Fundamentals of Nutrition and Metabolism

Carbohydrate Metabolism

Carbohydrates, such as sugars and starches, contain chemical bonds that power cellular processes. During digestion, complex carbohydrates are broken down into absorbable monosaccharides.

The body manages glucose levels through three distinct metabolic pathways:

1. Glycogenesis: When glucose levels are high, the body converts excess glucose into glycogen for storage in the liver and muscle cells.
2. Lipogenesis: If glycogen stores become full and glucose remains in excess, the body converts that glucose into fats, which are then stored in adipose tissue.
3. Gluconeogenesis: When glucose is scarce, the body can synthesize new glucose from noncarbohydrate sources, such as glycerol and amino acids.

Lipid Metabolism and Transport

Lipids (fats, oils, phospholipids, and cholesterol) are highly efficient energy sources, providing more than twice the chemical energy per gram compared to carbohydrates or proteins.

Saturated vs. Unsaturated Fats

The structure of a fatty acid determines its health impact:

• Saturated fats: These have no double bonds between carbon atoms. They are typically found in animal products (meat, eggs, milk) and some plant oils (palm, coconut) and are associated with an increased risk of cardiovascular disease.
• Unsaturated fats: These contain one or more double bonds. Found in nuts, seeds, and plant oils, they are generally considered healthier, with monounsaturated fats being identified as the healthiest option.

Lipoprotein Transport

Because lipids are not water-soluble, they must be transported through the bloodstream via lipoproteins, which combine lipids with proteins. They are classified by their density (as the proportion of lipid decreases, the density increases):

• VLDL (Very Low-Density Lipoprotein): High in triglycerides.
• LDL (Low-Density Lipoprotein): High in cholesterol.
• HDL (High-Density Lipoprotein): High in protein.

Protein Metabolism

Proteins are chains of amino acids used to build essential structures like enzymes, hormones, and antibodies.

Quality of Proteins

The "quality" of a protein is determined by its amino acid profile:

• Complete proteins: These provide adequate amounts of all essential amino acids. Examples include meat, milk, and eggs.
• Incomplete proteins: These lack sufficient amounts of one or more essential amino acids. Many plant proteins fall into this category, but they can be combined (e.g., beans and rice) to provide a complete amino acid profile.

Energy Utilization

While proteins are primarily structural, they can be used for energy if necessary. To do this, the amino groups must be removed through a process called deamination, which results in the formation of urea that is subsequently excreted in urine.

Energy Balance and Malnutrition

Metabolic health is largely determined by the relationship between caloric intake and caloric expenditure.

Energy Balance States

• Energy Balance: Caloric intake equals caloric output (Basal Metabolic Rate + muscular activity), resulting in a constant body weight.
• Positive Energy Balance: Caloric intake exceeds expenditure. The body stores the excess nutrients, leading to weight gain.
• Negative Energy Balance: Caloric expenditure exceeds intake. The body breaks down tissues for energy, leading to weight loss.

Malnutrition

Malnutrition refers to poor nutrition caused by a lack of essential nutrients or the body's inability to use them. It is categorized into two types:

1. Undernutrition: Caused by a deficiency of essential nutrients.
2. Overnutrition: Caused by an excess of nutrient intake.

Malnutrition can be primary (resulting solely from the diet) or secondary (where the diet is adequate, but an individual's physiological condition, such as a deficiency in bile salts, prevents nutrient absorption).