Macromolecules in Food

Overview of Major Macromolecules

Macromolecules are large, complex molecules essential for life, and they play critical roles in nutrition and physiology. The three main macromolecules found in food are:

• Carbohydrates: Organic compounds made of carbon, hydrogen, and oxygen, primarily serving as energy sources.

• Proteins: Polymers of amino acids that function in structure, transport, enzymes, and signaling.

• Lipids: Hydrophobic molecules including fats, oils, and cholesterol, important for energy storage and membrane structure.

Example: Bread contains carbohydrates (starch), cheese contains proteins (casein), and butter contains lipids (triglycerides).

Metabolism and the Mass Balance Model

Understanding Metabolism

Metabolism refers to all chemical reactions in the body that maintain life, including both the breakdown (catabolism) and synthesis (anabolism) of molecules. The mass balance model explains metabolism by accounting for the input, output, and storage of substances in the body.

• Mass Balance Model: The body maintains homeostasis by balancing intake, production, utilization, and excretion of substances.

• Equation:

Example: Glucose intake from food is balanced by its use for energy, storage as glycogen, or excretion if in excess.

Liver Function: Glycogenolysis and Glycogenesis

Role of the Liver in Carbohydrate Metabolism

The liver is central to regulating blood glucose through two key processes:

• Glycogenolysis: The breakdown of glycogen into glucose, increasing blood glucose levels. This is a catabolic process.

• Glycogenesis: The synthesis of glycogen from glucose, storing excess glucose. This is an anabolic process.

Example: After a meal, the liver performs glycogenesis; during fasting, it performs glycogenolysis.

Chemical Structure and Effects of Carbohydrates

Structure and Dietary Impact

• Chemical Structure: Carbohydrates are composed of monosaccharide units (e.g., glucose, fructose) with the general formula .

• Cellulose: A polysaccharide made of β(1→4) linked glucose units, forming rigid fibers.

• Effects of Cellulose Consumption: High cellulose intake increases dietary fiber, promoting bowel regularity but can cause bloating or gas because humans lack the enzyme to digest cellulose.

Example: Eating large amounts of leafy greens (high in cellulose) can lead to increased stool bulk.

3D Structures of Glycogen, Cellulose, and Starch

Comparison and Functional Implications

• Glycogen: Highly branched polymer of α(1→4) and α(1→6) linked glucose; allows rapid release of glucose.

• Starch: Mixture of amylose (linear α(1→4)) and amylopectin (branched α(1→6)); less branched than glycogen.

• Cellulose: Linear β(1→4) linked glucose; forms rigid, insoluble fibers.

Storage Function: Glycogen's branching allows quick mobilization of glucose (energy storage in animals). Starch is the main storage form in plants. Cellulose provides structural support in plant cell walls.

Enzymatic Catabolism: Glycogen is most rapidly catabolized due to its highly branched structure, providing many sites for enzyme action.

Lactose and Lactase: Digestion and Human Variation

Biochemistry and Health Implications

• Lactose: A disaccharide carbohydrate (glucose + galactose) found in milk.

• Lactase: An enzyme that hydrolyzes lactose into absorbable monosaccharides.

• Absorption: Lactose must be broken down by lactase before absorption in the small intestine.

• Lactase Persistence: The continued production of lactase into adulthood, a trait that varies among populations due to genetic adaptation to dairy consumption.

• Health Equity Issues: Populations with low lactase persistence may experience lactose intolerance, leading to dietary restrictions and potential nutritional disparities.

Example: Many people of East Asian descent have low lactase persistence, affecting their ability to digest dairy products.