Hydration levels are a concern for athletes and John Q. Public. In my office we use the Total Health System to accurately measure hydration levels. Dehydration has cognitive consequences, negatively affecting response time, coordination, tracking, short-term memory, attention, and mental focus (1). Feelings of fatigue take over faster and more recent research has even shown that dehydration increased brain activity related to painful stimuli (6).
Performance wise, dehydration can decrease strength by 2%, power by 3%, and high-intensity endurance by about 10% (1). Some of the reasons dehydration affects performance, especially for endurance activities combined with heat, include reduced plasma blood volume (leading to reduced stroke volume, increased heart rate), a decrease in blood flow to the skin (reducing the sweating response and heat dissipation), and an increase in core temperature (4,5).
So how much should an athlete consume to keep hydrated? The following chart lists recommendations from the American Dietetic Association, Dietitians of Canada, and the American College of Sports Medicine’s Fluid Replacement Guidelines (1):
If exercising less than an hour, water will generally meet hydration needs. For longer duration exercise, especially in the heat, a sports beverage will help replace the fluids and electrolytes lost.
Can water help as an intervention to weight loss? It can definitely help to reduce overall calorie consumption if it replaces high calorie beverages. It can also help by adding feelings of fullness, aiding in digestion, slightly increasing metabolism, or avenge feelings of thirst that can often be mistaken for hunger (9). More recently, a study aimed to discover if a pre-meal glass of water would be a successful intervention as part of a weight loss diet. The researchers found that the participants on a calorie restricted diet who drank +500 ml water before each meal lost more weight than those on the calorie restricted diet alone (10).
- Clark MA, Lucett SC. (2010). NASM’s Essentials of Sports Performance Training. Baltimore, MD: Lippincott Williams & Wilkins.
- National Research Council. (2005). Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. Washington, DC: The National Academies Press.
- Insel PM, Ross D, McMahon K, et al. (2011). Nutrition (4th edition). Sudbury, MA: Jones and Bartlett.
- Jeukendrup A, Gleeson M. (2010). Sport Nutrition: An Introduction to Energy Production and Performance (2nd Edition). Champaign, IL: Human Kinetics.
- Gonzalez-Alonso J, Mora-Rodriguez R, Below PR, Coyle EF. (1997). Dehydration markedly impairs cardiovascular function in hyperthermic endurance athletes during exercise. Journal of Applied Physiology, 82(4), 1229-1236.
- Ogino Y, Kakeda T, Nakamura K, Saito S. (2013). Dehydration enhances pain-evoked activation in the human brain compared with rehydration. Anesthesia and Analgesia. Advanced online publication. doi:10.1213/ANE.0b013e3182a9b028.
- Kenefick RW, Hazzard MP, Mahood NV, Castellani JW. (2004). Thirst sensations and AVP responses at rest and during exercise-cold exposure. Medicine and Science in Sports and Exercise, 36, 1528-1534.
- O’Brien C. (2003, December). Hydration in cold environments. RTO HFM Specialists’ Meeting on “Maintaining Hydration: Issues, Guidelines, and Delivery.” The RTO Human Factors and Medicine Panel (HFM) Specialists’ Meeting. NATO Science and Technology Organization, Boston, MA. RTO-MP-HFM-086, 9.1-9.8.
- Dennis EA, Dengo AL, Comber DL, Flack KD, Savla J, Davy KP, Davy BM. (2010). Water consumption increases weight loss during a hypocaloric diet intervention in middle-aged and older adults. Obesity, 18(2), 300-7.
- Boschmann, M. (2003) Water-induced thermogenesis. Journal of Clinical Endocrinology and Metabolism, 12(88), 6015-6019.