From:
http://www.pponline.co.uk/encyc/caffeine-dehydration.htm
How alcohol dehydrates
Alcohol promotes water loss by depressing production of the antidiuretic hormone called vasopressin, which acts on the kidneys, concentrating the urine by promoting the reabsorption of water and salt into the body. Vasopressin helps to regulate the concentration of fluids in the body, and interference with its action leads to an increased loss of body fluid from urination, which can contribute towards dehydration. To make matters worse, alcohol- induced water loss can lead to the additional loss of such minerals as magnesium, potassium, calcium and zinc, which are not only important nutrients but are also involved in the maintenance of fluid balance and nerve and muscle action.
Although alcohol use beyond the recommended limits is known to be detrimental to health (and thereby to athletic performance), what evidence is there that sensible use has any ill effects? In particular, can alcohol use impair hydration status? By contrast with caffeine, the evidence is quite damning.
A UK study examined the effect of alcohol consumption on the restoration of fluid and electrolyte balance after exercise-induced dehydration (8). In four separate trial conditions, six subjects consumed drinks containing 0, 1, 2, and 4% alcohol over 60-minute period, beginning 30 minutes after the end of a dehydrating exercise session. Although a different beverage was consumed in each condition, the volume remained constant at just over two litres (equivalent to 150% of body mass loss during exercise – the amount recommended for efficient hydration).
The researchers found that the total volume of urine produced in the six hours after rehydration tended to increase in line with the alcohol content of drinks. Furthermore, the increase in blood and plasma volume during this rehydration period was markedly slower with the 4% beverage and did not ultimately rise significantly above the dehydrated level! The researchers concluded that while alcohol has a negligible diuretic effect when consumed in dilute solution (2% or less), drinks containing 4% or more of alcohol tend to delay the recovery process.
Another study investigated the effects of alcohol on pulse rate and blood volume during orthostatic tilting (changing from lying down to an upright posture) (9). Ten men and as many women drank nonalcoholic beer mixed with either alcohol (1.1g per kg of body weight) or an equivalent volume of water. Pulse rate, blood volume and blood alcohol concentration were monitored for the next eight hours, and the alcohol group were found to develop a significant relative fluid deficit, beginning two hours after ingestion and averaging 0.5 litres by three hours.
Further evidence of the detrimental effects of alcohol on hydration comes from a more recent study examining the effects of alcohol after a cycle ergometer test (10). Eleven active men cycled on the ergo and one hour later drank either alcohol (0.7g per kg of body weight) mixed with flavoured water or an equivalent volume of flavoured water alone (placebo). Measurements of a range of blood markers were taken at one, five and 22 hours.
As expected, both groups showed a drop in plasma volume after training, but this had been restored in both groups an hour after the drinks were consumed. The alcohol group (unsurprisingly) experienced a big rise in blood alcohol after their drink, but after five hours, their blood alcohol levels had fallen back to zero.
However both groups were also tested for blood viscosity (stickiness). After cycling, viscosity was raised in both groups (probably through dehydration), but while the viscosity steadily returned to normal levels in the placebo group, levels in the alcohol group remained raised, even at 22 hours, despite the fact that their plasma volume values had returned to normal. In other words, although the alcohol had left the bloodstream and hydration appeared normal in terms of blood plasma volume, the blood of those who had consumed alcohol during the recovery phases was still behaving as though the body was dehydrated! The implication is that alcohol during post-exercise recovery may have longer lasting effects than simply impairing rehydration.
