Ch 24 pt 2

Lipid Metabolism

Lipids undergo various metabolic pathways, with glycerol and fatty acids taking distinct routes:

• Glycerol Metabolism: Glycerol can enter glycolysis and be converted into glucose or can be used for triglyceride synthesis depending on the energy needs of the body.

• Fatty Acid Oxidation: Fatty acids are broken down in a process called beta-oxidation, which occurs in the mitochondria, producing acetyl CoA, which then enters the Krebs cycle for energy production or ketogenesis.

Protein Metabolism

Learning Objectives:

• Describe the process of protein digestion.

• Explain the urea cycle's vital role in nitrogen management within the body.

• Discuss how proteins can be utilized for energy.

• Differentiate between glucogenic and ketogenic amino acids.

Importance of Proteins:

Proteins are not only essential structural components, such as collagen in bones and keratin in hair and nails, but they also serve functional roles as enzymes (catalysts for biochemical reactions) and transporters (like hemoglobin for oxygen transport). They participate in numerous physiological processes, including:

• Growth and repair of tissues.

• Serving as a source of energy, especially during periods of fasting or intense exercise.

• Regulating the body's pH and fluid balance.

• Providing a framework for cellular structures.

• Enabling immune responses as antibodies.

Excess protein intake can lead to conversion into glucose (via gluconeogenesis) or triglycerides, as the body lacks specialized mechanisms for protein storage.

Protein Digestion Process:

1. Stomach: Protein digestion begins in the stomach, where proteins are denatured by pepsin and hydrochloric acid (pH 1.5-3.5). Pepsinogen, an inactive proenzyme, converts to pepsin in the acidic environment, breaking proteins into smaller polypeptides.

2. Small Intestine: The presence of chyme (partially digested food) in the small intestine stimulates the release of pancreatic enzymes (like trypsin and chymotrypsin) and digestive hormones (such as secretin and CCK). These enzymes further degrade proteins into free amino acids and small peptides, which are absorbed by intestinal mucosa and used for protein synthesis or converted into fats.

3. Enzyme Activation: Pancreatic enzymes are secreted as inactive proenzymes to prevent self-digestion. Enterokinase, an enzyme in the intestinal lining, activates trypsin from trypsinogen, which then activates other proenzymes like chymotrypsinogen to continue the breakdown of peptides.

Urea Cycle:

The urea cycle is a crucial biochemical process occurring mainly in the liver and, to a lesser extent, in the kidneys, where ammonium ions (toxic byproducts of amino acid metabolism) are converted into urea for excretion in urine. The process involves the condensation of ammonium ions with carbon dioxide, and several enzymatic steps are required for this conversion. Proper functioning of this cycle is essential for nitrogen management and preventing ammonium toxicity in the body.

Energy Production from Amino Acids:

During starvation or prolonged fasting, amino acids can enter pathways for energy production, contributing to the Krebs cycle (e.g., converting into pyruvate and acetyl CoA).

• Glucogenic Amino Acids: Amino acids like alanine, glycine, and serine can convert to glucose and are vital for gluconeogenesis, particularly when dietary carbohydrates are unavailable, ensuring a steady supply of energy.

• Ketogenic Amino Acids: Such as phenylalanine, leucine, and lysine can convert into ketone bodies, providing an alternative energy source during extended fasting or low carbohydrate intake.

Metabolic States

• Absorptive State: This state occurs shortly after eating when anabolism exceeds catabolism. Elevated blood sugar levels stimulate insulin release, facilitating glucose uptake by tissues. Excess glucose is converted into glycogen in the liver and muscles, or into fats in adipose tissues.

• Postabsorptive State: A fasting state where blood glucose levels begin to drop, prompting the body to rely on glycogen stores. Glucagon is released, promoting glycogen breakdown and gluconeogenesis to maintain essential glucose levels to the brain and other vital organs.

• Starvation: In prolonged fasting, the body depletes glycogen stores and begins to produce ketones for energy. Eventually, proteins are broken down for glucose synthesis as fat stores become limited, leading to muscle breakdown and other physiological changes.

Energy and Heat Balance

Thermoregulation:

The human body maintains a core temperature between 36.5-37.5 °C despite environmental fluctuations through complex feedback mechanisms that include:

• Mechanisms of Heat Exchange:

  • Conduction: Transfer of heat through direct contact (e.g., touching a warm object).

  • Convection: Loss of heat to surrounding air or water.

  • Radiation: Emission of infrared waves, leading to heat loss without direct contact.

  • Evaporation: Loss of heat through sweat evaporation, a critical cooling mechanism in hot conditions.

Metabolic Rate:

The Basal Metabolic Rate (BMR) is the rate of energy expenditure at rest, influenced by factors such as lean body mass, sex, age, genetics, and overall activity level. It reflects the energy required to maintain vital functions, including respiration, circulation, and cellular metabolism.

Nutrition and Diet

Importance of Diet:

A balanced diet is crucial for overall health, influencing the risk of various diseases such as obesity, diabetes, and cardiovascular conditions. Daily caloric requirements vary by individual and are influenced by:

• Age

• Gender

• Activity level

• Body composition

Guidelines for Healthy Eating:

The USDA’s MyPlate provides a framework for a balanced diet, suggesting appropriate proportions of food groups including:

• Fruits

• Vegetables

• Grains

• Proteins

• Dairy

Managing calorie intake is essential: excess calories can lead to weight gain, whereas a deficit can impair essential metabolic functions.

Vitamins and Minerals:

Vitamins:

These are organic compounds essential for various metabolic processes. They are classified into:

• Fat-soluble vitamins (Vitamins A, D, E, K) that are stored in fatty tissues and the liver.

• Water-soluble vitamins (such as B-complex vitamins and Vitamin C) which typically are not stored and must be consumed regularly.

Minerals:

Minerals are inorganic nutrients crucial for various physiological functions. They are categorized into:

• Major minerals (e.g., calcium, potassium, sodium) required in larger amounts.

• Trace minerals (e.g., iron, zinc) needed in small quantities for physiological functions like enzyme activity and hormone production.

Deficiencies in essential vitamins or minerals can lead to significant health issues, such as anemia (from iron deficiency) or growth retardation (from inadequate intake of essential nutrients).