Different medical organizations, governments, and diet plans define “low fat” in slightly different ways. In general, a low-fat diet is one in which 30% or less of the total daily calories come from fats. A very low-fat diet is one in which 15% or less of the total daily calories come from fat. By comparison, in the average American diet, about 35%–37% of calories come from fat.
In the 1940s, medical professionals became concerned about a dramatic increase in heart disease. Investigations into the causes suggested that cholesterol in the blood was a risk factor for heart disease and that eating saturated fat increased blood cholesterol levels. This led to Ancel Keys' monumental Seven Countries Study (Yugoslavia, Italy, Greece, Finland, Netherlands, United States, and Japan), which ran from 1956 and was published in 1978, comparing the relationship between diet and other risk factors to the occurrence of coronary heart disease (CHD). Keys concluded that high intakes of animal foods, with the exception of fish, were associated with higher CHD death rates, based on an initial hypothesis correlating saturated fat, blood cholesterol, and heart disease. This study has often been cited as proof of the hypothesis that a high-fat diet causes CHD and became one of the foundations of the recommendation that all people should eat a low-fat diet. The United States government issued its dietary guidelines recommending a low-fat diet that year.
Fats are described as either saturated or unsaturated based on their chemical structure. Saturated fats are found in animal fats such as butter, the fats in milk and cream, bacon fat, the fat under the skin of chickens, lard, or the fat on a piece of prime rib of beef. These fats are usually solid at room temperature. Exceptions are palm oil and coconut oil, which are both liquid but high in saturated fats. Saturated fats are often referred to as the “bad” fats. When consumed in high amounts, they raise the level of LDL cholesterol (“bad” cholesterol) in the blood. High LDL cholesterol levels are associated with an increased risk of heart disease.
Unsaturated fats have a slightly different chemical structure that makes them liquid at room temperature. Unsaturated fats, which includes polyunsaturated and monounsaturated fats, are considered “good” fats that can help lower cholesterol levels. Olive oil, canola oil, and peanut oil are high in monounsaturated fats. Corn oil, soybean oil, safflower oil, and sunflower oil are high in polyunsaturated fats. Fish oils are a rich source of two particular polyunsaturated fats termed long-chain omega-3 fatty acids, which have beneficial health effects.
Another type of fat, trans fat, is the by-product of a manufacturing process that hydrogenates or partially hydrogenates vegetable oils to make them more solid at room temperature. Trans fats also raise LDL cholesterol levels. They are found in some margarines and in many commercially baked and fried foods, although many countries, including the United Kingdom, have banned the use of trans fats in foods. Trans fats are no longer considered to be an issue in the countries that have banned them. During the past ten years, much work has been done to reduce trans fats in foods. A number of cities, including New York City, have passed full or partial bans on the use of trans fats in restaurants, and in 2008, California became the first state to enact a statewide ban, which became effective in 2010.
The federal Dietary Guidelines for Americans 2015–2020 recommend that up to 35% of an adult's daily calories come from fat. Of that, no more than 10% of calories should come from saturated fat, and people should consume as little trans fat as possible. The American Heart Association's Nutrition Committee together with the American Cancer Society, the American Academy of Pediatrics, and the National Institutes of Health endorse these guidelines as part of a healthy diet. Some experts believe, however, that for heart health, the amount of fats consumed should be much lower.
Nathan Pritikin, originator of the Pritikin Diet Plan, developed a very low-fat diet for heart health. The Pritikin Plan calls for less than 10% of calories to come from fat. The diet is also low in protein and high in whole-grain carbohydrates. Respected independent research shows that this diet does cause weight loss and lower risk factors for heart disease such as cholesterol and blood triglycerides. Critics of the diet say that it is too difficult to stay on and that the low-fat component of the diet does not allow people to get enough beneficial fats such as omega-3 fatty acids.
The Ornish diet is another very low-fat diet where only about 15% of calories come from fat. The Ornish diet is an almost-vegetarian diet. It too is designed to promote heart health, and again critics claim that it does not provide enough essential fatty acids.
In the 1990s, scientists from the National Heart, Lung, and Blood Institute (NHLBI) conducted studies to look at the effects of diet on blood pressure. These trials, called DASH (Dietary Approaches to Stop Hypertension), showed that an eating plan low in saturated fat, cholesterol, and total fat as well as being high in fruits, vegetables, and low-fat dairy foods helped to lower blood pressure. DASH trials also followed the effects of a reduced-sodium diet on blood pressure. As a result, a comprehensive eating plan (the DASH diet) was developed with specific suggestions from each food group. As of 2018, the AHA still recommended the DASH diet and had published research indicating that this diet could dramatically lower blood pressure in hypertensive patients.
The DASH diet emphasizes lean meats, fish, chicken, low-fat dairy, fruits, vegetables, whole grains, legumes, nuts, and seeds. Vegetarian diets also may help keep blood pressure low, as may fiber and monounsaturated fats. The OmniHeart diet is similar to DASH but emphasizes a particular food group such as protein more. Other lifestyle changes such as quitting cigarette smoking and increasing physical activity are also recommended. U.S. News and World Report ranked the DASH diet as the best diet overall in 2018.
Other low-fat diets are designed for people who have digestive disorders. People who have gallstones or gallbladder disease often benefit from reducing the amount of fats they eat. Bile, a digestive fluid made in the gallbladder, helps break down fats. When the gallbladder is not functioning well, a low-fat diet can improve digestion. Symptoms of other gastrointestinal problems, such as diarrhea, irritable bowel disorder, various malabsorptive disorders, and fatty liver, often improve on a low-fat diet. People who have had weight-loss surgery usually have fewer digestive problems if they eat a low-fat diet.
People on low-fat diets need to avoid certain foods. High-fat foods include whole milk and whole-milk products such as ice cream or cheese, fried foods, marbled beef, chicken skin, meat with visible fat, fish canned in oil, regular salad dressings, potato chips, fried snack foods, and many baked goods such as cookies, cakes, pies, and doughnuts.
People wishing to reduce the fat in their diet must read food labels. Food labels in the United States are required to list in the nutrition information panel nutrition facts that include calories, calories from fat, total fat, saturated fat, trans fat, cholesterol, sodium, total carbohydrates, dietary fiber, sugars, protein, vitamin A, vitamin C, calcium, and iron. In addition, the following words have specific legal meanings on food labels:
When cooking at home, people can reduce fat in the diet in the following ways:
To reduce fat in meals when eating out:
Low-fat diets work as weight-loss diets because they reduce caloric intake. The difficulty with low-fat and very low-fat diets is that they are difficult to maintain. Often when people go off these diets they gain weight back, then diet again, then gain weight back in a pattern of weight cycling. This happens with many diets, but some research shows that people who go off low-fat diets tend to binge or overeat more than people who go off more moderate diets. Blaming obesity on overeating censures the victim for choosing bad habits, which is unreasonable; no one wants to be obese, and obese children certainly haven't chosen to overeat. In fact, obesity is caused by overconsumption of calories and expending too few of those calories (i.e., via activity).
In the 1990s and early 2000s, the public was encouraged to eat a low-fat diet, not just to lose weight, but also to lower cholesterol and triglyceride levels. This, people were told, would protect heart health and help prevent cardiovascular disease. By 2018, that premise had been disputed by more in-depth research into the health effects of different types of fat and carbohydrate.
Many experts now believe that recommending a low-fat diet does not offer health benefits if it is high in refined starches and sugar, which trigger the release of insulin, causing the body to store fat. This type of diet may also raise blood triglyceride levels, which can increase risk of heart disease. Low-fat diets can also be more difficult than moderate fat diets for many people to follow, which can influence weigh loss over the long term.
Many other experts claim that the low-fat low-calorie recommendations have been a failure; obesity and diabetes rates have skyrocketed, directly as a result of people reducing fat and increasing carbohydrates as these have typically been sugar and refined starch. Many researchers have observed that people who transition rapidly from a traditional diet to a modern processed one will display a rapid rise in high blood sugar, diabetes, and obesity; critics of low-fat diets point to this as proof that composition of the diet is key. It is more widely agreed that the quality of the fat and carbohydrate is key; a diet that is rich in unsaturated fatty acids and high fiber foods, and one that uses wholegrains as sources of carbohydrate is widely promoted in dietary guidelines around the world as of 2018.
The claimed benefits of low-fat diets include:
Young children, pregnant women, breastfeeding women, and the elderly have higher fat requirements and should not undergo low-fat diets unless advised to do so by a physician. Fat plays a key role in infant growth and development, and omega-3 fatty acids, in particular, may play a significant role in brain function and development.
Low-fat diets are difficult to maintain for long periods. They may increase the risk of yo-yo dieting or weight cycling.
Although many low-fat diets have been shown to be healthy, individual diets vary, and some low-fat diets are not nutritionally balanced. Some fat is needed in the diet to ensure an adequate supply of essential fatty acids and proper absorption of fat soluble vitamins.
Many health claims have been made for low-fat diets. One is that they help people lose weight better than other diets. Studies have shown, however, that low-fat diets are no better at helping people lose weight and keep that weight off than low-calorie diets that did not specifically tell people to lower fat. The DIETFITS Randomized Clinical Trial compared low-fat to low-carbohydrate diets and found no significant difference between them.
Critics of this study claim that the low-fat group did not reduce their fat significantly enough to make a difference in health and that the study did not cover enough time. Others said that eating unsaturated fat (a Mediterranean diet) was heart healthy and that this study did not distinguish between saturated and unsaturated fat intake. Supporters of the study have said that it shows that how much people eat and how much they exercise (their calorie balance) are more important than what they eat. This study is likely to stimulate more research into low-fat diets and the health differences between unsaturated and saturated fats.
Research in the late 2010s called into question the validity of recommendations to lower dietary fat. A 2018 study, for example, found that full-fat dairy products had a neutral effect on cardiovascular events, and fermented dairy products were actually beneficial. This suggests that the health effects of foods cannot be judged by one nutrient alone (e.g., saturated fat) because many other nutrients or nonnutrients can influence health outcomes. A 2016 Norwegian diet intervention study called FATFUNC suggested that a high intake of total and saturated fat did not increase the calculated risk of heart disease. In 2017, the European Society of Cardiology called for a reconsideration of dietary recommendations in light of research indicating that high carbohydrate diets (high in sugar and refined carbohydrates) are linked with worse mortality, and high fat intake is associated with lower risk of cardiovascular mortality.
See also Carbohydrate addict's diet; ChangeOne diet; DASH diet; Dean Ornish's Eat More, Weigh Less; Eating for Life; Fat flush diet; Heart-healthy diets; High-protein diet; Low-cholesterol diet; Medifast; Mediterranean diet; Pritikin diet; Protein; Scarsdale diet; Trans fats; Triglycerides.
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Tish Davidson, AM
Revised by Amy Blackwell