Nutrition and Metabolism Fundamentals

Hormonal Regulation of Hunger and Satiety

The brain is constantly monitoring the body's energy status to regulate appetite. This regulation occurs primarily in the hypothalamus, which contains specialized control centers for hunger and satiety. This process is managed through a sophisticated hormonal feedback loop involving several key players:

Exam Tip: If you are asked which hormone stimulates appetite, the answer is Ghrelin. All other major hormones mentioned (Leptin, Insulin, CCK, and PYY) act to suppress it.

The Building Blocks: Macronutrients and Micronutrients

Nutrients are broadly categorized based on the quantity the body requires. Macronutrients include carbohydrates, lipids, proteins, and water. Micronutrients consist of vitamins and minerals, which the body needs in smaller amounts.

Furthermore, we distinguish between essential nutrients, which must be obtained through the diet because the body cannot make them, and nonessential nutrients, which the body can synthesize on its own.

Carbohydrates: The Primary Energy Source

Carbohydrates are the body's preferred source of energy. All ingested carbohydrates are eventually converted into glucose. They exist in three main forms:

1. Monosaccharides: Simple sugars like glucose, fructose, and galactose that are absorbed directly into the bloodstream.
2. Disaccharides: Simple sugars like sucrose, lactose, and maltose that must be broken down into monosaccharides during digestion.
3. Polysaccharides: Complex carbohydrates or starches found in grains, legumes, and vegetables.

Dietary Fiber is a unique type of polysaccharide that resists digestion in the small intestine. It is vital for health because it absorbs water, adding bulk to the stool, which stretches the colon and promotes peristalsis (the movement of waste through the intestines). Additionally, gut bacteria ferment fiber to produce short-chain fatty acids (SCFAs), which nourish the intestinal lining and help prevent metabolic and immune disorders.

Lipids: Energy Reservoirs

Lipids (fats) serve as the body's primary reservoir for excess energy. They also provide cellular structure (as phospholipids and cholesterol), insulate the body, and enable the absorption of fat-soluble vitamins (A, D, E, and K).

• Saturated Fats: Typically solid at room temperature and often derived from animal sources or dairy.
• Unsaturated Fats: Liquid at room temperature, commonly found in nuts, seeds, and vegetable oils.
• Trans Fats: Unhealthy fats created by adding hydrogen to liquid oils; these can raise cholesterol and increase the risk of heart disease.
• Essential Fatty Acids: Fats that must be eaten, such as linolenic acid.

Proteins: Structure and Repair

Proteins are composed of amino acids and are primarily used to build and repair tissues, such as bone, skin, hair, and muscles. They also form vital substances like antibodies, hormones, and hemoglobin.

Amino acids are divided into two groups:

• Essential Amino Acids: Must be obtained through diet (e.g., histidine, lysine, leucine).
• Nonessential Amino Acids: Can be synthesized by the body (e.g., glutamine, tyrosine).

Proteins are categorized by their amino acid profile: Complete proteins (mostly from animal sources like meat, eggs, and soy) provide all essential amino acids, while incomplete proteins (mostly from plants like nuts and grains) lack one or more.

Vitamins and Minerals: The Micronutrients

While needed in smaller quantities, micronutrients are essential for physiological function.

• Vitamins: These are split into two categories based on how they are processed. Water-soluble vitamins are absorbed with water and excreted by the kidneys, meaning they must be consumed daily. Fat-soluble vitamins are absorbed with dietary fat and stored in the liver and tissues; because they are stored, excessive intake (mega-dosing) can lead to toxicity.
• Minerals: These are vital for numerous functions, with calcium and phosphorus being the most abundant in the human body.