Nutrition Basics: Fluid and Electrolyte Balance

Osmoregulation: The scientific term for the control of fluid balance and composition in the body.
Osmosis: The passive movement of water through a selectively permeable membrane from an area of high concentration to an area of lower concentration.
Fluid Compartments: Water and solutes are distributed inside cells (intracellular) and outside cells (extracellular).
Electrolytes: Substances that dissociate into charged ions when dissolved in water; cells control electrolyte movement to maintain water balance.

Transportation Vehicle: Acts as a universal solvent to transport nutrients and waste.
Medium for Chemical Reactions: Essential for enzyme formation and chemical processes.
Lubricant and Shock Absorber: Found in body fluids to protect joints and organs.
Temperature Regulator: Facilitates thermoregulation by storing heat and buffering the body against extreme temperature variations.

Adequate Intake (AI): $2.7\,L$ ( $11$ cups) for adult females and $3.7\,L$ ( $15.6$ cups) for adult males.
Daily Water Output: Averages $2.5\,L$ per day.
- Insensible Loss: Unaware loss through exhalation ( $400\,mL$ ) and skin ( $500\,mL$ ).
- Sensible Loss: Aware loss through urine ( $1,500\,mL$ ) and feces ( $100\,mL$ ).
Thirst Mechanism: Triggered by the hypothalamus in response to decreased fluid volume or increased sodium concentration.
Hormonal Regulation:
- Renin: Secreted by kidneys when blood volume is low.
- Antidiuretic Hormone (ADH): Released by the hypothalamus to decrease urinary water loss.
- Aldosterone: Released by adrenal glands to help reabsorb sodium and water.

Sodium:
- Function: Main regulator of fluid balance, nerve impulses, and nutrient absorption.
- Sources: $77\%$ comes from processed and prepared foods.
- Deficiency: Hyponatremia (nausea, muscle cramps, confusion).
- AI: $1,500\,mg/day$ for most adults; Upper Level (UL) is $2,300\,mg/day$ .
Chloride:
- Function: Aids fluid balance, maintains charge neutrality, and assists in fluid secretion (e.g., pancreatic juice and mucus).
- Clinical Note: Cystic fibrosis is caused by a mutation in chloride ion transport proteins.
Potassium:
- Function: Most abundant positively charged ion inside cells; essential for nerve impulses, muscle contraction, and acid-base balance.
- Deficiency: Hypokalemia (muscle weakness, respiratory distress).
- Toxicity: Hyperkalemia (affects heart function).

Dehydration: Water loss without adequate replacement; decreases blood volume and is a primary cause of heat stroke.
Heat Stroke: Life-threatening condition where body temperature exceeds $105.1^{\circ}F$ ( $40.6^{\circ}C$ ).
Hypertension: High blood pressure defined as $\ge 140/90\,mmHg$ ; a major risk factor for cardiovascular disease.
DASH Diet: "Dietary Approaches to Stop Hypertension"; emphasizes fruits, vegetables, and low-fat dairy to reduce sodium while increasing potassium, calcium, and magnesium.
Water Intoxication: Rare condition mainly affecting athletes, leading to hyponatremia when kidneys cannot excrete excess water (limit of $1\,L/hour$ ).

General Rule: Beverages satiate thirst but do not provide satiety (fullness); lower-calorie, nutrient-dense choices are preferred.
Alcohol: A psychoactive drug metabolized by the liver at a rate of one standard drink per hour.
- Excessive use causes Alcoholic Liver Disease (ALD) (fatty liver, hepatitis, cirrhosis) and micronutrient deficiencies (thiamine, folate, vitamin A).
Caffeine: A natural pesticide and psychoactive substance from xanthine.
- Intake Levels: Low-moderate ( $130\text{--}300\,mg/day$ ), High ( $\ge 400\,mg/day$ ).
- High doses can cause anxiety, shakiness, and insomnia, though coffee consumption may reduce risks for type 2 diabetes and Parkinson’s disease.