Nutrition, Metabolism, and Energy Balance: Carbohydrates, Lipids, and Proteins in Human Physiology

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Nutrient: A chemical substance in food that the body needs for growth, maintenance, and repair.
Essential nutrient: A nutrient that must be obtained from the diet because the body cannot synthesize it in sufficient quantities.
Calorie: A unit of energy; specifically, the amount of energy needed to raise the temperature of 1 kilogram of water by 1°C. In nutrition, it refers to the energy content of food.

The five major nutrient categories are:
1. Carbohydrates
2. Lipids (fats)
3. Proteins
4. Vitamins
5. Minerals
The three macronutrients that make up most of what we eat are carbohydrates, lipids, and proteins.

Simple carbohydrates: Sugars such as glucose, fructose, and sucrose, found in fruits, milk, and table sugar.
Complex carbohydrates: Polysaccharides such as starches and fibers, found in foods like potatoes, rice, whole grain bread, and vegetables.
Examples of complex carbohydrate sources: potatoes, rice, whole grain bread.

Saturated fats: Fats with no double bonds between carbon atoms; solid at room temperature. Sources include meat, coconut, and some dairy products.
Unsaturated fats: Fats with one or more double bonds; usually liquid at room temperature. Sources include olive oil, seeds, and nuts.

Complete proteins: Proteins that contain all essential amino acids in sufficient quantities. Sources include eggs, fish, and milk.
Incomplete proteins: Proteins that lack one or more essential amino acids. Sources include most plant proteins such as corn.

Nitrogen balance: The balance between nitrogen intake and nitrogen excretion, reflecting protein metabolism.
Positive nitrogen balance: Occurs when nitrogen intake exceeds loss (e.g., during growth, pregnancy, tissue repair).
Negative nitrogen balance: Occurs when nitrogen loss exceeds intake (e.g., during starvation, illness, or severe stress).

Table: Major Uses of Carbohydrates, Lipids, and Proteins

Use in the body	Carbohydrates	Lipids	Proteins
Provide insulation and reserve body fuel		X
Important structural materials of the body (e.g., keratin and collagen)			X
Form the basic structure of the plasma membrane		X
Regulate body functions (e.g., enzymes and some hormones)			X
The major energy fuel (glucose) for forming ATP	X
Form the glycocalyx on the outside of the plasma membrane	X

Vitamins: Organic compounds required in small amounts for normal metabolism, usually functioning as coenzymes.
Fat-soluble vitamins: Vitamins A, D, E, and K; stored in body fat and can accumulate to toxic levels.
Water-soluble vitamins: Vitamin C and B-complex vitamins; not stored in the body and excess is excreted in urine.

Table: Fat-Soluble vs. Water-Soluble Vitamins

Excessive intake of fat-soluble vitamins can lead to toxicity, while water-soluble vitamins are less likely to accumulate to dangerous levels.

Vitamin K: Found in green vegetables and produced by intestinal bacteria; important for blood clotting.
Vitamin C: Deficiency causes scurvy; important for collagen synthesis and wound healing.
Vitamin D: Deficiency causes rickets in children and bone softening in adults; aids in calcium absorption.
Vitamin E: Important antioxidant; deficiency is rare.
Vitamin B12: Important for red blood cell maturation; deficiency can cause anemia.

Major minerals required in moderate amounts include sodium (Na), potassium (K), calcium (Ca), iron (Fe), and zinc (Zn).
Functions and sources:
- Zinc: Component of thyroid hormones.
- Calcium: Bone and tooth formation, blood clotting, nerve and muscle function.
- Phosphorus: Part of ATP, nucleic acids, and phospholipids.
- Iron: Component of hemoglobin and myoglobin.
- Sodium: Involved in water balance and nerve function.

Metabolism: The sum of all chemical reactions in the body.
Catabolism: The breakdown of complex molecules into simpler ones, releasing energy.
Anabolism: The synthesis of complex molecules from simpler ones, requiring energy.

Oxidation: The loss of electrons (or hydrogen atoms) from a molecule.
Reduction: The gain of electrons (or hydrogen atoms) by a molecule.
In redox reactions, oxidized substances lose energy, while reduced substances gain energy.
Example equation for cellular respiration:

Coenzymes such as NAD+ and FAD accept hydrogen atoms and electrons during redox reactions, facilitating energy transfer.

Table: Substrate-Level vs. Oxidative Phosphorylation

Characteristic	Substrate-level phosphorylation	Oxidative phosphorylation
Occurs only in the mitochondrial matrix		X
Requires ATP synthase		X
Involves the creation of a proton gradient		X
Occurs in the cytosol and mitochondrial matrix	X
Involves the transfer of a phosphate group directly from a substrate to ADP	X

Cellular respiration involves glycolysis, the citric acid (Krebs) cycle, and oxidative phosphorylation.
Glycolysis occurs in the cytosol and produces pyruvate, ATP, and NADH.
The citric acid cycle occurs in the mitochondrial matrix and generates NADH, FADH2, and ATP.
Oxidative phosphorylation occurs in the inner mitochondrial membrane and produces the majority of ATP.

Mitochondria have an outer membrane, inner membrane, intermembrane space, and matrix.
Oxidative phosphorylation takes place in the inner mitochondrial membrane.

Glycogenesis: The formation of glycogen from glucose subunits; occurs when glucose supplies exceed demand.
Glycogenolysis: The breakdown of glycogen to glucose subunits; occurs when glucose is needed for ATP production.
Gluconeogenesis: The formation of glucose from noncarbohydrate sources, such as amino acids; occurs when dietary sources and glycogen reserves are depleted.