Water Requirements and Fluid Balance Chapter 8

Major Functions of Water Provides essential building material for cell protoplasm Protects key body tissues (spinal cord, brain) Maintains normal osmotic pressures Main constituent of blood Regulation of body temperature Important in hearing, vision, taste

Definitions Euhydration – normal body water content Hypohydration – body water deficit Dehydration – the dynamic loss of body water or the transition from euhydration to hypohydration

Daily Water Loss Urine – 1,300 ml Water in feces – 100 ml Exhaled air – 200 ml Skin (insensible perspiration) – 600 ml Sweat – 2-3 L/hr TOTAL – 2 - ??? liters

Daily Water Intake Approximate requirement – 1 ml/kcal Food – 700 ml Metabolism – 300 ml Fluids – everything else –Non-diuretic fluids –Diuretic fluids

Heat Production For every 1 liter O 2 consumed –4 kcal of heat dissipated –1 kcal of heat used to perform mechanical work, ie. contraction 1000 watt power output would increase core temp by 1 ° C every 4 minutes Body can only tolerate a rise of ° C before central fatigue occurs Above 40° C, heat exhaustion & stroke

Why Dehydration Impairs Performance Reduced blood volume Decreased skin blood flow Decreased sweat rate Decreased heat dissipation Increased core temp Increase in muscle glycogen use

Effects of Dehydration on Performance At 2% of body weight loss, aerobic performance declines 5-10%. At 5% of body weight loss, aerobic performance can decline up to 30%. At 2.5% of body weight loss, high intensity effort lasting several minutes can be impaired by 45%.

Fluid Replacement Recommedations oz of fluid 1 hour before exercise oz of fluid 30 min before exercise 6-8 oz every 15 minutes during exercise –Normal mouthful is ~ 1 oz 24 oz per pound lost after exercise (150% of weight loss)

What to Drink? Carbohydrate, electrolyte beverage is best immediately before and during exercise –6-8% CHO (glucose polymer/fructose) – mg Na per 8 oz –Increase palatability –Maintain thirst –Prevent hyponatremia –Increase water absorption –Decrease urine production Water is OK for normal re-hydration throughout the day

Heat Acclimatization Must exercise at >70% VO 2 max Significant changes occur in 7-10 days Benefits –Earlier onset of sweating –Sweat more –Larger sweat glands –Increase in total blood volume