- Drink early, drink often
- Even tiny levels of dehydration can result in a catastrophic drop in sports performance
- If you're thirsty, it's already too late
If you're thirsty, drink. Otherwise, don't worry about it.
In endurance sports, there is evidence from Triathlons in New Zealand and South Africa, and footraces in South Africa to suggest that the best performances are associated with higher levels of dehydration*. Further yet, there is significant evidence to suggest that there is no link between even moderate levels of dehydration and hyperthermia**. Going even further than this, the issuance of guidelines for athletes by the American College of Sports Medicine*** that they should "drink as much as is tolerable" is likely to have directly contributed to the rise in fatalities from exercise-associated hyponatraemia and exercise-associated hyponatraemia encephalopathy**** which were essentially unknown until the early 80s when the hydration fad started. In summary, the best evidence currently available says that athletes should drink to thirst. Evidence to the contrary is surprisingly limited, quite often flawed and mostly sponsored by the sports drink industry.
OK, that's great for marathons and triathlons but what about shooting?
As far as I am aware there has been no real study on hydration and performance in shooting sports for me to draw on. Most of the things I have managed to find online have quoted general guidelines and no actual backing evidence. The key difference in the case of shooting is going to be the effect of hydration on the eyes; however if you're drinking to thirst then I would suggest that this is unlikely to be a major issue.
It's still always worth having a bottle of water on the range to sip at when you get thirsty especially in warm conditions, but there's no value in trying to drink ahead of thirst. Desperately needing to pee while on the firing point is probably also not good for performance.
* Noakes TD. Waterlogged. Chapter 2. Wyndham CH, Strydom NB. The danger of an inadequate water intake during marathon running. South African Medical Journal. 1969; 43, 893-896.
** Adolph EF, Dill DB. Observations on water metabolishm in the desert. Americal Journal of Physiology. 1938; 123, 369-378. Jardon OM. Physiologic stress, heat stroke, malignant hyperthermia - a perspective. Military Medicine. 1982; 147, 8-14. Noakes TD, Adams BA, Myburg KH. The danger of an inadequate water intake during exercise. A novel concept re-visited. Euopean Journal of Applied Physiology & Occupational Physiology. 1988; 57, 210-219.
*** Convertino, V. A.; Armstrong, L. E.; Coyle, E. F.; Mack, G. W.; Sawka, M. N.; Senay, L. C.; Sherman, W. M. (1996-01-01). "American College of Sports Medicine position stand. Exercise and fluid replacement". Medicine and Science in Sports and Exercise. 28 (1): i–vii.
****Noakes TD. Waterlogged. Chapter 10. Nearman S. Drinking too much water can kill you; death of a marathon runner. Metro Competitor. 2003; 24 October. Noakes TD, Goodwin N, Rayner BI, et al. Water intoxication: A possible complication during endurance exercise. Medical Science Sports Exercise. 1985; 17, 370-375.