Human Nutrition and Digestion

Nutrition in Humans Overview

• Nutrition involves ingestion, digestion, absorption, assimilation, and egestion.
• Digestion breaks down large, insoluble food molecules into smaller, soluble, and diffusible ones for absorption.

Why Digestion is Important

• Food molecules vary in size; digestion is needed to make molecules small, soluble, and diffusible.
• Complex molecules like starch, proteins, and fats must be broken down into monosaccharides, amino acids, fatty acids, and glycerol.

Digestion Types

• Physical digestion involves breaking food into smaller pieces, increasing the surface area to volume ratio.
• Chewing, emulsification of fats, and stomach churning are examples of physical digestion.
• Chemical digestion involves using enzymes to break down food molecules into new substances.
• Salivary amylase breaks down starch to form maltose.

Human Digestive System

• The digestive system includes the alimentary canal and associated organs such as the pancreas and liver.
• The alimentary canal is a tube extending from the mouth to the anus.

Mouth (pH 7)

• Physical digestion in the mouth involves chewing, which reduces food size and increases surface area.
• Saliva from salivary glands softens food.
• Chemical digestion in the mouth: salivary amylase breaks down starch into maltose.

Oesophagus Function

• Swallowing enables food to pass from the pharynx into the oesophagus.
• The epiglottis closes the trachea during swallowing to prevent food from entering.

Oesophagus and Peristalsis

• The oesophagus is a muscular tube connecting the mouth to the stomach.
• Peristalsis, rhythmic muscle contractions, transports food through the oesophagus.

Stomach (pH 2)

• The stomach is a stretchable bag with muscular walls and gastric glands.
• Hydrochloric acid ensures an optimum pH for pepsin and kills pathogens.
• Physical digestion in the stomach: muscles churn food into chyme, increasing the surface area to volume ratio.
• Chemical digestion in the stomach: pepsin digests proteins into polypeptides.

Small Intestine

• The small intestine is composed of duodenum and ileum, and is about 6 metres long in humans.
• Digestion continues in the duodenum.
• Absorption of digested food molecules occurs mainly in the ileum.

Small Intestine - Duodenum

• The duodenum receives intestinal juice, pancreatic juice, and bile.
• Intestinal juice is secreted by the walls of the small intestine.
• Pancreatic juice is secreted by the pancreas.
• Bile is produced by the liver, stored in the gall bladder, and secreted into the duodenum via the bile duct.

Pancreas

• The pancreas is associated with the alimentary canal.
• The pancreatic duct connects the pancreas to the duodenum.
• The pancreas transports pancreatic juice containing enzymes into the duodenum.
• The pancreas produces insulin and glucagon.

Small Intestine - Digestion (pH 8-9)

• Intestinal lipase digests fats into fatty acids and glycerol.
• Maltase digests maltose to glucose.
• Sucrase digests sucrose into glucose and fructose.
• Intestinal protease (erepsin) digests polypeptides to amino acids.
• Pancreatic amylase digests starch into maltose.
• Pancreatic lipase digests fats into fatty acids and glycerol.
• Pancreatic protease (trypsin) digests proteins into polypeptides.

Digestion in Duodenum

• Bile emulsifies large fat globules into smaller ones, increasing the surface area for lipase.
• Emulsification is a physical process.

Small Intestine - Absorption in Ileum

• The ileum’s inner surface is folded extensively and has villi to increase surface area.
• Glucose, amino acids, fatty acids, glycerol, mineral salts, water and mineral salts are absorbed into the bloodstream via diffusion, osmosis and active transport.

Large Intestine

• At around 1.5 metres, the large intestine is shorter but wider than the small intestine.
• The colon, rectum, and anus make up the large intestine.
• The colon absorbs water and mineral salts from undigested material.
• Undigested materials are temporarily stored as faeces in the rectum and egested through the anus.

Assimilation

• Assimilation is the process by which absorbed nutrients are converted into new protoplasm or used to provide energy.

Absorbed Nutrients and the Liver

• Nutrients absorbed into blood capillaries form the hepatic portal vein.
• The hepatic portal vein transports mainly glucose and amino acids to the liver.

Carbohydrate Metabolism

• Glucose is a respiratory substrate for energy.
• Excess glucose is converted into glycogen and stored in the liver and muscle cells.

Protein Metabolism

• Amino acids are used to form protoplasm for growth and repair.
• Excess amino acids are deaminated in the liver to form urea.

Fat Metabolism

• Fats are used to form protoplasm like cell membranes.
• Excess fats are stored in adipose tissues beneath the skin and around vital organs.

Liver Functions

• Bile emulsifies fats.
• The liver deaminates excess amino acids, forming urea and glucose.
• Urea is removed from the body in urine.
• Glucose can be broken down by cells or converted to glycogen for storage.
• The liver breaks down hormones after they have exerted their effects.
• The liver detoxifies harmful substances, such as alcohol.

Effects of Alcohol Consumption

• Short-term alcohol effects include depression, reduced self-control, slurred speech, blurred vision, poor muscle coordination, and increased reaction time.
• Long-term alcohol effects include gastric ulcers, liver cirrhosis, brain damage, and social issues like addiction and violence.

Blood Glucose Regulation

• Insulin and glucagon regulate blood glucose levels.
• A hormone is a chemical substance produced by a gland and transported in the blood to alter the activity of a target organ.
• When blood glucose increases above normal, the pancreas produces insulin which is transported to the liver and muscles.
• Insulin increases cell membrane permeability to absorb glucose faster.
• Liver and muscle cells convert excess glucose to glycogen, decreasing blood glucose to normal.
• When blood glucose decreases below normal, the pancreas produces glucagon which is transported to the liver and muscles.
• Glucagon stimulates liver and muscle cells to convert glycogen to glucose, increasing blood glucose to normal.

Diabetes Mellitus

• Diabetes mellitus is a condition where the body cannot control blood glucose.
• Blood glucose levels can rise above normal and exceed the kidney's reabsorption capacity, leading to glucose excretion in urine.
• Type 1 diabetes is early-onset, where the pancreas cannot produce insulin.
• Type 2 diabetes is late-onset, where target cells do not respond well to insulin. Overweight individuals are at higher risk.
• Type 2 diabetes is characterized by persistently high blood glucose due to insulin resistance or insufficient insulin production.
• Risk factors for type 2 diabetes include obesity, sedentary lifestyle, blood lipid level, and age.
• Managing type 2 diabetes involves a low-calorie, high-fibre diet, increased physical activity, and maintaining a healthy weight.