16.3 The Basics of Carbohydrates

Carbohydrates Are Essential

Nutrition trends change over time, but our need for essential nutrients does not. Various fad diets make different claims about the amount of carbohydrate to consume, some suggesting they be drastically reduced or nearly eliminated from the diet. However, carbohydrates are essential, and eliminating them would mean depriving the body of a nutrient that it needs to survive and function properly, but can’t produce on its own; so, we need to consume carbohydrates (Institute of Medicine, 2006).

A primary role of carbohydrates is to provide energy via glucose, which is a form of sugar. Glucose from carbohydrates is the body’s preferred source of energy. Some cells can use only glucose for energy; the brain, red blood cells, and certain cells in the kidneys require a constant supply of glucose in order to function. While most of the glucose absorbed by the body is used as a direct source of energy, some of it is used to build important macromolecules that fuel critical life-supporting cellular processes and chemical reactions throughout the body. For these reasons, the RDA for carbohydrates is 130 grams per day (Institute of Medicine, Food and Nutrition Board, 2005).

When the body has more glucose than it needs in the moment, the excess is stored in the body’s liver and muscles as glycogen, which can then be quickly broken back down into glucose when the body needs it, such as during exercise. It also helps maintain normal blood glucose levels between meals. The body has a limit to how much glycogen it can store, after which point excess glucose gets converted to triglycerides, which are stored in the body’s fat cells and can also be released for energy between meals.

If not enough carbohydrates are consumed to meet the body’s glucose needs, the body can make glucose from amino acids, which come from protein. However, because the body doesn’t store excess amino acids, this diverts protein from performing its primary and necessary functions and causes breakdown of tissue—primarily muscle. Additionally, lack of sufficient carbohydrates interrupts normal fat metabolism, resulting in partially broken down fats called ketones. The body can use ketones for energy if it doesn’t have enough glucose available, but ketones are acidic and having high levels of them makes the blood acidic, a condition known as ketosis. Ketosis hinders normal body processes and can cause headache, dry mouth, and bad breath.

Another vitally important role of carbohydrate-rich foods is to provide fiber, which, while not an essential nutrient, is essential to health. Unfortunately, many Americans do not consume enough of it (U.S. Department of Agriculture, U.S. Department of Health and Human Services, 2015). Fiber needs vary depending on the individual, but the RDA for fiber for most adults is 25 to 40 grams per day (Institute of Medicine, Food and Nutrition Board, 2005). Fiber is classified into two types, depending on its level of water solubility. Both types of fiber are important. Soluble fiber dissolves in water and forms a gel; it absorbs fat, cholesterol, and glucose from the intestinal tract, reducing their absorption into the bloodstream. Insoluble fiber does not dissolve in water; it adds bulk to the diet, helping people feel full, and adds bulk to the stool, helping food move through the intestines and thereby regulating bowel movements and preventing constipation. Overall, sufficient fiber intake contributes to a decreased risk of constipation, hemorrhoids, certain gastrointestinal diseases, gastroesophageal reflux disease, duodenal ulcer, diverticulitis, colon cancer, obesity, diabetes, stroke, high blood pressure, and cardiovascular disease (Ötles & Ozgoz, 2014).

For these reasons, consuming carbohydrates is essential. 

The Type of Carbohydrates Matters

Carbohydrates exist in two forms: simple and complex.

Simple Carbohydrates

Simple carbohydrates—also known as simple sugars—are short chains of just one or two sugar molecules. Simple carbohydrates do not contain fiber.

Some simple sugars occur in some foods naturally, such as lactose in milk. Examples of other naturally occurring simple sugars include honey, maple syrup, and agave nectar.

Simple sugars are often refined into more concentrated forms, such as high fructose corn syrup, and added to many foods and beverages, such as soda, energy drinks, sports drinks, juice drinks, candy, baked goods, desserts, cereals, yogurts, and other pre-packaged foods. In fact, sugar is one of the most common food additives. Any simple sugar added to foods and beverages during processing or preparation is referred to as added sugar. Added sugars go by many names, including, but not limited to:

• agave syrup
• beet sugar
• brown sugar
• cane sugar, cane juice, and cane syrup
• confectioners’ sugar
• corn sweetener and corn syrup
• dextrose
• fructose
• fruit juice concentrates
• glucose
• granulated white sugar
• high-fructose corn syrup (HFCS)
• honey
• invert sugar
• lactose
• maltose
• malt syrup
• molasses
• raw sugar
• rice syrup and other syrups (caramel, carob, corn, malt, maple, sorghum)
• sucrose
• turbinado sugar

Complex Carbohydrates

Complex carbohydrates are made up of starch, which consists of long, interconnected chains of sugar molecules—the part of complex carbohydrates that provides energy—and fiber. Most whole plant foods contain some quantity of both soluble and insoluble fiber; for example, the fiber contained in the pulp of an apple is soluble and the fiber contained in its skin is insoluble.

Plants are excellent sources of complex carbohydrates. This includes:

• vegetables, such as peas, broccoli, and artichokes;
• fruits, such as berries, oranges, and mango;
• legumes, including beans, peas, and lentils;
• nuts;
• seeds; and
• whole grains.

Whole grains are those in which all parts of the grain seed—the bran, germ, and endosperm—remain intact. When these foods are processed or refined, manufacturers may remove the outer two layers of the grain seed—the germ and bran—leaving only the endosperm, which is the sugary part of the grain. For example, whole wheat flour becomes white flour; brown rice becomes white rice. Removing the germ and bran means removing important nutrients, such as essential fats, vitamins, minerals, and fiber.

To counteract this loss, some food manufacturers add some nutrients back into processed grains by enriching them. Unfortunately, fiber is often not added back. Thus, whole grains provide more fiber and other beneficial nutrients than refined grains do.

Some examples of grains include:

• Rice, including wild and brown rice
• Oats, including steel cut oats, oatmeal, and oat bran
• Quinoa
• Buckwheat
• Barley
• Amaranth
• Corn, including popcorn
• Rye
• Wheat, including varieties such as spelt, farro, Kamut, and durum and forms such as bulgur and wheatberries, as well as wheat bran

Benefits of Complex Carbohydrates Over Simple Carbohydrates

In the body, all forms of carbohydrate from foods and beverages—whether simple or complex—are broken down and converted into glucose. Because simple sugars are small, simple molecules, they are absorbed quickly into the bloodstream and cause a sudden spike in blood glucose level. It takes longer for the body to break apart complex carbohydrates, which results in a more gradual increase in blood glucose level. Regardless of its source, the body responds to an increase in blood glucose by producing and releasing insulin from the pancreas. Insulin is the hormone that helps transport glucose from the blood into cells so that it can be used for energy.

Continual drastic increases in blood sugar can lead to insulin resistance. Insulin resistance results when cells become less receptive to insulin and blood glucose levels remain high because the glucose can’t get into cells as efficiently. The pancreas responds to the high levels of blood glucose by producing even more insulin, in turn making the cells even less receptive to it. Unfortunately the pancreas can eventually get fatigued and stop producing a sufficient amount of insulin, or stop producing it altogether. Blood glucose is then elevated not just after meals, but continuously, and this constant elevation contributes to the development of type 2 diabetes.

High sugar intake is associated with not only insulin resistance and type 2 diabetes, but also hyperglycemia, high triglyceride levels, dyslipidemia, cardiovascular disease, obesity, hyperuricemia, fatty liver, dental cavities, and colorectal, breast, and prostate cancers (Stanhope, 2016; DiNicolantonio, Lucan, & O’Keefe, 2016; Makarem et al., 2018; Miles, Neuhouser, & Zhang, 2018; National Institute of Dental and Craniofacial Research, 2018).

Simple sugars don’t provide sustained energy the way that complex carbohydrates do. Although both forms of carbohydrates provide the same amount of energy, high consumption of simple sugars is generally followed by an eventual sugar “crash”—feelings of fatigue and low energy—whereas the more gradual absorption of complex carbohydrates results in a more consistent and sustained source of energy. More sustained glucose absorption and availability and better regulation of glucose has been associated with higher cognitive function and intellectual performance, such as memory (Bourre, 2006a). Foods high in complex carbohydrates generally provide other benefits that foods high in simple sugars don’t. For example, the fiber provided by complex carbohydrates helps maintain feelings of fullness for a longer period of time; consuming dietary fiber is also associated with higher alertness (Bourre, 2006b).

For these reasons, it’s recommended to prioritize consumption of complex carbohydrates while minimizing intake of simple sugars, added sugars, and refined grains.