Carbohydrates are molecules made up of carbon, hydrogen, and oxygen arranged as one or more sugar units. They are abundant in grains and other plant foods, as well as in milk and some milk products, and are key sources of fuel for the body, providing four kcal of energy per gram. Carbohydrates are one source of energy for exercising muscles; they are the exclusive source of energy for red blood cells and provide a significant portion of the energy needed by the brain. (INFOGRAPHIC 4.1)

Question 4.1

Depending on their size, carbohydrates can be classified as either simple or complex. Simple carbohydrates, also known as sugars, are short carbohydrates made up of one or two sugar units. Sugars that are made up of one sugar unit are called monosaccharides; sugars made up of two sugar units are called disaccharides.

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The three most abundant monosaccharides are fructose, glucose, and galactose. Fructose is the sweetest of the sugars and is found in fruits and vegetables, and in honey. Fructose is also used to sweeten foods and beverages by food manufacturers who process corn to create high-fructose corn syrup. Glucose is the most abundant sugar in our diet because it is part of every disaccharide, starches, and most fibers. Galactose is one of the two monosaccharides (the other is glucose) that make up the sugar found in milk.

The disaccharides are made up of pairs of monosaccharides and are called lactose, maltose, and sucrose. The mammary glands of humans and other mammals synthesize lactose (galactose and glucose), which is incorporated into milk to feed offspring. Maltose is composed of two glucose units and is produced when starch is broken down in digestion. Sucrose, or “table sugar,” is a disaccharide composed of glucose and fructose. We must digest dissacharides into their component monosaccharides before they can be absorbed by cells of the intestines.

Complex carbohydrates are composed of three or more monosaccharides linked together. They often take the form of long or branched chains. Humans and other animals break down some complex carbohydrates into individual monosaccharides during the process of digestion. These units are absorbed by the cells of the intestine and dumped into the blood for all body cells to pick up and use for energy. Oligosaccharides contain 3 to 10 linked monosaccharides, while polysaccharides are chains (or polymers) of more than 10 monosaccharides.

Most complex carbohydrates in our diet come from plants. Depending on how the sugars are bonded together, polysaccharides in plants may function as a source of stored energy (starch) or as structural material (fiber). Starches include amylose and amylopectin, found in grains, legumes, and starchy vegetables such as peas, potatoes, and corn. Starches are a form of stored energy for the plant. Fibers include cellulose and hemicellulose, which make up plant cell walls and impart structure to the plant. Fiber-rich foods are numerous and include whole grains and vegetables such as broccoli and green beans. Humans lack the digestive enzymes to break down plant fiber, so this material passes undigested through the digestive tract.

Humans and other animals also store glucose for later use. The storage form of carbohydrates in animals is called glycogen. Like starch, it is made up of linked glucose sugars, but is the most heavily branched of the polysaccharides. Glycogen is made and stored primarily in liver and muscle. When glucose is needed to maintain proper blood sugar levels, the liver breaks down the glycogen into glucose and releases it into the blood. Glycogen in skeletal muscles is broken down to supply fuel for contracting muscles during intense exercise. (INFOGRAPHIC 4.2)

Question 4.2

Carbohydrate-rich foods are a staple for most humans on earth. That’s because carbohydrate-rich plant foods are typically a plentiful and cheap source of energy. Plant foods supply most of the carbohydrates we consume. Milk and milk products and honey are the only significant animal source of carbohydrates in the American diet. (INFOGRAPHIC 4.3)

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Question 4.3

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The energy found within carbohydrates comes, ultimately, from the sun. Photosynthesis enables plants to capture the energy of sunlight and convert it into chemical energy in the form of glucose and fructose. Plants then link glucose molecules together to form starch and some dietary fibers. (INFOGRAPHIC 4.4)

Question 4.4

In grains, most of the starch is contained within the endosperm, which, if the seed was planted, would provide energy to the growing plant embryo, or germ. This energy is not readily accessible to humans who wish to consume the grain, however, since it is surrounded by the fibrous bran coating. That’s why, for millennia, humans have crushed, cracked, popped, pulverized, milled, or otherwise modified whole grains to expose the inner endosperm. Without such modifications, many grains would be either unpleasant to eat or would pass largely undigested through the digestive tract.

Beginning in the nineteenth century, “refining” grains became a common practice. Sophisticated milling machines allowed humans to separate the starchy endosperm from the tough bran and oily germ. The purified endosperm could then be ground into white flour. This practice helped keep flour from spoiling, since oil quickly turns rancid when exposed to air, and resulted in white bread that was soft and smooth in texture. (INFOGRAPHIC 4.5)

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Question 4.5

The downside of refining grains is that the bran and germ contain the majority of healthful vitamins, minerals, phytochemicals, and fiber. Not only are many modern grain-based foods made with nutrient-poor refined flour, they also often have a significant amount of added sugar. That’s why Mozaffarian was particularly interested in the relative amounts of starch, sugar, and fiber in the food products she and her colleagues studied. Their results were surprising, and may have you looking at grain products differently.

Whole grain hype can be traced to the mid-2000s, when nutrition guidelines started recommending that consumers eat more whole grains. Industry responded with an explosion of products marketed as containing whole grains. According to Mozaffarian, in 2010, the number of new products advertising their “whole grain” status was nearly 20 times higher than the number in 2000. But many of these products contain a mixture of whole grains, refined grains, and sugars. “That’s when you started seeing stuff like whole grain Goldfish crackers, whole grain Froot Loops, and so on,” Mozaffarian notes.

To help consumers make more informed choices, the USDA’s MyPlate program recommends that people look for whole grains listed first on the ingredient list (ingredients are listed in decreasing order of quantity). But that means being able to recognize any one of 29 ingredients classified as whole grains in the USDA’s MyPlate Servings Database, including oats, bulgur, whole wheat flour, brown rice flour, whole grain corn, and wheat bran.

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MyPlate also recommends that consumers choose products with fewer added sugars, but to do that consumers must be able to recognize as many as 21 different ingredients as a kind of sugar, including sugar, brown sugar, corn syrup, high-fructose corn syrup, malt syrup, maple syrup, fruit juice concentrate, honey, molasses, and dextrose.

To help address the confusion, the Whole Grains Council, a nongovernmental organization funded by food companies, created the Whole Grain Stamp. Used in 36 countries, the Whole Grain Stamp is a front-of-package icon that tells consumers that a product contains at least 8 grams of whole grains per serving.

A different approach, advocated by the American Heart Association, is to look for products with a total carbohydrate-to-fiber ratio of less than 10:1—the approximate ratio of carbs and fiber in whole wheat flour. Most white all-purpose flour, for example, has a ratio of over 20:1. This measure is intended to help consumers identify healthy whole grain foods without having to memorize long lists of specific ingredients. But with all these different recommendations out there, how is a consumer supposed to know which to use and trust?