Glossary
Hydration
Updated April 2, 2026
Hydration supports blood volume, temperature control, and performance. Use water intake goals as a baseline, then adjust for sweat rate and electrolytes. For the broader science, practical intake ranges, and beverage-specific hydration evidence, read The Complete Guide to Hydration. For marathon, triathlon, and ultra preparation, pair that with gut training so fluid intake and carbohydrate intake scale together.
Mechanism in plain terms
| System | Immediate role | Performance link |
|---|
| Blood plasma | Keeps transport and oxygen delivery stable | Better session output and less early fatigue |
| Body heat transfer | Helps sweating and skin blood flow | Better tolerance in warm sessions |
| Muscle function | Supports contraction timing and enzyme context | Lower cramp probability and steadier power |
| Gastrointestinal function | Maintains digestion pace | Better tolerance for food logging meals near training |
Imbalance signs and effects
| Pattern | Typical short-term signal | Likely performance effect |
|---|
| Early thirst with dark urine | Rising plasma concentration | Reduced focus, reduced session volume tolerance |
| Swollen hands with slow weight shifts | Possible excess intake with low output | Stomach heaviness, slower recovery feel |
| Persistent headache and dizziness | Electrolyte drift or severe hypohydration | Coordination changes and reduced effort quality |
Replacement timing
| Context | Rate guide | Why this window matters |
|---|
| Warm indoor or outdoor work | 400 to 800 mL per hour based on sweat output | Prevents steep temperature rise and early HR drift |
| Hard intervals | 150 to 250 mL every 15 to 20 minutes | Keeps intake practical, avoids stomach overload |
| Long resistance or endurance sessions | Combine water and sodium intake early in session | Preserves fluid retention and reduces later spikes |
Safe correction boundaries
| Goal | Method | Stop rule |
|---|
| Recover from mild deficit | Add fluid in small regular steps until urine lightens | Avoid huge boluses over short windows |
| Recover from exercise heat stress | Add both fluid and sodium, then reassess within 60 minutes | Stop if nausea or swelling rises |
| Correct recurrent low output | Use repeated 24-hour trend tracking with water intake goals | Escalate if dizziness, confusion, or repeated cramps persist |
Tie hydration logic to blood sugar control, electrolyte balance, and exercise recovery instead of using thirst alone, then use The Complete Guide to Hydration when you need the longer evidence-based playbook.