RGI.16 Introduction to Human Nutrition: The Macronutrients

Respiratory System, Gastrointestinal System (RGI)

Introduction

• The lecture will cover carbohydrates, fats, and proteins, including their digestion and absorption.
• The lecture is divided into three parts: Carbohydrates (Part 1), Fats (Part 2), and Proteins (Part 3).

Learning Outcomes

• Recognize the role of nutrition in disease prevention and management.
• Recall specific examples of nutrition and dietary-related diseases.
• Recall the basic structural features and roles of carbohydrates.
• Describe carbohydrate digestion and absorption processes.
• Define glycemic index and recall glycemic and non-glycemic effects of carbohydrates.
• Recall the basic structural features and roles of fats.
• Describe fat digestion and absorption processes.
• Discuss fat transport and storage in the body.
• Recall the basic structural features and roles of proteins.
• Describe protein digestion and absorption processes.
• Define essential amino acids.

Nutrition and Dietary Related Diseases

• Single nutrient treatments were discovered for many diseases, which kickstarted nutritional epidemiology and the understanding of the link between diet, health, and disease.
• Examples:
  • Scurvy and Vitamin C
  • Beri Beri and Thiamin (vitamin B1)
  • Iodine and Goitre
  • Iron and Anaemia
  • Vitamin D and Rickets

Nutrients

• A nutrient is a chemical that an organism needs to live and grow, or a substance used in an organism's metabolism.
• Macronutrients:
  • Required in large amounts (10's of grams/day).
  • Include carbohydrates, fats, and proteins.
  • All contain C, H, and O and have a structural role.
  • Carbohydrates and fats are the primary sources of energy.
• Micronutrients:
  • Required in very small amounts (mg or µg per day).
  • Include minerals (e.g., Ca, P), water-soluble vitamins, and fat-soluble vitamins.

Carbohydrates

Overview

• Carbohydrates range in complexity from simple sugars (monosaccharides and disaccharides) to larger, complex sugars (oligosaccharides and polysaccharides).
• Primary physiological roles:
  • Supply the body with energy.
  • Provide dietary fiber.

Monosaccharides

• Simple sugars ranging from 4-6 carbons (e.g., glucose, fructose, and galactose).
• Glucose:
  • The most prevalent sugar in the body.
  • Found in sugar, honey, fruit, vegetables, and confectionary.
  • Exists predominantly in a cyclic (pyranose) form.
  • If D- then OH at C-5 on RHS
• Fructose:
  • Found in honey, fruit, and some vegetables.
  • After absorption, it is metabolized in the liver to give glucose, glycogen, lactic acid, or fat.

Disaccharides

• Two monosaccharides joined together by an ether or glycosidic link:
  • Sucrose (table sugar): glucose + fructose
  • Lactose (milk): glucose + galactose
  • Maltose (germinating seeds): glucose + glucose

Oligosaccharides

• Composed of fewer than ten monosaccharide units, typically galactose or fructose linked to glucose units.
• Not absorbed; they pass to the colon where they are rapidly fermented by bacteria to give short-chain fatty acids and gases, leading to flatulence.

Polysaccharides

• Consist of more than ten monosaccharides arranged in straight, branched, or coiled chains.
• Starches:
  • Straight or branched chains of glucose units.
  • Amylose: contains glucose units linked by α-1,4-glycosidic bonds forming a secondary helical structure.
  • Amylopectin: contains glucose units linked by α-1,4-glycosidic bonds and additional α-1,6-glycosidic bonds, giving it a branched structure.
• Non-Starch Polysaccharides (NSPs):
  • Cellulose: contains glucose units linked by β-1,4-glycosidic bonds, making it very resistant to digestion.
  • Non-cellulose polysaccharides: include hemicelluloses, β-glucans, gums, and mucilage, which contain sugars such as arabinose, xylose, mannose, and glucose.
  • NSPs are either not metabolized (remain unchanged) or fermented by bacteria to short-chain fatty acids, hydrogen, methane, and carbon dioxide.

Carbohydrate Digestion and Absorption

Monosaccharides

• Do not need to be digested and are absorbed primarily in the small intestine (SI).
• Glucose and galactose are transported from the SI across the apical membrane and into the blood via a two-stage process facilitated by glucose transport proteins SGLT1 and GLUT2:
  • An Na^+/K^+ ATPase pump uses ATP molecules to move 3 sodium ions outward into the blood while bringing in 2 potassium ions, creating a downhill sodium ion gradient.
  • The sodium-linked glucose transport proteins SGLT1 use the energy from the downhill sodium ion gradient to transport glucose and Na^+ across the apical membrane.
  • The facilitative glucose transporters (GLUT-2) transport glucose to blood vessels by facilitated diffusion.

Disaccharides and Starches

• Disaccharides are split by specific enzymes into the corresponding monosaccharides as they pass through the small intestine (e.g., lactase acts on lactose).
• Cooked starch is acted upon by salivary amylase in the mouth.
• Pancreatic amylase breaks α-1,4-linkages in both cooked and raw starch in the duodenum.
  • Amylose is primarily broken down to maltose, maltotriose, and some glucose.
  • Amylopectin is metabolized by specific oligosaccharidases located on brush border cells in the SI to give glucose.
• Resistant starch can pass unchanged to the large intestine, where it is fermented to short-chain fatty acids and gases.

Glycemic Effects

• Glycemic Index (GI): provides an indication of how blood glucose levels change after ingesting carbohydrates.
  • Foods with carbohydrates that break down quickly during digestion and release glucose rapidly into the bloodstream tend to have a high GI.
  • Foods that break down more slowly, releasing glucose more gradually into the bloodstream, tend to have a low GI.
  • Glucose has a value of 100.
  • GI > 85 (High)
  • GI 60-85 (Moderate)
  • GI < 60 (Low)

Non-Glycemic Effects

• Diets with appropriate levels of non-starch polysaccharides encourage chewing, which slows eating and increases saliva flow, contributing to dental health.
• Soluble fiber provides a food source for beneficial microorganisms, increasing bulk and water content of stool mass and contributing to colon health.
• Insoluble fiber holds water, increases stool mass, and has a laxative effect.

Fats

Overview

• Fats, also known as lipids, are required in the diet as they perform the following essential roles:
  • Provide energy
  • Provide insulation
  • Provide structural components
  • Are functional constituents of many metabolic processes
  • Add flavor to food
• Types of Fats:
  • Triacylglycerols: Fatty acids and glycerol
  • Phospholipids: Fatty acids, glycerol, and phosphate
  • Sterols: Ring structure lipids (e.g., Cholesterol)

Fatty Acids

• Primary components of dietary fats.
• A hydrocarbon backbone capped at either end by a methyl group and a carboxylic acid group.
• Saturated Fatty Acids:
  • No double bonds.
  • Mostly contain 14, 16, and 18 carbons.
  • Often solids at room temperature.
  • All the bonds are single, so the chain is free to rotate.
  • Associated with increased plasma cholesterol levels and cardiovascular disease.
  • Sources include milk, milk products, meat, palm oils.
• Monounsaturated Fatty Acids (MUFAs):
  • Contain 1 double bond.
  • Sources include olive, mustard seed, and rapeseed oil, as well as oily fish.
• Polyunsaturated Fatty Acids (PUFAs):
  • Contain at least two double bonds.
  • Sources include dark green vegetables, seed oils, nuts, meat, and oily fish.
  • If the double bond is found on C-3, C-6, or C-9, the families are known as n-3, n-6, and n-9 (aka omega).
  • C18:2, n-6 (linoleic acid) is an 18-carbon fatty acid with two double bonds, the first of which starts at carbon 6 from the methyl end.
• Unsaturated Fatty Acids (UFAs):
  • Contain double bonds - cis-isomers predominate.
  • UFAs can contain 1 or more double bonds in the trans position, known as Trans Fatty Acids (TFAs).
  • TFAs are linked with negative health implications; sources include hydrogenated fats in manufactured foods.
  • The chains in trans-isomers are not bent, and the chains can pack closely together. The chains in cis- isomers are bent.

Triacylglycerols (TAGs or triglycerides)

• Consist of three fatty acids attached to a glycerol molecule.
• Store unused calories.

Phospholipids

• Consist of two fatty acids (nonpolar) and one polar phosphate group (or sugar or amino acids) attached to a glycerol molecule.
• Amphiphilic: contain hydrophilic and lipophilic properties and can, therefore, act at the interface of aqueous and lipid environments.
• Found in cell membranes, where they contribute to structural integrity.
• Sources include liver, eggs, soybean, and wheat germ.

Sterols

• Ring structures with associated side chains.
• Cholesterol:
  • The main sterol in the body.
  • Plays a role in (i) membrane structure, (ii) transport across membranes, and (iii) the synthesis of hormones and bile acids.
  • Principally carried by LDL and linked with risk of cardiovascular disease.
  • Sources include egg yolks, meats, and shellfish.

Digestion and Absorption of Fats

1. Large fat particles are broken down in the stomach by churning.
2. Coarse emulsion reaches the duodenum, where bile released from the liver via the gall bladder reduces emulsions to small micelles.
3. Pancreatic lipase splits fatty acids from TAGs to glycerol, fatty acids, and some monoacylglycerol.
4. Fatty acids are absorbed in the small intestine.

Transport of Fats

• Fats are hydrophobic and cannot circulate freely in blood.
• They are packaged into aggregate particles called lipoproteins.
• Lipoproteins primarily transport TAG and cholesterol:
  • Chylomicrons: transport TAG taken in from the diet and release fatty acids (action of lipoprotein lipase) as they travel through the body.
  • Very Low-Density Lipoproteins (VLDLs): transport TAG resynthesized in the liver.
  • Low-Density Lipoproteins (LDLs): transport cholesterol to tissues where it is needed in cell membranes or for the synthesis of metabolites.
  • High-Density Lipoproteins (HDLs): collect free cholesterol from peripheral tissues.

Storage of Fats

• Fat is stored in adipocytes in adipose tissue.
• White adipose tissue, where the cells store fat as a single droplet, predominates.
• Brown adipose tissue, found in children and young adults, contains many mitochondria and is thought to play a role in energy wasting (food to heat).

Proteins

Overview

• Proteins constitute the building blocks of all living things (e.g., cell membranes, organelles), enzymes, and chemical messengers.
• In addition to C, H, and O, they are made up of N and S.
• A protein molecule is a chain of amino acid units linked by peptide bonds.
• Folding via molecular interactions, including cross-linking of the chain, gives rise to higher orders of structure.

Structure of Proteins

• The exact sequence of the amino acids in the chain determines the identity and function of the protein.
• There are 20 amino acids, which differ by their side chains or R groups.
• Essential Amino Acids: The body cannot make nine of the amino acids used in protein synthesis (must come from food).

Digestion and Absorption of Proteins

• Dietary protein is nearly always completely digested.
• Cooking and acidity increase the digestibility of proteins.

1. Mouth - chewing action and saliva breaks food into smaller pieces and lubricates food respectively.
2. Stomach (pH < 4) - pepsin breaks polypeptide chains to smaller chains.
3. Duodenum (pH c. 7.5) - Trypsin and chymotrypsin are serine proteases that specifically target peptide bonds next to a basic side chain and aromatic side chain (respectively) within the peptide chain. Carboxypeptidase is a terminal amino acid peptidase.
4. Aminopeptidases complete the breakdown of small peptide chains.

• Absorption of amino acids occurs by passive diffusion or sodium-dependent active transport mechanisms.
• Endogenous amino acids (c. 80 g/day) are also absorbed.
• If intact protein is absorbed, it can give rise to allergic reactions (produced in the body).

Functions of Proteins

• There is a constant turnover of protein in the body between synthesis and breakdown.
• Protein synthesis is an energy-demanding process; 4.2 kJ (1 kcal)/g of protein.
• Certain amino acids are used for the synthesis of other molecules with vital functions in their own right.

Amino Acid Derived Products