The Facts About Fats
Saturated or unsaturated? It makes a big difference
Fats pack the most energy (that is to say, calories) per ounce of all the major nutrients. At times when food is scarce, eating more calories improves a person's chances of survival. Those two facts probably shaped the human affinity for fat over the ages. Plus, let's face it: Fats are generally delicious.
Today, dietary fat is rarely in short supply, at least not in developed countries like the United States. In fact, it's everywhere, from fast-food eateries to just about every supermarket aisle, which has very likely played a role in the obesity epidemic. Too much fat in the diet is also associated with an increased risk of diabetes, heart disease, cancer, and other diseases.
Yet fat is not without its virtues: It cushions organs, stores energy, and insulates the body against the elements. Some fats are better than others, though. Learning about this flavorful fuel can help in making healthy food choices.
Chain of Fuels
Fats are made up of the same basic building blocks as carbohydrates—carbon, hydrogen, and oxygen—but arranged to boost their energy-storage capacity. Most of the fat in plants and animals, including people, exists in a form called triglycerides. These fat storage molecules resemble a trident, the three-pronged spear of mythology, with the syrupy alcohol glycerol serving as a backbone for three prongs of fatty acid.
Fatty acids rarely roam the body alone, preferring to travel in triglycerides. A fatty acid is a string of carbon atoms with hydrogen limbs stemming outward from each. At the end of the fatty acid that joins up with glycerol are a couple of oxygen atoms that give the molecule its acidic properties.
All fatty acids share this same basic chain-like structure, but they come in several varieties, depending on how the carbons are linked, with dramatically different effects on human health. If single bonds connect the carbons along a fatty acid chain, the fat is said to be saturated; it's literally saturated with the most hydrogen atoms possible. Unsaturated fats are formed when a single bond is replaced by one or more double bonds, eliminating a couple of hydrogen atoms in the process. Then the fatty acid is monounsaturated or polyunsaturated.
Fat in Food
Meat and dairy are the main sources of saturated fat in the human diet, because the mammals whose meat and milk we eat store much of their fat in saturated form. Plants, on the other hand, usually favor unsaturated fat storage. Foods with a lot of saturated fat tend to be firmer; lard, butter, and meat trimmings are solid at room temperature because they contain a hefty amount of saturated fat. Unsaturated fats, like plant oils, have more flexible structures and are thus typically liquid. The length of the fatty acid chain also contributes to solidity; despite having high saturated fat content, coconut and palm kernel oil have relatively short fatty acid chains and so are liquids.
|Fats in Foods|
|Here are some sources of both saturated and unsaturated fats. It's a good idea to limit how much saturated fat you eat.|
|High in Saturated Fat||High in Monounsaturated Fat||High in Polyunsaturated Fat|
Saturation helps ward off rancidity, because saturated fats resist oxidation. Food scientists discovered that they could increase the shelf life and firmness of unsaturated fats through a chemical process called hydrogenation. As the word suggests, hydrogenation involves adding hydrogen atoms to unsaturated fats. The result sometimes is a trans fat, a molecule twisted into a zigzag pattern that isn't common in natural foods. In recent years, trans fats have been established as a hazard to human health, and food labels are required to list them. Yet if there is less than 0.5 gram of trans fat per serving in a food, manufacturers are allowed to round down the amount to zero on the label. Check ingredient lists for "partially hydrogenated" oils; if one is listed, that means the food contains trans fat, even if the nutrition facts say the product has zero grams.
Some of the most important fats for the human diet are monounsaturated and polyunsaturated. Omega-3 fatty acids and omega-6 fatty acids, which can be either monounsaturated or polyunsaturated, are essential, meaning that the body needs them to function normally but can't make them on its own. These fats are structural components of cell membranes and involved in cell signaling in the brain. Vegetable oils and meats supply enough omega-6 for most people, but omega-3 is harder to come by in a Western diet. Fatty fish and nuts are good sources of omega-3 fatty acids.
Fat in You
Fats aren't particularly soluble, making them a challenge to digest. The body enlists bile acids, emulsifiers made in the liver, to help dissolve fats in the small intestine. Bile acids make fats accessible to enzymes called lipases that chew fat up for absorption into the body.
Once in the body, fats are mostly reconstituted as triglycerides, which circulate constantly in the blood. Because of their insolubility, triglycerides travel in packages called lipoproteins (from lipid, a scientific word for fat). LDL and HDL are types of lipoproteins. In addition to triglycerides, lipoproteins carry cholesterol around the body, supplying cells with energy and building materials. LDL is the "bad" cholesterol and HDL the "good" one.
The amount and type of fat in the diet influences the relative amounts of LDL and HDL in the body, with saturated and trans fats bumping up LDL levels and polyunsaturated fats lowering them. Monounsaturated fats are thought to boost HDL levels. Low HDL and high triglyceride levels are common in people with type 2 diabetes.
Fat cells skim off circulating triglycerides and store them for later use by the body. There's no limit to how much fat these cells can store, and in these times of plenty, that has become a problem as excessive stored fat makes a person overweight or obese. Too much circulating triglyceride is a risk factor for heart disease.
People with diabetes are at a particularly high risk for heart disease, so keeping a sharp eye on dietary fat is a key part of healthy living. But that doesn't mean that tasty fats need to be eschewed entirely. So long as calories are kept in check, eating foods that contain mono- and polyunsaturated fats is a smart, and satisfying, move.
Recommended blood levels of triglycerides: < 150 mg/dl
Note: Corrections were made to this article on Oct. 12, 2010