Understanding Human Nutrition, Metabolism, and Energy Balance

Understanding Metabolism and Nutrition

Life depends on chemical reactions collectively known as metabolism. This process includes anabolism (the formation of tissues like muscle, bone, and blood) and catabolism (the breakdown of substances). Food provides the fuel for these reactions, which occur even at rest to support respiration, heartbeat, and circulation.

The Digestive Process

• Food: Must be digested in the gut and converted into a form that can be absorbed and utilized for metabolic chemical reactions.

Energy and Caloric Value

A calorie is the amount of heat needed to raise the temperature of one gram of water by one degree Celsius. A kilocalorie (1000 calories) is used to express the caloric value of foods, which consist of carbohydrates, lipids, proteins, vitamins, minerals, and water. Caloric value varies based on the ratios of fat, protein, and carbohydrates.

Feeding vs. Nutrition

• Feeding: The conscious and voluntary act of providing and ingesting food. Food quality depends on cultural and economic factors.
• Nutrition: A set of physiological processes where the organism receives, transforms, and uses chemical substances contained in food, including digestion, absorption, and transport of nutrients.

Metabolic Rates

• Metabolism: The set of chemical reactions occurring in the cells of a living organism.
• Metabolic Rate (TM): The speed at which the breakdown process occurs, measured in calories or kilocalories.
• Basal Metabolic Rate (BMR): The minimum energy required to stay alive, depending on body mass, fat-to-lean tissue ratio, and skin type.

Note: Eating is a conscious and voluntary act, whereas nutrient absorption (lipids, proteins, etc.) is an unconscious and involuntary process.

Body Mass Index (BMI)

The BMI is calculated as: Weight (kg) / Height (m)²

• Underweight: Less than 20
• Normal: 20–24.9
• Overweight: 25–27.8
• Obesity: More than 27.8

Liver Function and Glucose Regulation

Consumed carbohydrates are converted into glucose and absorbed. Glucose provides energy for the brain, muscles, and liver. Excess energy is stored as muscle and liver glycogen.

Hormonal Regulation of Glucose

• Glucagon: Increases blood glucose levels.
• Insulin: Decreases blood glucose levels.
• Adrenaline: Influences glucose mobilization.