Carbohydrates, Lipids, and Proteins: Core Concepts for Nutrition Students

Carbohydrates: Structure, Function, and Health Implications

Functions and Energy Value

Carbohydrates are a primary source of energy for the body, providing 4 kcal per gram. The minimum daily requirement for carbohydrates is 130 grams, which is essential for proper brain and red blood cell function.

• Energy Source: Especially important for the brain and red blood cells.

• Minimum Intake: 130 grams per day recommended.

Classification of Carbohydrates

• Simple Sugars:

  • Monosaccharides: Single sugar units.

    • Glucose: Also called blood sugar; preferred by brain and RBCs.

    • Fructose: Sweetest carbohydrate, found in fruits.

    • Galactose: Part of milk sugar.

  • Disaccharides: Two monosaccharides linked.

    • Sucrose: Glucose + Fructose (table sugar).

    • Lactose: Glucose + Galactose (milk sugar).

    • Maltose: Glucose + Glucose.

• Complex Carbohydrates (Polysaccharides):

  • Starch: Digestible, 4 kcal/g, found in plants.

  • Fiber: Indigestible, 0 kcal/g, found in plants.

    • Water-soluble fiber: Lowers cholesterol, moderates blood glucose.

    • Water-insoluble fiber: Reduces constipation.

    • Sources: Fruits, vegetables, legumes, whole grains.

  • Glycogen: Storage form in animals/humans, stored in liver and muscles. Excess glucose is converted to fatty acids.

Digestion and Absorption

• Most carbohydrate digestion and absorption occurs in the small intestine.

• Monosaccharides are converted to glucose in the liver after absorption.

Ketosis

• Occurs when carbohydrate intake is too low (<60-100g/day), leading to incomplete fat oxidation and production of acidic ketones.

• Keto Diet: Induces ketosis for weight loss, but weight is regained when carbohydrates are reintroduced.

• Symptoms: Increased ketones, decreased blood glucose, bad breath.

Hormonal Regulation: Insulin

• Insulin: Pancreatic hormone that facilitates entry of glucose, amino acids, and fatty acids into cells; helps store fat.

Blood Sugar Disorders

• Diabetes Mellitus: Hyperglycemia due to lack or ineffectiveness of insulin.

  • Type I: Autoimmune, little/no insulin, common in children.

  • Type II: Insulin resistance, associated with obesity and inactivity; managed with diet and exercise.

  • Complications: Heart attack, nerve damage, blindness, infections, kidney failure.

• Hypoglycemia: Low blood glucose, may result from excess insulin or insufficient carbohydrate intake with exercise.

  • Symptoms: Shakiness, light-headedness, hunger, headache.

Lipids: Types, Functions, and Health Effects

Overview and Energy Value

Lipids are a diverse group of hydrophobic molecules providing 9 kcal per gram. They are essential for energy storage, cell structure, and hormone production.

• 95% Triglycerides: Three fatty acids attached to glycerol.

• 5% Sterols & Phospholipids: Cholesterol (sterol) and lecithin (phospholipid).

Types of Fatty Acids

• Saturated Fatty Acids: No double bonds; increase heart disease risk. Sources: animal fats, palm/coconut oils.

• Monounsaturated Fatty Acids: One double bond; lower cholesterol. Sources: olive oil, peanut oil, avocado.

• Polyunsaturated Fatty Acids: Two or more double bonds; lower cholesterol. Sources: sunflower, safflower oils.

Essential Fatty Acids

• Omega-3 (Alpha-Linolenic Acid, ALA): Found in walnuts, flaxseed, vegetable oils. Only ~10% converted to DHA/EPA (active forms found in oily fish). Benefits: heart health, possible depression relief.

• Omega-6 (Linoleic Acid): Found in vegetable oils, dark green vegetables. Involved in blood pressure and clotting regulation.

Cholesterol and Phospholipids

• Cholesterol: Made by liver, found in animal products. Recommended max: 300 mg/day.

• Phospholipids (e.g., Lecithin): Key component of cell membranes, emulsifies fats, aids nutrient transport.

Atherosclerosis and Heart Disease

• Atherosclerosis: Fat accumulation in arteries, leading to heart disease.

• Risk Factors: Genetics, high blood pressure, high LDL cholesterol, smoking, diabetes, poor diet, lack of exercise.

Digestion and Absorption of Lipids

• Occurs mainly in the small intestine, aided by bile.

• Lipoproteins: Transport lipids in blood.

  • LDL (Low Density Lipoprotein): Delivers fat to arteries, increases heart disease risk.

  • HDL (High Density Lipoprotein): Removes fat from arteries, decreases heart disease risk.

Hydrogenation and Trans Fats

• Hydrogenation: Industrial process that increases fat saturation, creates trans fats.

• Trans fats raise LDL and lower HDL, increasing heart disease risk.

Proteins: Structure, Function, and Dietary Considerations

Structure and Functions

Proteins are made of 20 different amino acids containing nitrogen, carbon, oxygen, and hydrogen. They are essential for building, maintaining, and repairing tissues, and act as buffers to maintain pH balance.

• Essential Amino Acids: Must be obtained from diet.

  • Phenylalanine, tryptophan, leucine, isoleucine, valine, lysine, methionine, threonine, histidine

• Non-Essential Amino Acids: Can be synthesized by the body.

Protein Metabolism

• Deamination: Removal of the amine group from amino acids before they can be used for energy or stored as fat.

Protein Quality

• Complete Proteins: Contain all essential amino acids in adequate amounts. Sources: animal products, soy, quinoa.

• Incomplete Proteins: Lacking one or more essential amino acids. Most plant proteins (except soy, quinoa).

• Limiting Amino Acid: The essential amino acid in shortest supply, limiting protein synthesis.

Digestibility and Vegetarian Diets

• Protein digestibility affects amino acid availability.

• Vegan Diet: Excludes all animal products; risk of deficiencies in protein, iron, calcium, vitamin B12 if not well planned. Good sources: vegetables, whole grains, legumes, quinoa, tofu. Fruits are not significant protein sources.

Protein Denaturation and Digestion

• Denaturation: Unfolding of protein structure by heat or acids, altering function.

• Digestion: Begins in the stomach (hydrochloric acid and pepsin), completed in the small intestine.

Protein Requirements and Nitrogen Balance

• Recommended Intake: 0.8 grams per kilogram of body weight.

• Nitrogen Balance: Used to assess protein needs.

  • Equation: $\text{Nitrogen~in~diet} = \text{Nitrogen~excreted}$ (for healthy adults)

• Excess protein may increase risk of heart disease and kidney stones.

Protein Deficiency

• Kwashiorkor: Severe protein deficiency despite adequate caloric intake.

Summary Table: Key Properties of Macronutrients

Macronutrient	Energy (kcal/g)	Main Functions	Key Dietary Sources
Carbohydrates	4	Energy, brain/RBC fuel, GI health (fiber)	Grains, fruits, vegetables, legumes
Lipids	9	Energy storage, cell membranes, hormones	Oils, butter, animal fats, nuts, seeds
Proteins	4	Tissue building/repair, enzymes, hormones	Meat, dairy, legumes, soy, quinoa