The energy (measured in kilocalories [kcalories or kcal]) in the food we eat comes from carbohydrates, proteins, and fats, otherwise called macronutrients (Figure 2-1). Balanced meals contain all three macronutrients. The macronutrients provide the fuel for body functioning (although of the three, proteins serve this function the least efficiently). All three macronutrients contain the elements carbon, hydrogen, and oxygen. Proteins differ from carbohydrates and fats in that proteins also contain nitrogen. Carbohydrates and proteins provide 4 kcal/g of weight, and fats provide 9 kcal/g, as noted on many food labels (see Figure 1-4). Alcohol provides 7 kcal/g. Although alcohol is produced from a source of carbohydrate, the body uses it differently once the carbohydrates are fermented into alcohol.

FIGURE 2-1 Carbohydrate, protein, and fat content of the MyPyramid Food Guidance System.

There are only two dietary sources of carbohydrates: plant material and milk. Proteins are naturally found in all foods but are present in negligible amounts in fruit. There are no proteins in fats and sugars. The highest sources of protein include meat, or the muscle of animals, including poultry and fish; eggs, nuts, seeds, and milk are also high in protein content. Legumes contain significant amounts of both carbohydrates (as a plant source) and proteins (because of their ability to pull nitrogen from the air). Fats are naturally found within all protein foods and some fruits, but only in trace amounts in vegetables. Whole milk contains all three types of macronutrients. Through processing, food products can have altered content of macronutrients, either higher or lower.

EMPTY CALORIES

Both fats (see section below) and sugars are referred to as empty calories, meaning they contain a high amount of energy but few vitamins or minerals (see Chapter 3). Ordinary table sugar (available from a variety of sources such as beets or sugar cane), molasses, maple syrup, corn syrup, and honey are concentrated sources of carbohydrates but have virtually no vitamins or minerals. Blackstrap molasses is the one exception, in that it is relatively high in some nutrients such as iron, calcium, and potassium and it also contributes some B vitamins. Sugar can be noted on food labels as sugar, syrup, or any word that ends in -ose (except cellulose, which is a type of fiber). One teaspoon of sugar contains 4 g of carbohydrates (and therefore 16 kcal). When sugar is naturally found in foods, such as fruit and milk, it is not a source of empty kcalories.

WHAT ARE CARBOHYDRATES AND THEIR FUNCTIONS AND RECOMMENDATIONS?

Carbohydrates are made through the process of photosynthesis (in which the sun’s energy allows plant leaves to take in carbon dioxide [CO₂] from the air and water [H₂O] through the roots). Carbon (carbo-) and water (-hydrate) are formed into carbohydrates, and the plant gives off oxygen as a result. Hence carbohydrates are mainly found in foods of plant origin. Although we do not think of sugar as growing out of the ground, it does come from a plant. Even honey comes ultimately from plant matter, being produced from the nectar of flowers by honeybees. The only nonplant dietary sources of carbohydrates are milk and milk products. Cheese is negligible in carbohydrate because of the way it is produced, with the liquid whey being drained away.

BASIC FORMS OF CARBOHYDRATES

The current terminology of forms of carbohydrates used in the United States falls into three broad categories, which are used on food labels. However, there is controversy on how best to classify carbohydrates based on impact on nutritional status. To begin this discussion, it is helpful to recognize the basic categorization currently used.

1. Sugars, or simple carbohydrates, include a single or double molecule made up of carbon, hydrogen, and oxygen (C₆H₁₂O₆) and may also be referred to as a monosaccharide (single units) or disaccharide (double units or double sugars). There are two types of monosaccharide molecules: glucose (blood sugar) and fructose (fruit sugar). Disaccharides are found only in food and must break apart, through digestion, into monosaccharide molecules before they can be absorbed into the bloodstream. Lactose (milk sugar) is an example of a disaccharide. The inability to digest lactose into single monosaccharide molecules is a common digestive problem (see Chapter 4). Sucrose, common table sugar, is a combination of glucose and fructose (Table 2-1).

Table 2-1

Types and Sources of Carbohydrates

TYPE	DESCRIPTION	SOURCES
Monosaccharides (Simple Sugars)
Glucose (blood sugar)	The end product of most carbohydrate digestion. One form in which carbohydrates are absorbed, resulting from its being the only fuel the central nervous system can use.	Found in fruits, certain roots, corn, and honey. Also found in blood as the product of starch digestion.
Fructose (fruit sugar)	Gives honey its characteristic flavor. Combined with glucose in table sugar.	Found in fruit, honey, and vegetables.
Galactose	A byproduct of lactose digestion.	Naturally found only in mammalian milk.
Disaccharides (Double Sugars)
Sucrose (table sugar)	Composed of glucose and fructose. Commonly known as table sugar, which is made from sugar cane.	Found in sugar cane, sugar beets, molasses, maple sugar, maple syrup, many fruits and vegetables, and added to foods as table sugar.
Lactose (milk sugar)	Produced only by mammals. It is less soluble and less sweet than cane sugar and is digested more slowly. Composed of glucose and galactose.	Found in milk and unfermented milk products.
Maltose (malt sugar)	Formed when starch is changed to sugar during digestion. Composed of two glucose molecules.	Found in malt and malt products; not free in nature.
Polysaccharides (Starch, Complex Carbohydrates)
Complex carbohydrate (starch)	The reserve store of carbohydrates in plants; changed to glucose during digestion (through intermediate steps of dextrin and maltose).	Found in grains and grain products, seeds, roots, potatoes, green bananas, and other plants.
Dextrin	Formed from starch breakdown.	Cooked starch (toast).
Dietary fiber	Indigestible; provides bulk and stimulation for the intestines and helps prevent or manage many chronic illnesses.	Insoluble found in skins and seeds of fruits, vegetables, and grains.
Dietary fiber		Soluble found in large amounts in legumes, greens, citrus fruits, oatmeal, and barley.
Glycogen	The reserve store of carbohydrates in animals; changed to glucose as needed.	Stored in small amounts in the liver and muscles.

2. Starch, in the past called a complex carbohydrate, is a chain of at least three sugar links and is referred to as a polysaccharide (multiple units). It is now known that starch is digested as rapidly as sugar, with digestion completed within 1 hour. It was once believed that digestion of starch occurred through removal of sugar links off each end of the polysaccharide chain. It is now recognized that the chemical bonds holding each single sugar link together in a chain break apart simultaneously through digestion. Consequently starch and sugar have similar effects on blood glucose levels.

3. Dietary fiber is the most complex form of carbohydrate. Although it is a polysaccharide like starch, the complexity of the polysaccharide chain in fiber makes digestion by humans nearly impossible. For this reason, at least some of the fiber may be subtracted from the total carbohydrate content as listed on food labels (see below).

There are two broad groupings of fiber based on their solubility in water. Insoluble fiber (generally found in the skin and seeds of plant foods) tends to be crunchy, whereas soluble fiber (generally found in the pulp of plant foods) tends to be gummy. Insoluble fibers include cellulose and hemicellulose. Soluble fibers include gums, lignins, and pectins. Plant foods contain both soluble and insoluble fiber, but proportions vary depending on the food source. See Box 2-1 for a general listing of fiber content of various plant foods.

Box 2-1 Food Sources of Various Fiber Components

The basic classification of carbohydrates has to do with chemical structure in terms of the number of glucose molecules and the form of digestibility. There is growing debate that this classification is inadequate based on nutritional impact. The Food and Agriculture Organization/World Health Organization Expert Consultation in 1997 divided carbohydrates into three main groups: sugars (with 1 or 2 glucose molecules: monosaccharides and disaccharides), oligosaccharides (short-chain carbohydrates with 3 to 9 glucose molecules), and polysaccharides (10 or more glucose molecules). Additional terms used include polyols (sugar alcohols), modified starch (altered food starches used as a thickener, stabilizer, binder, or emulsifier), nonstarch polysaccharides (as found in plant cell walls), and total carbohydrate (the sum of total carbohydrate grams). Beyond the basic chemistry, physical properties affect how individual forms of carbohydrate impact nutritional status. This includes factors such as water solubility, gel formation, crystal formation, and association with other molecules such as proteins and lipids.

One nutritional classification includes a main group of available carbohydrates, meaning they are digested and absorbed in the small intestine for subsequent cellular metabolism (see Chapter 4), whereas resistant carbohydrates are those that resist digestion in the small intestine or are poorly absorbed/metabolized. The available carbohydrates are commonly referred to as starch and sugars. Resistant carbohydrates are commonly referred to as fiber. One definition and measurement proposed for dietary fiber is “intrinsic plant cell-wall polysaccharides” rather than simply determining indigestibility in the small intestine. Further characterization of carbohydrate forms has implications for nutrition claims and food labeling (Englyst, Liu, and Englyst, 2007).

Carbohydrate bioavailability is a term that has been coined to describe both the chemical identity and physical form of food taken into consideration. One proposed classification and measurement scheme divides dietary carbohydrates into glycemic carbohydrates (those having impact on blood glucose through being digested and absorbed in the small intestine) and nonglycemic carbohydrates (those that enter the large intestine undigested, examples being fiber or plant cell-wall nonstarch polysaccharides, resistant starch, and nondigestible oligosaccharides) (Englyst and Englyst, 2005). Three food energy systems are in use in food tables and for food labeling in different world regions based on selective interpretation of the digestive physiology and metabolism of food carbohydrates (Elia and Cummings, 2007).

Consequently there is confusion among the general public and among health care professionals on how best to utilize food labels for carbohydrates in the goal of meeting optimal health and blood glucose goals. The American Dietetic Association advocates that the total carbohydrate available for blood glucose is the sum of all sugars and starches, but only half of the fiber over 5 g and half of sugar alcohols. The outcomes of the Diabetes Control and Complications Trial (see Chapter 8) advised that total carbohydrate is the sum minus all of the fiber if it is over 5 g in a serving. Another method involves subtracting resistant starch from total starch (Granfeldt, Wu, and Björck, 2006). Available carbohydrate intake is a confounding factor in assessing impact from health changes in different levels of glycemic load. The role of fiber is not completely understood, and its impact on health remains to be determined (Livesey and colleagues, 2008).

On a practical level, it is generally agreed that intrinsic sugar content as found in fruit is beneficial to health and that the foundation of a person’s diet should include fruit, vegetables, whole grains to meet fiber goals, and milk, with minimal amounts of added sugars (extrinsic sources).

Teaching Pearl

An analogy for fiber is that it is like a twisted gold chain necklace. Just as it is difficult to unravel a gold chain necklace, the human digestive system cannot unravel a polysaccharide chain. Cows can digest fiber because they have the equivalent of four stomachs.

GLYCEMIC INDEX

Glycemic index refers to the effect of foods on blood glucose (sugar) levels. Carbohydrates have the primary impact on blood glucose. Of carbohydrates, those that are in liquid form, such as in fruit juice, carbonated soft drinks (soda pop), or milk, have a higher glycemic index than those that are in solid form, especially those high in fiber, such as legumes and vegetables. Whole grains have a lower glycemic index than white-flour food products. Whole fruits have a lower glycemic index than the equivalent fruit juices. Glycemic load is a relatively newer term that includes both how rapidly a food raises blood glucose and the total amount of carbohydrate.

A low glycemic index, by reducing the impact of a meal-related rise in blood glucose, called postprandial blood glucose, has been shown to have health benefits. High-fiber diets have been shown to lower postprandial glucose and insulin responses and improve blood lipid (see Chapter 7). They may also manage diabetes (see Chapter 8) by facilitating the cells’ use of insulin (Riccardi, Rivellese, and Giacco, 2008). The postprandial effect of one meal can affect the blood glucose outcome of a subsequent meal. This is referred to as the second-meal effect. This may be due to high-fiber foods causing fermentation in the colon (Brighenti and colleagues, 2006).

FUNCTIONS OF CARBOHYDRATES

Carbohydrates, in the form of sugars and starch, are generally easily converted to energy needed for physical activity and basic life processes (Table 2-1). The human body prefers carbohydrates for fuel. One gram weight of carbohydrates yields 4 kcal of physical energy.

Humans use carbon in carbohydrates as fuel, just as carbon is used for fuel in automotive vehicles and furnaces. The human body is made up of millions of microscopic body cells that act as small furnaces (specifically the mitochondria within the cells) to essentially burn our food energy. Each 1 g of carbohydrates in food can be considered like one nugget of coal. The higher the carbohydrate amount in a food or a meal, the more nuggets of coal, or the more fuel, available to the body cells.

Carbohydrates have the following functions, as well:

• Spare the burning of protein for energy (proteins have more important functions, such as building and repairing body structures).

• Allow for more efficient and complete oxidation (burning) of fats for energy.

• As sugar, can have a laxative effect.

• As starch, provide an economical and abundant source of energy after being digested into glucose. Sugar and starch are both digested quickly (in less than 1 hour).

• As lactose, aid in the absorption of calcium.

• As dietary fiber (insoluble and indigestible), promote normal functioning of the intestines by adding bulk. Soluble forms, in significant amounts, lower serum cholesterol levels (see Chapter 7). Fiber slows down the time of digestion, which helps promote satiety (the feeling of fullness or satisfaction after eating) and is related to low glycemic index (the rate at which blood glucose levels rise). Fiber is not considered part of a meal’s high glycemic load (a meal that is high in total carbohydrates and with a high glycemic index).