Carbohydrates: Sugars and Starches

Carbohydrates: Sugars, Starches, and Fibres

• Carbohydrates are the body's primary energy source.
• They're used as glucose or stored as glycogen.
• Dietary sources include grains (bread, cereals, pasta), beans, legumes, milk, alternatives, fruits, and starchy vegetables (potatoes, turnips, corn, carrots).

Types of Carbohydrates

• Monosaccharides: Single sugar units (glucose, galactose, fructose).
• Disaccharides: Two monosaccharides joined (sucrose, lactose, maltose).
• Polysaccharides: Many monosaccharides linked (starch, fiber, glycogen).

The Sugars (Simple Carbohydrates)

• Monosaccharides (single sugars):
  • Glucose (blood sugar, dextrose): Main energy source for brain and nerves; excreted in urine by diabetics.
  • Fructose: Found in fruit and honey; sweeter than regular sugar.
  • Galactose: Part of lactose (milk sugar); freed during digestion.
• Disaccharides (pairs of monosaccharides):
  • Sucrose (glucose + fructose): Table sugar (beet or cane).
  • Lactose (glucose + galactose): Sugar in milk; poorly digested by some (lactose intolerance).
  • Maltose (glucose + glucose): Produced during starch breakdown; important in brewing.

Polysaccharides (Complex Carbohydrates)

• Starch: Excellent glucose source in plants; long branched and unbranched chains.
• Fiber: Structural part of plants; indigestible, contributes little energy (e.g., cellulose, hemicellulose).
• Glycogen: Storage form of glucose in the liver and muscles in humans and animals.

Fibre

• Polysaccharides: Sugar units held together by bonds indigestible by human enzymes.
• Whole grains, fruits, vegetables, and legumes contain fiber.
• Two types: soluble (viscous) and insoluble (non-viscous).
• Functional fiber is extracted from plants and added to foods/supplements.
• Important to drink enough water when consuming fiber.

Soluble Fibre (Viscous Fibre)

• Dissolves in water, forming gummy gels.
• Fermentable.
• Adds thickness to foods (e.g., pectin).
• Found in apples, barley, carrots, legumes, oats, and psyllium.
• Promotes stool formation, significant cholesterol-lowering effect, and helps regulate blood glucose.

Insoluble Fibre (Non-Viscous Fibre)

• Does not dissolve in water, does not form gels.
• Less readily fermented.
• Found in bran, tough, stringy parts of foods (cellulose and hemicellulose in wheat bran, skin on fruits, vegetables and hulls of seeds).
• Helps move stool through the large intestine.

Carbohydrate Digestion in the Mouth

• Initiated by salivary amylase.
• Splits starch into maltose (sweet taste).

Carbohydrate Digestion in the Stomach

• Enzymes are deactivated by stomach acid.
• No further starch digestion occurs.
• Fiber is not digested.
• Delays gastric emptying, contributing to satiety.

Carbohydrate Digestion in the Small Intestine

• Majority of carbohydrate digestion occurs here.
• Pancreatic amylase digests starch into shorter glucose chains and maltose.
• Intestinal cells secrete enzymes (lactase, sucrase, maltase) to break down disaccharides.
• Fiber is not digested.
• Slows glucose absorption, preventing spikes in blood glucose.

Carbohydrate Digestion in the Large Intestine

• Fiber and resistant starch travel unchanged to the colon.
• Fiber attracts water to soften stool.
• Bacterial fermentation occurs, possibly producing odorous gases.
• Resistant starches may help prevent chronic diseases.
• Oligosaccharides and polyols are indigestible components.

Absorption

• Occurs in the small intestine.
• Monosaccharides (glucose, fructose, galactose) enter cells via active or facilitated transport.
• Liver converts galactose and fructose to glucose.

Lactose Intolerance

• Inadequate lactase to break down lactose in the small intestine.
• Intestinal cells produce the highest amount of lactase at birth, declining with age.
• Symptoms include bloating, abdominal discomfort, and diarrhea.
• Lactose travels undigested to the colon.
• Etiology: Primary (genetic) versus secondary (diet-related) lactose intolerance.
• Dietary changes/strategies may be needed, including partial elimination of lactose, foods containing live bacteria, or lactase tablets.

Glucose in the Body

• Used as energy for cells throughout the body.
• Stored as glycogen for later use in the liver and muscles.
• Brain relies on glucose for energy.
• Glycogen holds water, limiting the total amount stored.

Carbohydrate Storage as Glycogen

• Muscle: ~2/3 of body's glycogen - used during high-intensity exercise.
• Liver: ~1/3 of body's glycogen - released to maintain blood glucose for the brain and other tissues.
• Brain: Small amount of glycogen as an emergency reserve.

Using Glucose for Energy

• Glucose fuels most body cells.
• Glucose is split into two 3-carbon compounds, yielding energy (and water).
• The 3-carbon compounds can be reformed into glucose or converted to other compounds like fatty acids.
• Breakdown of glucose compounds to CO2 and water results in energy release in the Krebs cycle and electron transport chain.

Below Healthy Minimum

• Minimum 50-100g carbohydrates per day needed for brain and nerve function, to prevent ketosis.
• Insufficient carbohydrates can lead to reliance on fat as an energy source, and ketosis and ketone bodies.
• Glucose made from fat, or protein through gluconeogenesis is not ideal.

Handling Excess Glucose

• Excess glucose is typically converted to fat and stored in fat cells.

Regulation of Blood Glucose

• Glucose is regulated by two hormones:
  • Glucagon: Released when blood glucose levels fall; triggers the breakdown of liver glycogen to free glucose.
  • Insulin: Released when blood glucose levels rise; stimulates the uptake of glucose into cells and the storage of glycogen in the liver.
• Maintaining a proper balance relies on a regular schedule of balanced meals containing carbohydrates (starch), fiber, protein, and healthy fat.

Diabetes Mellitus

• Chronic disease where the body cannot regulate blood glucose levels normally.
• Types include Type 1, Type 2, Gestational, and Prediabetes (less common types: MODY - Maturity Onset Diabetes of the Young, and LADA- Latent Autoimmune Diabetes in Adults).
• Type 1 is insulin-dependent (autoimmune attack on pancreas, inadequate insulin produced).
• Type 2 is insulin-resistant (body cells do not respond to insulin, or pancreas does not produce enough insulin adequately).
• Prediabetes (impaired glucose tolerance - symptoms of increased sugar levels but not meet the criteria for diabetes).

Hypoglycemia

• Blood glucose level is below the normal range.
• Two types:
  • Reactive hypoglycemia: Low blood glucose after a meal (body secretes too much insulin).
  • Fasting hypoglycemia: Low blood glucose between meals due to prolonged time between meals or other factors (e.g., pancreatic damage, certain cancers, infection, uncontrolled diabetes).

Symptoms of Hypoglycemia

• Fatigue, weakness, dizziness, headache, rapid heartbeat, sweating, trembling, hunger, anxiety, irritability, moodiness, confusion.
• Extreme cases: Amnesia, seizures, unconsciousness.

The Glycemic Response

• How quickly glucose is absorbed after ingestion.
• How high blood glucose rises and how quickly it returns to normal.
• Glycemic index reflects the rate of absorption.

Carbohydrate Recommendations

• Canada's Food Guide (CFG) and the Dietary Reference Intakes (DRIs) suggest daily values of carbohydrates and dietary fiber.
• Minimum daily intake is 130 g of glucose to meet brain/nerve needs.
• Adults should aim for 45–65% of total calories from carbohydrates.

Practice Calculation

• Determine the grams of carbohydrates to consume based on total calories consumed daily and desired macronutrient distribution.

Guidelines Regarding Sugar

• Canada's Food Guide (CFG), the Dietary Reference Intake (DRI), and the World Health Organization (WHO) recommend limiting added sugars in the diet (less than 10% of total energy intake).

Sugar Substitutes

• Sugar alcohols: occur naturally, fewer calories than sugar, lead to GI issues in high doses (restricted on low FODMAP diets).
• Artificial sweeteners: calorie-free, approved daily intake values, some banned or not best for certain uses - (e.g., sucralose).

Health Effects of Sugar

• Sugar intake and obesity, heart disease, diabetes, behavior (in a restricted context).
• Excess sugar intake can lead to weight gain, increased risk of heart disease, type 2 diabetes, and dental problems. Food labels for carbohydrate/sugar content are shown on product labels.