This article discusses some of the popular mythology surrounding hydration from the point of view of people going running and hiking, as well as myths about the dangers of drinking water from wilderness sources. Most of what you've been told by pop culture is probably wrong.
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Highlights for the impatient:
Our popular culture has picked up some pervasive myths recently that wildly exaggerate the dangers of dehydration. There is a widespread belief that even if you're just sitting at a desk all day, you should drink at least eight 8-ounce glasses of water per day ("8x8"). A survey of the medical literature shows no evidence to support this claim.[Valtin 2002] In order to believe the 8x8 myth, you would have to believe that your body was always lying to you about how much water you needed, hence the subsidiary myth that "thirst is too late," i.e., that you need to drink when you're not thirsty, because by the time you're thirsty, you're already dehydrated. In fact, thirst is one of your body's most powerful instincts. Medically, dehydration is defined as a 5% increase in the concentration of solutes in your blood. (Often this can be more conveniently detected based on short-term weight loss.) Thirst sets in at about 2%, so you'll always feel strong thirst setting in long before you're dehydrated. It is also not true that dark urine indicates dehydration.
Because of the "thirst is too late" myth, many hikers carry huge amounts of water -- sometimes as much as several liters, often in a hydration pack. Hydration packs are heavy (about 9 oz for a large one), and water is very heavy. Three liters of water in a 9-oz hydration pack adds up to 7.2 pounds of extra pack weight, which is a vast amount of weight to be carrying based on a mistaken belief. In moderate summer weather in the Sierra, on a trail where streams are no more than a few miles apart, there is theoretically no reason to carry any water from point to point. In reality, you'll want to carry some, for a variety of reasons. You may be treating your water with tablets that take a certain amount of time to work; or camping at some distance from over-impacted areas near lakes and streams; or in late season you may be uncertain whether the creeks and streams on your map are actually running. Nevertheless, one liter per person is usually plenty of water to carry under these conditions, and you can carry that in a couple of lightweight plastic water bottles of the type that bottled water is sold in. This adds up to 2.3 lb immediately after refilling both bottles.
This disinformation about thirst is also affecting runners. Starting around 2012, I started seeing a lot of runners on my city's public trail system wearing vests or Batman-style utility belts with water bottles holstered in them. Carrying all that gear interferes with running, and it's totally unnecessary because the trail system has water fountains. More recently the holsters and vests seem to have gone out of style, but a lot of people are carrying water bottles in their hands, which is just as physically awkward when you're trying to run.
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It's generally not necessary to drink at all during your run, unless you feel thirsty, want to drink, and have the opportunity. Historically, coaches discouraged competitive runners from drinking during their races because they thought it would hurt performance, and in some cases race rules prohibited drinking because it might be harmful to the athletes' health. This didn't change until the 1970's, when running became a more mainstream form of recreation.[Noakes 2007] Studies with endurance athletes in real-world conditions have shown that post-race body temperatures were not reduced by drinking more water [Noakes 1988], and "there is not a single case report or clinical trial that unambiguously links exercise-induced dehydration with specific life-threatening, exercise-related disorders" [Curran 2004].
On the other hand, there have been multiple people who have died when running endurance races because they drank too much water. This can cause a serious condition called hyponatremia, where you dilute your body's electrolytes too much. One of the best-documented cases of a death from drinking too much water involved army recruits who were forced by a well-intentioned drill instructor to drink large amounts of water on a hot day because of very hot temperatures [Garigan 1999]. This unfortunate person drank something like 20 liters of water over the course of one morning.
Proponents of the dehydration myth commonly claim that alcoholic and caffeinated drinks "don't count," because alcohol and caffeine dehydrate you. In fact, beer consumed in moderation has a hydrating, rather than a dehydrating, effect,[Valtin 2002] and 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]
Because of this misplaced fear of a diuretic effect, one sometimes hears advice to abstain from caffeine at high altitude, the reasoning apparently being that if caffeine had a diuretic effect, it would dehydrate you, and dehydration would contribute to acute mountain sickness (AMS). In fact, 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 a 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.
Hackett writes: "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."
In contrast to dehydration, which people worry about too much, heat stroke is a condition that people don't worry about enough. Here's an account of a typical situation where hikers died of heat stroke.
On July 18, 2009, Robert Allen Liebler and a hiking partner started at 3:45 a.m. from the desert of Palm Springs, intending to summit San Jacinto Peak. This classic hike, well known to locals, involves an elevation gain of 10,000', and is normally undertaken only in fall or spring, not summer.
At 6 am, Liebler started cramping because of the heat and turned around, telling his partner to go ahead. Liebler was found dead the next day. He was off the trail, about 200 yards away from a tennis club, sitting upright, with water still remaining in his bottle.
This story illustrates the insidious nature of heat stroke. Liebler evidently didn't feel himself to be in distress, or he would have drunk his water and wouldn't have sat down so close to rescue. A victim of heat stroke may collapse like turning off a light switch, which is why it's common to find hikers dead from heat stroke when help was nearby. When heat stroke is coming on, the victim's brain starts malfunctioning. The fact that Liebler was found off the trail suggests that he may have become confused and disoriented, or that his judgment may have been impaired. It is common for victims of heat stroke to hallucinate or behave irrationally.
Because Liebler didn't drink the water he had, it seems that carrying large amounts of water would have done nothing to save his life. The key to his death was his bad decision to hike in the hot weather of Palm Springs that day: a high of 116 degrees Fahrenheit. Although he wisely started out long before dawn in order to avoid these temperatures, they were caught in a situation where retreating down the trail entailed descending into the heat in the middle of the day. The moral of this story is that if you're considering hiking in a low-elevation area where the temperature can get very high, you need to check the weather forecast, and cancel your hike if the temperatures are likely to be life-threatening. The medical evidence doesn't even support the idea that if you did drink more water, it would reduce your risk of heat stroke.
So-called "energy drinks" are a marketing phenomenon with little scientific justification. They're expensive, and they come in single-use plastic bottles, which are a huge environmental problem. People used to think that muscle cramps were caused by the loss of sodium from the body during intense endurance activity, which gave a reason for using these drinks to maintain one's electrolyte balance. However, studies show that's not true, and cramps really just occur because the nerves in the muscles have been repeatedly stimulated (Schwellnus 2008).
The same marketers have also pushed the idea that you need to "fuel" your exercise by constantly consuming sugary drinks or gels while you're doing the activity. Typically it takes quite a long time for your body to digest your food and turn it into muscle and liver glycogen, which are the main source of energy for endurance activity. If you're going on a morning run, your source of energy is glycogen from last night's dinner -- breakfast is still making its way through your gut. Hunger doesn't mean you're going to "bonk" (run out of glycogen), and you can bonk without being hungry. If you think you might get hungry during a 15-25k-ish run, bring some oreos or a piece of fruit in your pocket, and eat it if you feel hungry. It will be too late to contribute to your glycogen supply, but it will get rid of the sensation of hunger, which is unpleasant. (Marathon distances are a little different, and it can make sense to drink sugary drinks at that distance for the sake of their small amounts of relatively rapidly digestible carbs.) Fatigue is not generally an indication that you're out of "fuel;" especially for activities that involve less than about 2 hours of continuous, intense exertion, you have plenty of glycogen, and fatigue is more like an involuntary central nervous system response that prevents damage.
Some hikers add drink mixes to some of their water. This adds variety to the monotonous diet of a long hike, and for people who can taste iodine, the mix masks the taste. Mixes can be either zero-calorie (e.g., Crystal Light) or sweetened with sugar (e.g., Kool-Aid). If you use the ones sweetened with sugar, be scrupulous about cleaning the threads of your water bottles, because otherwise you may risk dysentery.[Mueser 1997, p. 100] If you're using chemical treatment, wait until it's complete before adding the mix.
A multi-day backpacking hike (what people in some countries call trekking or bushwalking) involves a change in your diet and daily routine, which can disrupt bowel function. Many backpackers get constipated,[Mueser 1997, p. 101] while others report "backpacker's diarrhea." Diarrhea hits about 10-20% of backpackers on short trips, more than 50% on long through-hikes.[Zell 1993,Mueser 1997]
Safe to drink! - https://commons.wikimedia.org/wiki/File:Lila_Lake_26527.JPG - CC-BY-SA
Many people who experience diarrhea while backpacking automatically attribute the problem to Giardia, which is the most popularly known microorganism that can contaminate backcountry water supplies. Among weekend backpackers, it is almost certain that any illness during the hike itself was not caused by wilderness-acquired Giardia, which has a typical incubation period of about a week or two.[Erlandsen 1984, Jokipii; see more detailed information in the footnotes] Fear of Giardia in backcountry water began, without any medical evidence, in the 1970's when large numbers of people began to participate in backpacking as part of a "get back to the land" idea. What seems to have really happened is that some people randomly got diarrhea and attributed it to Giardia, even in situations where that wasn't medically possible due to the long incubation period. The tale grew in the telling.
Giardia is a big problem in daycare centers and public swimming pools, but data show that its prevalence in the backcountry has been wildly exaggerated. In a surveys of sites in California's Sierra Nevada mountains, concentrations of Giardia cysts were much too low to be a realistic health risk.[Suk 1986] Studies on prison volunteers show that on the average, you have to swallow about 20 or 30 cysts to get an infection,[Cox 2002][Rose 1991] and even at the most contaminated site in the surveys, a hiker could only have swallowed this many of the bugs by drinking hundreds of liters of water. In fact, 67 out of the 69 sites in the survey had concentrations lower than 0.03 per liter, which was the typical concentration found in San Francisco and Los Angeles tap water at the time.[Rockwell 2002,sfwater 2009] Elsewhere in the U.S., similar testing[Jaret 2003] also failed to turn up any concentrations of cysts likely to cause disease at any of the backcountry locations tested: West Beaver Creek, AZ; Merced River, CA; Chattooga River, NC; Neversink River, NY; White Pine Lake, UT; Greenwater River, WA; and Renard Lake, WI.
Giardiasis is extremely common among the population of the U.S., with roughly a third of toddlers having it,[Ish-Horowicz 1989] and about 3-7% of adults.[Auerbach 2012] Most people who have giardiasis have no symptoms, and there is even some evidence that children who are asymptomatic carriers are actually healthier than those who aren't infected at all.[Ish-Horowicz 1989] Giardiasis cases peak in the summer, probably because people swim in public pools used by diaper-age children.[Furness 2000] Very few people who believe they contracted giardiasis from backcountry water ever get tested for it, and of those who get tested, most have negative results. Even when the results are negative, doctors often prescribe medication as a prophylactic. When the results are positive, the long incubation period makes it more likely that the hiker picked up the bug somewhere else, perhaps by changing a diaper or swimming in a public pool. A meta-analysis of the literature in 2000 concluded that "the evidence for an association between drinking backcountry water and acquiring giardiasis is minimal."[Welch 2000]
Obviously these can't be absolute statements of zero risk. For example, if you take your water from a puddle with a big heap of mule poop in it, you're taking a risk. Picking your water sources is discussed later in this article. I haven't seen any studies like Rockwell's for other mountains of the world such as the Andes or Himalayas.
Because the only common and well documented way that humans get sick with giardiasis is by catching it from other humans, it's not surprising that the only places epidemiologists have managed to detect disease-causing concentrations of giardia in water have been in the water supplies of cities, towns, and farms -- not uninhabited backcountry areas. A 1977 epidemiological study[Juranek 1977] of giardia outbreaks in Rome, NY; Camas, WA; and Berlin, NH found no strong evidence that the water supply was naturally contaminated. The outbreak in Berlin, HW, was initially blamed on beavers that built a dam in the town's reservoir, which is fed from the White Mountain National Forest.[Lippy 1977] However, there was "ample opportunity for human fecal contamination of raw water sources," and although giardia was found in a captured beaver, the beaver may have actually caught the disease from the human population.[Juranek 1977] In Camas, WA, it is unknown whether the giardia in the water supply came from humans or beavers. Another protozoan cyst, Cryptosporidium, has been studied in farming areas of Switzerland, and although the concentrations were high enough that they might theoretically cause disease,[Fuechslin 2012] again there seems to be evidence only that disease-causing protozoa in water supplies originate from humans in inhabited areas.
When people do actually contract backpacker's diarrhea from exposure during a hiking trip, by far the most common reason is hand-to-mouth contamination.[Welch 1995] Your gut contains so many bacteria that if your body was a democracy, the germs would outvote the human cells by a large margin. You've developed tolerance for your own gut flora, but not for other people's. If your hiking partner doesn't wash his/her hands properly after pooping, then you can ingest their bugs through shared food, food containers, or pots and pans. Hiking groups are extremely prone to contaminating each other with organisms such as E. coli and shigella. To guard against this, don't lower your standards of poop hygiene while hiking. (Don't rinse your hands in a stream, because the soap is environmentally damaging. Do your hand-washing in the same area where you do your pooping, i.e., as far as possible from lakes and streams.) If possible, avoid using cooking pots -- either by by going no-cook or by using foods that are cooked by pouring hot water into an individual-use bag. If you must use pots, wash them thoroughly after each use, and consider using a one-pot system, in which each person eats only from his/her own pot.
If you do decide that treating your water is worthwhile in a certain area, then tablets will save you about 5 ounces of pack weight compared to most filtering systems, and are less time consuming to use. But don't let water treatment distract you from the real issue, which is hand-to-mouth contamination; a 1997 study found that among a large sample Appalachian Trail through-hikers, a majority of whom experienced diarrhea at least once, there was no correlation between the use of water treatment and the risk of diarrhea.[Mueser 1997] A 2013-2014 survey of long-distance hikers on the PCT and AT had similar findings.[Feldman 2015]
Water treatment works differently for bacteria and viruses than for cysts of the protozoa Giardia and Cryptosporidium. Iodine doesn't work well against protozoans, but filters do, and chlorine dioxide tablets are effective if you wait long enough (up to four hours for very cold water). One low-weight, low-cost system is to use iodine to kill bacteria and viruses, plus a small 2-oz filter that fits on the mouth of the bottle to get rid of protozoa cysts. But remember, getting sick from Giardia in your water is little more than an urban folktale, and field surveys show that concentrations of Cryptosporidium are also much too low to be a health threat.[Jaret 2003] Since the techniques that work against protozoans all have drawbacks (inconvenience or higher cost), it may not be worth worrying about them. A survey of Appalachian Trail through-hikers showed that filters were one of the most failure-prone pieces of equipment, leading to comments such as "not worth the money or the effort."[Mueser 1997, p. 95]
The Steripen UV system is effective against all microbes, including viruses and protozoa cysts. It's worse than chemical treatments in terms of bulk and weight (3.6 oz with batteries), but it works in only a couple of minutes. The batteries will last for more days than you can hike without resupplying. There is an initial investment of about $90, and the lamp has a finite lifetime. It's definitely worth considering for day-hikes, for traveling in the third world, or for hikes in areas where protozoan-contaminated water is a big enough risk to be worth worrying about. Among backpackers on long through-hikes, there is anecdotal evidence that it is unreliable, but the reliability of more recent models is reputed to have been improved by a redesign that replaced electrodes with an optical sensor.
If you're boiling water for use in cooking, then the water is already pasteurized before it reaches the boiling point. It is not necessary to use filtering or chemical treatment, and iodine treatment may even cause foods like instant mashed potatoes to turn a funny color. It is not necessary to boil for a certain amount of time in order to kill microorganisms. Protozoa cysts are killed rapidly at about 55 C (131 F). Common bacteria and viruses such as E. coli, Shigella, and Hepatitis A are all killed rapidly at temperatures at or below 65 C (149 F). Even raw milk (which can be swarming with microbes compared to backcountry water) is normally pasteurized for only 15 seconds at temperatures of no more than 72 C (161 F), based on standards designed to kill the most heat-resistant disease-causing bacteria.[USDA 2004] Although the boiling point is depressed at higher altitudes, even at the 17,000-foot elevation of Everest base camp it's 82 C (180 F), which is high enough for complete pasteurization.
It's true that certain microorganisms can survive being boiled for a short time and still cause disease in humans. These are bacterial spores from the genera Bacillus and Clostridium. Examples of the diseases they cause are anthrax, tetanus, and botulism. However, these diseases are not transmitted by ingestion of the spores in drinking water, so they're not a concern here.[Ericsson 2002] For example, when people get botulism, it's because they ingest the growing organism, which has been proliferating in food; you don't get botulism by ingesting the dormant spores.
It is worthwhile to take your water from the cleanest possible place. The best source is a fast-running stream that is not below any trail or close to any area where you see droppings from livestock. (Calves, in particular, excrete a huge amount of cryptosporidium compared to wild animals or adult cows. Cattle are also prolific sources of E. coli.) Inlets of lakes are better than outlets, and outlets are better than the calm water of the lake itself. If you must collect calm water from a lake, avoid areas that seem like likely swimming spots for humans, or areas around beaver dams (beavers tend to carry a lot of giardia).
If you're forced to take water from a lake, I've heard conflicting statements about whether the safest water is at the surface or deeper down. I've seen some statements that the surface layer is the likeliest to harbor nasty bugs, so you should avoid the surface layer by covering the mouth of the bottle and plunging it in to arm's depth before letting any water in. I've also seen statements that the top layer of water is sanitized by solar UV, so it's the safest. I would be grateful if anyone could point me to a scientific paper that would clarify this point.
If you are forced to drink from a source that isn't running rapidly, be aware that blue-green algae can produce toxins that can make you sick. Treatment will not get rid of the toxins. Even after an algae bloom has died off, the toxins can remain in the water if it's stagnant. The only way to avoid this problem is to plan your hikes so that you never have to drink from such sources.
Ben Crowell, 2022 Nov. 29
This post is CC-BY-SA licensed.
Auerbach 2012 - Paul S. Auerbach, Wilderness Medicine (6th ed., 2012), ch. 68
Cox 2002 - Cox, F.E.G. (2002). History of Human Parasitology. Clin. Microbiol. Rev. 15(4): 595, referencing a 1954 study by Rendtorff. Rendtorff's data are summarized and analyzed in Rose 1991.
Curran 2004 - Curran, Rob, "Marathon Maladies & Dehydration Debunked," EMS World, June 2004, https://www.emsworld.com/article/10324701/marathon-maladies-dehydration-debunked , citing Noakes TD, "Hyponatremia in distance athletes," The Physician and Sportsmedicine 28(9):71, Sept. 2000.
Eddy 1928 - 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.
Ericsson 2002 - Ericsson and Steffen. Water Disinfection for International and Wilderness Travelers. Clin Infect Dis. (2002) 34 (3): 355-364. doi: 10.1086/324747, http://cid.oxfordjournals.org/content/34/3/355.full
Erlandsen 1984 - Erlandsen, Giardia and giardiasis: biology, pathogenesis, and epidemiology, 1984. When an animal is infected by a parasite, there's a prepatent period, which is the time from infection with a parasite to when the bugs reach a life stage where they can be detected by a lab test. There's also an incubation period, which is the time from infection to symptoms. For most parasites, the prepatent period is shorter than the incubation period, but for Giardia it's often longer. A 1954 study on prison volunteers showed an average prepatent period of 9 days, but there's a wide range of variation, and the incubation period can be as long as months. In a study of travelers to the Soviet Union, the typical time until acute symptoms occurred was found to be a couple of weeks.
Feldman 2015 - Dan Feldman, "Is Water Treatment Necessary?," http://distancehiking.com/tools/watertreatment/
Fuechslin 2012 - "Cryptosporidium spp. in drinking water: Samples from rural sites in Switzerland," Swiss Med Wkly. 2012;142:w13683, http://www.smw.ch/content/smw-2012-13683/
Furness 2000 - Furness, Beach, and Roberts, "Giardiasis surveillance--United States, 1992-1997," MMWR CDC Surveill Summ. 2000 Aug 11;49(7):1-13.
Garigan 1999 - Garigan, T. P., & Ristedt, D. E. (1999). Death from Hyponatremia as a Result of Acute Water Intoxication in an Army Basic Trainee. Military Medicine, 164(3), 234–238. doi:10.1093/milmed/164.3.234
Grandjean 2000 - 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) http://www.jacn.org/cgi/content/full/19/5/591
Hackett 2010 - Hackett, "Caffeine at high altitude: java at base camp," High Altitude Medicine and Biology 11 (2010) 1.
Ish-Horowicz 1989 - Ish-Horowicz et al., "Asymptomatic giardiasis in children," Pediatr Infect Dis J. 1989 Nov;8(11):773-9.
Jaret 2003 - Peter Jaret, "What's In the Water?," Backpacker, Dec. 2003, p. 45.
Jokipii, The Lancet, Volume 309:1095. In about two thirds of patients, the prepatency period was longer than the incubation period by a week or more.
Juranek 1977 - D. Juranek, Waterborne giardiasis, in Waterborne Transmission of Giardiasis: Proceedings of a Symposium, September 18-20, 1978, eds Jakubowski and Hoff
Mueser 1997 - Roland Mueser, Long-Distance Hiking: Lessons from the Appalachian Trail, International Marine/Ragged Mountain Press, 1st edition, 1997, p. 96
Noakes 1988 - Noakes et al., "The danger of an inadequate water intake during prolonged exercise," European Journal of Applied Physiology and Occupational Physiology 57 (1988) 210.
Noakes 2007 - Noakes, "ration in the marathon : using thirst to gauge safe fluid replacement," ports Med . 2007;37(4-5):463-6. doi: 10.2165/00007256-200737040-00050 - https://pubmed.ncbi.nlm.nih.gov/17465636/
Rockwell 2002 - Robert L. Rockwell, Sierra Nature Notes, Volume 2, January 2002, http://web.archive.org/web/20051026030831/www.yosemite.org/naturenotes/Giardia.htm
Rose 1991 - Rose, Haas, and Regli, "Risk assessment and control of waterborne Giardiasis," Am J Public Health 81 (1991) 709, http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1405147/pdf/amjph00206-0039.pdf
Schwellnus et al., 2008, "Cause of Exercise Associated Muscle Cramps (EAMC) — altered neuromuscular control, dehydration or electrolyte depletion?," https://sci-hub.se/http://dx.doi.org/10.1136/bjsm.2008.050401
Scott 2004 - Scott et al, "The effect of drinking tea at high altitude on hydration status and mood," Eur. J. Appl. Physiol. 91(4):498.493
sfwater 2009 - http://sfwater.org/Files/FactSheets/2009-WQB_annualreport_LowRes.pdf
Suk 1986 - Map cited as reference 17 in S.C. Zell, "Epidemiology of wilderness-acquired diarrhea: implications for prevention and treatment," Wilderness and Environmental Medicine 3 (1992) 241, http://www.wemjournal.org/article/S0953-9859(92)71235-2/abstract
USDA 2004 - National Advisory Committee on Microbiological Criteria for Foods: Requisite Scientific Parameters for Establishing the Equivalence of Alternative Methods of Pasteurization, USDA , 2004
Valtin 2002 - Heinz Valtin, "'Drink at least eight glasses of water a day.' Really? Is there scientific evidence for '8x8'?," Am J Physiol Regul Integr Comp Physiol 283: R993-R1004, 2002. http://ajpregu.physiology.org/cgi/content/full/283/5/R993
Welch 1995 - Thomas R. Welch and Timothy P. Welch, "Giardiasis as a threat to backpackers in the United States: a survey of state health departments," Wilderness and Environmental Medicine, 6 (1995) 162, http://www.wemjournal.org/article/S1080-6032%2895%2971046-8/abstract
Welch 2000 - Welch, T.P. "Risk of giardiasis from consumption of wilderness water in North America: a systematic review of epidemiologic data," Int J Infect Dis. 2000;4:103100, http://download.journals.elsevierhealth.com/pdfs/journals/1201-9712/PIIS1201971200901024.pdf?refuid=S1080-6032(04)70498-6&refissn=1080-6032&mis=.pdf
Zell 1993 - S.C. Zell and S.K. Sorenson, "Cyst acquisition rate for Giardia lamblia in backcountry travelers to Desolation Wilderness, Lake Tahoe," Wilderness and Environmental Medicine 4 (1993) 147, http://www.journals.elsevierhealth.com/periodicals/jwm/article/S0953-9859%2893%2971172-9/abstract
