products: rich sources of carbohydrates Carbohydrate consumed in food yields 3.87 kilocalories of energy per
gram for simple sugars, and 3.57 to 4.12 kilocalories per gram for complex carbohydrate in most other foods. Relatively high levels of carbohydrate are associated with processed foods or refined foods made from plants, including sweets, cookies and candy, table sugar, honey, soft drinks, breads and crackers, jams and fruit products, pastas and breakfast cereals. Refined carbohydrates from processed foods such as white bread or rice, soft drinks, and desserts are readily digestible, and many are known to have a high glycemic index, which reflects a rapid assimilation of glucose. By contrast, the digestion of whole, unprocessed, fiber-rich foods such as beans, peas, and whole grains produces a slower and steadier release of glucose and energy into the body. Animal-based foods generally have the lowest carbohydrate levels, although milk does contain a high proportion of
lactose. Organisms typically cannot metabolize all types of carbohydrate to yield energy. Glucose is a nearly universal and accessible source of energy. Many organisms also have the ability to metabolize other
monosaccharides and
disaccharides but glucose is often metabolized first. In
Escherichia coli, for example, the
lac operon will express enzymes for the digestion of lactose when it is present, but if both lactose and glucose are present, the
lac operon is repressed, resulting in the glucose being used first (see:
Diauxie).
Polysaccharides are also common sources of energy. Many organisms can easily break down starches into glucose; most organisms, however, cannot metabolize
cellulose or other polysaccharides such as
chitin and
arabinoxylans. These carbohydrate types can be metabolized by some bacteria and protists.
Ruminants and
termites, for example, use microorganisms to process cellulose, fermenting it to caloric short-chain fatty acids. Even though humans lack the enzymes to digest fiber, dietary fiber represents an important dietary element for humans. Fibers promote healthy digestion, help regulate postprandial glucose and insulin levels, reduce cholesterol levels, and promote satiety. The
Institute of Medicine recommends that American and Canadian adults get between 45 and 65% of
dietary energy from whole-grain carbohydrates. The
Food and Agriculture Organization and
World Health Organization jointly recommend that national dietary guidelines set a goal of 55–75% of total energy from carbohydrates, but only 10% directly from sugars (their term for simple carbohydrates). A 2017
Cochrane Systematic Review concluded that there was insufficient evidence to support the claim that whole grain diets can affect cardiovascular disease. Carbohydrates are one of the main components of insoluble
dietary fiber. Although it is not digestible by humans, cellulose and insoluble dietary fiber generally help maintain a healthy digestive system by facilitating
bowel movements. Other polysaccharides contained in dietary fiber include
resistant starch and
inulin, which feed some bacteria in the
microbiota of the
large intestine, and are
metabolized by these bacteria to yield
short-chain fatty acids.
Classification The term
complex carbohydrate was first used in the
U.S. Senate Select Committee on Nutrition and Human Needs publication
Dietary Goals for the United States (1977) where it was intended to distinguish sugars from other carbohydrates (which were perceived to be nutritionally superior). However, the report put "fruit, vegetables and whole-grains" in the complex carbohydrate column, despite the fact that these may contain sugars as well as polysaccharides. The standard usage, however, is to classify carbohydrates chemically:
simple if they are sugars (
monosaccharides and
disaccharides) and
complex if they are
polysaccharides (or
oligosaccharides). Carbohydrates are sometimes divided into "available carbohydrates", which are absorbed in the
small intestine and "unavailable carbohydrates", which pass to the
large intestine, where they are subject to
fermentation by the
gastrointestinal microbiota. Expressed numerically as GI, carbohydrate-containing foods can be grouped as high-GI (score more than 70), moderate-GI (56–69), or low-GI (less than 55) relative to pure glucose (GI=100). A "meta-analysis, of moderate quality," included as adverse effects of the diet
halitosis,
headache and
constipation. Carbohydrate-restricted diets can be as effective as low-fat diets in helping achieve weight loss over the short term when overall calorie intake is reduced. An
Endocrine Society scientific statement said that "when calorie intake is held constant [...] body-fat accumulation does not appear to be affected by even very pronounced changes in the amount of fat vs carbohydrate in the diet." The reasoning of diet advocates that carbohydrates cause undue fat accumulation by increasing blood
insulin levels, but a more balanced diet that restricts refined carbohydrates can also reduce serum glucose and insulin levels and may also suppress lipogenesis and promote fat oxidation. However, as far as energy expenditure itself is concerned, the claim that low-carbohydrate diets have a "metabolic advantage" is not supported by
clinical evidence. Further, it is not clear how low-carbohydrate dieting affects
cardiovascular health, although two reviews showed that carbohydrate restriction may improve lipid markers of
cardiovascular disease risk. Carbohydrate-restricted diets are no more effective than a conventional
healthy diet in preventing the onset of
type 2 diabetes, but for people with type 2 diabetes, they are a viable option for losing weight or helping with
glycemic control. The
American Diabetes Association recommends that people with diabetes should adopt a generally healthy diet, rather than a diet focused on carbohydrate or other macronutrients. An extreme form of low-carbohydrate diet – the
ketogenic diet – is established as a medical diet for treating
epilepsy. The
British Dietetic Association named it one of the "top 5 worst celeb diets to avoid in 2018". ==Sources==