Nutrition 331: Nutrition for Health
Study Guide: Unit 5
The Carbohydrates: Sugar, Starch, and Fibre
Carbohydrates are the most abundant and readily available nutrient in foods. Nutrition and health professionals consistently encourage higher carbohydrate intakes.
In this unit, we examine the classification of carbohydrates and give examples of food sources of each type. We review the processes of digestion and absorption of carbohydrates, and discuss the problem of lactose intolerance. We then discuss the functions of carbohydrates, the health effects of dietary fibre, and the regulation of blood glucose. Finally, we look at trends in carbohydrate consumption and the Canadian recommendations for carbohydrate intake.
After completing this unit you should be able to
- discuss the classification systems for carbohydrates and fibre.
- describe how carbohydrates are chemically digested.
- explain what is meant by lactose intolerance, and recommend suitable remedies for this health problem.
- discuss the six functions of carbohydrates.
- explain the roles of hormones that regulate blood glucose levels.
- identify major dietary sources of different monosaccharides, disaccharides, and polysaccharides, including viscous and nonviscous dietary fibre.
- describe the actions of fibre in the body and at least five health benefits of ingesting adequate dietary fibre.
- explain how ingesting fibre helps to prevent constipation, diverticulosis, hemorrhoids, and appendicitis.
- discuss the Canadian recommendations for carbohydrate and fibre intake, and identify the types of foods that will help Canadians achieve the recommendations.
Section 1 Carbohydrate Chemistry and Classification
Read pages 115–120 of Chapter 4: “The Carbohydrates: Sugar, Starch, Glycogen, and Fibre.”
Carbohydrates can be classified into three main categories: simple carbohydrates, complex carbohydrates, and dietary fibre.
You need not memorize any chemical structures (e.g., Fig. 4.2, p. 117), but you should know the component atoms (C, H, O) and the general chemical formula for a simple carbohydrate molecule (glucose). Also, you should remember that a carbon atom can bond with four atoms; a nitrogen with three; an oxygen with two; and a hydrogen with one. Carbon, nitrogen, oxygen, and hydrogen are the four major elements found in nutrients. Nitrogen occurs only in proteins and some B vitamins. You should be familiar with the terms listed below.
Simple sugars and simple carbohydrates can be used interchangeably. Specific monosaccharides include glucose, fructose, and galactose; and specific disaccharides are sucrose, maltose, and lactose.
Blood sugar refers to glucose in the blood.
Refined sugars are sugars such as white table sugar, brown sugar, or corn syrup; they are pure, simple carbohydrates, and are nutritionally empty-calorie foods.
Refined cereals are grains that have been milled to remove the bran and germ, leaving only the endosperm. Examples are white flour, polished (white) rice, and cornstarch. Refined cereals consist mainly of starch, and have much less dietary fibre than unrefined (whole grain) cereals. Some refined cereals have vitamins and minerals added back in.
Refined carbohydrates include refined sugars and refined cereals.
Section 2 Digestion and Absorption
Read “From Carbohydrates to Glucose,” pages 130–133 of Chapter 4: “The Carbohydrates: Sugar, Starch, Glycogen, and Fibre.”
When we consume carbohydrate-containing foods, our bodies must digest the carbohydrates into the basic units of glucose, fructose, or galactose. Carbohydrates are absorbed in the form of monosaccharides—primarily glucose and fructose—plus a small amount of galactose. Absorption takes place mainly in the intestinal mucosa of the jejunum and the ileum, although some glucose can be absorbed through the lining of the mouth. Absorbed fructose and galactose are converted to glucose in the liver. Ultimately, glucose will be circulated in the bloodstream to the liver and other parts of the body. Once inside the cell, glucose is metabolized according to body needs. It can be stored as glycogen or fat, used to produce other body compounds, or oxidized to produce energy, carbon dioxide, and water.
Starch is digested mainly by amylase, an enzyme present in saliva and pancreatic juice. Other important enzymes are the following disaccharidases: lactase (converts lactose to glucose and galactose), maltase (converts maltose to glucose), and sucrase (converts sucrose to glucose and fructose).
The textbook describes lactose intolerance on pages 132–133. In many cases, people with lactose intolerance can tolerate some lactose without symptoms. Foods such as cheese, which contains very small amounts of lactose, often do not pose any problem. However, the amount of lactose in cottage cheese may vary, because milk solids are an optional ingredient in the creaming mixture of cottage cheese. Yogourt contains lactose, but most has been degraded to lactic acid during fermentation by the bacterial culture, so it can be tolerated by people with lactose intolerance. Low lactose milks are commonly available in supermarkets.
Section 3 Functions and Regulation of Carbohydrates
Read the following sections from Chapter 4: “The Carbohydrates: Sugar, Starch, Glycogen, and Fibre”:
“The Need for Carbohydrates,” pages 120–129. “The Body’s Use of Glucose,” pages 133–138.
“If I Feel Dizzy between Meals, Do I Have Hypoglycemia?” page 142.
The primary function of carbohydrates is to provide energy. Many people believe that carbohydrates are fattening. In fact, gram for gram, carbohydrates have the same energy content as proteins (four kilocalories per gram) and far fewer kilocalories than fats (nine kilocalories per gram).
Body tissues require a constant supply of glucose for all metabolic functions. The most important function of carbohydrates is to supply energy to the body. Body storage of carbohydrates is relatively limited: only about 110 grams of glycogen is stored in the liver and about 225 grams in the muscles, giving a total storage of about 335 grams, plus about 10 grams as blood glucose. As each gram of carbohydrate yields four kilocalories, total glycogen storage—equivalent to about 1,340 kilocalories—would not meet most people’s daily energy requirement. Therefore, carbohydrates must be consumed throughout the day to meet energy needs and to maintain a sufficient glycogen storage.
Some functions of carbohydrates are listed below.
- Carbohydrates maintain blood glucose levels by circulating in the bloodstream and supplying energy to all body cells. Optimal functioning of the body cells requires a normal range of blood glucose. Signs of low blood sugar are tiredness, shakiness, and hunger. Signs of high blood sugar are sleepiness, frequent urination, and thirst. Extreme cases of high or low blood sugar can lead to coma. Generally, in a healthy individual, the body can regulate the blood glucose level within the normal range. The regulatory mechanisms are discussed further on.
- Carbohydrates are essential to the normal functioning of the specialized body cells that make up the nerve tissue of the brain and centralnervous system, the lungs, and the red blood cells. These cells require glucose as their sole energy source. In prolonged fasting, some brain cells partially adapt by using a small amount of ketones (intermediate products in the breakdown of fats) to fuel the brain. However, other brain cells still rely exclusively on glucose. This will be discussed further in Unit 8.
- Carbohydrates “spare” protein. In the body, energy needs take precedence over other purposes. Body proteins can be converted to glucose as a source of energy when there is insufficient carbohydrate. However, the primary function of proteins in the body is synthesis and maintenance of body tissues and fluids. To allow optimal functioning of the proteins, sufficient carbohydrates are needed to spare proteins from being used for energy. This requirement is particularly important in low-calorie diets; enough carbohydrates should be included to prevent muscle loss.
- Carbohydrates prevent ketosis. When there is insufficient carbohydrate, fats are broken down for energy—sometimes more than the body is equipped to handle. As a result, fats are incompletely oxidized, and ketones are produced. The ketone bodies are not excreted rapidly enough, and accumulate; this accumulation causes ketosis, which we discuss in Unit 8 when we consider fasting.
- Carbohydrates help to maintain the body’s normal balance of water and sodium. As the body tries to excrete ketone bodies produced during the breakdown of fat, sodium is also excreted, because sodium binds with acidic ketones so that they can be excreted as sodium salts in the urine. Since sodium and water are excreted together, the loss of sodium automatically leads to loss of body fluid, resulting in dehydration and sodium imbalance. These symptoms, plus fatigue and muscle wasting, are typical of low carbohydrate diets. The fluid loss partly explains why low carbohydrate diets cause rapid weight loss. However, most of the weight returns when carbohydrate intake is normalized.
- Foods providing complex carbohydrates also contain significant amounts of protein, vitamins, minerals, and phytochemicals, and are generally low in fat.
Normally, blood glucose levels are maintained within a narrow range. The following table outlines the ideal ranges of blood glucose levels at different intervals before and after food consumption.
|Interval||Blood glucose level (mmol/L)|
|Fasting (before breakfast)||3.3–5.0|
|After meals (1 hour)||7.7 or less|
|After meals (2 hours)||6.6 or less|
Homeostasis of blood glucose is maintained and controlled by the hormones insulin, glucagon, and epinephrine. Homeostasis is the body’s ability to regulate itself. It’s comparable to the way a thermostat works, turning the heat on or off in response to the desired set temperature’s relation to the actual room temperature.
Insulin: After a meal, blood glucose level rises. Insulin is secreted by the pancreas to reduce the blood glucose level by
- increasing the permeability of cell membranes, which allows glucose to enter the cells. Insulin serves as a “key,” allowing glucose to go through the cell “door.”
- stimulating the production of energy from glucose in the cells.
- facilitating the conversion of glucose to glycogen in the muscle and liver cells.
- facilitating the conversion of glucose to fat in the liver cells. Insulin also stimulates fat cells to take up and store fat.
Glucagon: Between meals, the blood glucose level drops. Glucagon is secreted by the pancreas to increase the blood glucose level by
- facilitating the conversion of glycogen stored in the liver to glucose, which enters the bloodstream and circulates to the brain and other parts of the body.
- stimulating the conversion of body protein to glucose (gluconeogenesis) and energy if the glycogen store in the liver is exhausted.
Epinephrine (adrenalin): During “fight-or-flight” or emergency situations, epinephrine is secreted to ensure that sufficient energy is available. It stimulates the conversion of liver and muscle glycogen to glucose and energy. Epinephrine works in a manner similar to that of glucagon, except that it is stimulated by different circumstances.
To help the body regulate blood glucose, it is best to avoid extremes of food intake by
- eating when hungry and not waiting until famished. When life gets busy, people often skip meals.
- eating balanced meals with a combination of protein and complex carbohydrates.
- spreading food intake evenly over the day by eating three or four meals and avoiding overconsumption at any one meal.
Section 4 The Health Effects of Dietary Fibre
Reread the section titled “Why Do Nutrition Experts Recommend Fibre-rich Foods?” pages 122–125 of Chapter 4: “The Carbohydrates: Sugar, Starch, Glycogen, and Fibre.”
Table 4.2 (p. 122) provides a succinct summary of two types of fibre, their sources, actions, and health benefits. Most fibre, except for lignin, is made up of carbohydrates. In the colon, anaerobic bacteria (bacteria which do not require oxygen) ferment soluble fibre (primarily) to produce short-chain fatty acids, gasses, and acids. The acids are absorbed and contribute one to five per cent of energy intake.
There are two sets of names given to the two types of fibre. The older names, which are still commonly used, are soluble and insoluble fibre. The more modern names are viscous and nonviscous fibre. Both sets of names are widely used and considered acceptable.
Figure 4.4 (p. 123) breaks down foods according to their content of viscous and nonviscous fibre. Two additional whole grains containing predominantly nonviscous fibre are whole wheat bread (1.6g nonviscous fibre vs. 0.3g viscous fibre per slice) and brown rice (1.6g nonviscous fibre vs. 0.1g viscous fibre per half cup).
The textbook refers to the benefit of a large quantity of stool and fast transit times through the intestine (p. 124). Fibre contributes to this by causing an increase in fecal weight. The amount of the increase varies considerably with the type of fibre. Wheat bran causes a large increase in fecal bulk, which explains its strong laxative action.
The increase in fecal weight induced by fibre speeds up the intestinal transit time. Transit time is the time it takes for fecal matter to move through the entire gastrointestinal tract. Transit time increases (takes longer) with less fecal bulk and decreases (takes less time) with more fecal bulk. Studies demonstrate a definite relationship between fecal weight and transit time. At low fecal weights (50–150 grams/day, which is typical of the fecal output in Western countries), there is considerable person-to-person variation in transit time, with most transit times being above 50 hours and a significant number being above 100 hours. As the fecal weight reaches or exceeds 150–200 grams/day, the great majority of people have transit times ranging from 24 to 48 hours. At this point further increases in fecal weight do not shorten the transit time.
The amount of fibre required to produce a fecal weight of 150–200 grams/day is about 35–45 grams/day, with the majority coming from nonviscous fibre. This amount is considerably above the DRI (see textbook, p. 125, margin).
As a result of the shorter transit time, there is less time for the colon to reabsorb water with a high fibre diet. The increased water content makes the feces softer and they are passed with little discomfort, whereas on a low fibre diet the feces are commonly dry and hard, and are passed with strain. Increased water content is an additional reason why fibre increases fecal output.
The above information demonstrates that fibre has a strong laxative action, and it has often been stated that constipation results from a deficiency of fibre. However, this statement is not entirely true. While fibre certainly plays a major role in the prevention and treatment of the disorder, it is not the only factor involved. Not all people with constipation experience relief when treated with fibre. There are, apparently, other factors that affect colonic motility.
The textbook mentions the role of fibre in preventing diverticula (p. 124). Diverticular disease (or diverticulosis), the development of “pockets” in the colon, is a very common condition in people older than 50. In most people it is symptomless. A more serious complication is diverticulitis, a disease where the pockets become infected or inflamed. The causes of the condition are not fully understood, but high pressures within the colon generated by a low fibre diet seem to play an important role. Conversely, a high fibre diet has been shown to bring much relief.
Hemorrhoids and appendicitis are also mentioned in the textbook (p. 124). High fibre diets are routinely recommended to prevent hemorrhoids.
As is the case for any nutrient, excessive intake of fibre may be undesirable. This is especially true of nonviscous fibre, which may impede absorption of minerals such as calcium, copper, magnesium, potassium, iron, and zinc, and possibly some vitamins. However, when adequate water is consumed, the risk of excessive fibre intake is low for healthy adults.
In summary, the type of fibre that affects colon function is mainly nonviscous fibre. The best source for such fibre is wheat bran. Viscous fibre, especially plentiful in oats, beans, and fruit, has a much smaller effect on the colon, but is valuable for helping to lower blood cholesterol and slowing the digestion and absorption of carbohydrates in the small intestine. We will come back to this subject several times in later units.
Review objectives 1, 6, 7, 8, and 9 at the beginning of this unit. Be sure that you understand the information in this section in relation to those objectives.
Section 5 Dietary Carbohydrate Recommendations
As mentioned above, the diet should have a generous fibre content. This means it should be plentiful in whole grains, whole fruits, and vegetables (rather than juices), and legumes. In addition to the benefits of fibre, such a diet has a high content of both nutrients and phytochemicals (non-nutrients) that are vital for health. These effects will be discussed in later units.
The RDA for carbohydrate is 130 grams per day for people over one year of age. This is the amount of glucose the brain requires to function properly. This is viewed as a minimal amount of carbohydrates for adults; fortunately, most diets provide over 130 grams of carbohydrate per day.
In Section 2 of Unit 3, we discussed the Accepted Macronutrient Distribution Ranges. These state that at all ages above one year carbohydrates should provide 45% to 65% of energy.
Nutrition surveys show that Canadians are receiving about 50% of their calories from carbohydrates. What types of foods provide carbohydrates? Grain products are the single largest source of carbohydrates in diets, followed by beverages (including fruit juices, fruit drinks, and soft drinks) and then vegetables (Forster-Coull et al., 2004; Mendelson et al., 2003).
Fibre intakes of Canadians are consistently lower than the DRI recommendations. Men consume about 19 grams per day on average; women consume about 15 grams per day (Kirkpatrick & Tarasuk, 2008).
The average Canadian consumes about 12% of energy as sugar.
Read pages 143–147, “Food Feature: Finding the Carbohydrates in Foods.”
Note: You will not be tested on specific carbohydrate contents of different foods (the contents of Tables 4.9 and 4.10). Rather, it is important to understand what foods contain different types of carbohydrates and which foods are relatively high or low in various carbohydrates. Another way to learn more about carbohydrate sources is to read the Nutrition Facts on various food packages.
In Canada, use of the term whole grain is not specifically regulated. As a result, the term whole grain includes foods that include the endosperm and bran, but the germ section may or may not be included in the food (see textbook, Fig. 4.7, p. 126). Whole grain foods including the germ are more nutritious than those without, as the germ is rich in vitamins and minerals.
Many grain products leave the impression that they are made with whole grains. Multi-grain and wheat breads and buns, however, may or may not be made of whole grains. Check the ingredient list. If enriched flour is the first ingredient, this is a strong indicator that the food is made with refined grain. For oat-based products, oatmeal is considered a whole grain but in some foods (like oatmeal cookies) oatmeal may be the second or third ingredient.
For addition review questions, see textbook page 140, “Self-Check” questions 1, 2, 3, 4, 5, 6, 8, and 10.