A typical American diet contains somewhere between 35 to 40 percent of calories from fat. The 2005 U.S. Department of Agriculture (USDA) Dietary Guidelines for Americans recommend that healthy adults keep total dietary fat intake between 20 to 35 percent of calories from fat. They define low fat intake as no more than 20 percent of calories from fat. There are many different types of low-fat diets, each with their own benefits and drawbacks. Studies show that low-fat diets may reduce the risk of coronary heart disease and may help people lose weight.

Dietary fat is an essential macronutrient that supplies energy and essential fatty acids and acts as a carrier for fat-soluble vitamins such as A, D, E, and K. Fat provides 9 calories per gram which is more than twice the caloric density of carbohydrates or protein. In adipose (fat) tissue, it serves as an important energy storage source. When the body has depleted energy from carbohydrates, it begins to burn fat in a process called fatty acid oxidation. This occurs during long fasts and after about 20 minutes of exercise.

Certain segments of the population need to consume a low fat diet due to health concerns. These groups include people with gallbladder disease, fatty liver, fat absorption disorders, and fatty acid oxidation disorders. People with elevated cholesterol, hypertension (high blood pressure), and/or a history of heart disease are advised by the American Heart Association (AHA) to reduce saturated fat and cholesterol intake and keep total fat intake below 30 percent of total calories.

There are several popular low-fat diets, many of which have been around for decades. Vegan, vegetarian, and macrobiotic diets are plant-based diets that are usually low in fat because most animal products are avoided, but they do not intentionally restrict calories from fat. Very low-fat diets (no more than 15 percent calories from fat) have been criticized as being "too restrictive" and depriving the body of essential fatty acids like omega-3 polyunsaturated fatty acid. It is possible that intake of large amounts of fiber from very low-fat diets diet could cause gastrointestinal distress such as diarrhea. Also, very restrictive diets are difficult to maintain for Americans who travel often and frequently eat out at restaurants. However, low-fat diets have documented clinical studies showing reversal of coronary blockage and weight loss. They are one of the few popular diet plans with a strong scientific basis.

The Ornish Diet is a well-known and well-studied very low fat diet. Dean Ornish is a cardiologist who is Clinical Professor of Medicine at the University of California, San Francisco. He created the Ornish Lifestyle Modification Program, based on: (1) a very low fat, high complex carbohydrate diet rich in fruits, vegetables, whole grains, beans, and legumes, (2) regular exercise, (3) stress management, and (4) family/community support systems to maintain desired healthy behaviors. The Ornish Diet restricts fat intake to 10 percent of total calories and promotes consumption of complex carbohydrates. According to Ornish's disease and may help people lose weight. book Eat More, Weigh Less, the typical American diet contains 40 percent calories from fat, 20 percent from protein, and 40 percent from carbohydrates. Ornish recommends eating beans, fruits, grains, and vegetables whenever you are hungry. Nonfat dairy products, egg whites, and nonfat processed foods are allowed in moderation. Foods to be avoided include simple sugars, alcohol, meats, oils, avocados, olives, nuts, seeds, fat-containing dairy products, and any processed food containing more than 2 grams of fat per serving.

Similar very low-fat diets have been developed by Drs. John McDougall, Terry Shintani, and Gabe Mirkin. McDougall developed the 12-Day Diet and the McDougal Plan for Maximum Weight Loss (MWL). Both diets are almost vegan, emphasizing grains, vegetables, fruits, and beans. The 12-Day Diet restricts high-fat foods and caffeinated beverages for 12 days. After 12 days, some of these foods may be consumed occasionally. The MWL program has the same restrictions but also restricts fruit juices and dried fruits. It advocates consumption of whole grains, vegetables, legumes, fruit (limit to two servings per day) and sparing use of salt and sweeteners.

Terry Shintani's diet is based on the USDA food pyramid, but the top three tiers (oils, meats, and dairy) are replaced with nondairy calcium foods and noncholesterol protein/iron foods. A daily diet would consist of 8-13 servings of whole grains, 3-5 servings of vegetables, 2-4 servings of fruits, 2-3 servings of nondairy calcium foods, and 2-3 servings of noncholesterol protein/iron foods. Examples of the nondairy calcium group include calcium-rich greens, seaweed, tofu, and sesame seeds. Examples of noncholesterol protein/iron foods include lentils, beans, high-iron greens, seaweed, nuts, and seeds. Gabe Mirkin recommends no more than 20 grams of fat per day (under 10 percent calories from fat). About 15 grams of fat ("the basic 15") should come from unlimited servings of fruits, vegetables, whole grains, and beans, and up to five servings of low fat dairy, seafood, or reined grains. The remaining 5 grams can come from personal favorite foods that may not otherwise be part of the diet. High fat foods should be avoided.

The Dietary Approaches to Stop Hypertension (DASH) clinical studies showed that dietary modification could reduce hypertension (high blood pressure). It is recommended by both the AHA and National Cancer Institute. Like a low fat version of the Food Guide Pyramid, the DASH diet is rich in fruits, vegetables, complex carbohydrates and low fat dairy products. It is lower in fat, saturated fat, cholesterol, and sodium, and higher in potassium, magnesium, and calcium than the typical American diet. High levels of potassium, magnesium, and calcium are thought to be partially responsible for the observed benefits. A typical daily DASH diet would include 7-8 servings of grains, 4-5 servings of fruits, 4-5 servings of vegetables, 2-3 servings of low fat or fat-free dairy products, under 2 servings of meat, and 4-5 servings of nuts and seeds per week. The therapeutic Lifestyle Changes (TLC) diet was initially developed by the National Heart, Lung, and Blood Association's National Cholesterol Education Program (NCEP) as the Step I and Step IFI diets. The TLC diet is designed for people with elevated cholesterol. It recommends restricting saturated fat intake to under 7 percent of total calories, and reducing total cholesterol to less than 200 milligrams per day. Total fat intake can be maintained from 25 to 35 percent, with 15 percent of calories from protein and 50 to 60 percent from carbohydrates. This range of allowable total fat intake puts the TLC diet on the border of being low fat, even though saturated fat and cholesterol are restricted. It should be noted that the 25 to 35 percent fat recommendation is intended to allow for increased intake of unsaturated fat in place of carbohydrates in people with metabolic syndrome or diabetes.

Populations with diets naturally low in total fat and saturated fat have a reduced risk for heart disease. For example, native Japanese living in Japan derive about 15 percent of calories from fat, migrant Japanese living in Honolulu derive about 33 percent of calories from fat, and migrant Japanese living in California derive about 38 percent of calories from fat. The latter two groups have higher incidences of cardiovascular disease and increased body weight. Several studies have shown low-fat diets along with other lifestyle modifications, such as regular exercise and dietary counseling, result in weight loss and reduced risk of cardiovascular disease. Low-fat diets have reduced caloric density compared to high fat diets and often provide less total calories even when caloric intake is unrestricted. This factor may explain why many people lose weight on low-fat diets. However, some people on long-term low-fat diets may eventually adjust their caloric intake to that of a high fat diet, resulting in no weight loss or even weight gain. This is especially true when people consume highly processed fat-free products that contain large amounts of simple sugars with equal amounts of calories as their high fat counterparts. Some people in the medical community believe the low-fat diets and fat-free products developed in the 1990s contributed to the current obesity epidemic in America. Despite recent trends toward low carbohydrate diets that are high in fat, including Atkins, Mediterranean, and South Beach diets, low-fat diets are still the mainstay of most commercial weight loss plans like Jenny Craig and Weight Watchers. They are still highly recommended by the medical community for weight management and reducing the risk of cardiovascular disease. Low-fat diets are not a cure for obesity, but when accompanied by exercise and other lifestyle changes, they may be a valuable tool for weight management.

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