Digestion and Absorption, BMR, Nutritional Disorders

Digestion and Absorption

Digestion is the process by which complex food substances are broken down into simpler, soluble forms that can be absorbed into the bloodstream. This process involves both mechanical and chemical mechanisms. Absorption is the movement of these digested nutrients from the digestive tract into the circulatory or lymphatic systems.

Mechanical Digestion

Mechanical digestion begins in the mouth with mastication (chewing), which breaks down food into smaller pieces, increasing the surface area for enzyme action. The tongue aids in mixing food with saliva and forming a bolus for swallowing. In the stomach, churning action mixes food with gastric juices. Peristalsis, the rhythmic contraction and relaxation of muscles in the digestive tract, propels food along.

Chemical Digestion

Chemical digestion involves the enzymatic breakdown of food molecules. This process starts in the mouth with salivary amylase, which begins the digestion of carbohydrates. In the stomach, hydrochloric acid (HCl) denatures proteins and activates pepsinogen into pepsin, an enzyme that breaks down proteins into smaller polypeptides. The small intestine is the primary site for chemical digestion and absorption. Here, secretions from the pancreas, liver, and the intestinal wall work together.

Digestive Juices and Enzymes

Salivary Glands

Saliva contains salivary amylase (ptyalin) and lingual lipase. Salivary amylase breaks down starch into maltose. Lingual lipase is activated in the acidic environment of the stomach and digests fats.

Gastric Juice (Stomach)

The stomach secretes gastric juice, which contains:

• Hydrochloric Acid (HCl): Kills bacteria, denatures proteins, and activates pepsinogen.
• Pepsinogen: Activated by HCl to pepsin, which breaks down proteins into proteoses and peptones.
• Mucus: Protects the stomach lining from the acidic environment.
• Intrinsic Factor: Essential for the absorption of vitamin B12.

Pancreatic Juice

The pancreas secretes pancreatic juice into the duodenum. It contains:

• Amylase: Continues carbohydrate digestion, breaking down starch into maltose, sucrose, and lactose.
• Trypsinogen: Activated by enterokinase (secreted by the intestinal wall) into trypsin. Trypsin further breaks down polypeptides and can activate other proteases.
• Chymotrypsinogen: Activated by trypsin into chymotrypsin, which digests proteins.
• Carboxypeptidase: Digests peptides, releasing amino acids.
• Lipase: Digests fats (triglycerides) into fatty acids and glycerol.
• Nucleases: Digest nucleic acids (DNA and RNA).

Bile

Bile, produced by the liver and stored in the gallbladder, is released into the small intestine. Bile salts emulsify fats, breaking large fat globules into smaller droplets, increasing the surface area for lipase action. Bile does not contain digestive enzymes but is crucial for fat digestion and absorption.

Intestinal Juice (Succus Entericus)

The intestinal wall secretes intestinal juice, which contains:

• Disaccharidases (Maltase, Sucrase, Lactase): Break down disaccharides into monosaccharides (glucose, fructose, galactose).
• Peptidases (Aminopeptidase, Dipeptidase): Break down small peptides into amino acids.
• Lipase: Aids in fat digestion.
• Enterokinase: Activates trypsinogen.

Absorption of Nutrients

Absorption primarily occurs in the small intestine, specifically the jejunum and ileum. The small intestine has a large surface area due to folds (plicae circulares), villi, and microvilli, which are finger-like projections of the epithelial cells.

• Carbohydrates: Absorbed as monosaccharides (glucose, fructose, galactose) through active transport or facilitated diffusion into the capillaries of villi.
• Proteins: Absorbed as amino acids, dipeptides, and tripeptides through active transport into the capillaries of villi.
• Fats: Digested fats (fatty acids and glycerol) are absorbed into the epithelial cells, where they are re-synthesized into triglycerides and packaged into chylomicrons. These enter the lacteals (lymphatic vessels) of the villi.
• Water and Electrolytes: Absorbed by osmosis and active transport throughout the small intestine.
• Vitamins: Fat-soluble vitamins (A, D, E, K) are absorbed along with fats. Water-soluble vitamins (B vitamins, C) are absorbed by diffusion or active transport. Vitamin B12 requires intrinsic factor for absorption in the ileum.

The large intestine absorbs most of the remaining water and electrolytes, forming feces.

Basal Metabolic Rate (BMR)

Basal Metabolic Rate (BMR) is the minimum amount of energy (calories) the body needs to maintain basic life functions at rest. These functions include breathing, circulation, cell production, nutrient processing, protein synthesis, and ion transport. BMR represents the largest portion of a person's total daily energy expenditure.

Factors Affecting BMR

Several factors influence an individual's BMR:

• Body Size and Composition: Larger individuals generally have a higher BMR. Muscle tissue is more metabolically active than fat tissue, so individuals with more muscle mass have a higher BMR.
• Age: BMR is highest in infancy and childhood when growth is rapid. It declines gradually with age, primarily due to a decrease in muscle mass.
• Sex: Men typically have a higher BMR than women due to generally larger body size and higher muscle mass.
• Hormones: Thyroid hormones (thyroxine and triiodothyronine) play a significant role in regulating BMR. Hyperthyroidism increases BMR, while hypothyroidism decreases it. Other hormones like growth hormone and adrenaline can also influence BMR.
• Body Temperature: Fever increases BMR. For every degree Celsius increase in body temperature, BMR increases by about 13%.
• Environmental Temperature: Exposure to extreme cold or heat can increase BMR as the body expends energy to maintain its core temperature.
• Nutritional Status: Prolonged fasting or starvation can lower BMR as the body conserves energy.
• Sleep: BMR is slightly lower during sleep.
• Physical Activity: While BMR is measured at rest, regular physical activity can increase muscle mass, thereby increasing resting BMR over time.

Measurement of BMR

BMR is typically measured under strict conditions:

• The person must be awake but resting completely.
• The measurement should be taken in a comfortably warm room (around 20-25°C).
• The person should have fasted for at least 12 hours (usually overnight).
• No strenuous physical activity should have been performed for at least an hour before measurement.

BMR is measured using indirect calorimetry, where oxygen consumption and carbon dioxide production are monitored. The amount of oxygen consumed is directly proportional to the metabolic rate.

Relationship to Total Energy Expenditure

Total Daily Energy Expenditure (TDEE) is the total number of calories a person burns in a day. It is composed of:

• Basal Metabolic Rate (BMR): Typically 60-75% of TDEE.
• Thermic Effect of Food (TEF): The energy used to digest, absorb, and metabolize food; accounts for about 10% of TDEE.
• Physical Activity Energy Expenditure (PAEE): The energy burned during exercise and other physical activities; this is the most variable component, ranging from 15-30% of TDEE.

Nutritional Disorders

Nutritional disorders, also known as malnutrition, occur when a person's diet does not provide adequate or excessive amounts of nutrients, leading to adverse health effects. These disorders can arise from deficiencies, excesses, or imbalances of macronutrients (carbohydrates, proteins, fats) and micronutrients (vitamins, minerals).

Protein-Energy Malnutrition (PEM)

PEM is a severe form of malnutrition characterized by a deficiency of protein and/or energy. It is common in developing countries, particularly among infants and young children.

Kwashiorkor

Kwashiorkor is a form of PEM caused primarily by a severe deficiency of protein, even if sufficient calories are consumed. It typically occurs when a child is weaned from breast milk to a diet high in carbohydrates but low in protein.

• Symptoms: Edema (swelling, especially in the legs and abdomen), stunted growth, apathy, irritability, skin changes (darkened, dry, peeling skin), hair changes (thin, brittle, reddish-brown), fatty liver, increased susceptibility to infections.
• Cause: Insufficient protein intake leading to a decrease in plasma proteins (like albumin), which causes fluid to leak from blood vessels into tissues.

Marasmus

Marasmus is another form of PEM resulting from a severe deficiency of both protein and calories. It occurs when an infant or child is deprived of food for an extended period.

• Symptoms: Severe emaciation (wasting of muscle and subcutaneous fat), wrinkled skin, "old man" appearance, normal or near-normal plasma protein levels (no edema), severe growth retardation, alertness may be preserved but the child is irritable.
• Cause: Insufficient intake of both protein and calories, leading to the body breaking down its own tissues for energy.

Vitamin Deficiencies

Deficiencies in essential vitamins can lead to specific diseases.

Vitamin A Deficiency

• Disease: Xerophthalmia (night blindness, dry eyes, corneal ulcers, blindness).
• Function: Essential for vision, immune function, and cell growth.
• Sources: Liver, eggs, dairy products, orange/yellow fruits and vegetables (carotenoids).

Vitamin B1 (Thiamine) Deficiency

• Disease: Beriberi (neurological symptoms like confusion, difficulty walking, paralysis; cardiovascular symptoms like edema, heart failure).
• Function: Crucial for carbohydrate metabolism and nerve function.
• Sources: Whole grains, pork, legumes.

Vitamin B3 (Niacin) Deficiency

• Disease: Pellagra ("the 3 Ds": Dermatitis, Diarrhea, Dementia).
• Function: Involved in energy metabolism.
• Sources: Meat, poultry, fish, whole grains, legumes.

Vitamin B12 Deficiency

• Disease: Pernicious anemia (megaloblastic anemia), neurological damage (tingling, numbness, memory loss).
• Function: Essential for DNA synthesis and nerve function.
• Sources: Animal products (meat, fish, eggs, dairy).

Vitamin C Deficiency

• Disease: Scurvy (bleeding gums, easy bruising, poor wound healing, fatigue).
• Function: Antioxidant, collagen synthesis, immune function.
• Sources: Citrus fruits, berries, peppers, leafy greens.

Vitamin D Deficiency

• Disease: Rickets (in children, soft bones, bowed legs), Osteomalacia (in adults, bone pain, muscle weakness).
• Function: Calcium and phosphorus absorption, bone health.
• Sources: Sunlight exposure, fatty fish, fortified dairy products.

Mineral Deficiencies

Iron Deficiency

• Disease: Iron-deficiency anemia (fatigue, weakness, pale skin, shortness of breath).
• Function: Component of hemoglobin, crucial for oxygen transport.
• Sources: Red meat, poultry, fish, beans, spinach.

Iodine Deficiency

• Disease: Goiter (enlargement of the thyroid gland), hypothyroidism, cretinism (in children born to deficient mothers, severe mental and physical retardation).
• Function: Essential for thyroid hormone production.
• Sources: Iodized salt, seafood, dairy products.

Calcium Deficiency

• Disease: Osteoporosis (weak, brittle bones), muscle cramps.
• Function: Bone health, muscle function, nerve signaling.
• Sources: Dairy products, leafy greens, fortified foods.

Overnutrition and Obesity

Overnutrition occurs when an excessive intake of calories, particularly from fats and sugars, leads to weight gain and obesity. Obesity is a complex disease characterized by an excessive accumulation of body fat, increasing the risk of numerous health problems, including:

• Heart disease and stroke
• Type 2 diabetes
• Certain types of cancer
• Osteoarthritis
• Sleep apnea
• High blood pressure

Obesity results from a chronic energy imbalance, where calorie intake consistently exceeds calorie expenditure.

Dietary Guidelines and Prevention

Preventing nutritional disorders involves consuming a balanced diet that provides all essential nutrients in appropriate amounts. Key dietary principles include:

• Eating a variety of foods from all food groups.
• Consuming plenty of fruits, vegetables, and whole grains.
• Choosing lean protein sources.
• Limiting intake of saturated and trans fats, added sugars, and sodium.
• Ensuring adequate hydration.
• For specific populations (e.g., pregnant women, children, elderly), tailored nutritional advice is essential.