Carbohydrates are an essential component of human nutrition that enable the body to produce the energy required for normal function. In addition to being an important source of substances that are metabolized into energy producing sugars, carbohydrates are essential for normal brain function, and are also needed for gastrointestinal function and integrity, glucose and insulin metabolism, cholesterol and triglyceride metabolism, and as an important source of numerous essential nutrients, including vitamins, minerals, and protein.
      In recent years, there has been a great deal of scientific inquiry directed toward the role of carbohydrates in human nutrition. Much has been learned that has forced nutritional scientists and dietitians alike to modify their view of this important dietary component and the role it plays in human nutrition.

      Like most food elements, carbohydrate structure and metabolism can be a complex subject. In describing it as simply as possible, carbohydrates are compounds made up of individual chemical units, that are chained together - building blocks, as it were - in a manner that determines their structure and, more important, how they are metabolized in the body.
      Monosaccharides/Disaccharides. The simplest carbohydrates are made up of single saccharide units, more commonly referred to as individual sugars. The most important of these are glucose, fructose and galactose. When two such sugars are combined into a single compound, they are referred to as disaccharides, the most important of which include maltose (glucose-glucose), sucrose (glucose-fructose), and lactose (glucose-galactose). Ordinary table sugar is sucrose.
      Oligosaccharides. This intermediary group consists of compounds made up of three to nine sugars, and are often breakdown products of more complex carbohydrates.
      Polysaccharides. Otherwise known as starch (if from plant sources) or glycogen (if from animal sources), this group is comprised of complex carbohydrates consisting of ten or more sugars, though typically numbering in the hundreds or even thousands.
      In the U.S., food labels usually distinguish only between sugars, dietary fiber, and total carbohydrates, the latter representing all other carbohydrates. (Please see our separate section on dietary fiber.)

      In normal diets, complex carbohydrates are broken down into shorter chains, which in turn are broken down into still shorter chains, ultimately reducing the ingested compounds into individual sugars which are absorbed into the blood stream as an energy source for the cells. It follows then, that the simple carbohydrates - monosaccharides and disaccharides - are more readily digested and metabolized into energy producing sugars than are their more complex brethren, oligosaccharides and polysaccharides.
      The impact carbohydrate ingestion has on glucose metabolism - referred to as glycemic response - is significant. In healthy individuals, those carbohydrates that are digested and absorbed rapidly, cause a rapid rise in blood sugar (glucose) to a high level, with a corresponding rapid fall. Other carbohydrates which are digested and absorbed more slowly, create a glycemic response that is manifested by a relatively slow rise to a lower level, longer duration and slower fall in blood sugar.
      In most situations, not all such glucose is consumed as energy, so the body converts the unused portion into low-density lipids - in this case triglycerides - for future use. This process is referred to as lipogenesis. The nature of the glycemic response - that is, whether it is fast or slow - dictates, in part, the amount and rate that this excess glucose is converted into fat. This is an important consideration when selecting the most suitable carbohydrates for a well balanced diet.

      The most common food sources for complex carbohydrates are vegetables, cereals, grains, rice, beans, fruits, nuts and seeds. Because of their diverse nutrient value - apart from carbohydrates - it is suggested that sources include a selected assortment of different carbohydrates that best suits the dietary goal of the individual.
      Glycemic Index. The term glycemic index, often abbreviated simply as "GI," is a relative reference used to compare one carbohydrate food source to another. Simply stated, the higher the GI, the faster the carbohydrate is digested and absorbed, and converted into blood sugar. Conversely, the lower the GI, the slower the carbohydrate is digested, absorbed, and converted into blood sugar.
      It follows then, for a given individual in a given situation, that carbohydrates with a high glycemic index - if not consumed as energy - are more readily converted to triglycerides than are those with a low glycemic index. In most cases, an excess of the former will ultimately lead to higher triglyceride levels. It is for this reason that the popular media has elected to distinguish these forms of carbohydrates as, "bad carbohydrates," and "good carbohydrates," respectively.
      Glycemic Load. While the glycemic index serves well as a scale by which one can compare various carbohydrates relative to one another, it does not take into account the amount of carbohydrate in a serving actually consumed. The glycemic load was created to take into account the amount of carbohydrate in a measured portion, and what impact it will actually have on a typical individual's glycemic response.
      Two often sited food sources with a high glycemic index are parsnips and watermelon. Both, however, have relatively low amounts of carbohydrate in a typical portion, so both create a low glycemic load. The actual amount of carbohydrate consumed is the most important factor, so the glycemic load is a more accurate measure of the effect a particular food will have on any given individual, other factors considered.
      The glycemic index and, more importantly the glycemic load, of various carbohydrates should be one of the most important criteria in the selection of food sources for most individuals.

      The percent daily value (%DV) currently used on U.S. food labels is calculated using a Daily Reference Value (DRV) of 300 grams of total carbohydrates for an average adult (25 grams of which is dietary fiber), based on a reference intake of 2000 calories. Until recently, nutritionists had generally agreed that total carbohydrates should make up some 50-60% of the total caloric intake per day. Recent science has significantly altered this view.
      An extensive evaluation of the most recent science on dietary carbohydrates was completed by the National Academy of Sciences (NAS) in September 2002 as a component of its study of Dietary Reference Intakes (see our summary of this study). In brief, their evaluation recommends a significantly lower amount of daily carbohydrate consumption for both juveniles and adults than has previously been recommended.

Reference Values for Carbohydrates
Current RDI¹ is 300 mg/day for a 2000 Calorie Diet

	Age	DRI2 EAR3 (g)     RDA4 (g)       AI 5 (g)
Children	0-  6 mo			60
	6-12 mo			95
	1- 3 yrs	100	130
	4- 8 yrs	100	130
Males	9-13 yrs	100	130
	14-18 yrs	100	130
	19+ yrs	100	130
Females	9-13 yrs	100	130
	14-18 yrs	100	130
	19+ yrs	100	130
pregnant	14-18 yrs	135	175
	19-50 yrs	135	175
lactating	14-18 yrs	160	210
	19-50 yrs	160	210

1.  Reference Daily Intake referenced on current food labels [Title 21 CFR]   2.  DRI = Dietary Reference Intakes          (EARs, RDAs, and AIs from recent NAS DRI Study)   3.  EAR = Estimated Average Requirement. The intake that meets the          estimated nutrient needs of half of the individuals in a group.   4.  RDA = Recommended Dietary Allowance. The intake that meets the          nutrient need of almost all (97-98%) of the individuals in a group.   5.  AI = Adequate Intake. This parameter is used when scientific          data are insufficient to establish an RDA. AI is not equivalent to RDA.

Updated: Sep 2002

Abbreviations, Units and Measurements

 Role in Human Nutrition

      Carbohydrates are an important source for many essential nutrients that are required for normal health. Equally as important, is the maintenance of proper energy balance. High carbohydrate foods promote satiety in the short term, limiting the consumption of less desirable foods such as fat. Additionally, carbohydrates are less efficiently stored than fat, resulting is less stored energy, particularly important for less active individuals.
      Unfortunately, these fundamentals have been shown to work against a significant percentage of individuals in developed countries, particularly those who are sedentary, and who have lifestyles that place them at risk for common health disorders such as diabetes and heart disease.
      Stated simply, an excess of carbohydrates - particularly those with a high glycemic load - in less active individuals can contribute to excess body weight, and higher levels of cholesterol and triglycerides, as easily as an excess of fat in the diet. It is essential for all, and particularly so for those in this latter group, to get nutritional guidance from a qualified healthcare professional.