I've heard from a tour leader that you shouldn't drink tea in high altitude because it makes you thirsty and cause problem in blood circulation. Is this true? Should'nt we drink tea in high altitude?
The idea that caffeinated drinks dehydrate you or "don't count" toward your body's water requirement is a myth. Laboratory studies have shown that caffeinated soda is just as hydrating as water, i.e., the diuretic effect of the caffeine is too small to measure.[Grandjean 2000] Even in the case of coffee, which has much higher concentrations of caffeine than soda, studies going back as far as 1928 have shown that the diuretic effect vanishes for people whose bodies have learned to tolerate the caffeine.[Eddy 1928]
It's extremely common for mountaineers to drink tea at high altitudes. For example, I traveled with several mountain guides in Kenya and Tanzania who drank the stuff in massive quantities. For someone who's a caffeine addict, not drinking caffeine at high altitude is a really silly thing to do. It will give you a headache and make you feel terrible -- exactly the same symptoms as mild altitude sickness.
Reviews of the effects of caffeine at high altitude [Hackett 2010], and specifically of tea at Everest base camp [Scott 2004], showed that there were multiple positive effects of caffeine and no detectable negative effects. The positive effects included a decrease in high-altitude lassitude, as well as an improvement in athletic performance that was bigger at high altitude than at low altitude. Dehydration is difficult to avoid at very high altitudes such as the altitude of Everest base camp; however, the Everest study showed that the hydration status of tea drinkers was the same as that of a control group who abstained from caffeine.
While symptoms of dehydration are similar to AMS, no compelling evidence suggests that dehydration contributes to AMS.
Similar to caffeine’s successful use for headaches at low altitude, owing to its cerebral vasoconstriction properties, it is likely that caffeine will help prevent or treat altitude headaches and therefore AMS [...]
[T]he symptoms [of caffeine withdrawal] and their timing (12 to 24 h) mimic acute mountain sickness. [...] One can only guess at how many cases of caffeine withdrawal are misdiagnosed as AMS and at what role caffeine withdrawal might play in promoting AMS.
Eddy NB, Downs AW. Tolerance and cross-tolerance in the human subject to the diuretic effect of caffeine, theobromine and theophylline. J Pharmacol Exp Ther. 1928;33:167-174.
Grandjean et al., "The Effect of Caffeinated, Non-Caffeinated, Caloric and Non-Caloric Beverages on Hydration," Journal of the American College of Nutrition, Vol. 19, No. 5, 591-600 (2000)
Hackett, "Caffeine at high altitude: java at base camp," High Altitude Medicine and Biology 11 (2010) 1.
Scott et al, "The effect of drinking tea at high altitude on hydration status and mood," Eur. J. Appl. Physiol. 91(4):493–498.