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.