Maintaining Adequate Hydration

Looking around one might think that there is an epidemic of dehydration plaguing Americans, who tote drinks with them to all locales--cars and strollers armed with cups holders, hydration stations every mile of a running race, exercise belts studded with water bottles marketed to the recreational runner. It seems we might all shrivel up and dehydrate without this. 

We all know that drinking sugary soda is bad for us and has helped lead to the current obesity epidemic.  We have been led to believe that incessant water drinking is healthy, but how much and what does one really need to drink to feel good and prevent problems?

As summer approaches in Hotlanta, my home, I’ve become interested in learning more of the specifics of what constitutes healthy fluid consumption. In my practice I see patients both who over-hydrate and those who under-hydrate, each of which can contribute to medical morbidity.

Humans and other animals are adapted to have finely tuned physiologic mechanisms to protect against dehydration. The perceived result of these mechanisms is thirst.  Studies have shown that as we age our thirst mechanism becomes less effective, making older adults particularly sensitive to dehydration.  According to tables of normative water requirements for men and women by age and energy expenditure, an average middle aged man  should consume about 3.7 liters of fluid daily (125 oz. or 15 cups). An average middle aged woman should consume about 2.7 liters of fluid daily (91 oz. or 11 cups). Of course, fluid requirements also depend on fluid losses through sweating, which can range from .3 L/hour to 2 L/hour. I found this reference Water, Hydration and Health to be very informative.

What constitutes healthy fluid replenishment in the context of exercise? The American Academy of Sports Medicine recommends that athletes pre-hydrate 4 hours prior to physical activity with 2-3 mL per pound of body weight. For me, at 130 pounds, that’s 325 ml of liquid (11 oz., or about a cup and a half). For a man weighing 200 pounds that’s about 500 ml of liquid (17 oz., or about 2 cups).  Here’s a very good brochure on the subject:

During exercise, adequate hydration is defined as preventing a loss of more than 2% of one’s body weight. Again, using myself as an example at 130 pounds, that’s about 2.5 lbs. I know my usual “hydrated” weight, and typically weigh myself after exercise and before showering so it’s fairly easy to monitor my fluid loss. 

According to recent guidelines, rapid and complete recovery from excessive dehydration can be accomplished by drinking at least 16-24 oz. (450-675 mL) of fluid for every pound (0.5 kg) of body weight lost during exercise. What’s the best liquid to consume? At least some of one’s rehydration should include fluid with electrolytes---sodium and potassium. In addition, carbohydrate (sugar) in fluid actually helps to replenish glycogen stores, and has been found to improve athletic performance. Of course, for those who are trying to lose weight through physical activity, consuming beverages with carbohydrates also contributes calories, so one needs to be careful about overdoing it with sugary beverages, including sports drinks like Gatorade or PowerAde. My personal practice is to consume one bottle or can of salty beverage (500 mL) and approximately 16 oz. of water after a summer work-out of 45 minute duration (my most common exercise duration). Drinking a sports drink 30-45 minutes into a race that’s destined to last more than 60-90 minutes may enhance performance, followed by drinking water as one is able every 15-30 minutes thereafter. One of my personal favorite beverages with which to help rehydrate with is V-8 or tomato juice, which is loaded with sodium and potassium, though the carbohydrate content is less than Gatorade or PowerAde. I typically drink this along with my usual 2-3 cups of water after a vigorous 60 minute work-out in the summer.

The risk of excessive water drinking is hyponatremia, which is known to occur in athletes who exercise in the heat and then re-hydrate with large amounts of water without salt or electrolyte content quickly. In my office I also see hyponatremia as a problem for some patients who consume large amounts of water for health reasons or weight loss (“psychogenic polydipsia”), or who are on diuretics, which cause loss of sodium. 

The Institute of Medicine recommends the following composition of sports drinks for prolonged (>60 minutes) physical activity in hot weather: 20-30 meq/L of sodium, 2-5mEq/L potassium, and 5-10% carb (>8% may delay gastric emptying).

Here is a comparison of the nutritional content of various drinks along with an approximate range for the electrolyte content of sweat (which can vary considerably in sodium content).  Something interesting that I learned through reading on this subject is that muscle cramps in athletes seem to correlate more with sodium loss than potassium loss. As you can see, Gatorade does approximate the electrolyte content of sweat.

Nutritional Content per : 1 cup, 8 oz.,  237 ml	Gatorade	Chocolate Milk	V-8	ZICO: Coconut Water	Sweat (estimate)
Sodium (mg)	104.4 mg	152.5 mg	640 mg	64 mg	~100-300 mg
Potassium (mg)	30 mg	425 mg	445 mg	471.36 mg	~40-60 mg
Carb (gm)	14 gm	26 gm	9.7 gm	10.72 gm
Protein (gm)	0	8 gm	0	0
Fat	0	2.5 gm	0	0

My conclusion--while the contrarian in me used poke fun at the ever water-toting health nut, I’ve now become a believer. Personally, I’ve probably been running on the dry side.