CARBOHYDRATES IN FOODS

10.2 CARBOHYDRATES IN FOODS

Carbohydrates are the predominant part of the Western diet. Generally, we consume at least 50% of our calories as carbohydrates. Carbohydrates are popular since they taste good, represent the core of many eating patterns (grains, breads, pasta), are inexpensive, and are convenient. The primary role of carbohydrates is to provide energy to the body, particularly the brain, which is the only carbohydrate-dependent organ in the body. Additionally, recommendations for carbohydrate intake for endur- ance athletes are higher, as much as 70% of calories as carbohydrates.

The Nutrition Facts label in the U.S. divides carbohydrates into total carbohy- drates, sugars, and dietary fiber. Carbohydrates can also be subdivided based on the

number of sugar units present. Monosaccharides consist of one sugar unit, such as glucose or fructose. Few monosaccharides occur naturally in foods. Disaccharides such as sucrose, lactose, and maltose are considered sugars. Sucrose and lactose are commonly consumed in foods, and maltose is a digestive breakdown product of starch. Sugars improve palatability of foods, but are also added to foods for viscosity, texture, and food preservation. Sugars enter our food supply in fruits and fruit juices, as sucrose in baked foods, and as sweeteners, usually isolated from corn. Corn syrups contain some trisaccharides and longer-chain carbohydrates, but they should still be considered sugars.

Attempts to further categorize sugars have been fraught with difficulty and dissent. In the U.K., sugars were separated into intrinsic and extrinsic sugars. Intrin-

sic sugars are present within the cell walls of plants (naturally occurring), while extrinsic sugars are typically added to foods. The U.S. Department of Agriculture (USDA) has defined “added sugars” for the purpose of analyzing the nutrient intake of Americans using nationwide surveys. Added sugars are sugars and syrups added to food during processing or preparation, including soft drinks, cakes, other desserts, and candy. Added sugars do not include naturally occurring sugars such as lactose in milk or fructose in fruits. Added sugars are not chemically different from naturally occurring sugars, but many foods and beverages that are sources of added sugars have low nutrient densities, and intakes of such foods need to be limited with the current obesity epidemic.

Oligosaccharides such as raffinose and stachyose are found in legumes, onions, and garlic and are generally 3 to 10 sugar units. Oligosaccharides are poorly digested

in the small intestine, escape to the large intestine, and are fermented by the micro- flora. Polysaccharides include digestible carbohydrates such as starch and nondi-

gestible carbohydrates termed dietary fiber. Even starch is much more complicated since it exists in different chain lengths and is also composed of amylose and

amylopectin. Amylose is a straight-chain starch, while amylopectin is branch chained. Additionally, not all starch is digested, and the portion that is not digested, called resistant starch, is considered dietary fiber by new dietary fiber definitions. Glycogen, animal starch, is not consumed as food, but has a structure similar to that of amylopectin.

Glycemic response, glycemic index, and glycemic load have also been suggested as terms to assist in putting carbohydrates into categories that could improve food intake. 1 Glycemic index (GI) is defined as the area under the curve for the increase

Dietary Carbohydrates and Risk of Cancer

in blood glucose after the ingestion of a set amount of carbohydrates in an individual food in the 2-h post-ingestion period, compared with ingestion of the same amount of carbohydrates from a reference food (white bread or glucose) tested in the same individual under the same conditions using the initial blood glucose concentration as a baseline. The average glycemic load is derived the same way as the GI, but without dividing by the total amount of carbohydrate consumed. Glycemic load is an indicator of glucose response or insulin demand that is induced by total carbo- hydrate intake. The two main factors that influence GI are carbohydrate type and the rate of digestion, the latter factor being affected by grain granulation (whether grains are intact or ground into flour), and food firmness resulting from cooking, ripeness, and soluble fiber content. Intrinsic factors such as the amylose–amylopectin ratio and particle size, and extrinsic factors such as food preparation and processing all affect GI. When the glycemic indices of common foods are compared, they range from 126 in low-amylose white rice to 32 in fructose.

The Panel on Dietary Reference Intakes for Macronutrients was responsible for reviewing the research on dietary fiber and disease prevention and deciding whether

to set a recommended intake level for dietary fiber. Prior to this report, there was no Recommended Dietary Allowance (RDA) for dietary fiber. The panel also found in its deliberations that there was no official definition of dietary fiber. Thus, a Panel on the Definition of Dietary Fiber was formed to review existing literature on dietary

fiber and determine the best scientific definition. 2 New definitions for dietary fiber and recommendations for fiber intake were published in the Dietary Reference Intakes (DRIs). 3 Dietary fiber consists of nondigestible carbohydrates and lignin that are intrinsic and intact in plants. Added fiber consists of isolated, nondigestible carbohydrates that have beneficial physiological effects in humans. Total fiber is the sum of dietary fiber and added fiber. Two categories of fiber are described: dietary fiber , fiber in its natural state, and functional fiber, fiber that is isolated, manufactured, synthetic, or enzyme-produced. Functional fiber does not have to be plant based. Other important recommendations of the committee were that functional fiber must show a beneficial physiological effect to be classified as functional fiber. Additionally, the committee recommended phasing out the terms soluble and insoluble dietary fiber . Two properties, viscosity and fermentability, were recommended as meaningful alternative characteristics for the terms soluble and insoluble fiber.

All fiber is not created equal. Dividing dietary fiber into soluble and insoluble fiber was an attempt to assign physiological effects to chemical types of fiber. Scientific supports that soluble fibers lower serum cholesterol while insoluble fibers increase stool size are inconsistent at best. A meta-analysis testing the effects of pectin, oat bran, guar gum, and psyllium on blood lipid concentrations found that

2 to 10 g/day of viscous fiber was associated with small but significant decreases in total and LDL cholesterol concentrations. 4 Oat bran lowers serum lipids, while

wheat bran does not. 5 Resistant starch, generally a soluble fiber, does not affect serum lipids. 6 Thus, not all soluble fibers are hypocholesterolemic agents, and other traits such as viscosity of fiber play a role and must be considered. The insoluble fiber association with laxation is also inconsistent. Fecal weight increases 5.4 g/g of wheat bran fiber (mostly insoluble), 4.9 g/g of fruits and vegetables (soluble and insoluble), 3 g/g of isolated cellulose (insoluble), and 1.3

Functional Food Carbohydrates

g/g of isolated pectin (soluble). 7 Many fiber sources are mostly soluble but still enlarge stool weight, such as oat bran and psyllium. Not all insoluble fibers are

particularly good at relieving constipation, for example, isolated cellulose. The disparities between the amounts of soluble and insoluble fiber that are measured

chemically and the physiological effects led a National Academy of Sciences panel to recommend that the terms soluble and insoluble fibers gradually be eliminated and replaced by specific beneficial physiological effects of a fiber, perhaps viscosity and fermentability.

The DRI committee used the new definitions for dietary, functional, and total fiber in their report. Additionally, they set adequate intakes (AIs) for total fiber in foods of 38 and 25 g/day for young men and women, respectively, based on the intake level observed to protect against coronary heart disease. AI is the recom- mended average daily intake level based on observed or experimentally determined approximations or estimates of nutrient intake by a group of apparently healthy people that are assumed to be adequate — used when an RDA cannot be determined. There was insufficient evidence to set a tolerable upper intake level (UL) for dietary fiber or functional fibers. The committee concluded that the recommended intake of dietary fiber should also provide protection against cancer, but there is not enough research to set a recommended fiber intake based on cancer prevention. The median intake of dietary fiber ranged from 16.5 to 17.9 g/day for men and 12.1 to 13.8 g/day for women. Thus, there is a large fiber gap to fill between usual intake of dietary fiber and recommended intakes.

Besides the AI for dietary fiber, the DRIs set a Recommended Dietary Allowance (RDA) for carbohydrates of 130 g/day for adults and children. Additionally, the

committee recommended that carbohydrate intake as a percentage of calories range from 45 to 65% of kilocalories. The committee suggested a maximal intake of 25% of energy from added sugars, based on ensuring sufficient intakes of essential micronutrients that are for the most part present in relatively low amounts in foods and beverages that are major sources of added sugars in North American diets.