Simple sugars contain only one (mono-) or two (di-) sugar (saccharide) molecules; they vary in sweetness and sources (Table 2.1). Monosaccharides, such as glucose, fructose, and galactose, are absorbed “as is” without undergoing digestion; disaccharides, such as sucrose (table sugar), maltose, and lactose, must be split into their component monosaccharides before they can be absorbed. Glucose, also known as dextrose, is the simple sugar of greatest distinction: It circulates through the blood to provide energy for body cells, it is a component of all disaccharides and is virtually the sole constituent of complex carbohydrates, and it is the sugar to which the body converts all other digestible carbohydrates.

Complex carbohydrates, also known as polysaccharides, are composed of hundreds to thousands of glucose molecules linked together. Despite being made of sugar, polysaccharides do not taste sweet because their molecules are too large to fit on the tongue’s taste bud receptors that sense sweetness. Starch, glycogen, and fiber are types of polysaccharides.

This group is synonymous with “carbs” and consists of grains (e.g., wheat, barley, oats, rye, corn, and rice) and products made with flours from grains (e.g., bread, crackers, pasta, and tortillas).

Grains are classified as “whole” or “refined” (Box 2.1). Whole grains consist of the entire kernel of a grain (Fig. 2.3). They may be eaten whole as a complete food (e.g., oatmeal, brown rice, or popcorn) or milled into flour to be used as an ingredient in bread, cereal, pasta, and baked goods. Even when whole grains are ground, cracked, or flaked, they must have the same proportion of the original three parts:

Whole Grains contain the entire grain, or seed, which includes the endosperm, bran, and germ.

Phytonutrients also known as phytochemicals, are bioactive, nonnutrient plant compounds associated with a reduced risk of chronic diseases.

Refined Grains consist of only the endosperm (middle part) of the grain and therefore do not contain the bran and germ portions.

Bran cereals and wheat germ are not whole grains because they come from only one part of the whole.

Enrichment adding back certain nutrients (to specific levels) that were lost during processing.

Fortified adding nutrients that are not naturally present in the food or were present in insignificant amounts.

“Refined” grains have most of the bran and germ removed. They are rich in starch but lack the fiber, B vitamins, vitamin E, trace minerals, unsaturated fat, and most of the phytonutrients found in whole grains (International Food Information Council, 2014). The process of enrichment restores some B vitamins (thiamin, riboflavin, and niacin) and iron to levels found prior to processing. Other substances that are lost, such as other vitamins, other minerals, fiber, and phytonutrients, are not replaced by enrichment. Enriched grains are also required to be fortified with folic acid, a mandate designed to reduce the risk of neural tube defects. Examples of refined grains include white flour,
white bread, white rice, flour tortillas, and grits. Whether whole or refined, an ounce-equivalent of grain (e.g., one slice of bread or ½ cup of pasta) is estimated to provide 15 g of carbohydrates. Fiber content can range from 0 to 1 g in refined grains to 10 g or more per serving of high-fiber cereals. Some items in this group, such as sweetened ready-to-eat cereals, muffins, and pancakes, have added sugar.

Starch and some sugars provide the majority of calories in vegetables, but the content varies widely among individual vegetables. Generally, a ½ cup serving of starchy vegetables, such as corn, peas, potatoes, and yams, provides approximately 15 g carbohydrates. In comparison, “watery” vegetables, such as asparagus, broccoli, carrots, and green beans provide 5 g carbohydrate or less per ½ cup serving.

Generally, almost all of the calories in fruit come from the natural sugars fructose and glucose. (The exceptions to this are avocado, olives, and coconut, which get the majority of their calories from fat.) According to the American Diabetes Association’s Food Lists, a serving of fruit, defined as ½ cup of 100% juice, 1 small fresh fruit, ½ cup of canned or frozen fruit, or 2 tbsp of dried fruit, provides 15 g carbohydrate (American Diabetes Association & Academy of Nutrition and Dietetics, 2014). Because the skin of fruits provides fiber, fresh whole fruits provide more fiber than do fresh peeled fruits, canned fruits, or fruit juices. The effect of processing on fiber content is demonstrated in the examples on the left.

Although milk is considered a “protein,” more of milk’s calories come from carbohydrate than from protein. One cup of milk, regardless of the fat content, provides 12 g of carbohydrate in the form of lactose. Flavored milk and yogurt have added sugars, as do ice cream and frozen yogurt. With the exception of cottage cheese, which has about 6 g of carbohydrate per cup, natural cheese is virtually lactose free because lactose is converted to lactic acid during production. The carbohydrate content, including both natural and added sugars, of various dairy foods is listed in the box on the left.

Added sugars are sugars and sweeteners used as an ingredient in a food or beverage, such as white sugar, maple syrup, honey, corn syrup, or agave syrup. Sugar has many functional roles in foods including taste, physical properties, antimicrobial purposes, and chemical properties. Sugar adds flavor and interest. Few would question the value brown sugar adds to a bowl of hot oatmeal. Besides its sweet taste, sugar has important functions in baked goods, such as promoting tenderness in cakes. In jams and jellies, sugar inhibits the growth of mold; in candy, it influences texture.

However, added sugars are considered empty calories because they provide calories with few or no nutrients. Sometimes, 100% of the calories in a food are from added sugar, such as in sweetened soft drinks, pancake syrup, and hard candies. In other products, added sugars account for only some of the calories. For instance, in the chocolate milk listed earlier, added sugars provide 14 g (56 empty calories) of the 26 g total carbohydrate content, with the remaining 12 g coming from the natural sugar lactose. Only the calories of the added sugar are considered “empty.”

An added sugar that generates a lot of controversy is high-fructose corn syrup (HFCS), a commercial sweetener made from enzymatically treated corn syrup. HFCS is composed of glucose and either 42% or 55% fructose, making it similar in composition to sucrose, which is 50% glucose and 50% fructose (Fig. 2.4). HFCS is widely used in food and beverages not only because it provides the same sweetness as white sugar but also because it has other desirable functional properties, such as enhancing spice and fruit flavors. A review of short-term randomized controlled trials, cross-sectional studies, and review articles consistently found little evidence that HFCS differs uniquely from sucrose and other nutritive sweeteners in metabolic effects (e.g., levels of circulating glucose, insulin, postprandial triglycerides), subjective effects (e.g., hunger, satiety, calorie intake at subsequent meals), and adverse effects such as risk of weight gain (Fitch & Keim, 2012).

Cooked starch begins to undergo digestion in the mouth by the action of salivary amylase, but the overall effect is small because food is not held in the mouth very long (Fig. 2.5). The stomach churns and mixes its contents, but its acid medium halts any residual effect of the swallowed amylase. Fibers delay gastric emptying and thus provide satiety.

Most carbohydrate digestion and absorption occurs in the small intestine. Pancreatic amylase, secreted into the intestine by way of the pancreatic duct, reduces polysaccharides to shorter glucose chains and maltose. Disaccharidase enzymes (maltase, sucrase, and lactase) on the surface of the intestinal cells split maltose, sucrose, and lactose, respectively, into monosaccharides. Monosaccharides, whether consumed as monosaccharides or resulting from the digestion of disaccharides or polysaccharides, are absorbed through intestinal mucosa cells and travel to the liver via the portal vein.

Normally, most starches and all sugars are digested within 1 to 4 hours after eating. Small amounts of starch that have not been fully digested pass into the colon and are excreted in the stools. Fibers, which are nondigestible, advance to the large intestine where they attract water, which softens stool and promotes laxation. Some fibers are fermented by gut microbiota, which yields water, methane, hydrogen, and shortchain fatty acids. These fatty acids are used by the colon for energy or are absorbed and metabolized by liver cells.

Satiety the feeling of fullness and satisfaction after eating.

Gut Microbiota also known as gut flora, is the collective term for the microorganisms that inhabit the gut.