or sunflower oil. It's liquid at room temperature. Medical studies have shown that polyunsaturated fats actually lower the amount of cholesterol in the blood.

Monounsaturated fat is found in some nuts and in olives. It doesn't seem to have much effect on cholesterol levels.

According to Dr. Anne Goldberg of the Washington University Medical School in St. Louis, Mo., everyone should watch what they eat to keep their cholesterol level low. Dr. Goldberg has conducted several studies on cholesterol and treats patients with cholesterol problems. She advises cutting down on very high-fat foods.

Dr. Goldberg says that saturated fats make up about 14 to 18 percent of the typical American diet. To cut back, she says, you should reduce the amount of meat you eat, and use very lean red meat. Take the skin off chicken and turkey when you eat them, and eat lots of fish. Avoid fried foods. Change from whole milk dairy products to 2 percent or skim milk.

Dr. Goldberg says, "If you set up good eating habits for children when they're young, they'll continue throughout their lives. If you set bad eating habits -- lots of snacking, sugar, junk food -- they'll keep eating that way."

Tips for Parents

Anne Goldberg, MD, of the Lipid Research Clinic at Washington University in St. Louis advises parents to give children 2 percent milk rather than whole milk. Include fewer servings of hot dogs and bacon and more turkey, chicken and fish in their meals. Serve fruits, vegetables and whole-grain products, and avoid sugary snacks and junk food. If you can already hear the groans coming from your family, remember this: An occasional meal, says Dr. Goldberg, of hamburgers and french fries and pizza and ice cream is allowable. "If you're following a reasonable low-fat diet every day, a once-a-week meal that's high in fat is not a problem."

Catherine O'Neill is a free-lance writer in Silver Spring.