What is the most abundant substance in the body?

• A. Water
• B. Blood
• C. Proteins
• D. Lipids

Answer: (A)

Water is the most abundant substance in the body, making up about 60% of an average adult’s body weight. This high water content sits in two main compartments: roughly two-thirds of body water is inside cells (intracellular fluid) and the remaining one-third is outside cells (extracellular fluid, including interstitial fluid and plasma). Water serves as the universal solvent and medium for nearly all biochemical reactions, supports transport of nutrients and wastes, aids temperature regulation, and helps cushion and lubricate tissues. While proteins and lipids are important components of tissues, their total mass is much smaller than the mass of body water, so water remains the largest constituent.