Macronutrients: Chemistry, Types & Structure

Basic Chemistry of Macronutrients

• Macronutrients share carbon, hydrogen, and oxygen atoms.
• Proteins are unique because they also contain nitrogen.
• Atoms form bonds based on the number of electrons needed to fill their outer valence shell.
• Carbon can form 4 bonds.
• Nitrogen can form 3 bonds.
• Oxygen can form 2 bonds.
• Hydrogen can form 1 bond.

Carbohydrates: Types & Structure

• Monosaccharides are simple sugars.
• Glucose is the preferred energy source for the brain and red blood cells.
• Fructose, found in fruit, is the sweetest sugar.
• Galactose is a component of lactose in milk.
• Disaccharides are formed when two monosaccharides bond via condensation.
• Maltose consists of glucose + glucose.
• Sucrose consists of glucose + fructose.
• Lactose consists of glucose + galactose.
• Polysaccharides are complex carbohydrates.
• Starch is the plant storage form of glucose, found in grains and potatoes.
• Glycogen is the human storage form of glucose, stored in the liver and muscles.
• Fiber is a structural component of plants that cannot be broken down by human enzymes.

Carbohydrates: Digestion & Absorption

• In the mouth, salivary amylase initiates starch digestion.
• No enzymatic carbohydrate digestion occurs in the stomach; amylase is inactivated.
• In the small intestine, pancreatic amylase breaks down starch into maltose.
• Brush border enzymes (maltase, sucrase, lactase) break down disaccharides into monosaccharides.
• Glucose and galactose are absorbed via active transport (SGLT-1).
• Fructose is absorbed via facilitated diffusion (GLUT5).
• The liver converts fructose and galactose into glucose for energy or glycogen storage.

Carbohydrates: Metabolism & Functions

• Glycolysis is the breakdown of glucose into ATP.
• Glycogenesis is the storage of glucose as glycogen.
• Glycogenolysis is the breakdown of glycogen into glucose.
• Gluconeogenesis is the production of new glucose from amino acids.
• Insulin lowers blood sugar levels.
• Glucagon raises blood sugar levels.

Carbohydrates: Health Effects

• Lactose intolerance is caused by a deficiency of the lactase enzyme.
• Type 1 diabetes is an autoimmune condition with no insulin production.
• Type 2 diabetes is characterized by insulin resistance.
• Fiber consumption lowers the risk of heart disease, cancer, and diabetes.

Lipids (Fats): Types & Structure

• Triglycerides (glycerol + 3 fatty acids) are the most abundant lipid in food and the body.
• Phospholipids form cell membranes and act as emulsifiers (e.g., lecithin).
• Sterols, like cholesterol, are precursors for bile, vitamin D, and hormones.

Lipids (Fats): Fatty Acids

• Saturated fatty acids have no double bonds and are found in butter, coconut oil, and animal fat.
• Monounsaturated fatty acids have one double bond and are found in olive oil and avocado.
• Polyunsaturated fatty acids have two or more double bonds.
  • Omega-3 fatty acids (linolenic acid) are found in fish and flaxseeds.
  • Omega-6 fatty acids (linoleic acid) are found in vegetable oils.
• The number of fatty acids and the types of bonds affect structure and function.
• Longer, saturated fats are solid at room temperature.
• Shorter or less saturated fats are liquids at room temperature.
• Omega-3 and omega-6 are polyunsaturated essential fatty acids.

Lipids (Fats): Digestion & Absorption

• Hard fats melt upon reaching body temperature in the mouth.
• Stomach churning breaks fats into smaller particles. Gastric lipase aids in fat breakdown, but minimal fat digestion occurs here.
• Fat in the intestines triggers the release of CCK, which signals the gallbladder to release bile for emulsification.
• Bile emulsifies fats, breaking them into smaller droplets of free fatty acids and monoglycerides.
• Pancreatic lipases break down triglycerides into free fatty acids and monoglycerides.
• Glycerol and short- and medium-chain fatty acids are absorbed directly into intestinal cells.
• Micelles, formed from monoglycerides and long-chain fatty acids, diffuse into intestinal cells.
• Inside intestinal cells, micelles break apart, and fatty acids, monoglycerides, cholesterol, proteins, and phospholipids are reassembled into chylomicrons.
• Chylomicrons, containing triglycerides, cholesterol, phospholipids, and proteins, are transported via lymph and enter the bloodstream at the thoracic duct.

Lipids (Fats): Liver and Lipoproteins

• LDL receptors control blood cholesterol levels.
• LDL ("bad" cholesterol) delivers cholesterol to tissues.
• HDL ("good" cholesterol) removes excess cholesterol from tissues.
• Chylomicrons are the largest and least dense lipoproteins; deliver fats to cells, remnants go to liver.
• Very low-density lipoproteins (VLDL) are created in the liver from chylomicron remnants repackaged with proteins; deliver triglycerides to cells, eventually becoming LDL.
• LDL is about 50% cholesterol; LDL receptors on the liver remove LDL from circulation to control blood cholesterol levels.
• HDL, made by the liver, is mostly protein; it carries cholesterol from cells back to the liver, prevents plaque buildup, and has anti-inflammatory properties.

Lipids (Fats): Health Effects

• Replacing saturated fats with mono- and polyunsaturated fats reduces inflammation, lowers LDL cholesterol, and lowers the risk of heart disease and high blood pressure.
• Saturated and trans fats increase LDL cholesterol, raising the risk of heart disease.

Proteins: Structure & Digestion

• In the mouth, proteins are crushed and moistened.
• In the stomach, hydrochloric acid (HCl) denatures proteins, and pepsin hydrolyzes peptide bonds.
• In the small intestine, pancreatic proteases (trypsin, chymotrypsin, carboxypeptidase) break down proteins into amino acids.
• Amino acids are actively transported into the bloodstream and then to the liver.

Protein Absorption

• Specific carriers transport amino acids (and di- and tripeptides) into intestinal cells.
• Amino acids are used for energy or synthesized into compounds.
• Remaining amino acids are transported across the membrane to capillaries and travel to the liver.
• Enzymes that help the body break down proteins to amino acids are proteins themselves; they are broken down and digested when optimal pH is altered.

Proteins: Functions

• Structural proteins include collagen, keratin, and muscle fibers.
• Enzymes catalyze reactions (e.g., amylase, DNA polymerase).
• Hormones include insulin, glucagon, and thyroid hormones.
• Transport proteins include hemoglobin (oxygen) and albumin (nutrients).
• Immune function is provided by antibodies.

Proteins: Metabolism & Nitrogen Balance

• Positive nitrogen balance occurs during growth, pregnancy, and recovery.
• Negative nitrogen balance occurs during starvation and severe burns.
• Deamination converts amino acids into ammonia (NH₃), which is then converted to urea and excreted in urine.
• Transamination creates new non-essential amino acids plus a keto acid.
• High protein diets increase water needs due to high urea production.
• Liver failure results in high ammonia levels (the liver cannot convert ammonia to urea).
• Kidney failure results in high urea levels (the kidneys cannot excrete urea).

Dietary Recommendations: Macronutrient Intake

• Carbohydrates: 45-65% of daily kcal (≥130g/day).
• Fats: 20-35% of daily kcal.