High-fiber-high-fruit-high-vegetable-low-fat: Dietary fiber or roughage is the indigestible portion of food derived from plants. It has two main components.

• Soluble fiber, which dissolves in water, is readily fermented in the colon into gases and physiologically active by-products, and can be prebiotic and viscous. This delays gastric emptying which, in humans, can result in an extended feeling of fullness.

Insoluble fiber, which does not dissolve in water, is metabolically inert and provides bulking, or it can be fermented in the large intestine. Bulking fibers absorb water as they move through the digestive system, easing defecation.

Dietary fibers can act by changing the nature of the contents of the gastrointestinal tract and by changing how other nutrients and chemicals are absorbed. Some types of soluble fiber absorb water to become a gelatinous, viscous substance which is fermented by bacteria in the digestive tract. Some types of insoluble fiber have bulking action and are not fermented. Lignin, a major dietary insoluble fiber source, may alter the rate and metabolism of soluble fibers. Other types of insoluble fiber, notably resistant starch, are fully fermented. Some but not all soluble plant fibers block intestinal mucosal adherence and translocation of potentially pathogenic bacteria and may therefore modulate intestinal inflammation, an effect that has been termed contrabiotic.

Chemically, dietary fiber consists of non-starch polysaccharides such as arabinoxylans, cellulose, and many other plant components such as resistant starch, resistant dextrins, inulin, lignin, chitins, pectins, beta-glucans, and oligosaccharides. A position has been adopted by the US Department of Agriculture to include functional fibers as isolated fiber sources that may be included in the diet. The term "fiber" is something of a misnomer, since many types of so-called dietary fiber are not actually fibrous.

Food sources of dietary fiber are often divided according to whether they provide (predominantly) soluble or insoluble fiber. Plant foods contain both types of fiber in varying degrees, according to the plant's characteristics.

Advantages of consuming fiber are the production of healthful compounds during the fermentation of soluble fiber, and insoluble fiber's ability (via its passive hygroscopic properties) to increase bulk, soften stool, and shorten transit time through the intestinal tract. A disadvantage of a diet high in fiber is the potential for significant intestinal gas production and bloating.

A low-fat diet is one that restricts fat and often saturated fat and cholesterol as well. Low-fat diets are intended to reduce diseases such as heart disease and obesity, but have been shown to cause obesity. Reducing fat in the diet can make it easier to cut calories. Fat provides nine calories per gram while carbohydrates and protein each provide four calories per gram, so choosing low-fat foods makes it possible to eat a larger volume of food for the same number of calories. The Institute of Medicine recommends limiting fat intake to 35% of total calories to help prevent obesity and to help control saturated fat intake.

Low Fat Vegan Diets have been shown to reduce both weight and cholesterol levels. In an analysis controlling for medication changes, a low-fat vegan diet appeared to improve glycemia and plasma lipids more than did conventional diabetes diet recommendations.

Reducing total fat intake leads to reductions in caloric intake, resulting in weight loss or less weight gain. The overall benefit is small but beneficial. With respect to weight loss low-fat diets do not appear to differ from other diets that also reduce overall calories.

Low-fat diets have been promoted for the prevention of heart disease. Lowering fat intake from 35-40% of total calories to 15-20% of total calories has been shown to decrease total and LDL cholesterol by 10 to 20%; however, most of this decrease is due to a reduction in saturated fat intake. Saturated fat has been shown to raise total and LDL cholesterol in a large number of studies^] and has also been correlated with a higher risk of heart disease.

A 2013 meta-analysis of randomized controlled trials of low- and high-fat diets showed low-fat diets decreased total cholesterol and LDL, but these decreases were not found when only considering low-calorie diets. It also showed HDL increases and triglyceride decreases in high-fat diets. Furthermore, lower total cholesterol was associated with lower intake of saturated fat and higher intake of polyunsaturated fat, HDL increases were associated with high monounsaturated fat intake and triglycerides associated with high carbohydrate intake. Decrease in saturated fat intake was only marginally related to decrease in LDL cholesterol. The meta-analysis concluded that neither high-fat nor low-fat diets could be unequivocally recommended.

According to the National Academies Press, a high-fat diet can contain "unacceptably high" amounts of saturated fat, even if saturated fats from animal products and tropical oils are avoided. This is because all fats contain some saturated fatty acids. For example, if a person chose fats with only 20% saturated fatty acids, setting fat intake at 35% of total calories would mean that 7% of calories would come from saturated fat. For this reason, the Institute of Medicine recommends consuming no more than 35% of calories from fat.