ASIN500 - Introduction to Nutrition - Lecture 3 - Carbohydrates

Lecture Objectives

• Outline the three major types of carbohydrates, including their structures and sources.

• Discuss the different types of dietary fibre, their specific health benefits, and recommended intake levels.

• Explain the detailed hormonal regulation of blood glucose by insulin and glucagon, including the mechanisms of action.

• Discuss how the glycemic index (GI) is used to classify foods and identify factors that significantly affect GI values.

• Describe the comprehensive impact of carbohydrates on health, distinguishing between the effects of simple sugars and complex carbohydrates.

Types of Carbohydrates

• Monosaccharides: Single sugar units; the simplest form of carbohydrates.

  • Glucose: Found in fruits, vegetables, honey; known as "blood sugar" because it is the primary sugar used by the body for energy. Its chemical formula is C6H{12}O_6.

  • Fructose: Found in fruits, honey, and high fructose corn syrup (HFCS); known as "fruit sugar." It is the sweetest monosaccharide.

  • Galactose: Found as part of lactose in milk and dairy products; typically not found in nature alone.

• Disaccharides: Two monosaccharides linked by a glycosidic bond.

  • Sucrose: Glucose + Fructose; commonly known as "table sugar"; extracted from sugar cane and sugar beets.

  • Lactose: Glucose + Galactose; known as "milk sugar"; found in milk and dairy products, providing energy to newborns.

  • Maltose: Glucose + Glucose; found in germinating cereal grains; a product of starch breakdown, important in brewing and baking.

• Polysaccharides: Long chains of monosaccharides (primarily glucose) linked together; also known as complex carbohydrates.

  • Starch (plants): Long chains of glucose units used by plants to store energy.

    • Amylose: Straight chains of glucose molecules connected by α(1-4) glycosidic bonds. It digests more slowly than amylopectin.

    • Amylopectin: Branched chains of glucose molecules with α(1-4) glycosidic bonds and α(1-6) glycosidic bonds at branch points. It digests more quickly due to more ends for enzymes to attack.

    • Found in grains (wheat, rice, corn), vegetables (potatoes, peas), and legumes (beans, lentils).

  • Glycogen (animals): Highly branched chains of glucose units, similar to amylopectin but more branched.

    • Storage form of carbohydrate in liver and muscle tissue. It is rapidly broken down to provide glucose when energy is needed.

Dietary Fibre

• Recommended intake: 25 – 30g per day for general health; optimal intake may vary based on individual needs and health conditions.

• Two forms:

  • Soluble Fibre: Includes pectins, gums, mucilages, and some hemicelluloses; dissolves in water to form a gel-like substance.

    • Lowers blood cholesterol by binding to bile acids and increasing their excretion. It also slows the rate of glucose absorption, helping to stabilize blood sugar levels.

    • Found in fruits (apples, citrus), oats, barley, and legumes (beans, peas).

  • Insoluble Fibre: Includes lignins, cellulose, and hemicelluloses; does not dissolve in water and adds bulk to the diet.

    • Decreases transit time through the gut, promoting regular bowel movements and preventing constipation. It also increases feelings of fullness, which can aid in weight management.

    • Found in vegetables (celery, carrots), wheat bran, and whole grains.

Fibre in Selected Foods (Examples)

• Grains: Whole-grain products provide about 1 to 2 grams (or more) of fiber per serving.

  • Examples: 1 slice whole-wheat, pumpernickel, rye bread; 1 oz ready-to-eat cereal (100% bran cereals contain 10 grams or more); ½ c cooked barley, bulgur, grits, oatmeal.

• Vegetables: Most vegetables contain about 2 to 3 grams of fiber per serving.

  • Examples: 1 c raw bean sprouts; ½ c cooked broccoli, brussels sprouts, cabbage, carrots, cauliflower, collards, corn, eggplant, green beans, green peas, kale, mushrooms, okra, parsnips, potatoes, pumpkin, spinach, sweet potatoes, swiss chard, winter squash; 1/2 c chopped raw carrots, peppers.

• Fruit: Fresh, frozen, and dried fruits have about 2 grams of fiber per serving.

  • Examples: 1 medium apple, banana, kiwi, nectarine, orange, pear; ⚫c applesauce, blackberries, blueberries, raspberries, strawberries. Note: Fruit juices contain very little fiber because the fiber is removed during processing.

• Legumes: Many legumes provide about 6 to 8 grams of fiber per serving; some provide about 5 grams per serving.

  • Examples: ½ c cooked baked beans, black beans, black-eyed peas, kidney beans, navy beans, pinto beans (6-8 grams); ½ c cooked garbanzo beans, great northern beans, lentils, lima beans, split peas (5 grams).

• Increasing Fibre Intake at Every Meal:

  • Aim for 30g of fibre and beyond daily to maximize health benefits.

  • Women: Recommended minimum daily requirement = 28g.

  • Men: Recommended minimum daily requirement = 38g.

  • Examples of fibre-rich foods and their fibre content:

    • Chia seeds: 1 tbsp = 4g

    • Wholegrain cereal: ¾ cup = 6g

    • Pear: 1 medium = 3g

    • Weet-Bix: 2 biscuits = 3g

    • Soy-linseed bread: 2 slices = 5g

    • Baked beans: ¼ cup = 3.5g

    • Frozen berries: 1/4 cup = 2g

    • Wholegrain rolled oats: ¼ cup = 4g

    • Banana: 1 medium = 2g

    • Four-bean mix: ¼ cup = 7g

    • Salad greens: 1/4 cups = 1g

    • Seed mix: 2 tbsp = 1g

    • Wholemeal wrap: 1 wrap = 2g

    • Ryvita: 2 crackers = 3g

    • Vita-Weats 9 Grains: 4 crackers = 3g

    • Avocado: 14 medium = 2g

    • Potatoes: 2 small = 2.5g

    • Brown rice (cooked): 1 cup = 3.5g

    • Lentils (cooked): ½ cup = 3.5g

    • Wholemeal pasta (cooked): 1 cup = 6.5g

    • Kumara: ½ cup = 3.5g

    • Vegetables (cooked): 1 cup = 2.5g

    • Peanut butter: 1 tbsp = 3g

    • Apple: 1 medium = 3.5g

    • Dried fig: 1 fig = 8.5g

    • Almonds: 20 nuts = 3g

    • Hummus: 3 tbsp = 3g

    • Carrot/celery sticks: 1 cup = 2.5g

    • Quinoa (cooked): 1/2 cup = 2g

    • Chickpeas (cooked): ½ cup = 6.5g

    • Plain popcorn: 2 cups = 3g

    • Psyllium husk: 1 tbsp = 6g

Fate of Glucose

• Pancreatic islet beta cells: Sense high glucose levels and secrete insulin.

• Fat cells: Take up glucose for storage as triglycerides.

• Muscle: Uses glucose for energy or stores it as glycogen.

• Liver: Stores glucose as glycogen or converts it to fatty acids.

• Small intestine: Absorbs glucose into the bloodstream.

• Insulin facilitates glucose uptake by binding to receptors on cell membranes, triggering glucose transporters to move glucose into cells.

Blood Glucose Disposal

• Skeletal muscle is a primary site of glucose disposal and storage due to its large mass and insulin-stimulated glucose uptake.

Hyperglycemia

• High blood sugar, typically defined as a fasting blood glucose level above 125 mg/dL or a postprandial (after meal) blood glucose level above 180 mg/dL.

• High levels of insulin can lead to insulin resistance and are linked with coronary heart disease (CHD).

• Prolonged hyperglycemia increases the risk of medical problems such as nerve damage, kidney damage, and vision problems.

• May cause reactive hypoglycemia, a condition where blood sugar drops too low after a meal due to an overreaction of insulin.

Hypoglycemia

• Low blood sugar, typically defined as a blood glucose level below 70 mg/dL.

• Symptoms: lethargy, confusion, anxiety, poor concentration, pale skin, dizziness, headache, shaking, sweating, and irritability.

• Reasons: Low energy or carbohydrate intake, high activity levels, illness, stress, type I diabetes, or side effects of diabetes medications.

• Remedy: Use sugars (High GI CHO) to increase blood glucose level quickly, such as glucose tablets, fruit juice, or candy.

• Can be life threatening if left untreated, leading to seizures, loss of consciousness, or coma!

Regulation of Blood Glucose

• Blood glucose regulation is primarily controlled by insulin and glucagon:

  • Insulin: Secreted by pancreatic beta cells in response to high blood glucose levels, promoting glucose uptake by cells and storage as glycogen.

  • Glucagon: Secreted by pancreatic alpha cells in response to low blood glucose levels, stimulating the breakdown of glycogen in the liver to release glucose into the bloodstream.

• (Refer to Understanding Nutrition, 2008, Fig 4.12, p 114 for a visual representation)

Glycemic Index (GI)

• GI = A ranking (0-100) given to carbohydrate foods that indicates the rate at which that CHO increases blood sugar compared to a standard (pure glucose or white bread). The GI is calculated by measuring the area under the blood glucose response curve after consuming a food containing 50 grams of carbohydrates.

• High GI: Foods with a GI of 70 or more increase blood glucose quickly, causing a rapid spike in blood sugar levels.

• Low GI: Foods with a GI of 55 or less increase blood glucose slowly, resulting in a more gradual and sustained release of glucose into the bloodstream.

Factors Affecting GI

• Glucose vs. fructose: Fructose has a lower GI because it is metabolized differently than glucose.

• Type of starch (amylose vs. amylopectin): Amylose digests more slowly than amylopectin, resulting in a lower GI.

• Cooking method / processing: Cooking and processing can break down starch molecules, increasing the GI.

• Fibre (soluble vs. insoluble): Soluble fibre slows down glucose absorption, lowering the GI, while insoluble fibre has less of an effect on GI.

• Fat and protein: Fat and protein slow down gastric emptying, which can reduce the rate of glucose absorption and lower the GI.

• Acidity: Acidic foods can slow down starch digestion, resulting in a lower GI.

Carbohydrates in the Diet

• CHO requirements vary greatly depending on gender, body size, activity level, and individual tolerance.

• The minimum intake an individual should consider is 80 – 100g per day to provide adequate glucose for brain function and prevent ketosis.

• For minimal activity or light exercise (e.g., walking), 2 – 4g/kg/d is sufficient to maintain energy balance.

• Prolonged CHO restriction (<50g per day) can result in many adverse side effects such as fatigue, muscle loss, and nutrient deficiencies.

20g Carbohydrate Portions (Examples)

• ½ cup corn

• 1 medium potato or sweet potato

• 1 cup pumpkin

• 2/3 cup kidney or baked beans

• 1 medium banana

• 1 large apple, pear, orange, or peach

• 2 large apricots, kiwifruit, or grapefruit

• 200ml fruit-flavored yogurt

• 400ml milk

• 2 T raisins or sultanas

• 20 cherries or grapes

• 300 – 400ml sports drink

• 2 T milo

• 1 slice of thick bread

• ½ long roll

• 1 dinner roll

• ½ bagel

• 2 thin pancakes

• 8 Arnott’s water crackers

• 2 slices mountain bread

• 1 cup cooked porridge (4 T raw)

• 1 cup Kellogg’s Sustain

• 2 Weetbix or Vita-Brits

• 1/3 – ½ cup muesli

• 1½ cups couscous

• ½ cup cooked pasta, rice, or noodles

Carbohydrates and Health

• High Sugar Intake Causes:

  • Tooth decay: Bacteria in the mouth feed on sugars, producing acid that erodes tooth enamel.

  • Obesity: Excess sugar intake contributes to weight gain and increases the risk of obesity.

  • Elevated insulin and glucose: Leads to insulin resistance, increased risk of platelet aggregation, and cardiovascular disease (CVD).

• Reduce Sugar Intake By:

  • Using less added sugar in foods and beverages.

  • Avoiding soft drinks, fruit juice, lollies, and other sugary snacks.

  • Limiting intake of sugary cereals, muesli bars, cakes, biscuits, and sports drinks.

• High Fibre Intake Leads To:

  • Improved control of blood glucose: Slows down glucose absorption and helps regulate blood sugar levels.

  • Reduced risk of bowel cancer: Promotes healthy bowel movements and reduces exposure to carcinogens.

  • Reduced risk of heart disease: Lowers blood cholesterol levels and reduces inflammation.

  • Healthier GI tract: Promotes the growth of beneficial bacteria.

• Increase Fibre Intake By:

  • Increasing fruit and vegetable intake, especially those with edible skins and seeds.

  • Using wholegrain breads and cereals instead of refined grains.

Summary

• Carbohydrates are categorised as either monosaccharides, disaccharides or polysaccharides, each with distinct structures and roles in the body.

• Dietary fibre is either soluble or insoluble, each offering unique health benefits such