Hydration and wellness are married for life! Total Body Water (TBW) is the amount of water in your body, measured in liters or as a percentage of body weight, and is a key component of body composition scans (such as SECA mBCA or InBody). TBW includes both intracellular water (inside cells, mostly muscle) and extracellular water (outside cells, in blood plasma and tissues). In women, TBW is influenced by sex hormones, stress hormones, thyroid function, and day-to-day hydration status. Below is a clear explanation of how each hormone affects TBW and how much hydration at the time of testing can change the results.
We use the SECA mBCA (medical Body Composition Analyzer) scale, which provides detailed results including Total Body Water (TBW), Extracellular Water (ECW), and the ECW/TBW ratio. Here’s a clear explanation of what these mean and how to calculate Intracellular Water (ICW) and Extracellular Water (ECW) when you’re given the ECW/TBW percentage on your personalized reading.
Standard recommendation: Be well-hydrated but avoid excessive fluid intake 1–2 hours before the scan. Avoid testing immediately after heavy exercise, saunas, or alcohol, as these cause fluid shifts that are temporary and not reflective of overall health. SECA and similar devices are sensitive to hydration, so results can vary 1–3 kg (2–6 lbs) or 2–6% of TBW based on hydration alone.
Key Definitions
– **TBW** = Total Body Water = all water in your body (both inside and outside cells).
– **ECW** = Extracellular Water = water outside your cells (in blood plasma, interstitial spaces, etc.).
– **ICW** = Intracellular Water = water inside your cells (muscle cells, organ cells, etc.).
– **ECW/TBW ratio** = the percentage of your total body water that is extracellular (reported as a % on SECA reports, typically 38–42% is considered healthy in adults).
Simple Formula to Calculate ICW and ECW
The SECA scale gives you:
– TBW in liters (or kg, since 1 liter of water ≈ 1 kg)
– ECW/TBW ratio as a percentage (e.g., 40%)
Use these two steps:
- **ECW (Extracellular Water) = TBW × (ECW/TBW ratio ÷ 100)**
Example: If TBW = 35.0 liters and ECW/TBW = 40%,
ECW = 35.0 × (40 ÷ 100) = 35.0 × 0.40 = 14.0 liters
- **ICW (Intracellular Water) = TBW − ECW**
Using the same example:
ICW = 35.0 − 14.0 = 21.0 liters
Reference Example
| TBW (liters) | ECW/TBW Ratio (%) | ECW (liters) | ICW (liters) |
|————–|——————-|————–|————–|
| 35.0 | 40% | 14.0 | 21.0 |
| 30.0 | 38% | 11.4 | 18.6 |
| 40.0 | 42% | 16.8 | 23.2 |
| 28.5 | 45% | 12.8 | 15.7 |
What the ECW/TBW Ratio Tells Us
– Normal range in healthy adults: approximately 38–42% (slightly lower in muscular individuals, slightly higher in older adults or those with inflammation/edema).
– Higher than 42–43%: often indicates excess extracellular fluid (edema, inflammation, poor cell membrane health, or increased visceral fat/inflammation common in menopause).
– Lower than 38%: may reflect very high muscle mass or dehydration (less common).
Why This Matters in Menopause
Menopause can shift body water balance due to loss of estrogen (which supports cell membrane integrity and muscle mass), increased visceral fat, and sometimes mild inflammation or insulin resistance. A rising ECW/TBW ratio (e.g., moving from 39% to 44%) often signals early sarcopenic obesity or fluid shifts that contribute to midsection weight gain, fatigue, and metabolic changes. Tracking ICW (mostly muscle-related) and ECW helps us see if treatments (exercise, protein, hormone therapy, etc.) are preserving muscle and reducing unhealthy fluid retention.
We encourage tracking TBW, ICW, ECW, to help you achieve your overall body composition goals.
