Chapter 4 – Comprehensive Carbohydrate Notes

Carbohydrates – Chemical Nature & Energy Cycles

• Elements: carbon, hydrogen, oxygen → often abbreviated as CHO.
• Plants: photosynthesis combines CO2 + H2O + sunlight → \text{glucose} (see Fig 4-1).
• Humans: catabolize glucose; release energy + CO_2 (Fig 4-11).

Classification of Carbohydrates

• Simple CHO ("Sugars").
  • Monosaccharides.
  • Disaccharides.
• Complex CHO ("Polysaccharides").
  • Plant starches & fibres.
  • Animal glycogen.

Simple Carbohydrates – Monosaccharides

• Glucose
  • Primary fuel; brain depends almost exclusively on it.
• Galactose
  • Same formula as glucose; differing orientation of a hydroxyl on C-4.
• Fructose
  • Same formula as glucose; different ring structure.
  • Natural sources: fruits, honey, sugarcane.
  • Added to foods via high-fructose corn syrup → soft drinks, cereals, etc.

Simple Carbohydrates – Disaccharides

• General: two monosaccharides linked; must be enzymatically split before absorption.
• Sucrose = glucose + fructose
  • Table sugar; abundant in many fruits & vegetables (beets, cane).
• Lactose = glucose + galactose
  • “Milk sugar” in milk, yogurt, cheese.
• Maltose = glucose + glucose
  • Product of starch breakdown; gives sweet taste while chewing bread; rarely present freely in foods.

Complex Carbohydrates (Polysaccharides)

• Long (sometimes branched) chains of glucose.
• Plant starch
  • Digestible by human amylases.
• Plant fibre (e.g., cellulose)
  • Bonds resistant to human enzymes.
• Glycogen (animal storage)
  • Highly branched; stored in liver & muscle; fully digestible.

Dietary Recommendations for Carbohydrate Intake (Table 4-1)

• DRI: 45–65 % of total energy from CHO with emphasis on complex/whole-grain forms.
• Minimum: 130\,\text{g CHO/day} (spares protein & prevents ketosis).
• Added sugars: <25 % of total energy.
• Typical sources: pasta, rice, beans, cereals, breads, milk, fruit, potatoes, corn, peas, squash.

Dietary Fibre – Quantitative Goals

• WHO: >25 g/day (all adults).
• DRI
  • <50\,\text{yrs}: males 38 g, females 25 g.
  • >50\,\text{yrs}: males 30 g, females 21 g.
• Refer to Fig 4-4 for food values.

Water-Soluble Fibre

• Types: gums, mucilages, pectins, psyllium, some hemicellulose.
• Sources: oats, oat bran, barley, rye, seeds, legumes, fruits & vegetables.
• Physiological benefits
  • ↓ blood cholesterol (binds bile; Fig 4-5).
  • Slows glucose absorption → blunts post-meal peaks.
  • Delays gastric emptying & small-intestinal transit.
  • Holds water in stool → softer feces.
  • Fermented by colonic bacteria → short-chain fatty acids (energy for colonocytes).
  • ↓ risk of heart disease & diabetes.

Water-Insoluble Fibre

• Types: cellulose, lignin, some hemicellulose.
• Sources: wheat bran, brown rice, whole grains, seeds, fruits, vegetables, legumes.
• Physiological benefits
  • ↑ fecal bulk & weight.
  • Speeds colonic transit.
  • Relieves constipation.
  • ↓ risk of diverticular disease (Fig 4-6), hemorrhoids, appendicitis.
  • Promotes satiety → aids weight management.

Consumer Corner – Flour/Bread Label Terms

• Refined flour: husk, germ, bran removed → ↓ nutrients & fibre.
• Enriched flour: specific nutrients lost in refining added back (not fibre).

Digestion & Absorption of Carbohydrates

• Mouth: salivary amylase ~5 % starch hydrolysis.
• Stomach: mechanical churn; acid halts amylase activity → chyme.
• Small intestine
  • Pancreatic amylase: starch → disaccharides.
  • Brush-border enzymes: disaccharides → monosaccharides.
• Fibre passes intact to colon.
• Absorption: glucose, fructose, galactose enter portal blood → liver converts galactose & fructose → glucose.
• Glucose uses in body
  • Immediate energy (ATP).
  • Glycogen synthesis (liver & muscle).
  • Structural: glycoproteins (mucus, cell membranes).

Post-Prandial CHO Metabolism

• Rise in blood glucose depends on
  • Quantity ingested.
  • Digestion/absorption rate.
  • Cellular uptake rate (insulin dependent).

Blood-Glucose Homeostasis

• Normal fasting range: 4!\text{ – }!6\,\text{mmol/L}.
• Hypoglycemia: <4 mmol/L.
• Hyperglycemia: >6 mmol/L (fasting).

Hormonal Regulation

• Low glucose → pancreas secretes glucagon.
  • Stimulates liver glycogenolysis → glucose.
  • Promotes gluconeogenesis (AA → glucose); chronic use causes muscle wasting.
• High glucose → insulin secretion (mentioned later in diabetes context).

Ketosis

• Need ≥130\,\text{g CHO/day} to prevent.
• Normal intake: 200–400 g/day.
• Liver glycogen depleted in 4–6 h without intake → fat breakdown → ketone bodies.

Diabetes Mellitus

Type 1 (T1D)

• <10 % of cases; onset early/mid-life.
• Autoimmune destruction of pancreatic β-cells → little/no insulin.
• Untreated: hyperglycemia, glucosuria, weight loss.
• Management: match injected insulin to diet & activity.

Type 2 (T2D)

• >90 % of cases; now common in children/teens.
• Insulin resistance (often obesity-related); pancreas secretes plenty of insulin but tissues unresponsive.
• Chronic hyperglycemia → ↑ risk: CVD, nephropathy, retinopathy, neuropathy, amputations.

Risk Factors

• Family history, sedentary lifestyle.
• Overweight/obesity; moderate adult weight gain.
• History of gestational diabetes.
• Ethnicity: Aboriginal, African American, Hispanic, Asian, Pacific Islander.

Warning Signs

• Excessive urination & thirst; glucosuria.
• Weight loss, nausea, weakness, irritability.
• Craving for sweets; easy fatigue, drowsiness.
• Vision changes.
• Frequent infections; slow wound healing.
• Pain/tingling in extremities.
• Abnormal oral glucose-tolerance test.

Management

• Diet modification & physical activity; ~10 % weight loss beneficial.
• Oral hypoglycemic agents; insulin injections if needed.

Glycemic Effect of Food (Glycemic Index, GI)

• Definition: incremental area under blood-glucose curve for test food vs glucose solution/white bread.
• Low GI (desirable): legumes, peas.
• High GI: white bread, fruit juice.
• Influencing factors
  • Food type: whole apple < apple juice.
  • Preparation: baked potato < mashed.
  • Mixed-meal context lowers composite GI (glycemic load concept).
• Limitations
  • Ignores carbohydrate quantity.
  • Nutrient-dense foods may show higher GI (e.g., oatmeal) than less healthy high-fat foods (e.g., chocolate).

Carbohydrate Counting (Diabetes Meal-Planning Tool)

• Goal: keep CHO content consistent meal-to-meal.
• Standard exchanges (≈15 g CHO each)
  • 1 Grain/Starch = 15 g.
  • 1 Fruit = 15 g.
  • 1 Milk/Alternative = 15 g.
• Meat/Alternative & Fat choices: essentially 0 g CHO.

Lactose Intolerance vs Milk Allergy

Lactose Intolerance

• Low lactase activity (common outside Northern-European ancestry).
• Symptoms: nausea, pain, diarrhea, gas (osmotic & bacterial fermentation effects).
• Management strategies
  • Small milk portions (½–1 cup) with meals.
  • Yogurt, aged cheeses.
  • Lactase-treated products (Lactaid® milk/ice-cream) or enzyme drops/pills.

Milk Allergy

• Immune response to milk proteins.
• Need alternative calcium sources
  • Fortified soy beverage, fortified OJ.
  • Canned sardines/salmon with bones.
  • Certain vegetables (broccoli, bok choy).
  • Possible calcium supplements.

Sugars & Health – Controversies

• Investigations into links with obesity, diabetes aggravation, heart disease risk, behaviour issues, dental caries.

Intake Guidelines

• WHO: <10 % of calories from added sugars.
• DRI/IOM: <25 % of energy.

Added vs Natural Sugars

• Added: syrups/sugars introduced during processing or home prep (brown sugar, corn syrup, sucrose, maple syrup, juice concentrates, etc.).
• Natural: intrinsic sugars in whole foods (fruits, milk) – chemically identical but accompanied by fibre, vitamins, minerals.

Sugar Alcohols (Polyols)

• Sorbitol, mannitol, xylitol, isomalt, maltitol, lactitol.
• 3–5 kcal/g; incompletely absorbed → possible laxative effect.
• Not fermented by oral bacteria → lower cariogenicity.

Artificial (Non-nutritive) Sweeteners

• Aspartame (≈200× sweeter than sucrose) – \text{phenylalanine} + \text{aspartic acid}; people with PKU must limit.
• Acesulfame-K (≈200×).
• Sucralose (≈600×).
• Cyclamate & saccharin: restricted uses in Canada.
• Calorie-free because minuscule amounts used.
• Acceptable Daily Intake (ADI) concept
  • Aspartame ADI Canada: 40\,\text{mg·kg}^{-1} body weight⋅day⁻¹.
  • 68 kg (150 lb) adult → ≈72 Equal® packets or ≈13 diet sodas per day below ADI.