Carbohydrates and Metabolism

• Carbohydrates are essential nutrients in human health and are classified as macronutrients

Functions of Carbohydrates

• Carbohydrates serve as an energy source
• Glucose is the body's main fuel
• Glucose is the only energy source for the brain, red blood cells, and part of the kidney
• Adequate glucose intake spares proteins
• Adequate glucose intake prevents ketosis
• Carbohydrates allows for complete fat breakdown
• Excess glucose is stored as glycogen in muscle and liver

Carbohydrate Metabolism

• The body stores glucose as glycogen in the liver and muscle cells
• The body uses glucose for energy if glycogen stores are available
• When glycogen stores are depleted, the body produces glucose from protein, fat byproducts, and other waste products
• Gluconeogenesis is the conversion of protein to glucose
• Consuming enough carbohydrates in the diet has a protein-sparing action to prevent the breakdown of protein for energy
• When glycogen stores are depleted, the body can produce ketone bodies from fat fragments
• Ketones serve as an alternative fuel source during starvation
• The accumulation of ketone bodies in the blood results in ketosis
• Ketosis upsets the acid-base balance in the body
• When glycogen stores are full, the body can use glucose to make body fat

Glucose Homeostasis

• Normal blood glucose range is 3.5-8 mmol/L
• The body keeps blood glucose levels within the normal range by taking glucose into cells or releasing it from the liver
• Insulin moves glucose into cells and helps lower blood sugar levels
• Glucagon brings glucose out of storage and raises blood sugar levels
• Epinephrine brings glucose out of storage quickly during times of stress

Glycemic Index (GI)

• The glycemic index measures the incremental rise of blood glucose after ingesting a food containing 50g of carbohydrate.
• The measurement is expressed as a percentage.
• This measurement is in comparison to the same amount of carbohydrate from a reference food like white bread or glucose, and absorbed by the same person
• To determine the GI of a food, 25 or 50 grams of carbohydrate of the test food is given to a subject
• Blood samples are taken at 15-minute intervals for the first hour and then every 30 minutes for the second and third hours
• Values are plotted and the Area Under the Curve, AUC, is calculated
• The test food response is compared to the response produced after the reference food is ingested.
• The mean GI of 8-10 volunteers serves as the GI of the test food
• GI depends upon the rapidity of digestion and absorption of carbohydrates, which is influenced by physical and chemical properties.
• GI depends on the ratio of amylose to amylopectin
• High amylose/amylopectin ratio = low GI = slow absorption
• GI for Glucose = 100%
• Low GI = ≤ 55
• Medium GI = 56-69
• High GI = 70 and above
• Low GI foods result in a smaller and more gradual rise in blood glucose, which helps control diabetes
• Low GI foods can help people lose weight and lower blood cholesterol however, be mindful of fat content
• Low GI diets will improve the body's sensitivity to insulin
• High GI foods help refuel carbohydrate stores after exercise

Factors Influencing GI Ranking

• Degree of chewiness
• Type of starch
• Physical entrapment
• Viscosity of fiber
• Type of sugar & sugar content
• Fat content
• Protein content
• Acid & salt content
• Degree of ripeness
• Cooking
• Food processing

Type of Starch

• Amylose is an unbranched polysaccharide that absorbs less water and forms tight clumps, resulting in a slower rate of digestion
• Kidney beans, 28, and low GI rice, 50, are low-GI foods
• Amylopectin is a branched sugar which absorbs more water and forms more open molecules, resulting in a faster rate of digestion
• Russet potato, 85, and glutinous rice, 98, are high-GI foods

Physical Entrapment

• Bran acts as a physical barrier that slows down enzymatic activity on the internal starch layer
• All Bran, 38, and pumpernickel bread, 50, are low-GI foods
• A bagel, 72, and corn flakes, 92, are high-GI foods

Viscosity of Fiber

• Viscous, soluble fibers transform intestinal contents into a gel-like matter that slows down enzymatic activity on starch
• Apples, 40, and rolled oats, 51, are low-GI foods
• Whole wheat bread, 73, and Cheerios, 74, are high-GI foods

Sugar content

• Sucrose yields glucose, GI 100, + fructose, GI 19
• Starch yields maltose, GI 105, yields glucose + glucose
• Frosted Flakes, 55 and Raisin Bran, 61, are low-GI foods
• Golden Grahams, 71, and Rice Krispies, 82, are high-GI foods

Fat & Protein Content

• High fat and protein content delays entry of food to the intestine
• Fat and protein slow down gastric emptying and slow down the digestion of starch
• Peanut M&M's, 33, potato chips, 54, and Special K, 69, are low-GI foods
• Jelly beans, 78, baked potato, 85, and corn flakes, 92, are high-GI foods

Acid Content

• Acid slows down gastric emptying and slows the digestion of starch
• Sourdough wheat bread, 54, is a low-GI foods
• Wonder white bread, 73, is a high-GI food

Food Processing

• Highly processed foods require less digestive processing
• Old fashioned, rolled oats, 51, are low-GI foods
• Quick, 1-minute oats, 66, are high-GI foods

Cooking

• Cooking swells starch molecules and softens foods, which increases the rate of digestion
• Al dente spaghetti boiled for 10–15 minutes, 44 is a low-GI food
• Over-cooked spaghetti boiled for 20 minutes, 64 is a high-GI food

Degree of Ripeness

• The riper the fruit, the higher the GI
• Under ripe banana, 30, is a low-GI food
• Ripe banana, 51, is a high-GI food

Benefits of a Low-GI Diet

• Lowers blood glucose levels
• Lowers cholesterol levels
• Aids in weight loss
• Lowers type 2 diabetes risk
• Lowers heart disease risk

Glycemic Index Factor

• Rank in order of GI highest to lowest: Yoghurt, Lucozade, Honey, Pumpkin, White bread, Skim milk, Whole milk, Peanuts, Basmati rice, Brown rice, Sausages, Banana, Chocolate, Spaghetti

High vs Low GI Meals and Snacks

• High GI Snack: Corn Flakes and Banana, GI = 60
• Low GI Snack: Oat Bran and Strawberries, GI = 42
• High GI Meal: Cooked potato and Salmon, GI = 80
• Low GI Meal: Sweet potato and Salmon, GI = 61
• High GI Snack: Processed bread good, GI = 85
• Low GI snack, GI = 39

Energy Density

• Energy density in a food is the amount of energy stored in the food per unit weight.
• Energy density is influenced by the total energy, especially from fat content
• The energy density formula is: Total food energy/Total food weight
• 1 g of fat = 37 KJ
• 1 g of protein = 17 KJ
• Fat contains twice the weight in KJ than protein
• Foods that contain a lot of fat are classified as high energy density foods
• If the goal is to reduce energy intake, the best way is to reduce the amount of fat consumed from food
• Fibre and water content in food decreases energy density

Energy Density Calculation example

• 8x boiled potatoes by weight has the same energy density as 2x fried chips by weight

Energy Density Examples

• Croissant: 67 g, 1042 KJ = 15.6kJ/g
• 2 slices wholemeal bread: 64 g, 584 KJ = 9.1 kJ/g
• 2 slices white bread: 64 g, 639 KJ = 10 kJ/g
• Average energy density x Total food weight eaten/day (constant) = Total food energy (KJ)
• Fat in foods increases energy density
• High fat and high starch/sugar foods have a high energy density.
• McDonald's Muffin Lite: 97% fat-free
• Each muffin: 165 g
• 3.3 g fat (8% E) - low fat
• Total energy: 1650 KJ
• Energy density: 10 KJ/g
• Salad ED 0.8 kJ/g, Apple ED 1.7 kJ/g, Skim milk ED 1.4 kJ/g
• To achieve a low energy density diet: dilute high energy dense foods with low energy dense foods
• Dilute oil and meat with vegetables
• Dilute cereals with skim milk
• 30 g natural muesli (426 KJ) = (ED 14.2 kJ/g)
• 200 g skim milk (290 KJ) = (ED 1.5 kJ/g)
• muesli + milk (717 KJ) = (ED 7.2 kJ/g)
• Western Cafeteria meal: 212 g, ED 12.5 KJ/g
• Cretan or Mediterranean meal: 770 g, ED 3.4 KJ/g