A Primer on Fats
An understanding of the different types of dietary fat is critical to giving well-informed dietary counsel. Advice on the intake of dietary fat has a checkered history. As recently as the 1990s, diets low in total fat were very popular as a means to better health and resulted in the development of many commercial nonfat and low-fat products that were high in processed carbohydrates -- a combination that did not prove to be healthy.
We know now that a more nuanced approach to fat intake is necessary to take advantage of the health benefits from certain types of fats -- monounsaturated and polyunsaturated -- while avoiding the more harmful fats, saturated and trans-unsaturated fat. Saturated fat is the principal dietary contributor to low-density lipoprotein (LDL) cholesterol and is abundant in high-fat animal products including nonlean cuts of beef and full-fat dairy products. Minimizing intake of saturated fat is key to a healthy diet, and intervention studies have shown that doing so can dramatically reduce markers of disease such as LDL cholesterol or coronary stenosis.
Replacing saturated fat with monounsaturated and polyunsaturated fats rather than eliminating fat intake altogether is preferred and results in reductions in cardiovascular disease risk as well as other disease outcomes such as diabetes. Foods, such as olive and canola oils, avocados, and certain nuts, such as almonds and cashews, are high in monounsaturated fats. Monounsaturated fat has been shown to lower LDL cholesterol, insulin resistance, and thrombotic potential. Foods that are high in omega-3 polyunsaturated fat include fatty fish (such as salmon) and certain nuts (such as walnuts). Omega-6 polyunsaturated fat is found in safflower, sunflower, and corn oil and may lower healthy high-density lipoprotein (HDL) cholesterol, although its overall effect is probably beneficial. Health benefits include lowering LDL cholesterol, triglycerides, and platelet aggregation as well as reducing coronary heart disease-related death. A recent intervention study on newly diagnosed patients with diabetes showed that patients who followed a Mediterranean-style diet (which included increased intake of poly- and monounsaturated fats in place of carbohydrates) had a markedly lower need for diabetic medications than those who consumed an American Heart Association low-fat diet. Those who were on the Mediterranean diet consumed at least 30% of calories as fat, primarily from olive oil, which is high in monounsaturated fat.
Trans Fat Enters, and Exits, the Dietary Scene
Trans fat is the most unhealthy of the dietary fats and its source is primarily manufactured. Trans fat is created by partial hydrogenation of vegetable oils, a process first used in the early 1900s to increase the stability of certain oils. In 1911, Crisco was the first product made with trans fat, and in the 1950s trans fat was used increasingly in manufactured food products owing to its ability to increase the shelf life of store-bought foods. By the 1970s its use in margarine was promoted as a healthier alternative to the saturated fat in butter, and not until the 1980s-1990s were its health hazards fully appreciated. Trans fat has been shown to increase LDL cholesterol, reduce HDL cholesterol, and play a role in inflammation. It is found in fried foods as well as in many store-bought cookies and crackers. Trans fat received recent press attention when it was banned in New York City restaurants and other cities soon followed, resulting in its removal from products of major national food chains such as McDonald's and Arby's. Risk models have shown that its elimination in the food supply and its replacement with healthy fats could substantially reduce the burden of heart disease.
Although it is difficult to measure our daily intake of types of fat by counting percentages, it is useful to counsel patients to read the Nutrition Facts labels on manufactured foods. Advise patients to avoid all food with the ingredient "partially hydrogenated oil," even if the Nutrition Facts label reports 0 grams of trans fat. The Nutrition Facts label is permitted to list 0 grams of trans fat as long as the product contains less than 0.5 gram per serving. Trans fat could be inadvertently consumed in significant levels if enough of these less-than-0.5 gram-per-serving foods are consumed, and this consumption may be harmful even in small amounts. Also advise consuming foods very low in saturated fat and with higher amounts of mono- and polyunsaturated fats.