Chapter 4 – Carbohydrates
Section 1: Carbohydrates in Food
Section 2: Carbohydrate Metabolism, Blood Glucose and Diabetes
Section 3: Carbohydrates and Health
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Section 1: Carbohydrates In food
Photosynthesis: Plants biosynthesize glucose using the energy of the sun
u Energy from sunlight used to convert carbon dioxide and water to the monosaccharide glucose
u Glucose can then be converted to starch which is a polymer of glucose (polysaccharide) or other monosaccharide, disaccharides and polysaccharides
Chemistry of Carbohydrates: Common
monosaccharides
Derived from lactose. Animal source carbohydrate: milk
Disaccharides are 2 monosaccharide units covalently
2 glucose units
1 galactose and 1 glucose unit
The most common disaccharides are
1 glucose and 1 fructose unit
shown here.
Hydrolysis breaks down the linkage or covalent bond between sugars, with the addition of water.
Condensation
forms a linkage or
covalent bond
between sugars, with the release of water.
Oligosaccharides
u Contain 3 to 10 monosaccharide units
u Commonly found in beans
u Fermentation of these oligosaccharides in the large intestine cause the flatulence often experienced with the consumption of beans
4.2 Polysaccharides
Contains more than 10 monosaccharide units
Polysaccharides fall into 2 main categories
Digestible
(Are broken down by digestive enzymes and absorbed)
Indigestible
Resistant starch (Cannot be broken down by digestive enzymes)
4.2 Total Fibre
Functional Fibre
Fibre isolated from plant sources
Dietary Fibre Fibre in intact foods
falls into 2 categories
Both dietary fibre and functional fibre can be are made up of a mixture of:
Insoluble fibre, which does not dissolve in water and Soluble fibre, which absorbs water and can form viscous solutions
Examples of Polysaccharides of glucose:
Cellulose is one example of dietary
fibre other forms of dietary fibre are composed of other monosaccharides beside glucose.
Glycogen: form of polysaccharide stored in the liver.
Amylopectin is a branched polysaccharide;
Amylose is a linear polysaccharide
Carbohydrates are also categorized as unrefined or refined.
4.1 Carbohydrates in Whole Foods
e.g. fruits, vegetables, whole grains, legumes (beans, peas, lentils) “Unrefined”
“Refined” Carbohydrates
Carbohydrates as ingredients in highly processed foods
White flour, starches (cornstarch, potato starch)
Free sugars:
• Added sugars: monosaccharides and
disaccharides added during processing
e.g. glucose, fructose, sucrose
• Monosaccharides and disaccharides in
honey, syrups, juices
Total Sugars
Total sugars refers to sugars intrinsic to foods e.g. fruits contain sugars and free sugars.
Free Sugars (monosaccharides and disaccharides)
Sugars intrinsic to food
Added Sugars
Sugars in honey, syrups, juices
e.g. sucrose
The many names for sugars: all the ingredients listed below are sources of monosaccharides or disaccharides
Sugar (sucrose)
Invert sugar (1:1 mixture of glucose and fructose)
Dextrose (same as glucose)
Maltose or malt sugar
Corn syrup (glucose in syrup form)
Fruit juice concentrates
Glucose/fructose or high fructose corn syrup ( a mixture of glucose and fructose in syrup form) 13
Whole grain cereals:
Whole Grains Council Stamps
u Food products that contain whole grains can voluntarily put this stamp on their label.
u This stamp acts as a signal to consumers that the product contains whole grains.
These breads are different:
100% Whole wheat bread
u Made from flour that contains: u Endosperm
u Germ is removed because it contains oil and
spoils easily
u Some nutrients are missing, as a result of missing germ, but still a good source of fibre
100% Whole grain wheat bread
u Made from flour that contains the whole grain: u Endosperm – contains starch
u Bran –contains fibre uGerm -containsvitaminE
White bread
• Made from white flour that contains endosperm only; bran and germ removed
• Fortified with iron, thiamin, niacin, riboflavin to replace some, but not all nutrients,
removed when bran and germ removed • Low in fibre, vitamin E
How are Canadians doing with respect to fibre intake?
u Dietary Reference Intake for dietary fibre is an Adequate Intake (AI)
u AI is assigned to a nutrient when there is not enough research to determine an EAR
u How to interpret the intake results for a nutrient with an AI:
u If 50% or more of the population is consuming above the AI, the population intake is adequate
u If less than 50% of the population is consuming above the AI, the population intake may or may not be adequate
u Results suggest that adult Canadians’ intake of fibre may or may not be adequate, as less than 50% are consuming above the AI, in all age categories shown.
Section 2: Carbohydrate Metabolism, Blood Glucose and Diabetes
Digestion of Carbohydrates
Brush border enzymes include:
u Sucrase-breaks down sucrose u Maltase-breaks down maltose u Lactase-breaks down lactose
u All human infants produce lactase to process lactose in human milk
u Many individuals loss the ability to produce sufficient quantities of lactase as they age, leading to lactose intolerance
u Lactose intolerance
u Because lactose cannot be digested when lactase is not present, lactose enters the colon where it is fermented by microflora, causing intestinal gas bloating, diarrhea.
u Lactose intolerance varies in severity
u Many can tolerate lactose in small quantities e.g. in small doses or in fermented milk products, where most but not all lactose is converted to lactic acid
Indigestible Carbohydrates:
Insoluble and soluble fibre have different physiological effects. Both are beneficial.
Insoluble Fibre
• Does not dissolve in water
• Not fermentable by microflora
• Increases fecal bulk à promotes
GI motility à reduced constipation
• e.g. bran-containing & whole-
grain foods
Soluble Fibre
• Some is fermented by microflora to short chain fatty acids
• Absorbs water à softens fecal bulk
à reduced constipation
• e.g. psyllium-containing foods
Consume fibre with water
Role of Fibre
u Dietary fibre, especially soluble fibre dilutes the contents of the stomach and small intestine and “slows” the absorption of nutrients as shown (green dots)
u An example, of where this process is important, is the reduction blood glucose levels, which will be discussed later in this chapter
Glucose Metabolism
After absorption glucose is delivered to cells in the body, where it can be used as a source of energy.
Glucose is broken down to carbon dioxide and water. In the presence of oxygen, this happens with the associated production of the high energy compound ATP, which is used to fuel many cellular processes. This process is called cellular respiration.
Glycemic response:
Blood glucose response curve
u Shown opposite is a blood glucose response curve. It shows the rise in blood (or plasma) glucose that occurs after a meal. The characteristics of the blood glucose curve, the rate of rise, its magnitude and its duration is called the glycemic response.
u The rise in blood glucose immediately after a meal stimulates the release of the hormone insulin from
the pancreas. Insulin stimulates the uptake of glucose from the blood into the cells, causing the blood glucose to return to normal levels. In healthy individuals blood glucose levels returns to fasting levels in about two hours.
u The next slide describes the action of insulin in more detail.
The action of insulin
u After a meal, blood glucose rises and stimulates the secretion of insulin from the beta-cells of the pancreas
1. In the pancreas, glucose (in a multi-step process), stimulates the synthesis of insulin, so there is always a supply of the hormone in the beta-cell
2. In the liver, insulin stimulates the uptake of glucose for the synthesis of glycogen and other metabolic processes
3. In the muscle, insulin stimulates the uptake of glucose as a source of energy to support physical activity and for the synthesis of muscle glycogen
4. In adipose tissue, insulin stimulates the uptake of glucose where it is used in the synthesis of lipids
The action of insulin at the cellular level in muscle and adipose tissue.
The action of glucagon
u When carbohydrates have not been consumed for several hours, blood glucose levels begin to decline.
1. In the pancreas, the drop in blood glucose stimulates the release of glucagon, from the alpha cells of the pancreas.
2. In the liver, glucagon stimulates the breakdown of glycogen.
3. In the liver, glucagon also stimulates gluconeogenesis, the synthesis of glucose from amino acids (See next slide)
4. Both steps 2 & 3 return blood glucose levels to normal.
Gluconeogenesis:
When blood glucose levels drop, protein is broken down. Some of the amino acids in protein are glucogenic amino acids, meaning they can be converted to glucose, a process called gluconeogenesis.
Ketogenic amino acids are converted to Acetyl CoA and used as a source of energy or for the synthesis of fatty acids.
Ketone formation
When carbohydrates (and hence oxaloacetate-red arrow) is present, acetyl Co-A, which is formed from the breakdown of fatty acids can enter the citric acid cycle.
When carbohydrate intake is low or absent, such as during fasting, acetyl Co-A cannot enter the citric acid cycle and is diverted into metabolic pathways that produce ketones.
Glycemic response varies with the food
Shown here are two glucose response curves. The rise in blood glucose is more rapid in the absence of soluble-fibre (red line) (1) . This steep rise stimulates the release of high levels of insulin which in turn causes the large drop in blood glucose (2), compared to the fibre rich meal (blue line) where the rise was not as high, so less insulin is released and the decline is more gradual. This gradual response is considered the healthier response because it does not stress the pancreas as much as steep increases in blood glucose, which require more insulin secretion.
Glycemic Index (GI)
The reason that the glycemic response of a high soluble-fibre rich meal is more blunted than a low- fibre meal is shown opposite. Foods high in soluble fibre absorbs water and form a viscous solution that dilutes glucose (green dots) in the intestine. This slows the release of glucose from the this food in the digestive tract and into the blood.
This effect, of food on blood glucose, is characterized by the glycemic index.
Definition of Glycemic Index
• A ranking of the effect, on blood glucose, of a food of a certain carbohydrate content relative to an equal amount of carbohydrate from a reference food, usually 50g of glucose or white bread containing 50 g of digestible (available) carbohydrate.
Determining Glycemic Index
• A person is asked to consume a test food and the glucose response curve is obtained, by measuring blood glucose at various time points. This is repeated with the reference food and the index is the ratio of the two areas under the glucose response curves.
• Area 2 (from glucose) has a value of 100, and the index for other foods is expressed relative to this.
• The glycemic index is the average result obtained from 10 healthy individuals.
Effect of GI on Nutrient Absorption: Low glycemic index foods are believed to slow the rate at which glucose is absorbed from the digestive tract. This is similar to the previous description of the effect of soluble fibre. The factors that influence whether a food has a low glycemic index include, but are not limited to fibre content. In fact some high-fibre foods, e.g. whole wheat bread, have a moderately high glycemic index. The way in which the food is processed, the presence of other components such as fats and proteins, and the physical and chemical properties of the food may all influence its glycemic index.
Low glycemic index foods can be helpful in the prevention or treatment of diabetes.
u Diabetes is a disease characterized by high blood glucose levels and is linked to the inability of the body to produce sufficient insulin and/or the inability of the body to respond to insulin or a combination.
u Three are three main types of diabetes: u Type-1-Diabetes
u This disease is characterized by the body’s inability to produce insulin due to an autoimmune disease that destroys the pancreas. u It is treated with insulin injections.
u About 10% of diabetes cases are type-1-diabetes.
u Gestational Diabetes
u This form of diabetes occurs during pregnancy. If untreated in can result in a high birthweight baby, which may cause
complications during delivery.
u Babies born to mothers with gestational diabetes may also be at increased risk of obesity and type-2-diabetes later in life. u Mothers who develop gestational diabetes are also at increased risk of developing type-2-diabetes later in life.
u Gestational diabetes can be treated, reducing the risk to both mother and infant.
u Type-2-Diabetes
u The most common type of diabetes representing 90% of cases.
Type-2-Diabetes
u Type-2-diabetes is characterized by high blood glucose levels and results from a combination of:
uinsulin resistance in muscle, liver, and adipose tissue
ureduced insulin secretion by the pancreas.
u Insulin resistance: refers to the state when the muscle and adipose tissue do not respond to stimulation by
insulin to take up glucose.
u As a result glucose does not readily enter muscle or adipose tissue but instead remains in the blood, elevating blood glucose levels and causing damage to blood vessels which increase the risk of heart disease
u In the insulin-resistant liver, gluconeogenesis, which is normally suppressed by insulin, continues, contributing to elevated blood glucose
u In the early stages of insulin resistance, the pancreas responds to the lack of responsive to insulin, by liver, muscle and/or adipose tissue, by increasing insulin secretion. This initially results in a lowering of blood glucose levels, but eventually the pancreas becomes exhausted and secretion of insulin is insufficient to maintain normal blood glucose levels.
The figure opposite illustrates the changes in blood glucose levels after an overnight fast and after a meal.
The normal blood glucose levels are shown in blue.
As insulin resistance develops and/or pancreatic function begins to decline, blood glucose rises to the pre-diabetes state.
Treatment of pre-diabetes, with lifestyle changes, can often successfully normalize blood glucose levels.
If however insulin resistance and pancreatic function continue to worsen blood glucose levels rise to levels that meet the definition of type-2-diabetes.
Risk factors for type-2-diabetes
u One of the major risk factors for the development of type-2-diabetes is obesity.
u When we discuss the Consequences of Obesity later in the term, how obesity contributes to the
development of diabetes will be described.
u Type-2-diabetes normally develops during adulthood, typically after age 40, but, because of the rise of obesity among children and teens, it is being seen increasingly in this population and in younger adults.
Treatment of Type-2-Diabetes
u The main steps in the treatment of type-2-diabetes include: u Weight loss – obesity is a major risk factor
u Exercise – exercise is effective because it promotes the uptake of glucose by muscle tissue, independent of the need for insulin
u Diet- dietary patterns that are generally well-balanced are effective for type-2-diabetes
u Medication – medication in combination with the three steps above can be effective
Dietary Patterns Effective in the Prevention or Treatment of Type- 2-Diabetes:
A number of dietary patterns have been linked to reduced risk for type-2-diabetes.
Generally, dietary patterns that are nutritionally balanced, with an emphasis on whole grains, fruits and vegetables and plant-based proteins are suitable dietary patterns.
The Mediterranean diet has been widely studied and has been shown to be effective in lowering blood glucose levels.
Dietary Patterns Effective in the Prevention or Treatment of Type-2-Diabetes:
Canada’s Food Guide also promotes a nutritious diet.
Dietary Patterns Effective in the Prevention or Treatment of Type- 2-Diabetes:
Just the Basics
Diabetes Canada (formerly the Canadian Diabetes Association) has developed its own plate- based dietary pattern called Just the Basics. It is similar, but not identical to Canada’s Food Guide.
Diabetes Canada also recommends selecting low and medium glycemic index foods, like those listed opposite. Including at least one with each meal helps to slow glucose absorption. This results in lower blood glucose levels and less demand on the pancreas to secrete insulin, helping in the prevention and or treatment of diabetes.
Textbook Required Reading
u To read about the impact of both GI and dietary fibre on the development of type-2-diabetes see the required reading for this chapter:
u Critical Thinking: Dietary Fibre, Glycemic Index, and Type-2-Diabetes
u Be sure you can answer the questions in the textbook, that are included with the reading. The answers to these questions are located in A-45 of the appendix.
April 2020
Low–carb <45% kcal carbs Very low carb: < 50g carbs /day
Diabetes Canada: Position Statement
u Limitations of scientific evidence
u Unclear whether benefits are due to macronutrient composition or reduction in kcal intake that results in weight loss
u RCTs of short duration; < 12 months
uLong term impact unclear
uStudies have high drop out rates; diet not sustainable for many participants
Section 3: Carbohydrates and Health
Carbohydrates and Weight Management
Good carbohydrate-containing foods
u Low GI foods may help modestly with weight loss
u Slower absorption rates extend the feeling of fullness, which may reduce food intake
Bad carbohydrate containing foods.
u Sugar-sweetened beverages are directly associated with obesity in systematic reviews of cohort studies,
probably due to the ease with which one can overconsume kcalories from beverages.
u This is why Canada’s Food Guide recommends drinking water.
u Sugar-sweetened beverages, such as soft drinks, are sweetened with high fructose corn syrup.
u There is some concern that fructose metabolism promotes the synthesis of fat in the body, but this may only occur at doses higher than what most people consume.
u This remains an area of controversy 51
Carbohydrates and Weight Loss: Comparing High-Carb vs Low Carb diets
u High carb VS low carb diets
uLow carb diets tend to result in faster initial
weight loss (first 6 months)
uby one year, there is no significant difference between the two diets
u Healthy weight management requires sustainability, i.e. diets that can be maintained long term without compromising health,
u diets should be selected that are nutritionally-balanced and match the personal preferences of the individual
Carbohydrates and Cardiovascular Disease:
Good Carbs: Foods high in soluble fibre help lower blood cholesterol levels.
Bile acids, which aid in the digestion of fats, are biosynthesized from cholesterol, in the liver.
In the small intestine the some of the bile acids are recycled as shown by the blue arrow in figure (a).
Soluble fibre tends to bind bile acids, causing more of the bile acids to be excreted an
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