Eight FAQs About Hydration

Water is the most essential nutrient for human life. It is possible to go up to 30 days without food, but only a few days without water. Water is present in every cell in the body and is involved in almost every biological process including temperature regulation, digestion, absorption, circulation, and excretion. Our bodies are composed of 50 to 70 percent water and the brain is up to 85 percent water.

Despite the obvious importance of water for well being and performance, there are a lot of misconceptions when it comes to hydration. This article will address some of the most common questions to help you achieve optimal hydration and better performance.

Question #1: Will drinking eight glasses of water a day keep me hydrated?

Answer: Not necessarily. This recommendation was first put forth by a 1945 paper that recommended eating 1 ml of water for every calorie consumed. Since most people are recommended to eat 2,000 calories a day, this adds up to 2,000 ml or roughly 64 ounces, which translates to eight 8-ounce glasses. In 2002, a research review published in the American Journal of Physiology found that there was zero scientific evidence to support the 8 glasses of water a day recommendation. So, how much should you drink?

Total daily fluid intake recommendations from the Institute of Medicine for men in the general population are 3.7 liters per day of which 3 liters should be consumed as beverages (the rest can come from food such as fruits and vegetables) and 2.7 liters for women of which 2.2 liters should be beverages. People who are exercising intensely in the heat may need more.

Question #2: Is thirst is the best guide for maintaining hydration during exercise?

Answer: Although it does a fine job at keeping you hydrated during normal, non-exercising conditions, the thirst mechanism is not capable of keeping up with water losses during heavy sweating. When drinking ad libitum, athletes will only drink sufficient water to replace about 60 percent of the water lost during exercise. This makes a strategic hydration plan essential for athletes with a high sweat rate or who are training intensely in the heat.

Question #3:How does the body regulate fluid levels?

Answer: The body has an intricate system for maintaining water balance, which is all centered around the kidneys. The majority of water in your body is in the blood. The brain has specialized sensors that detect changes in blood pressure, sending a message to the kidneys to either conserve water or eliminate water in urine to maintain water balance.

As with most things in the body, this system is mediated by hormones. The hormone vasopressin (also known as anti-diuretic hormone or ADH) is released by the posterior pituitary to tell the kidneys to conserve water and reabsorb sodium. A second, more complex hormonal cascade called the RAAS system leads to the release of aldosterone from the adrenal glands, which also tells the kidneys to hold onto sodium and water. Together these systems restore water balance during dehydration. They work on negative feedback loops so that as water levels increase in the blood, raising plasma volume, release of aldosterone and vasopressin is shut down and homeostasis is achieved.

Question #4: Are electrolytes necessary for hydration?

Answer: To maintain hydration, the body requires water and sufficient levels of electrolytes: Sodium, potassium, and chloride. Electrolytes help the body regulate how much water a cell can hold. Sodium is an electrolyte that controls how much water is outside the cells, whereas potassium controls how much water is inside cells. Consuming excessive water without electrolytes can dilute the sodium concentration, leading cells to take on too much water and swell. This actually cause dehydration by diluting the body’s levels of electrolytes.

In recreational athletes who eat a standard American diet that is high in sodium, extra electrolytes probably aren’t necessary, but for athletes sweating profusely and consuming large volumes of water, electrolyte supplementation will improve cellular hydration. They also reduce urine output from the kidneys, which helps the body gain water quickly when dehydrated.

Question #5:Does dehydrating yourself prior to an endurance competition make you lighter and faster?

Answer: This is not a good approach. Mental and physical performance plunge when you pass 2 percent dehydration, so any advantage from a decrease in body weight would be offset by a decrease in power and mental strength. Additionally, it can put you at risk of more severe fluid loss since dehydration hinders judgement and the thirst mechanism is impaired during exercise. Loss of 3 to 5 percent body weight causes cardiovascular strain and heat exhaustion. At loss of 7 percent body weight, collapse is likely, and fluid loss of greater than 10 percent is likely to result in death.

Question #6: Is it better to drink a lot all at once or sip fluids little by little?

Answer: It might seem like small sips over the course of the workout would provide for steady hydration, however, research shows that consuming larger volumes of fluid at a time will accelerate gastric emptying, or the rate at which fluids move through the stomach and into the small intestine. This translates into faster, more efficient delivery of nutrients, sodium, and fluids to your cells.

One review found that when athletes consumed 20 ounces of fluid at a time, the fluid moved faster through the stomach than when they ingested 13.5 ounces. The same was found for 13.5 ounces over 7 ounces. The increased pressure in the stomach from more liquid signals to the body that it’s time to get digestion moving. Interestingly, the effect caps out at 30 ounces, with subjects experiencing a decrease in absorption rate when they chugged volumes above this threshold.

Question #7: Sports drinks: Good or bad?

Answer: The problem with sports drinks is that they provide liquid carbohydrates—a delivery form of sugar that the body readily deposits as visceral belly fat when excess calories are present. In this sense, sports drinks are the equivalent of drinking soda. For anyone trying to lose body fat or who is suffering from metabolic disorders like diabetes, pre-diabetes, or inflammation, plain filtered water should be your workout drink of choice.

For intense long-duration efforts lasing more than 90 minutes, sports drinks provide carbs that the body can use for energy and may improve GI absorption and cellular hydration. This pays off in terms of increased endurance performance when using sports drinks. One large review found the greatest benefit of a 6.5 percent increase in performance from taking 0.9 g/kg/hour of carbs. For high-intensity sports, a carb mouth rinse can be used. For high-intensity efforts longer than an hour, try a dose of 60 g/hour.

Question #8: How can I get enough fluids during training?

Answer: The simplest way to assess hydration status is to record body weight before and after training. Sweaty clothes should be removed for accuracy because sweat trapped in clothing will increase weight recordings. Each pound (0.45 kg) lost equals 1 pint (0.5 L) of fluid loss. A body weight log can identify both acute hydration needs after single training sessions as well as chronic dehydration, such as the progressive loss of 5 to 10 pounds over the course of a week.

Another tool is to calculate your Sweat Rate. This is a calculation based on body weight and fluids in/out over the exercise time period. It requires you to weigh the athlete before and after exercise, and account for all fluid intake and urine output during training:

A	Date	June 1
B	Body Weight Before Exercise	61.7 kg
C	Body Weight After Exercise	61.0 kg
D	Change in Body Weight (B-C)	0.7 kg = 700 g
E	Drink Volume During Exercise	420 ml
F	Urine Volume	90 ml
G	Sweat Loss (D+E-F)	1,030 ml
H	Exercise Time	90 min
I	Sweat Rate Per Minute (G/H)	11.4 ml/min
J	Sweat Rate Per Hour (I X 60)	684 ml

This calculation tells us that you should consume a bit over half a liter per hour to keep up with sweat rate and maintain a good hydration status.

Final Words: Hopefully, this article clears up some of the most pervasive misconceptions regarding fluid replacement and gives you practical tools to ensure optimal hydration and peak performance.