All About Fat in Your Diet
The body needs fat. It’s a major energy source and also helps you absorb certain vitamins and nutrients. Certain fats from plant sources and fish turn out to be the most beneficial. These unsaturated fats — which come mainly in the form of vegetable oils, nuts, and whole grains — are good for you. There are two types of unsaturated fat: polyunsaturated fat and monounsaturated fat. These good fats can help lower LDL (bad) cholesterol, prevent abnormal heart rhythms, and prevent heart disease. (For more information on fats and specific illnesses, see “The food-health connection”.)
Only some fats are bad for you: trans fats (found in some manufactured foods and margarines) and some saturated fats (found mainly in meat and whole-fat dairy foods). Bad fats boost your chances of developing heart disease by increasing blood levels of two of its main risk factors, LDL cholesterol and triglycerides. Trans fats also lower levels of protective HDL, the “good” cholesterol.
The DRI for fat gives considerable leeway: 20% to 35% of your daily calories can come from fat. This means you can get up to 35% of your calories from fat and still have a diet that’s good for your heart, helps reduce your risk of hypertension, and lets you maintain your weight or even lose weight. Even more may be fine, so long as it’s mostly healthy fats from fish and vegetables (see “Eating plans for good health”).
Although research results are conflicting, it’s best to limit red meat, particularly processed meat, and full-fat dairy products like butter, cheese, and ice cream. Try to choose non-fat or low-fat dairy products and lean cuts of meat, poultry, fish, soy, and beans. Processed and cured meats are particularly linked to an increased risk of cardiovascular disease. For trans fats, there is no safe level, and the guidelines recommend eating as little as possible.
What’s the difference between a good fat and a bad fat? All fats have a similar chemical structure: a chain of carbon atoms bonded to hydrogen atoms. What differs is the length and shape of their carbon chains and the number of hydrogen atoms connected to the carbon atoms. Seemingly slight differences in structure translate into crucial differences in the body.
The two forms of unhealthy fat, trans fats and saturated fats, share a physical trait: they’re solid at room temperature. Think of butter, margarine, shortening, or the marbled fat in a steak. But bad fats abound in some liquids, too, including whole milk, cream, and palm and coconut oils. These fats drive up your total cholesterol, tipping the balance toward LDL cholesterol, the destructive type that prompts blockages to form in the coronary arteries, the hallmark of heart disease. The expert panel of the National Cholesterol Education Program recommends cutting back on saturated and trans fats as a means of preventing and controlling heart disease.
Saturated fat. The word “saturated” here refers to the number of hydrogen atoms in this type of fat. In saturated fat, the chain of carbon atoms holds as many hydrogen atoms as possible, making it literally saturated with hydrogen atoms. Each carbon atom in the chain is connected to the next by a single bond, leaving the maximum number of bonding points available to hold hydrogen. By contrast, in unsaturated fats, the carbon atoms have fewer than the maximum number of hydrogens.
Saturated fat, which is concentrated in full-fat dairy products and meat, has received some positive publicity in the media lately. Maybe you’ve seen reports that it doesn’t cause heart disease after all. Unfortunately, many media stories oversimplified the conclusions of the scientists who conducted a 21-study meta-analysis published in The American Journal of Clinical Nutrition in 2010.
The study really said that there was not enough evidence to conclude that saturated fat increases the risk of heart disease, but that replacing saturated fat with polyunsaturated fat may indeed reduce risk of heart disease.
Two other major studies in the same academic journal narrowed the prescription slightly, concluding that replacing saturated fat with polyunsaturated fats like vegetable oils or high-fiber carbohydrates is the best bet for reducing the risk of heart disease. All told, if you are trying to reduce saturated fat, your best choice is to replace it with polyunsaturated oils, like corn oil.
In regards to cholesterol, a diet high in saturated fats boosts total cholesterol by elevating harmful LDL. Like all dietary fat, saturated fat also raises the protective HDL. But eating unsaturated fat is a better choice because it lowers the bad cholesterol and raises the good.
Trans fats (partially hydrogenated oils). Trans fats are rapidly fading from the scene, thanks to the government requirement that they be listed on food labels. Trans fats were created in the laboratory to provide cheap alternatives to butter. Food chemists learned how to solidify vegetable oil by heating it in the presence of hydrogen and a heavy-metal catalyst such as palladium. The process, called hydrogenation, gives the carbon atoms more hydrogen atoms to hold, making polyunsaturated fat (a good fat) more like saturated fat in structure. That’s how solid vegetable fats such as shortening and margarine came into being. On food label ingredient lists, this manufactured substance is typically listed as “partially hydrogenated oil.”
Trans fats are even worse for you than saturated fats. Not only do they increase your LDL cholesterol, but they also reduce your beneficial HDL cholesterol. The Institute of Medicine expert panel stated that trans fats have no known health benefits and that there is no safe level of consumption.
Since 2006, the FDA has required trans fat content to be listed as a separate line item on food labels. As a result, the food industry has reduced trans fats in many foods, and some local governments have banned trans fats in restaurant foods. Happily, these mainly man-made fats, which used to be in everything from commercial cookies to fast-food fries, are now much less prevalent. Harvard researchers and the Center for Science in the Public Interest, a consumer advocacy group, analyzed 83 reformulated supermarket and restaurant foods and found that nearly all of them were free or mostly free of trans fat, and that saturated fat hadn’t replaced it, yielding products and menu items with a healthier balance of fats.
Be aware that although many labels now state “0 trans fat,” manufacturers can still sneak them in. According to the labeling law, if a product has no more than half a gram of trans fat per serving, the label can list it as having 0 grams of trans fat. By making the serving size very small (two small cookies, for example), the manufacturer may thus be able to label a product as having 0 grams trans fat per serving. But if you eat six cookies, you may have consumed nearly 3 grams of trans fat. Check the label for hydrogenated oils and be wary of foods with small serving sizes. And keep in mind that just because a cookie is listed as having no trans fat, it’s still a cookie, which can be loaded with saturated fat, sugar, and empty calories.
Good fats come mainly from vegetable, nut, and fish products. They differ from bad fats by having fewer hydrogen atoms bonded to their carbon chains. They are liquid, not solid, at room temperature. There are two broad categories of beneficial fats: polyunsaturated and monounsaturated.
Polyunsaturated fats. When you pour liquid cooking oil into a pan, there’s a good chance you’re using polyunsaturated fat. Corn oil, sunflower oil, and safflower oil are common examples. (The exceptions are sunflower and safflower oils labeled “high-oleic,” which come from crops intentionally bred to produce mostly monounsaturated fats.) Polyunsaturated fats are essential fats. That means they’re required for normal body functions, but your body can’t manufacture them and so you must get them from food. Polyunsaturated fats help build cell membranes, the exterior casing of each cell, and the sheaths surrounding nerves. They’re vital to blood clotting, muscle movement, and inflammation. They reduce LDL more than other types of fats, improving your cholesterol profile. Even better, they also lower triglycerides.
A polyunsaturated fat has two or more double bonds in its carbon chain. There are two types of polyunsaturated fats: omega-3 (n-3) fatty acids and omega-6 (n-6) fatty acids. (The numbers refer to the distance between the end of a carbon chain and the first double bond.) Both types offer health benefits.
Research has shown that omega-3s in dietary fish and fish oil supplements help prevent and even treat heart disease and stroke. The reasons are several: these fats help reduce blood pressure, raise HDL, lower triglycerides, and — perhaps most importantly — prevent lethal heart-rhythm disorders. Evidence also suggests they may help reduce the need for corticosteroid medications in people with rheumatoid arthritis. Unfortunately, most of the other studies linking omega-3s to a wide range of other health improvements, including reducing risk of dementia, are inconclusive, and some of them have major flaws, according to a systematic review of the evidence by the Agency for Healthcare Research and Quality.
Omega-3s come mainly from fish, but you can also find them in flaxseeds, walnuts, canola oil, and unhydrogenated soybean oil. Fatty fish such as salmon, mackerel, and sardines are especially good sources of omega-3s. The DRI for alpha-linolenic acid, the omega-3 in vegetable oils, is 1.6 grams per day for men and 1.1 for women.
Omega-6 fatty acids are even more protective against heart disease. High levels of linoleic acid, an omega-6, are in such vegetable oils as safflower, soybean, sunflower, walnut, and corn oils. (Be aware, however, that high-oleic safflower and sunflower oils, often used in chips, have much lower omega-3 and omega-6 content.) The DRI for linoleic acid is 17 grams per day for men ages 19 to 50 and 12 grams for women in this age group. For adults ages 51 to 70, the DRI is 14 grams for men and 11 grams for women.
Monounsaturated fats. When you dip your bread in olive oil at an Italian restaurant, you’re getting mostly monounsaturated fat. Unlike a polyunsaturated fat, which has two or more double bonds of carbon atoms, a monounsaturated fat has just one. The result is that it has more hydrogen atoms than a polyunsaturated fat, but fewer than a saturated fat. Although there’s no DRI for monounsaturated fats, the Institute of Medicine recommends using them as much as possible along with polyunsaturated fats to replace saturated and trans fats. Good sources of monounsaturated fats are olive oil, peanut oil, canola oil, avocados, and most nuts, as well as high-oleic safflower and sunflower oils.
The discovery that monounsaturated fat could be healthful came from the Seven Countries Study during the 1960s. This study revealed that people in Greece and other parts of the Mediterranean region enjoyed both a low rate of heart disease and a high-fat diet. The fat in their diet, however, was not the animal fat common in other countries with higher rates of heart disease: it was olive oil, which has mainly monounsaturated fat. This finding produced a surge of interest in olive oil and the “Mediterranean diet,” a style of eating regarded as a healthful choice today.
Building your plate
At mealtime, simplify the process of healthy eating by dividing your plate in half. Fill one half with vegetables. Divide the other half into two quarters, filling one quarter with lean protein such as fish or skinless chicken. Fill the remaining quarter with whole grains such as brown rice, whole-wheat pasta, barley, or quinoa.