We love to eat it, hate it in the mirror! But dietary fat is essential for life, just like glucose and protein. In fact, if glucose is unavailable, the body will break fat down by ketosis as a backup brain fuel. Fat is crucial to maintaining cell membranes, brain tissue, and nerve sheaths, increasing immunity, maintaining energy reserves, stabilizing blood sugar, and controlling hunger. Digestion breaks fat down into fatty acids and lipids, one of which, cholesterol, forms the basic building block of our sex hormones.

A couple of fatty acids that we cannot synthesize ourselves—omega-3 (linoleic acid) and omega-6 (alphalinoleic acid), called the essential fatty acids (EFAs)—have been linked with decreases in inflammation, hypertension, and LDL cholesterol and improvements in mood, cognition, and hormonal balance. More important than the EFAs themselves are the components they break down into, many of which serve as precursors to our all-important hormones. Since we don’t make them, we must get these EFAs in our diet. The ratio between them is important: In our Westernized diet, we tend to get too much omega-6, found in many common vegetable oils like soybean, safflower, and corn, compared to our intake of omega-3, which is found abundantly in fatty and freshwater fish and in primrose, flaxseed, and hemp.

When cells have easy access to glucose in the blood, they rely less on the long-term stored energy in fat. Instead of flowing in and out of fat cells, then, fatty acids accumulate there as triglycerides, forming tissue called white adipose tissue—body fat. The kind we love to hate. When one fat cell is full and you still have extra energy to store, your body makes another, and so on until you can’t zip up your skirt. But the key thing to realize here is that it’s extra glucose making you fatter faster, not extra dietary fat. Dietary fat has more calories per gram than protein or carbs, but it is metabolized differently. And you can’t eat as much in one sitting as you can of carbohydrates, so you often end up eating fewer calories overall (not that we’re counting calories).

Does this mean you can march right out and order a triple bacon cheeseburger with fries? No, because the dietary fat I’m talking about is healthy fat—real fat, found in organic dairy products, fish, vegetables, nuts, seeds, and their oils. Depending on the structure of the fat molecule, it is monounsaturated, polyunsaturated, or saturated. All three types are fine to eat in moderation. These healthy fats, found in nature, are made of long chains of carbon molecules bonded together. Fats become unhealthy—damaged—when these bonds are broken, which occurs during cooking, charbroiling, artificial processing, or poor storage. Next time you’re in a fast-food restaurant, take a whiff—that smell is rancid, damaged fat!

You’ve probably heard about the dangers of trans fats—artificially hydrogenated oils used mainly to extend the shelf life of food. To make these substances, extra hydrogen is pumped into polyunsaturated vegetable oils to saturate the carbon molecules in them. Margarine, butter substitutes, and any foods containing hydrogenated or partially hydrogenated oils have trans fats in them, even if it’s not listed on the label. (The government allows products with less than 0.5 percent trans fat per serving to be labeled trans-fat-free. The problem, of course, is that most people don’t eat just one serving.)

The USDA can’t determine a healthful limit for trans-fat intake, because even very small amounts appear to be harmful. Researchers are now in nearly universal agreement that trans fats and damaged fats, rather than saturated fats, are the culprits in atherosclerosis and high cholesterol in most healthy individuals. Studies indicate that if we didn’t eat these fats, more than 30,000 deaths from cardiovascular disease could be avoided every year.