Dietary Guidance For Complex Carbohydrates

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In the U.S. the term complex carbohydrates was used in the McGovern report and included digestible carbohydrates or starch (2). The British Nutrition Foundation Report (13) included both starch and non-starch polysaccharides as complex carbohydrates. The British report defined polysaccharides as carbohydrates with 20 or more sugar residues. The British report stated that there are a variety of foods which are rich in complex carbohydrates and display great diversity in their physical and biological properties. Thus, it is difficult to make generalizations about properties of complex carbohydrates. They also noted that there is more overlap between the behavior and properties of the starches and non-starch polysaccha-rides than was previously thought.

Resistant starch is probably metabolized similar to some dietary fiber components in the gut. Thus, it appears that complex carbohydrate should contain both dietary fiber and starches and we must recognize that substances (such as phytate, phenolic compounds) and micronutrents (such as zinc and magnesium) are important nutritional components associated with complex carbohydrates. Dietary recommendations to increase intake of complex carbohydrate assume that the carbohydrates are not isolated, but come from intact foods. ^

Most dietary guidance is offered as a general endorsement of starches and complex carbohydrate to replace dietary fat. For example the U. S. Department |

of Agriculture food pyramid recommends 6-11 servings from the grain group, 3-5 servings from the vegetable group, and 2-4 servings from the fruit group (1). Such a diet would contain significantly more complex carbohydrate than the typical American diet. Dietary Guidelines for Americans also support increased intake of starch and dietary fiber in the form of bread, cereal, rice, pasta and foods from the vegetable and fruit group (3). The Surgeon General's Report on Nutri-

Slavin tion and Health (7) recommends increased consumption of whole grain foods and cereal products, vegetables (including dried beans and peas), and fruits. In 1989 the National Research Council examined the relationship between diet and health and issued new recommendations (8). They recommend that Americans consume five or more servings of a combination of vegetables and fruits (especially green and yellow vegetables and citrus fruits) and increase starches and other complex carbohydrates by eating six or more daily servings of a combination of breads, cereals, and legumes daily.

There is growing acceptance that isolated carbohydrates provide different physiological responses than whole foods. Thus, it is difficult to study complex carbohydrates since they must be fed in their refined state. It is then difficult if not impossible to generalize the results to the unrefined complex carbohydrates fed in their native state. This is compounded further by the fact that many other nutritional ingredients cloud the physiological results of the experiment. If an exciting physiological response is seen with intake of a whole grain high in complex carbohydrate the question arises as to the true origin of the response: is it due to the carbohydrate, a specific component of the carbohydrate or an associated substance such as an antioxidant, phytoestrogen, trace mineral, and so on.

Another important consideration in devising nutrition guidance for carbohydrates is how they are metabolized. As discussed by Hirsch (14), more research is needed on the roles and benefits of carbohydrates in the diet. To determine how carbohydrates fit into dietary guidelines, he suggests we consider three issues:

1. The role of carbohydrate in the control of food intake in humans.

2. The effect of carbohydrate on energy metabolism as measured by long-term feeding experiments in small numbers of human subjects.

3. Whether a large carbohydrate intake leads to increased lipogenesis in humans.

Obviously, these are all important issues when considering the metabolic effects of carbohydrates. But such experiments are costly and dietary recommendations will have to be made before all experiments on these topics have been conducted.

Determining the appropriate metabolic tests to compare complex carbohy- j drates will also be difficult. Metabolic studies use human subjects and results tend to vary greatly among subjects. If we accept a physiological definition of ^

dietary fiber as complex carbohydrate that escapes digestion in the small intestine, do we measure dietary fiber by gut fermentation? If we are convinced that slow digestion and absorption determines a complex carbohydrate from a simple carbohydrate, do we use glycemic index as the physiological determinant of a complex carbohydrate? As more research is available it may be easier to answer these questions, but meanwhile consumers need the best dietary guidance on complex carbohydrate intake.

Complex carbohydrates also need further definition. Should complex carbo-

Dietary Guidelines

hydrates be only long chain (>10 DP) carbohydrates or should shorter carbohydrates that resist digestion in the smaller intestine be included in complex carbohydrates, especially if we accept a physiological definition for complex carbohydrates. If carbohydrates are isolated from non-traditional food sources or are manufactured in laboratory, can they qualify as complex carbohydrates if they act similarly to long chain carbohydrates in the gut? If we as an industry accept a chemical definition for complex carbohydrates it would be difficult to argue that we only accept complex carbohydrates that are in their native state and will not accept chemically altered or manufactured carbohydrates if they meet our chemical definition. If consumers are unwilling or unable to increase consumption of complex carbohydrate and dietary fiber with usual food sources, should we fortify the diet with isolated carbohydrates that have been shown to have the desired physiological effects? The questions are many but need answers if consumers are to be able to select high complex carbohydrate foods that are convenient or if they are to have access to useful data on complex carbohydrate content of foods to help in food selection.

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