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Water Requirements and Fluid Balance

This chapter covers the major functions of water, daily water loss and intake, heat production, the effects of dehydration on performance, fluid replacement recommendations, and the benefits of heat acclimatization.

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Water Requirements and Fluid Balance

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  1. Water Requirements and Fluid Balance Chapter 8

  2. 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

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

  4. 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

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

  6. Heat Production • For every 1 liter O2 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 2-2.5 ° C before central fatigue occurs • Above 40° C, heat exhaustion & stroke

  7. 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

  8. 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%.

  9. Fluid Replacement Recommedations • 12-16 oz of fluid 1 hour before exercise • 12-16 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)

  10. What to Drink? • Carbohydrate, electrolyte beverage is best immediately before and during exercise • 6-8% CHO (glucose polymer/fructose) • 50-100 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

  11. Heat Acclimatization • Must exercise at >70% VO2max • Significant changes occur in 7-10 days • Benefits • Earlier onset of sweating • Sweat more • Larger sweat glands • Increase in total blood volume

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