2.2: Carbohydrates
By the end of this section, you should be able to:
- 2.1.1 Describe the nutritional function of carbohydrates.
- 2.1.2 Identify the impact of carbohydrates on wellness promotion and illness prevention.
Carbohydrate Function and Metabolism
Carbohydrates are a large food category containing starches , cellulose , and sugars. They are composed of organic carbon, hydrogen, and oxygen molecules and are found naturally in fruits, vegetables, legumes, grains, and dairy products. The term carbohydrate refers to both simple and complex carbohydrates.
Converting carbohydrates found in food to cellular energy begins with enzyme action in the mouth and is completed by enzymes formed in the small intestine and auxiliary organs. Table 2.1 lists some enzymes and their breakdown products. The end product of carbohydrate metabolism is a simple sugar; the three main ones involved in human nutrition are glucose, galactose, and fructose. Glucose , a simple sugar, provides energy to cells and is the preferred fuel for the brain and nervous system. It is a monosaccharide , which is a sugar molecule that cannot be further metabolized.
| Enzyme | Produced By | Site of Action | Substrate Acted Upon | Breakdown Product |
|---|---|---|---|---|
| Salivary amylase | Salivary glands | Mouth | Starch |
|
| Pancreatic amylase | Pancreas | Small intestine | Starch |
|
| Oligosaccharides | Small intestine | Small intestine | Disaccharides |
|
Monosaccharides link with each other in different formations to create disaccharides , compounds made up of two monosaccharide molecules. The disaccharide sucrose is formed from one glucose molecule and one fructose molecule; lactose is formed from glucose and galactose , and maltose is formed from two glucose molecules. Disaccharides are present in a wide variety of foods: lactose in milk products, maltose in molasses and other fermented foods , and sucrose in a variety of foods as table sugar. Once a disaccharide is broken into monosaccharides, a rise in blood glucose can be seen in 10–15 min. These rapid spikes in blood glucose can be dangerous for individuals with diabetes.
Complex Carbohydrates and Fiber
Complex carbohydrates , also called polysaccharides , comprise long chains of glucose molecules, creating starch or fiber. The chains of glucose molecules in starch are digestible and are slowly metabolized to a monosaccharide. Fiber is made up of plant substances, such as cellulose, and moves through the digestive system in an undigested form. Fiber is best known for its gastrointestinal health benefits. Research shows that fiber protects against colorectal cancer, diabetes, diverticulitis, heart disease, inflammatory bowel syndrome, and obesity (Precker, 2022; USDA, 2022). Although the recommended fiber amount varies with age and sex, an estimated 95% of American adults and children do not consume recommended amounts of fiber (Quagliani & Felt-Gunderson, 2016). The most prudent advice on increasing dietary fiber is to increase high-fiber foods gradually to avoid cramping, gas, and bloating, while ensuring adequate fluid intake. Table 2.2 lists some common foods and their fiber content.
| Food Item | Standard Portion Size | Fiber Content (Grams) |
|---|---|---|
| Guava | 1 cup | 8.9 |
| Pinto beans | ½ cup | 7.7 |
| Brussels sprouts, cooked | ½ cup | 6.4 |
| Sweet potato, cooked | 1 cup | 6.3 |
| Pumpkin seeds, whole | 1 oz | 5.2 |
| Green peas, cooked | ½ cup | 4.4 |
| Spinach, cooked | 1 cup | 4.3 |
| Baked potato with skin | 1 medium | 3.9 |
| Orange | 1 medium | 3.7 |
| Ready-to-eat cereal, toasted oat | 1 cup | 3.0 |
| Crackers, whole wheat | 1 oz | 2.9 |
| Edamame, cooked | ¼ cup | 2.1 |
Ultraprocessed Foods
Food not eaten in its original state is considered processed. Processed foods can include something as minimally processed as wheat milled into flour or as complex as a hot dog made from five or more ingredients. It is estimated that ultraprocessed foods (UPFs), commercially produced food products with a long shelf life and usually high in sodium and fat, provide 50–60% of individuals’ total daily energy consumption in some high-income countries (Beslay et al., 2020). During ultraprocessing of grains, they are refined, thereby losing essential nutrients, and sugar and salt are added. Therefore, UPFs are much more likely to have lower amounts of fiber and higher amounts of sugar and salt than their unrefined versions, as well as chemical preservatives, such as nitrates and sulfides, to extend shelf life. Table 2.3 includes examples of how whole foods , or foods in their natural state, are transitioned into UPFs. Research studies on UPFs and type 2 diabetes have found that participants with the highest UPF consumption had a higher risk for type 2 diabetes, with a statistically significant dose–response relationship (Llavero-Valero et al., 2021; Seal et al., 2021; Wood et al., 2023).
| Whole Food Item | Minimally Processed Food Item | Ultraprocessed Food Item |
|---|---|---|
| Peanuts | Natural peanut butter | Peanut butter pretzels |
| Apple | Natural applesauce | Apple danish |
| Soybeans | Tofu | Soy-based meat alternatives |
| Corn | Old-fashioned grits | Corn chips |
| Grapes | 100% grape juice | Grape jelly |
| Wheat | 100% whole wheat bread | White bread |
Nurses can help increase food literacy by holding meaningful discussions with clients about where food comes from and how it is changed as it enters the consumer marketplace. Clients should be able to identify UPFs and understand their relationship to health issues such as obesity, hypertension, and cancer (Fiolet et al., 2018). Creating meal plans with whole and minimally processed foods ensures the consumption of more complex carbohydrates instead of simple sugars.
Carbohydrates and Nutrition-Related Disease
When discussing carbohydrate-related diseases with clients, the nurse should always consider the type of carbohydrate and the amount of sugar, fat, and salt added to carbohydrate-rich foods during processing. Carbohydrates should make up 50–65% of caloric intake. However, there is a significant difference in health impact if 50% of carbohydrates come from sugar-sweetened beverages and salty snacks instead of whole grains, fruits, and vegetables. Individuals with a plant-forward philosophy typically consume less meat and more fresh and minimally processed grains, fruits, and vegetables. A plant-forward meal plan provides more fiber, less sodium, and, consequently, lower spikes in blood glucose. Research suggests that carbohydrates found in whole foods, especially fiber, promotes health by contributing to good gut microbiota (Clemente-Suarez, 2022). Overconsumption of any macronutrient can contribute to weight gain. A regular diet of ultraprocessed carbohydrates has been associated with overweight, obesity, heart disease, and type 2 diabetes (Wood et al., 2023).
Two diets rich in fruits, vegetables, whole grains, and lean meats are the Mediterranean diet and the Dietary Approaches to Stop Hypertension diet (DASH) . The Mediterranean diet is an excellent example of a diet high in complex carbohydrates because it is rich in fruits, vegetables, beans, legumes, nuts, and seeds. The diet also includes fish, seafood, and poultry, as well as dairy products in moderate amounts. This diet is high in omega-3 fatty acids (from fish) and low in fat. The Mediterranean diet can be beneficial because it reduces heart disease, improves serum lipid profiles, and decreases hypertension (Hauser et al., 2022).
Research has shown that the DASH diet can substantially lower blood pressure (Hauser et al., 2022). The DASH diet includes minimally processed foods, and sugary beverages, sweets, and red and processed meats are discouraged. Other research has found that rates of diabetes, cancer, metabolic syndrome, and obesity are lower in individuals following diets like the Mediterranean and DASH diets (Shoaibinobarian et al., 2023).
Athletes need increased molecular energy in the form of carbohydrates to sustain their energy supply during exercise. When someone is exercising, circulating glucose is the first source of energy. This energy source is quickly depleted, and the body then relies on stored glycogen , a large polysaccharide stored in limited amounts in the skeletal muscle and liver. When these stores are depleted, the body metabolizes fat for additional energy. Fat releases energy slower than carbohydrates do and requires over twice as much oxygen to complete the breakdown. Recent research indicates that manipulating fat intake before an athletic event may further increase endurance (Iwayama et al., 2023). Endurance athletes have practiced “ carbohydrate loading ” since the 1960s, which means they deliberately maximize the amount of carbohydrate in their diet approximately three days before a competition (Thomas et al., 2016), thereby enlarging their glycogen stores. A diet higher in carbohydrates than usual, coupled with days of no exercise, allows for the maximum buildup of stored energy.
Milk Carbohydrate and Lactose Intolerance
People are often surprised to discover that dairy products contain sugar. Lactose, often called “milk sugar,” is a disaccharide composed of galactose and glucose. Breaking down lactose into a monosaccharide requires the enzyme lactase . Lactase peaks at birth so that infants can digest breast milk. If lactose intolerance is diagnosed, it is not recommended to change to a completely dairy-free diet, as many intolerant patients can tolerate up to 12–15 g of lactose per day without side effects (Catanzaro et al., 2021). Some people do not have this enzyme due to injury or a defect in the small intestine. Lactose intolerance appears to occur more frequently in Africa and Asia, while northern Europeans show better tolerance of lactose-containing foods. In the United States, it is estimated that about 33% of the population has lactose intolerance (Alzahrani et al., 2022).
Without lactase, individuals will develop lactose intolerance symptoms such as bloating, gas, and diarrhea. They can take steps to help minimize discomfort and still consume milk-based foods by consuming lactose-free dairy products or taking lactase enzymes before meals to aid digestion. Additional calcium and vitamin D sources should be pursued if clients limit their dairy intake. A possible replacement for these nutrients is plant-based dairy products such as calcium- and vitamin D-fortified almond, rice, soy, hemp, and coconut milk if food allergies are not a concern (Katoch et al., 2022).
Carbohydrates and Blood Glucose
For individuals attempting to stabilize their blood sugar , intentional monitoring of food intake is critical, as sugars, starches, and fiber can affect blood glucose levels. Monosaccharides can cause spikes in blood glucose, requiring the hormone insulin to move it into the cells. Starch and fiber cause a less dramatic rise in blood glucose. For healthy adults, whole or minimally processed carbohydrates should supply 50–65% of daily calories. The American Diabetes Association (ADA, 2019) recommends that the percentage of daily carbohydrates be individualized for those with diabetes. Current recommendations include a focus on self-management of diabetes, including individualization of carbohydrate levels based on food preferences and metabolic goals.
The glycemic index is a tool for better understanding the impact of certain foods on blood glucose. More than 4,000 foods have been ranked based on the availability of highly absorbable monosaccharides (Atkinson et al., 2021). For example, processed foods made with refined flour and sugar, including cookies, cakes, and candy, have a high glycemic index, whereas whole foods, such as unrefined grains and nonstarchy vegetables, have a lower glycemic index. It is important to note that the glycemic index evaluates only individual foods, not entire meals, and no nutrient qualities are included in the glycemic index rankings (Charles, 2022).
The glycemic load is a newer and more comprehensive approach for assessing the impact of carbohydrates on blood glucose. The glycemic load uses the glycemic index and the number of total carbohydrates in a food serving to predict the timing and rise of blood glucose. It is a more accurate assessment of healthy food intake than the glycemic index alone (Charles, 2022).