Chapter 4. Carbohydrates
Digestion and Absorption of Carbohydrates
From the Mouth to the Stomach
The mechanical and chemical digestion of carbohydrates begins in the mouth. Chewing, also known as mastication, crumbles the carbohydrate foods into smaller and smaller pieces. The salivary glands in the oral cavity secrete saliva that coats the food particles. Saliva contains the enzyme salivary amylase. This enzyme breaks the bonds between the monomeric sugar units of disaccharides, oligosaccharides, and starches. The salivary amylase breaks down amylose and amylopectin into smaller chains of glucose called dextrins and maltose. The increased concentration of maltose in the mouth that results from the mechanical and chemical breakdown of starches in whole grains is what enhances their sweetness. Only about five percent of starches are broken down in the mouth (which is a good thing as more glucose in the mouth would lead to more tooth decay). When carbohydrates reach the stomach, no further chemical breakdown occurs because the amylase enzyme does not function in the acidic conditions of the stomach. However, mechanical breakdown is ongoing—the strong peristaltic contractions of the stomach mix the carbohydrates into a more uniform mixture of chyme.
Figure 4.7 illustrates the salivary glands in the human mouth, including the parotid, submandibular, and sublingual glands. These glands are responsible for producing saliva, which aids in the digestion process. Additionally, the diagram shows the chemical breakdown of starch. Starch molecules are depicted as being broken down into smaller sugar molecules, such as maltose, with the help of the enzyme amylase found in saliva. This initial digestion of starch occurs in the mouth, setting the stage for further breakdown in the digestive system.
Salivary glands secrete salivary amylase, which begins the chemical breakdown of carbohydrates by breaking the bonds between monomeric sugar units.
From the Stomach to the Small Intestine
The chyme is gradually expelled into the upper part of the small intestine, where most carbohydrate digestion occurs. Upon entry of the chyme into the small intestine, the pancreas releases pancreatic juice through the pancreatic duct. This pancreatic juice contains the enzyme pancreatic amylase, which starts again the breakdown of dextrins and any remaining polysaccharides into shorter and shorter carbohydrate chains (e.g., maltose). Additionally, the enzymes sucrase, maltase, and lactase are secreted by the intestinal cells that line the villi. Sucrase breaks sucrose into glucose and fructose molecules. Maltase breaks the bond between the two glucose units of maltose, and lactase breaks the bond between galactose and glucose. Once carbohydrates are chemically broken down into single sugar units, they are then transported to the inside of intestinal cells and into the bloodstream.
Lactose Intolerance
When people do not have enough of the enzyme lactase, lactose is not sufficiently broken down resulting in a condition called lactose intolerance. The undigested lactose moves to the large intestine where bacteria can digest it. The bacterial digestion of lactose produces gases leading to symptoms of diarrhea, bloating, and abdominal cramps. Lactose intolerance usually occurs in adults. Most people with lactose intolerance can tolerate some amount of dairy products in their diet. The severity of the symptoms depends on how much lactose is consumed and the degree of lactase deficiency.
Absorption: Going to the Blood Stream
The cells in the small intestine have membranes that contain many transport proteins to get the monosaccharides and other nutrients into the blood where they can be distributed to the rest of the body. The first organ to receive glucose, fructose, and galactose is the liver. The liver takes them up and converts galactose to glucose, breaks fructose into even smaller carbon-containing units, and either stores glucose as glycogen or exports it back to the blood. How much glucose the liver exports to the blood is under hormonal control and you will soon discover that even the glucose itself regulates its concentrations in the blood. If we don’t immediately need glucose for energy, it is stored in the form of glycogen in the muscle (approx. 300 g) and liver (approx. 100 g). [1] Liver stores serve to top up blood glucose through glycogen breakdown (glycogenolysis) and muscle glycogen is mainly accessed during exercise. If glycogen levels drop, our bodies undergo gluconeogenesis (the generation of glucose from a non-carbohydrate source) to generate glucose.
Maintaining Blood Glucose Levels: The Pancreas and Liver
Glucose levels in the blood are tightly controlled, as having either too much or too little glucose in the blood can have negative health consequences. Glucose regulates its levels in the blood via a process called negative feedback. An everyday example of negative feedback is in your oven because it contains a thermostat. When you set the temperature to cook a delicious homemade noodle casserole at 375°F, the thermostat senses the temperature and sends an electrical signal to turn the elements on and heat up the oven. When the temperature reaches 375°F, the thermostat senses the temperature and sends a signal to turn the element off. Similarly, your body senses blood glucose levels and maintains the glucose “temperature” in the target range. The glucose thermostat is located within the cells of the pancreas. After eating a meal containing carbohydrates, glucose levels rise in the blood.
Insulin-secreting cells in the pancreas sense the increase in blood glucose and release the hormone insulin into the blood. Insulin sends a signal to the body’s cells to remove glucose from the blood by transporting it into different organ cells around the body and using it to make energy. In the case of muscle tissue and the liver, insulin sends the biological message to store glucose away as glycogen. The presence of insulin in the blood signifies to the body that glucose is available for fuel. As glucose is transported into the cells around the body, the blood glucose levels decrease. Insulin has an opposing hormone called glucagon. Glucagon-secreting cells in the pancreas sense the drop in glucose and, in response, release glucagon into the blood. Glucagon communicates to the cells in the body to stop using all the glucose. More specifically, it signals the liver to break down glycogen and release the stored glucose into the blood, so that glucose levels stay within the target range and all cells get the needed fuel to function properly. Glucagon also stimulates gluconeogenesis. Catecholamines, such as epinephrine and norepinephrine, as well as cortisol and growth hormone, also work to increase blood glucose.
Figure 4.9 shows how the pancreas regulates blood glucose. When levels are high, insulin is released, lowering glucose. When levels are low, glucagon is released, raising glucose, and maintaining it within the normal range.
Typical Blood Glucose Levels
Hypoglycemia is defined as a blood glucose level below 4mmol/L. Blood glucose can be measured during a fasting state or following a glucose tolerance test, which involves drinking a carbohydrate-rich solution and measuring blood glucose 2 h later (see Table 4.1 for typical values).
| Fasting Blood Glucose Levels | Blood Glucose Levels Following a Glucose Tolerance Test |
| Normal range: 4-6 mmol/L | Normal: 6.7-7.8 mmol/L |
| Pre-diabetes: 6.1-6.9 mmol/L | |
| Diabetes: > 7 mmol/L | Diabetes: > 11.1 mmol/L |
Hypoglycemia
Have you ever felt shaky if you haven’t eaten in a while? If so, you may be experiencing hypoglycemia, which is when blood sugar falls below 4 mmol/L.
Reactive hypoglycemia can occur when the pancreas secretes too much insulin after a high carbohydrate meal whereas fasting hypoglycemia occurs when the pancreas secretes too much insulin, even when someone has not eaten.
As previously mentioned, hypoglycemia may cause shakiness, but it can also cause sweating, anxiety, weakness, tiredness, dizziness, and a fast heart rate.
Diabetes
Diabetes is a disease where your body is unable to regulate blood glucose levels, either because your body can’t produce insulin or the insulin produced is ineffective.
- Type 1 diabetes is known as insulin-dependent diabetes. It’s an autoimmune disease where the pancreas can’t produce insulin because the body attacks the pancreas. People with type 1 diabetes need to inject insulin (or use an insulin pump) to regulate insulin blood sugar levels. According to Diabetes Canada, approximately 10% of people living with diabetes have type 1 diabetes.
- In those with type 2 diabetes, either the body can’t properly utilize the insulin produced or cannot produce enough insulin. This type of diabetes more commonly occurs in adulthood, however, it is becoming more common in children and adolescents. This type of diabetes can sometimes be managed through a healthy diet and exercise, however, it may also require medications or insulin. According to Diabetes Canada, approximately 90% of people living with diabetes have type 2 diabetes.
- Gestational diabetes is a temporary form of diabetes that occurs during pregnancy. According to Diabetes Canada, approximately 3-20% of pregnant people develop gestational diabetes.
Complications associated with having high blood sugar
Uncontrolled diabetes can damage arteries in the eyes and kidneys and result in retinopathy, blindness, high blood pressure, heart attack and stroke. It can also cause peripheral cell death which can play a role in wounds that don’t heal/ become infected and can potentially result in amputation. It can also cause nerve damage.
Leftover Carbohydrates: The Large Intestine
Almost all of the carbohydrates, except for dietary fibre and resistant starches, are efficiently digested and absorbed into the body. Some of the remaining indigestible carbohydrates are broken down by enzymes released by bacteria in the large intestine. The products of bacterial digestion of these slow-releasing carbohydrates are short-chain fatty acids and some gases. The short-chain fatty acids are either used by the bacteria to make energy and grow, are eliminated in the feces, or are absorbed into cells of the colon, with a small amount being transported to the liver. Colonic cells use short-chain fatty acids to support some of their functions. The liver can also metabolize short-chain fatty acids into cellular energy. Dietary fibre yields about 2 kilocalories per gram of energy for humans but is highly dependent upon the fibre type, with soluble fibres and resistant starches yielding more energy than insoluble fibres. Since dietary fibre is digested much less in the gastrointestinal tract than other carbohydrate types (simple sugars, many starches), the rise in blood glucose after eating them is less and slower. These physiological attributes of high-fibre foods (i.e. whole grains) are linked to a decrease in weight gain and reduced risk of chronic diseases, such as Type 2 diabetes and cardiovascular disease.
A Carbohydrate Feast
You are at your grandma’s house for family dinner and you just consumed ham, white rice, sweet potatoes, mac salad, chicken long rice and a hot sweet bread roll dripping with butter. Less than an hour later, you top it off with a slice of apple pie and then lie down on the couch to watch TV. The “hormone of plenty,” insulin, answers the nutrient call. Insulin sends out the physiological message that glucose is abundant in the blood so that cells can absorb it and either use it or store it. The result of this hormone message is the maximization of glycogen stores and all the excess glucose, protein, and lipids are stored as fat.
A typical Thanksgiving meal contains many foods that are dense in carbohydrates, with the majority of those being simple sugars and starches. These types of carbohydrate foods are rapidly digested and absorbed. Blood glucose levels rise quickly, causing a spike in insulin levels. Contrastingly, foods containing high amounts of fibre are like time-release capsules of sugar. A measurement of the effects of carbohydrate-containing food on blood glucose levels is called the glycemic response.
When exploring the effects of a carbohydrate-rich meal like Thanksgiving dinner, it is important to consider that individuals with different health conditions or dietary needs may have distinct responses. For instance, individuals with diabetes should be mindful of their blood glucose levels and may need to make adjustments to their carbohydrate intake or monitor their insulin levels more closely after consuming such a meal. Similarly, individuals who follow specific dietary restrictions based on their cultural or religious beliefs, such as those observing kosher or halal practices, may need to adapt the traditional dishes to align with their dietary requirements.
Glycemic Index
The glycemic responses of various foods have been measured and then ranked in comparison to a reference food, usually a slice of white bread or just straight glucose, to create a numeric value called the glycemic index (GI). Foods that have a low GI do not raise blood-glucose levels either as much or as fast as foods that have a higher GI. A diet of low-GI foods has been shown in epidemiological and clinical trial studies to increase weight loss and reduce the risk of obesity, Type 2 diabetes, and cardiovascular disease.[2]
| Foods | GI Value |
| Low GI Foods (< 55) | |
| Apple, raw | 36 |
| Orange, raw | 43 |
| Banana, raw | 51 |
| Mango, raw | 51 |
| Carrots, boiled | 39 |
| Taro, boiled | 53 |
| Corn tortilla | 46 |
| Spaghetti (whole wheat) | 37 |
| Baked beans | 48 |
| Soy milk | 34 |
| Skim milk | 37 |
| Whole milk | 39 |
| Yogurt, fruit | 41 |
| Yogurt, plain | 14 |
| Icecream | 51 |
| Medium GI Foods (56–69) | |
| Pineapple, raw | 59 |
| Cantaloupe | 65 |
| Mashed potatoes | 70 |
| Whole-wheat bread | 69 |
| Brown rice | 55 |
| Cheese pizza | 60 |
| Sweet potato, boiled | 63 |
| Macaroni and cheese | 64 |
| Popcorn | 65 |
| High GI Foods (70 and higher) | |
| Banana (over-ripe) | 82 |
| Corn chips | 72 |
| Pretzels | 83 |
| White bread | 70 |
| White rice | 72 |
| Bagel | 72 |
| Rice milk | 86 |
| Cheerios | 74 |
| Raisin Bran | 73 |
| Fruit roll-up | 99 |
| Gatorade | 78 |
For the dietary glycemic index on different foods, visit Mendosa online.
The type of carbohydrate within a food, along with its fat and fibre content, affects the GI. Increased fat and fibre in foods increases the time required for digestion and delays the rate of gastric emptying into the small intestine, which ultimately reduces the GI. Processing and cooking also affect a food’s GI by increasing its digestibility. Advancements in the technologies of food processing and the high consumer demand for convenient, precooked foods in Canada have created foods that are digested and absorbed more rapidly, independent of the fibre content. Modern breakfast cereals, breads, pastas, and many prepared foods have a high GI. In contrast, most raw foods have a lower GI. However, the more ripened a fruit or vegetable is, the higher its GI.
The GI can be used as a guide for choosing healthier carbohydrate choices, but it has some limitations. The first is that GI does not take into account the amount of carbohydrates in a portion of food, but only takes into account the type of carbohydrate. Another is that combining low- and high-GI foods changes the GI for the meal. Also, some nutrient-dense foods have higher GIs than less nutritious food (ie. oatmeal has a higher GI than chocolate because the fat content of chocolate is higher). Lastly, meats and fats do not have a GI since they do not contain carbohydrates. Cultural diversity plays a significant role in shaping dietary practices and preferences. Recognizing and valuing diverse food cultures is crucial in promoting dietary patterns that are both health-promoting and culturally relevant. In some cultures, traditional diets include complex carbohydrates like quinoa, millet, or traditional whole grain products, which offer a variety of health benefits.
More Resources
Visit GIlisting to discover the glycemic indices of foods. Foods are listed by category and by low, medium, or high glycemic index.
