Carbohydrates, Lipids, and Proteins: Structure, Function, and Health Implications

Carbohydrates (CHO)

Functions and Energy Value

• Primary Function: Carbohydrates provide energy for the body, yielding 4 kcal per gram.

• Minimum Daily Requirement: The body requires at least 130 grams of carbohydrates per day to support brain function and red blood cell metabolism.

Classification of Carbohydrates

• Simple Sugars

  • Monosaccharides: Single sugar units.

    • Glucose: The main energy source for the brain and red blood cells.

    • Fructose: The sweetest naturally occurring carbohydrate.

    • Galactose: Found in milk and dairy products.

  • Disaccharides: Composed of two monosaccharides.

    • Sucrose: Glucose + Fructose (table sugar).

    • Lactose: Glucose + Galactose (milk sugar).

    • Maltose: Glucose + Glucose (product of starch digestion).

• Complex Carbohydrates (Polysaccharides)

  • Starch: Digestible storage form in plants; provides 4 kcal/g.

  • Fiber: Indigestible by human enzymes; provides 0 kcal/g.

    • Water-Soluble Fiber: Lowers cholesterol and moderates blood glucose spikes.

    • Water-Insoluble Fiber: Promotes regularity and reduces constipation.

    Health Benefits: Decreases gastric emptying, normalizes blood glucose, lowers serum cholesterol.

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

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

Digestion and Absorption

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

• After absorption, monosaccharides are transported to the liver, where they are converted to glucose.

Ketosis

• Occurs when carbohydrate intake is very low (<60–100 g/day), leading to incomplete fat oxidation.

• Results in the production of acidic ketone bodies, which can accumulate in the blood (ketonemia) and lower blood glucose.

• Keto Diet: Designed to induce ketosis for weight loss, appetite suppression, but may cause side effects such as bad breath. Weight is typically regained when carbohydrate intake increases.

Insulin and Blood Sugar Regulation

• Insulin: A hormone produced by the pancreas that facilitates the uptake of glucose, amino acids, and fatty acids into cells, and promotes fat storage.

Blood Sugar Disorders

• Diabetes Mellitus: Characterized by hyperglycemia (high blood glucose) due to insufficient insulin or insulin resistance.

  • Type 1 Diabetes: Autoimmune destruction of pancreatic beta cells; little or no insulin produced; more common in children.

  • Type 2 Diabetes: Most common form; insulin resistance often associated with obesity and sedentary lifestyle; managed with diet and exercise.

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

• Hypoglycemia: Abnormally low blood glucose, may result from excessive insulin, inadequate carbohydrate intake, or excessive exercise.

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

Lipids

Overview and Classification

• Energy Value: Lipids provide 9 kcal/g, making them the most energy-dense macronutrient.

• Types of Lipids:

  • Triglycerides (95%): Composed of three fatty acids attached to a glycerol backbone. Fatty acids can be saturated, monounsaturated, or polyunsaturated.

  • Sterols (5%): Includes cholesterol, found in animal products and synthesized by the liver. Recommended intake is <300 mg/day.

  • Phospholipids: Such as lecithin, important for cell membrane structure and nutrient transport; also act as emulsifiers.

Fatty Acids: Structure and Types

• Saturated Fatty Acids: No double bonds; typically solid at room temperature; can increase cholesterol and heart disease risk. Sources: animal fats, tropical oils (palm, coconut).

• Monounsaturated Fatty Acids: One double bond; may lower cholesterol and heart disease risk. Sources: olive oil, peanut oil, avocado.

• Polyunsaturated Fatty Acids: Two or more double bonds; may lower cholesterol and heart disease risk. Sources: sunflower oil, safflower oil.

Essential Fatty Acids

• Omega-3 (Alpha-Linolenic Acid, ALA): Found in walnuts, flaxseed, and vegetable oils. Only about 10% is converted to active forms (DHA & EPA), which are abundant in cold-water fish (e.g., salmon). Health benefits include reduced heart disease risk and possible improvement in depression.

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

Atherosclerosis and Heart Disease

• Atherosclerosis: Accumulation of fat in arterial walls, restricting blood flow and increasing heart disease risk.

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

Lipid Digestion and Transport

• Digestion occurs mainly in the small intestine with the aid of bile.

• Lipoproteins: Complexes of lipids and proteins that transport fats in the blood.

  • Low-Density Lipoprotein (LDL): Delivers cholesterol to tissues; high levels increase heart disease risk.

  • High-Density Lipoprotein (HDL): Removes cholesterol from arteries; high levels decrease heart disease risk.

  • Note: Stearic acid (in dark chocolate) does not appear to raise LDL cholesterol.

Hydrogenation and Trans Fats

• Hydrogenation: Industrial process that adds hydrogen to unsaturated fats, increasing saturation and producing trans fats.

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

Proteins

Structure and Functions

• Proteins are composed of 20 different amino acids containing nitrogen, carbon, oxygen, and hydrogen.

• Functions: Build, maintain, and repair tissues; act as buffers to maintain pH balance.

Essential and Non-Essential Amino Acids

• Essential Amino Acids (9): Must be obtained from the diet.

  • Phenylalanine, Tryptophan, Leucine, Isoleucine, Valine, Lysine, Methionine, Threonine, Histidine

• Non-Essential Amino Acids (11): Can be synthesized by the body.

Protein Metabolism

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

Protein Quality

• Complete Proteins: Contain all essential amino acids in adequate amounts; sources include animal products.

• Incomplete Proteins: Lacking one or more essential amino acids; most plant proteins (except soy and quinoa) are incomplete. The "limiting amino acid" is the one present in the lowest amount relative to need.

Protein Digestibility

• Digestibility affects the availability of amino acids for the body.

Vegetarian Diets

• Vegan Diet: Excludes all animal products; may risk deficiencies in protein, iron, calcium, vitamin B12 if not carefully planned.

• Fruits are not significant sources of protein; good sources include vegetables, whole grains, legumes, quinoa, and tofu.

Protein Denaturation

• Denaturation is the uncoiling of protein structure, altering its function; caused by heat or acids.

Protein Digestion and Absorption

• Begins in the stomach with hydrochloric acid (HCl) and pepsin, which denature and break down proteins.

• Digestion and absorption of amino acids are completed in the small intestine.

Protein Requirements and Nitrogen Balance

• Recommended Intake: 0.8 g protein per kilogram of body weight per day.

• Nitrogen Balance: Used to assess protein needs; balance occurs when nitrogen intake equals nitrogen excretion, indicating adequate protein intake.

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

Protein Deficiency

• Kwashiorkor: Severe protein deficiency despite adequate caloric intake; symptoms include edema, fatty liver, and muscle wasting.

Table: Comparison of Macronutrients

Macronutrient	Energy Value (kcal/g)	Main Functions	Key Dietary Sources
Carbohydrates	4	Energy, especially for brain and RBCs	Fruits, vegetables, grains, legumes
Lipids	9	Energy, cell membranes, hormone synthesis	Oils, butter, nuts, animal fats
Proteins	4	Tissue building, enzymes, immune function	Meat, dairy, legumes, grains

Key Equations

• Protein Requirement Calculation: $\text{Protein requirement (g)} = 0.8 \times \text{body weight (kg)}$

Additional info: Academic context and definitions have been expanded for clarity and completeness. Table and equations have been added for exam preparation.