Monday, June 9, 2008

Dean's Nutrition Advice: Round 4

N.C. Randonneur Dean Furbish, who teaches physiology at a local college, has contributed three recent columns on cycling and nutrition. That has generated several questions from readers. Here's the latest, with Dean's answer.

hspatz said...
I told my doctor of problems I was having in the heat and he said not to ride so much in the heat (not a helpful answer.) I put my symptom set into Google and it was a perfect match to hyponatremia and I informed my doctor of such. He copied off 7 pages about hyponatremia from a Dr. Burton Rose that said that exercise induced hyponatremia comes from excess water consumption and the proof came from a Boston Marathon where many hyponatremic runners had weight gain from water, as opposed to a New Zealand marathon with fewer water stations where there was no hyponatremia. Dr. Rose's advice was to drink only when thirsty and to avoid weight gain by drinking less. From my experience, the advise should be to increase electrolytes and not worry about drinking too much. If the electrolytes are OK the athlete will urinate excess fluids instead of holding on to them. It seems to me that Dr. Rose's advice will lead to dehydration and not fix the hyponatremia. What do you think?
May 28, 2008 5:31 PM

Dean's response:

HS is correct to increase water intake supplemented with electrolytes to stave off hyponatremia and maintain performance. The only remaining question regarding intake is: “How much?” While “more is better,” there are limits to water and electrolyte intake. To my knowledge, no expert on the subject recommends consuming more than one loses during an endurance event. Moreover, one school of thought questions whether one should even try to replace all that is lost. The reason is that there are limits regarding how much water, food, and electrolytes can be absorbed during an endurance event. Fortunately, estimates of water and sodium loss during endurance events abound, giving athletes a place to start in calculating replenishment needs. I’ve provided one such link below.

With that said, I feel it worthwhile to expound on HS’s situation. Given the competing schools of thought HS has dug up, an explanation of why I believe the rider’s proposed strategy is correct may benefit other riders.

Let’s start with something with which everyone agrees. Excessive water intake can dilute blood sodium, resulting in hyponatremia (i.e., low blood sodium). Hyponatremia should be avoided. According to current thought, hyponatremia leads to nausea, vomiting, muscular cramping, and cerebral edema (i.e., brain swelling). HS points out two strategies for avoiding hyponatremia. The first strategy would be to drink less water, the position ascribed by HS to the good doctor Rose. The second strategy would be to supplement water intake with electrolytes, HS’s position.

The first strategy of drinking less water to avoid hyponatremia would be good advice for athletes who are, in fact, drinking excessive amounts. But what is excessive? According to HS, the good doctor argues that athletes should be guided by thirst so as not to drink too much. But is this a workable strategy? I shall argue that it is not. While the strategy prevents hyponatremia, it ultimately leads to dehydration and underperformance.

Our sense of thirst is not a sufficient indicator of water need. Although the brain’s “thirst center” monitors blood-sodium directly, it is slow to send the thirst signal. If this weren’t bad enough, the thirst signal shuts off prematurely, that is, after we’ve taken only a few gulps of water! This is completely counter to the needs of endurance athletes. Elaine Marieb (2007, p. 1040) in Human Anatomy & Physiology writes:

As effective as thirst is, it is not always a reliable indicator of need. This is particularly true during athletic events, when thirst can be satisfied long before sufficient liquids have been drunk to maintain the body in top form.

My advice for endurance athletes regarding hydration is as follows:

First, begin hydrating at least a day in advance of an endurance contest. Urination is not necessarily a good indicator of one’s hydration state, however. The kidneys will immediately begin to flush extra water from the blood plasma before it has had a chance to move into both extra- and intracellular spaces, resulting in complete hydration. Hydration takes time, given the passive, slow processes of osmosis and diffusion. The athlete should not consume excessive electrolytes during this hydration period.

Second, taper the usage of diuretics such as caffeine and alcohol during this hydration period. Caffeine causes the body to excrete sodium, thus upsetting both fluid and electrolyte balance. This does not include the pre-event cup of coffee, which, by many accounts, may boost performance. Some athletes are even able to consume caffeine during events without any adverse (i.e., diuretic) consequences.

Third, begin drinking fluid at the start of an endurance event, long before the onset of thirst, and continue drinking throughout the event enough water for replenishment.

Fourth, in order to avoid hyponatremia, make sure to supplement water intake with electrolyes. Food provides some electrolytes. One should be aware that some popular sports drinks do not supply all the electrolytes one needs and can lead to hyponatremia. The possibility of hyponatremia increases in the late stages of endurance events when athletes begin diluting sports drinks due to a heightened ability to taste sugar and the perception such drinks are too sweet.

In his blog entry on hyponatremia, Douglas Stoddard, a physician—with a specialty in sports medicine—and an endurance athlete, himself, makes this very point when he quotes an authoritative source on sweat loss, Dr. Burton Rose’s (1994, p. 656) Clinical Physiology of Acid-Base and Electrolyte Disorders,

. . . these losses are almost entirely replaced by . . . solutions (including Gatorade) that have a much lower salt concentration. The net effect is water retention and, in some cases, symptomatic hyponatremia, with a fall in plasma sodium concentration . . .

But simply adding more sodium to one’s water or sports drink may not be the answer endurance athletes seek. Although a major player in fluid and electrolyte balance, sodium is not the only electrolyte important for good health. Overloading on sodium may cause a shift in the hormonal controls that maintain proper water and electrolyte balance during an endurance event to competing mechanisms that focus more on sodium overload.

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