Simple and complex carbohydrates, sugar, starch, glycogen, glycemic index, and fibre. Downloadable worksheet available.
CARBOHYDRATES
Simple and Complex Carbohydrates
Carbohydrates can be generally classified as simple carbohydrates (sugars)—the monosaccharides and disaccharides—and complex carbohydrates, the polysaccharides. (Saccharide is derived from the Greek saccharin, which means sweet.) All carbohydrates have a ratio of 1:2:1, referring to the ratio of carbon: hydrogen:oxygen. The carbon-hydrogen bonds are what make carbohydrates particularly useful for energy production (Chapter 4). The molecular formula for each monosaccharide is C6H12O6. However, the structures are slightly different for each of the three monosaccharides: glucose, fructose, and galactose (Figure 2.15). Fructose gives fruit its sweet taste. Honey consists of both glucose and fructose monosaccharides, and it is sweeter than table sugar, which is composed of glucose-fructose disaccharides.
Did You Know?
Fructose is the sweetest-tasting sugar. As high-fructose corn syrup (HFCS), it is added to many foods, particularly soft drinks, canned fruits, and concentrated juices. Fructose is processed by the liver and converted directly into fat. It does not decrease the hunger response and is a significant contributor to weight gain, cardiovascular disease, and type II diabetes. HFCS is 55% fructose and 45% glucose. Sucrose is 50% fructose and 50% glucose.
Disaccharides
Each of the monosaccharides can be paired by covalent glycosidic bonds to form disaccharides. Table sugar, or sucrose, is formed from the dehydration synthesis reaction between glucose and fructose. Maltose, found in grains, is formed from two glucose molecules, and lactose, milk sugar, is formed from glucose and galactose. These are the most common disaccharides (Figure 2.16).
Lactose Intolerance
Why do some people become bloated and have stomach pains and diarrhea after drinking milk? Lactose is a disaccharide that requires a specific enzyme, lactase, to break it down into glucose and galactose. Some people lose the ability to produce this enzyme as they get older. The lactose stays in the digestive tract instead of being broken down and absorbed. The lactose molecules are then taken up by the normal bacteria that reside in the digestive tract; these bacteria break down the lactose to make ATP and produce methane gas as a waste product. The lactose remaining in the intestine also increases osmotic concentration in the intestine, which causes water to move into the intestine, and this causes diarrhea.
Polysaccharides
Polysaccharides, the complex carbohydrates, are long chains of glucose. Three major complex carbohydrates are relevant to humans: starch, glycogen, and cellulose (Figure 2.17). Starch, a polysaccharide that we can eat, is the stored energy in plant cells. Starchy plant foods include rice, oatmeal, potatoes, yams, and grains such as corn, wheat, barley, rye, millet, and buckwheat. The carbohydrates in our diet are an important source of energy. The digestive system breaks down the long chains of glucose molecules through hydrolysis, and individual glucose molecules are absorbed into the bloodstream. The body’s cells then take up these glucose molecules and use them to make ATP (Chapter 4). When we eat more carbohydrates than needed for ATP production, the excess is stored as either glycogen or fat. The polysaccharide glycogen is the stored energy in animal liver and muscle cells; it can be used later when blood sugar levels start to decrease with exercise or between meals.
Another important plant polysaccharide is cellulose. Cellulose is a structural plant carbohydrate and is not used to produce energy. When we eat plants such as fruit, vegetables, whole grains, and legumes, we cannot digest the cellulose. However, cellulose is important in our diet as fibre. The consumption of fibre contributes significantly to our health: it regulates bowel movements and prevents constipation; it plays a role in the prevention of bowel cancer, hemorrhoids, and diverticulitis; it lowers blood cholesterol; and it helps regulate blood sugar.
Did You Know?
Chitin is a structural polysaccharide found in crustaceans and insects; it gives them a tough exoskeleton. Carrageenan is a gelatinous polysaccharide extracted from edible red seaweed that is often added to processed foods as a thickening agent.
Polysaccharides are formed by dehydration synthesis between glucose molecules. However, each polysaccharide has very different properties due to the way the bonds are formed. For example, in cellulose the glucose molecules form bonds between the first carbon of one glucose and the fourth carbon of the next glucose, which leads to a straight chain structure; in glycogen, the glucose molecules form bonds between the first and fourth or the first and sixth carbons of adjacent glucose molecules, which leads to a branched structure (Figure 2.17).
Dietary Carbohydrates
Since starches are made from sugar, why isn’t it just as healthy to eat chocolate bars and candy that are made from sugar? One reason why candy, a simple carbohydrate, is less healthy than complex carbohydrates relates to the rate of digestion and absorption from the intestines into the bloodstream. Sugar can be readily absorbed into the blood, and this causes a rapid increase in blood sugar level. When this occurs, the pancreas secretes insulin, causing a lot of that sugar to be converted into fat, rather than used for energy production. Starch takes much longer to digest because every bond must be broken by enzymes in the digestive tract before each sugar molecule can be absorbed. A second reason why sugar is not as healthy as starch relates to the composition of the sugar and its usefulness in energy production. Most simple carbohydrates are a combination of glucose and fructose, whereas starch is made up of only glucose. The body’s cells prefer to use glucose for ATP production, and the liver converts most fructose into fat. Therefore, eating a lot of simple sugars results in weight gain.
Did You Know?
People who have high amounts of fructose in their diet can develop fatty liver disease, which has symptoms that are similar to cirrhosis of the liver caused by alcoholism.
Glycemic Index
The glycemic index (GI) is a measure of how rapidly various foods increase blood sugar. Higher GI foods increase blood sugar more than lower GI foods, which causes more insulin to be produced. Insulin stimulates cells to convert sugar into fat. Excess insulin production leads to insulin resistance and eventually to type 2 diabetes. Eating lower GI foods helps you to maintain more stable blood sugar levels, control appetite, control blood LDLs (fats made by the liver from excess sugar and that increase risk of heart disease), and lower your risk of developing type 2 diabetes. Sugars and starchy carbohydrates have a higher glycemic index than proteins or fats. Table 2.4, with data from the Canadian Diabetes Association, provides information on the glycemic index of certain foods.
Fibre
Complex carbohydrates that are not broken down by our digestive enzymes are known as fibre. Dietary fibre is important for intestinal health.
- Fibre adds bulk to large intestine. Bulk stimulates the large intestine to contract and causes bowel movements, called defecation. It is important for undigested materials to be defecated on a daily basis so that unwanted molecules are not absorbed into the bloodstream. Build-up of waste materials in the large intestine and difficulty defecating is called constipation. Frequent episodes of constipation increase the risk of inflammatory bowel diseases and colon cancer.
- Fibre acts as a prebiotic. Prebiotics are nutrients that feed our intestinal bacteria. Probiotics are foods, such as yogurt, that contain healthy bacteria. It is important to have healthy bacteria in the intestines because they help in the digestion of food and absorption of nutrients, play a role in regulating immune functions, stimulate cell regeneration in the GI tract, and fight off pathogenic organisms.
Fibre is classified into soluble and insoluble types. Soluble fibre is gelatinous and dissolves in water, thereby increasing the fluid content of the stool and providing lubrication that makes bowel movements easier. Examples of soluble fibres include psyllium, pectin in fruits, and chia seeds. Insoluble fibres do not dissolve in water. They make up plant cell walls, including cellulose and hemicellulose, which is found in whole grains, legumes, and bran.
It is recommended that adults consume at least 25g of fibre per day. Here are some examples of fibre content in foods.
- ½ cup spinach—6g
- ½ cup bran cereal—7g
- ½ cup almonds—8g
- 1 medium apple—4g
- 1 medium pear—5g
- 1 cup baked beans—16g
- 1 slice whole wheat bread—3g
- 2 tbsp chia seeds—11g
- 2 tbsp flaxseeds—4g
- 2 tbsp hemp seeds—2g
- 1 cup lentils—16g
- 1 cup peas—9g
- 1 cup broccoli—5g
- ½ avocado—6g
- 1 cup cooked oatmeal—4g
Did You Know?
Fibre slows the absorption of sugars and therefore lowers the glycemic index of starchy foods. Whole grain bread has a lower GI than white bread because of the fibre content. If you are craving something sweet, you could lower the GI by adding fibre, such as adding bran, oatmeal, and chia seeds to your chocolate chip cookie recipe.
