Total Body Water
Total body water is the fluid held in your cells and tissues — day-to-day scale swings are mostly water, not fat.
Plain English
Patient-Friendly Explanation
Total body water is all the fluid inside your cells, in the spaces between them and in your bloodstream — typically about 50 to 60 percent of body weight. It changes day to day with salt, carbohydrates, hormones and exercise, which is why the scale can swing by a kilo or two without any fat gain. Tracking trends over weeks, rather than single readings, gives the true picture.
Medical
Clinical Definition
Total body water (TBW) comprises intracellular and extracellular compartments and makes up roughly 50 to 60 percent of adult body mass, with the fraction lower in people with more adipose tissue because fat is relatively anhydrous. It is estimated in body-composition analysis by bioelectrical impedance analysis and measured precisely by isotope-dilution methods such as deuterium oxide. Acute changes in TBW — driven by sodium, glycogen, hormonal cycles and medications — can shift body weight by several kilograms and confound short-term weight-loss tracking, which is why clinical programmes assess trend rather than single weigh-ins.
Key Insight
Why It Matters
The scale cannot tell water from fat — and most of the 'I gained 2 kg overnight' moments are fluid, not failure. Knowing that helps patients stop reacting to daily noise and stay consistent with what actually moves fat.
Sources
Reviewed against standard medical references.
Medically reviewed by
Family Physician & Registered Acupuncturist, Accord Medical Clinic. MBBS (NUS), Graduate Diploma in Family Medicine (NUS), Graduate Diploma in Acupuncture (Singapore College of TCM).
Last reviewed: September 2026
This page is general health education, not medical advice. Whether any treatment is appropriate for you is a decision made in consultation after assessment.
