Stuff Fats are sources of energy that add flavor to food — the sizzle on the steak, you can say. However, as anyone who’s spent knows, fats may also be hazardous to your health. The trick is separating the good from the bad. Trust me. It can be done.

Here’s a sentence that you probably never thought you’d read: A healthy body needs fat. Your body uses dietary fat (the fat that you get from food) to make tissue and manufacture biochemicals, such as hormones. Some of the body fat made from food fat is visible.

Even though your skin covers it, you can see the fat in the adipose (fatty) tissue in female breasts, hips, thighs, buttocks, and belly or male abdomen and shoulders. This visible body fat:

• Provides a source of stored energy.
• Gives shape to your body.
• Cushions your skin (imagine sitting in a chair for a while to reading book without your buttocks to pillow your bones)
• Acts as an insulation blanket that reduces heat loss.

Other body fat is invisible. You can’t see this body fat because it’s tucked away in and around your internal organs. This hidden fat is:

• Part of every cell membrane (the outer skin that holds each cell together)
• A component of myelin, the fatty material that sheathes nerve cells and makes it possible for them to fire the electrical messages that enable you to think, see, speak, move, and perform the multitude of tasks natural to a living body; brain tissue also is rich in fat.
• A shock absorber that protects your organs (as much as possible) if you fall or are injured.
• A constituent of hormones and other biochemicals, such as vitamin D and bile.

Although fat has more energy (calories) per gram than proteins and carbohydrates, your body has a more difficult time pulling the energy out of fatty foods. Imagine a chain of long balloons — the kind people twist into shapes that resemble dachshunds, flowers, and other amusing things.

When you drop one of these balloons into water, it floats. That’s exactly what happens when you swallow fat-rich foods. The fat floats on top of the watery food-and-liquid mixture in your stomach, which limits the effect that lipases — fat-busting digestive enzymes in the mix below — can have on it. Because fat is digested more slowly than proteins and carbohydrates, you feel fuller (a condition called satiety) longer after eating high-fat food.

When the fat moves down your digestive tract into your small intestine, an intestinal hormone called cholestokinin beeps your gallbladder, signaling for the release of bile. Bile is an emulsifier, a substance that enables fat to mix with water so that lipases can start breaking the fat into glycerol and fatty acids.

These smaller fragments may be stored in special cells (fat cells) in adipose tissue, or they may be absorbed into cells in the intestinal wall, where one of the following happens:

• They’re combined with oxygen (or burned) to produce heat/energy, water, and the waste product carbon dioxide.
• They’re used to make lipoproteins that haul fats, including cholesterol, through your bloodstream.

Glucose, the molecule you get by digesting carbohydrates, is the body’s basic source of energy. Burning glucose is easier and more efficient than burning fat, so your body always goes for carbohydrates first.

But if you’ve used up all your available glucose — maybe you’re stranded in a cabin in the Arctic, you haven’t eaten for a week, a blizzard’s howling outside, and the corner deli 500 miles down the road doesn’t deliver — then it’s time to start in on your body fat.

The first step is for an enzyme in your fat cells to break up stored triglycerides (the form of fat in adipose tissue). The enzyme action releases glycerol and fatty acids, which travel through your blood to body cells, where they combine with oxygen to produce heat/energy, plus water — lots of water — and the waste product carbon dioxide.

As anyone who has used a highprotein/ high-fat/low-carb weight-loss diet such as the Atkins regimen can tell you, in addition to all that water, burning fat without glucose produces a second waste product called ketones.

In extreme cases, high concentrations of ketones (a condition known as ketosis) alter the acid/alkaline balance (or pH) of your blood and may trip you into a coma. Left untreated, ketosis can lead to death. Medically, this condition is most common among people with diabetes. For people on a low-carb diet, the more likely sign of ketosis is stinky urine or breath that smells like acetone (nail polish remover).

Food contains three kinds of fats: triglycerides, phospholipids, and sterols. Here’s how they differ:

• Triglycerides: You use these fats to make adipose tissue and burn for energy.

• Phospholipids: Phospholipids are hybrids — part lipid, part phosphate (a molecule made with the mineral phosphorus) — that act as tiny rowboats, ferrying hormones and fat-soluble vitamins A, D, E, and K through your blood and back and forth in the watery fluid that flows across cell membranes.

• Sterols (steroid alcohols): These are fat and alcohol compounds with no calories. Vitamin D is a sterol. So is the sex hormone testosterone. And so is cholesterol, the base on which your body builds hormones and vitamins.

Getting the right amount of fat in your diet is a delicate balancing act. Too much, and you increase your risk of obesity, diabetes, heart disease, and some forms of cancer. (The risk of colon cancer seems to be tied more clearly to a diet high in fat from meat rather than fat from dairy products.)

Too little fat, and infants don’t thrive, children don’t grow, and everyone, regardless of age, is unable to absorb and use fat-soluble vitamins that smooth the skin, protect vision, bolster the immune system, and keep reproductive organs functioning.

In the fall of 2002, the National Academies’ Institute of Medicine (IOM) recommended that no more than 20 to 45 percent of daily calories should come from fat. On a 2,000-calorie daily diet, that’s 400 to 900 calories from fats a day. The Dietary Guidelines for Americans 2005 lowers that to 20 to 30 percent of total calories.

Translation: 400 to 600 of the calories on a 2,000-calorie/day regimen. Because your body doesn’t need to get saturated fats, cholesterol, or trans fats from food, neither IOM nor the Dietary Guidelines for Americans 2005 have set levels for these nutrients, except to say, “Keep them as low as possible, please.”

This advice about fat intake is primarily for adults. Although many organizations, such as the American Academy of Pediatrics, the American Heart Association, and the National Heart, Lung, and Blood Institute, recommend restricting fat intake for older children.

They stress that infants and toddlers require fatty acids for proper physical growth and mental development, and that’s why Mother Nature made human breast milk so high in fatty acids. Never limit the fat in your baby’s diet without checking first with your pediatrician.

Essential Fatty Acids

An essential fatty acid is one that your body needs but cannot assemble from other fats. You have to get it whole, from food. Linoleic acid, found in vegetable oils, is an essential fatty acid. Two others — linolenic acid and arachidonic acid — occupy a somewhat ambiguous position.

You can’t make them from scratch, but you can make them if you have enough linoleic acid on hand, so food scientists can work up a good fight about whether linolenic and arachidonic acids are actually “essential.” In practical terms, who cares?

Linoleic acid is so widely available in food, you’re unlikely to experience a deficiency of any of the three — linoleic, linolenic, or arachidonic acids — as long as 2 percent of the calories you get each day come from fat. In 2002, the Institute of Medicine (IOM) published the first daily recommendations for two essential fatty acids, alpha-linolenic acid and linolenic acid.

The former is an omega-3 fatty acid that’s found in fish oils, milk, and some veggie oils. The latter is an omega-6 fatty acid (ditto), found in safflower and corn oil. IOM recommends that:

• Women get 12 grams linolenic acid and 1.1 grams alpha-linolenic acid per day.
• Men get 17 grams linolenic acid and 1.6 grams alpha-linolenic acid per day.

Finding fat in all kinds of foods

As a general rule:

• Fruits and vegetables have only traces of fat, primarily unsaturated fatty acids.

• Grains have small amounts of fat, up to 3 percent of their total weight.

• Dairy products vary. Cream is a high-fat food. Regular milks and cheeses are moderately high in fat. Skim milk and skim milk products are low-fat foods. Most of the fat in any dairy product is saturated fatty acids.

• Meat is moderately high in fat, and most of its fats are saturated fatty acids.

• Poultry (chicken and turkey), without the skin, is relatively low in fat.

• Fish may be high or low in fat, primarily unsaturated fatty acids that — lucky for the fish — remain liquid even when the fish is swimming in cold water. (Saturated fats harden when cooled.)

• Vegetable oils, butter, and lard are high-fat foods. Most of the fatty acids in vegetable oils are unsaturated; most of the fatty acids in lard and butter are saturated.

• Processed foods, such as cakes, breads, canned or frozen meat, and vegetable dishes, are generally higher in fat than plain grains, meats, fruits, and vegetables.

Here’s a simple guide to finding which foods are high (or low) in fat. Oils are virtually 100 percent fat. Butter and lard are close behind. After that, the fat level drops, from 70 percent for some nuts down to 2 percent for most bread. The rule to take away from these numbers? A diet high in grains and plants always is lower in fat than a diet high in meat and oils.

Fatty acids are the building blocks of fats. Chemically speaking, a fatty acid is a chain of carbon atoms with hydrogen atoms attached and a carbon-oxygenoxygen- hydrogen group (the unit that makes it an acid) at one end. All the fats in food are combinations of fatty acids.

Nutritionists characterize fatty acids as saturated, monounsaturated, or polyunsaturated, depending on how many hydrogen atoms are attached to the carbon atoms in the chain. The more hydrogen atoms, the more saturated the fatty acid.

Depending on which fatty acids predominate, a food fat is likewise characterized as saturated, monounsaturated, or polyunsaturated.

• A saturated fat, such as butter, has mostly saturated fatty acids. Saturated fats are solid at room temperature and get harder when chilled.

• A monounsaturated fat, such as olive oil, has mostly monounsaturated fatty acids. Monounsaturated fats are liquid at room temperature; they get thicker when chilled.

• A polyunsaturated fat, such as corn oil, has mostly polyunsaturated fatty acids. Polyunsaturated fats are liquid at room temperature; they stay liquid when chilled.

So why is margarine, which is made from unsaturated fats such as corn and soybean oil, a solid? Because it’s been artificially saturated by food chemists who add hydrogen atoms to some of its unsaturated fatty acids.

This process, known as hydrogenation, turns an oil, such as corn oil, into a solid fat that can be used in products such as margarines without leaking out all over the table. A fatty acid with extra hydrogen atoms is called a hydrogenated fatty acid.

Another name for hydrogenated fatty acid is trans fatty acid. Trans fatty acids are not healthy for your heart. Because of those darned extra hydrogen atoms, they are, well, more saturated, and they act like — what else? — saturated fats, clogging arteries and raising the levels of cholesterol in your blood.

To make it easier for you to control your trans fat intake, the Food and Drug Administration now requires a new line on the Nutritional Facts label that tells you exactly how many grams of trans fats are in any product you buy.

In the meantime, the same smart food chemists who invented hydrogenation have now come up with trans fat–free margarines and spreads, including some that are made with plant sterols and stanols.

Plant sterols are natural compounds found in oils in grains, fruits, and vegetables, including soybeans, while stanols are compounds created by adding hydrogen atoms to sterols from wood pulp and other plant sources.

Sterols and stanols work like little sponges, sopping up cholesterol in your intestines before it can make its way into your bloodstream. As a result, your total cholesterol levels and your levels of low-density lipoproteins (otherwise known as LDLs or “bad cholesterol”) go down.

In some studies, one to two 1-tablespoon servings a day of sterols and stanols can lower levels of bad cholesterol by 10 to 17 percent, with results showing up in as little as two weeks.

Fats are characterized according to their predominant fatty acids. Nevertheless, because corn oil has more polyunsaturated fatty acid, corn oil is considered a polyunsaturated fatty acid.