A few good links

Twisty, of I Blame the Patriarchy, talks about aggregating harvestmen.

Science Daily summarizes a psychology experiment wherein women who are told that there is a genetic explanation for women's lack of ability to do math perform worse on math tests than women who are told there is no difference between how men and women perform on math tests.

Between 2003 and 2006, Dar-Nimrod and Heine conducted their research with more than 220 female participants. Their study provided participants with bogus scientific explanations for alleged sex differences in math.

Some women received a genetic account of inborn traits to explain the difference while others received an experiential account -- such as math teachers treating boys preferentially during the first years of math education. Other participants were reminded of the stereotype about female math underachievement, or were told that there are no sex differences in math.

Heine and Dar-Nimrod found the worse math performances belonged to women who received a genetic explanation for female underachievement in math or those who were reminded of the stereotype about female math underachievement. Women who received the experiential explanation performed better -- on par with those who were led to believe there are no sex differences in math.

The Times Union posts a news story about airport screeners failing to detect bomb-making materials in carry on luggage (published July 4, 2007):

The unannounced inspection by TSA officials took place [at Albany International Airport] early last week. The airport's security measures failed in five of seven tests, most of the problems occurring at the passenger checkpoint, the sources said.

In one test, TSA inspectors hid the components of a fake bomb in carry-on luggage that also contained a bottle of water. Passengers are prohibited from carrying containers holding more than three ounces of liquids, gels or aerosols through airport checkpoints.

The screeners at Albany International confiscated the water bottle but missed the bomb. In all, the inspectors slipped four banned items through the main checkpoint during the test, sources said.

The New York Times has an article on how the ability of the Surgeon General to be an independent office is being compromised:

Former Surgeon General Richard H. Carmona told a Congressional panel Tuesday that top Bush administration officials repeatedly tried to weaken or suppress important public health reports because of political considerations.

The administration, Dr. Carmona said, would not allow him to speak or issue reports about stem cells, emergency contraception, sex education, or prison, mental and global health issues. Top officials delayed for years and tried to “water down” a landmark report on secondhand smoke, he said. Released last year, the report concluded that even brief exposure to cigarette smoke could cause immediate harm.

Dr. Carmona said he was ordered to mention President Bush three times on every page of his speeches. He also said he was asked to make speeches to support Republican political candidates and to attend political briefings.

And administration officials even discouraged him from attending the Special Olympics because, he said, of that charitable organization’s longtime ties to a “prominent family” that he refused to name.

The Independent has an article by a reporter who traveled on a cruise ship packed with readers of the National Review. Read it for a view into the mindset of a few American conservatives.

Teaching link: Wellcome Images

One of the problems of developing an online course (or an in-person course, for that matter) is finding good artwork. Sure, there are lots of images available on the web, but relatively few of these are completely legal to use (i.e., most are copyrighted works with no clear license to allow educators to use them). However, works licensed under Creative Commons licenses are freely usable by educators¹, and thus I now attempt to use only Creative Commons licensed works when I develop teaching materials.

There are many places where you can find Creative Commons licensed materials (e.g., Flickr's advanced search lets you filter by license, and all PLOS journal articles are Creative Commons licensed), but BoingBoing just linked to an amazing resource: Wellcome Images. This website, run by the Wellcome Trust, contains images depicting "two thousand years of human culture," and everything on it has been released under a Creative Commons license².

Wellcome Images is one of the world's richest and most unique collections, with themes ranging from medical and social history to contemporary healthcare and biomedical science.

All our images are available on demand in digital form. Search online or use the expertise of our professional scientific and historical researchers.

Whether it's medicine or magic, the sacred or the profane, science or satire - you'll find more than you expect.

This unrivalled collection contains historical images from the Wellcome Library collections, Tibetan Buddhist paintings, ancient Sanskrit manuscripts written on palm leaves, beautifully illuminated Persian books and much more.

The Biomedical Collection holds over 40 000 high-quality images from the clinical and biomedical sciences. Selected from the UK's leading teaching hospitals and research institutions, it covers disease, surgery, general healthcare, sciences from genetics to neuroscience including the full range of imaging techniques. (quote from here)

I've only been browsing for a short while, but have already found a ton of images I think I'll use in my course. Who wouldn't want pictures of a malaria parasite in a mosquito's gut, an opium poppy, a human embryo implanting at 6 days, a picture of male bodybuilders pre-testosterone-injections, or drawings of morels? Go find some for yourself!

¹ As long as the educators are creating non-commercial works, and even then some Creative Commons licenses allow commercial works.
² All images are either under a Creative Commons Attribution, Non-commercial Licence 2.0 or a Creative Commons Attribution, Non-Commercial, No Derivatives 2.0 license.

My new favorite band

Nunatak.

They were in Live Earth yesterday. Of course, they were playing from the British Antarctic Survey's Rothera Research Station, so there weren't too many people in the audience (17), but who wouldn't love a band made up of members like this:

• Matt Balmer – electronics engineer with the physics and meteorology team.
• Tris Thorne – communications engineer
• Ali (Alison) Massey – marine biologist
• Rob Webster – meteorologist
• Roger Stilwell – Field General Assistant (polar guide)
  

Go watch the videos of their songs here, and then go read about the band here.

Update: They've got their songs posted on YouTube, so here's one to whet your appetite:

Even limited exercise helps

One of the exercise mantras commonly floated about is that adults should get 30 minutes of moderate activity exercise at least five days a week. While this is a good goal, the majority of Americans don't get this amount of exercise¹. And, for a lot of beginning exercisers, exercising for 30 minutes a day probably seems like a huge commitment, if not an insurmountable challenge.

My SO and I have long viewed exercise in the "some is better than none" category, and thus try to do even little amounts whenever we can (e.g., walking to the store instead of driving, always taking the stairs at work, doing our own gardening). A recent paper (Church et al., 2007) has shown, via a randomized, controlled trial, that even small amounts of exercise are better than no exercise at all (at least for the subset of people they tested).

Church et al. divided overweight, post-menopausal women into four groups:

• Control: Did no exercise a week (other than normal walking)
  
• 4kcal/kg: Exercised about 70 minutes a week
  
• 8kcal/kg: Exercised about 135 minutes a week
• 12 kcal/kg: Exercised about 190 minutes a week

Participants were randomly assigned to a condition, and all exercise was performed in a lab². The participants exercised at their given level for six months, at which point physiological data were compared to data taken at the start of the study.

Fitness improvement correlated linearly with the amount of exercise:



Figure 3 from Church et al. (2007)."Percent Change in Fitness Data for Each Study Group. The data represent the least-squares means adjusted for age, ethnicity/race, weight, and peak heart rate. The P values for pairwise comparisons of control with 4-kcal/kg, 8-kcal/kg, and 12-kcal/kg per week groups are P .001 for each variable. P for linear trend across groups .001 for each outcome. Error bars indicate 95% confidence intervals."
So, what this means is that even if you work out for only 20 minutes a day three days a week, you'll almost certainly see fitness benefits. And, once you start working out and getting used to doing it regularly, if you increase your workout durations (say, to 40 minutes a day three days a week), you'll see even more improvements. And, to help scare you into exercising, note that the people who didn't do any exercise actually had a small decrease in fitness³.

So, if you're one of the many sedentary Americans out there, take a look at those graphs and start exercising, even if it's only for a few minutes a day. It'll do ya good!

¹ As in "More than 60 percent of U.S. adults do not engage in the recommended amount of activity (30 minutes a day, 5 days a week)" and "Approximately 25 percent of U.S. adults are not active at all." (data from the 1996 Surgeon General's Report on Physical Activity and Health, specifically from here)
² "Women in the exercise groups alternated training sessions on semi-recumbent cycle ergometers and treadmills." The control (non-exercising) women tracked their daily steps with a pedometer and were "asked to maintain their level of activity during the 6-month study period."
³ In fact, it was the realization that our fitness was slowly declining (and would continue declining until we would no longer be able to walk when we were 70) that finally motivated my SO and me to start regularly exercising (and tracking our exercise goals and progress here) a few years ago.

Church, TS., DP Earnest, JS Skinner, and SN Blair. 2007. Effects of Different Doses of Physical Activity on Cardiorespiratory Fitness Among Sedentary, Overweight or Obese Postmenopausal Women With Elevated Blood Pressure. JAMA. 297:2081-2091. Abstract.

In the news

Looks like I was right; CNN has finally decided that 100% fruit juice won't make your kids fat after all.

Carbon dating of chicken remains in South America has provided evidence that Polynesians sailed to the Americas at least a hundred years before the Spanish did. Chickens are not native to the Americas, yet the chicken remains date to AD 1321 to 1407.

And, don't expect widespread Mad Cow testing to occur anytime soon in the US; the Bush administration is fighting to prevent a company from testing all their cows for the disease:

The Bush administration said Tuesday it will fight to keep meatpackers from testing all their animals for mad cow disease .

The Agriculture Department tests less than 1 percent of slaughtered cows for the disease, which can be fatal to humans who eat tainted beef. But Kansas-based Creekstone Farms Premium Beef wants to test all of its cows.

Larger meat companies feared that move because, if Creekstone tested its meat and advertised it as safe, they might have to perform the expensive test, too.

The Agriculture Department regulates the test and argued that widespread testing could lead to a false positive that would harm the meat industry.

A federal judge ruled in March that such tests must be allowed. U.S. District Judge James Robertson noted that Creekstone sought to use the same test the government relies on and said the government didn't have the authority to restrict it.

The ruling was to take effect June 1, but the Agriculture Department said Tuesday it would appeal -- effectively delaying the testing until the court challenge plays out.

Fungi that live off radiation

Some of you may have already read this on BoingBoing or /., but if you haven't, here's the big news: there is now evidence that fungi may be able to obtain metabolic energy from ionizing radiation. By ionizing radiation I don't mean that stuff that we see as light, and that plants use to obtian energy through photosynthesis; I mean nuclear radiation. As in, the stuff that kills you (e.g., what Caesium 137 emits). This is cool.

The study (Dadachova et al. 2007) was published in PLoS One, meaning that it's completely open access. So, while I'll summarize a few of the biological details here, if you want all the nitty gritty, head over there and read away.

The researchers started their search by noticing that fungi growing in radiation-intense environments (e.g., around the Chernobyl reactor) tended to have extremely high levels of melanin in their cells (yes, the same type of pigment that humans have in their skin). While this melanin has been hypothesized to function in a protective role (by absorbing radiation and dealing with the free radicals that are produced), the researchers wondered if the fungi might be able to use melanin, and the high-energy electrons it produces, for more than just protection.

To start, Dadachova et al. did a number of biochemical experiments: they showed that the fungal species they were working with (Cryptococcocus neoformans, Cladosporium sphaerospermum, and Wangiella dermatitidis) expressed melanin, analyzed the fungal melanin via HPLC and ESR, and determined that melanin exposed to radiation could speed up other oxidation reduction reactions (i.e, that radiation could affect the metabolic reactions of the cell). To show that melanin could affect oxidation reduction reactions, Dadachova et al. isolated melanin from fungal cells, mixed it with NADH and ferricyanide, and then showed that the rate of the NADH\ferricyanide oxidation reduction reaction increased when the solution was exposed to radiation. While Dadachova et al. state that the mechanism by which radiation energy is absorbed by melanin and transfered to biochemical reactions is unknown, based on their work it sounds like what might be going on is that melanin is eventually reducing NAD to NADH. This would be elegantly simple metabolically, as one of the primary products of the Krebs cycle (i.e., the cycle in fungi that metabolizes sugar) is NADH; NADH from the Krebs cycle is then used to power the electron transport chain, which ends up producing ATP. So, if melanin was reducing NAD to NADH using energy from radiation, it would be extremely simple to turn that NADH into usable ATP¹.

Dadachova et al. were able to obtain a mutant strain of C. neoformans [Lac(-)] that was unable to produce melanin. Thus, to test the hypothesis that melanin is the pigment that absorbs radiation, and that this radiation absorption provides useful energy, Dadachova et al. grew both melanin-producing and non-melanin-producing C. neoformans in either irradiated or non-irradiated conditions². The results are below:


Growth of normal C. neoformans (left) or non-melanin-producing C. neoformans (right) in either irradiated or non-irradiated conditions. Modified from figure 6 of Dadachova et al. (2007).
This is exactly what we'd expect to see if melanin was providing energy for growth: the melanin-producing fungi grew faster when exposed to radiation (the red bar in the left graph), but when the fungi were unable to produce melanin, there was not much of a difference between the irradiated and non-irradiated fungal growth (right graph).

Dadachova et al. were able to do essentially the same experiment with another species of fungus, W. dermatitidis, which they were also able to obtain a non-melanin-producing mutant of. Again, the results show that the fungi grew better when exposed to radiation:


Growth of normal W. dermatitidis (left) or non-melanin-producing W. dermatitidis (right) in either irradiated or non-irradiated conditions. Modified from figure 8 of Dadachova et al. (2007).
One thing to note here is that radiation does not appear to be required for these fungi to live; the mutant strains that don't produce melanin (and thus presumably cannot use this energy-gathering pathway) still grew, and the fungi that did produce melanin were able to grow even in the absence of extra radiation. Thus, this mechanism is not directly analogous to plant photosynthesis (as photosynthesis is typically the sole mechanism by which plants obtain energy, while radiation is not the sole energy source for these fungi).

When we think of organisms growing based on electromagnetic radiation, we think of plants, which get energy from light and carbon from carbon dioxide. Plants use the energy they get from light to take carbon dioxide from the atmosphere and assemble it into sugars (which nicely store the energy they've captured from light in a chemical form). Plants are therefore known as photoautotrophs (photo: getting energy from light; autotroph: obtaining carbon from an inorganic source, such as carbon dioxide). Animals, along with fungi and many other non-photosynthesizers, typically get their energy from organic molecules (i.e, sugar, fat, protein), and their carbon from those same organic molecules. Thus, animals and fungi are known as chemoheterotrophs.

A question with these radiation-using fungi follows: are these fungi fixing carbon dioxide from the atmosphere, like plants, or are they getting carbon from organic molecules, like most other fungi do? In other words, are these fungi heterotrophs (like most other fungi) or autotrophs (like plants)? To partially test this, the researchers added acetate (an organic form of carbon) to the fungal growth medium, and labeled that acetate with carbon-14. They then exposed both the normal fungi and the non-melanin-producing fungi to radiation, and observed that the normal fungi incorporated significantly more carbon-14-labeled acetate into their cells when they were exposed to radiation. Since the fungi were absorbing the acetate at higher rates when exposed to radiation, it seems as though the fungi are still using heterotrophic mechanisms of carbon uptake (i.e., they're not autotrophs, though note that they didn't directly test for absorption of carbon dioxide).

So, as Dadachova et al. say in their discussion,

[W]e cautiously suggest that the ability of melanin to capture electromagnetic radiation combined with its remarkable oxidation-reduction properties may confer upon melanotic organisms the ability to harness radiation for metabolic energy. The enhanced growth of melanotic fungi in conditions of radiation fluxes suggests the need for additional investigation to ascertain the mechanism for this effect.

Looks like it's time to go add another line to that "sources of energy for growth" slide in my lecture.

¹ Note that this is pure speculation on my part, and I'm most certainly not a biochemist.
² For the irradiated growth conditions, Dadachova et al. exposed the fungi "to a radiation field of 0.05 mGy/hr created by ¹⁸⁸Re/¹⁸⁸W isotope generator".

Dadachova E., RA Bryan, X Huang, T Moadel, AD Schweitzer, P Aisen, JD Nosanchuk, and A Casadevall. 2007. Ionizing Radiation Changes the Electronic Properties of Melanin and Enhances the Growth of Melanized Fungi. PLoS ONE 2(5): e457. doi:10.1371/journal.pone.0000457. Full-text.

Good thing I'm revising my lab manual this summer

PZ Myers has just posted a detailed writeup of some new evo-devo work looking at the evolutionary relationship between ascidians (sea squirts), cephalochordates (Amphioxus), and vertebrates. Sea squirts and cephalochordates always confuse my students, as they're in the same phylum as us (Chordata), yet they're not vertebrates (i.e., they don't have a skull or backbone). Sea squirts are particularly cool, as they're sessile filter feeders, and thus look nothing like vertebrates.

PZ reports that the evolutionary relationship of these three groups is being revised: those wacky sea squirts are now thought to be evolutionarily closer to us than cephalochordates are. PZ does more than just say that, though: he also goes into lots detail about the developmental biology of ascidians¹. It's all good, so go read the post already.

¹ Be sure to look for the figure showing that some ascidians develop adult characteristics well before they metamorphose!

Corals on dry land

National Geographic has pictures from an island in the South Pacific whose elevation was increased 10 feet by a recent earthquake. This probably won't harm the terrestrial organisms too much, but has wreaked havoc with the sub-tidal organisms, as many of the large sessile ones (e.g., corals) are now poking up out of the water. Poor corals. (Via BoingBoing)

Testing for mad cow

I've written about bovine spongiform encephalopathy (BSE; mad cow disease) testing in cattle before. As the disease is extremely hard to detect in cattle (see here), and we don't know how easily it is transmitted to humans, it was thus somewhat surprising to learn that a few months ago the USDA decided to drastically reduce the number of cattle being tested for the disease.

Given that people are worried about the disease, one might expect that private beef companies would step in and start voluntarily testing their cattle before selling it (labeling the resulting beef as "mad-cow free" or some such). You've probably noticed that no such products are on the market.

Why? The USDA has threatened to sue any company that tests for mad cow separately from the government program (most notably Creekstone Farms). Just a few days ago this ridiculous prohibition was stopped by a federal judge, and, assuming that the government doesn't appeal the decision, private companies will now be free to test their own cattle for the disease.

Here's an excerpt from the New York Times article that has a bit more background on the specific case:

Creekstone Farms Premium Beef, a meatpacker based in Arkansas City, Kan., wants to test all of its cows for [mad cow] disease, which can be fatal to humans who eat tainted beef. Larger meat companies feared that move because if Creekstone tested its meat and advertised it as safe, they could be forced to do the expensive test, too.

The Agriculture Department currently regulates the test and administers it to less than 1 percent of slaughtered cows. The department threatened Creekstone with prosecution if it tested all its animals.

U.S. District Judge James Robertson ruled that the government does not have the authority to regulate the test. Robertson put his order on hold until the government can appeal. If the government does not appeal by June 1, he said the ruling would take effect.

...

The Agriculture Department argued that widespread testing could lead to a false positive that would harm the meat industry. Robertson said he was concerned by that possibility but noted that Creekstone sought to use the same test the government relies on.

A few good links

Weights Build Muscles, But Not the Manly Kind - A New York Times article debunking many of the myths surrounding weightlifting women. Refer female friends to this whenever they say ridiculous things like, "Oh, I only use 5-pound weights at the gym because I don't want to get big muscles."

Florida Girls Lift Weights, and Gold Medals - While we're on the topic of weightlifting women, this is a neat article about weightlifting teams of high school girls.

The confessions of a leading psychic - And, for those who enjoyed yesterday's James Randi's videos, here's a post about one of the featured psychics (James Hydrick) confessing that he was just a fraud.

Teaching link: James Randi videos

James Randi is a debunker extraordinaire and host to the million-dollar challenge, a challenge where any psychic who can convincingly demonstrate that they have supernatural powers will win a million dollars. BoingBoing just linked to an 18-minute video wherein James Randi embarasses James Hydrick, a psychic who claimed to be able to move objects with his mind.



Of course I couldn't stop at just one James Randi video, so I ended up pestering my SO with an hour or two of James Randi videos from YouTube. The videos had excellent footage of psychics in action, and while most of the psychics simply gave up or refused to be tested when confronted, I got to thinking that these would still be a good vehicle for starting discussions of skepticism and pseudoscience in the classroom. Thus, I thought I'd post a few of the videos here as the most recent installment of my teaching link series.

James Randi's classic experiment in horoscopes:



James Randi discussing Uri Geller and Peter Popoff:



James Randi performing psychic surgery (caution: filled with fake blood):



James Randi demonstrating spoon bending and discussing the preliminary test a psychic healer will undergo to earn the million-dollar prize:



James Randi discussing Doris Collins, a cold reader:



James Randi explaining homeopathy (contains no footage of psychics, but is a great lecture on the ridiculosity of homeopathy):

Filing Papers

As long-term readers will know, for the past few years I’ve been involved in a faculty-driven attempt to start a field research program at my campus. At the start of this academic year we were told that the program might be shut down due to the possible sale of our field site, and we’ve spent the last few months fighting to prevent that.

We lost the fight.

Of course there’s currently lots of spin that we didn’t actually lose, and lots of talk about how excited everyone still is about field experiences, but that doesn’t change the fact that our current program is being shut down¹.

This morning I’m spending some time cleaning up my office, and I keep coming across papers related to the program. Even though they’re just generic paperwork (budgets, plans for future courses, lists of past courses, summaries of research findings, etc.), they’re filled with hope and excitement. They represent hundreds (and probably thousands) of hours of work by many people in just the past few months, most of which was spent trying to document our successes and justify the program. Now they represent dead weight that needs to be moved out of my current-documents pile and into an archived folder. Who knew that sorting papers could be so depressing?

¹ This decision also makes most of the research that I (and my students) have been doing for the past few years meaningless.

Teaching link - flowchart of the scientific method

BoingBoing recently linked to a flowchart comparing how the scientific method and religious faith build knowledge. The contrast between science and religion is great, but could be somewhat offensive to religiously inclined students¹. However, the scientific method flowchart is clear and informative by itself; I think it would be a great aid for explaining the scientific method to introductory students.

The image is by Wellington Grey, and is Creative Commons licensed.

[Update just before posting: PZ Myers has posted the images as separate figures; his versions should be easier to use in class.]

¹ I'm torn about using the faith portion of this in my classes; on the one hand it is a decently accurate representation of how faith-based systems build knowledge, but on the other hand religion is such a taboo topic that it seems easier to leave that part of the figure out.

Science badges!

The Order of the Science Scouts of Exemplary Repute and Above Average Physique (OOTSSOERAAAP) has a collection of merit badges for scientists. I was never a boy scout, so I've never gotten any badges; now I can finally get some!



Badges I easily qualify for:


Talking science: I'm a biology professor. It's my job.



I blog about science: Um, where am I posting this?



Sexing up science: Drosophila, baby. I've done selective breeding, stock breeding, and worked in a lab that studied fly mating behavior.



Has frozen stuff just to see what happens (LEVEL III): One cannot work with liquid nitrogen and fail to get this badge. The stuff is just so cool.



Inordinately fond of invertebrates: Need I explain why I deserve this? (and that really should be an insect, not a mollusk, on the badge)



I've done science with no conceivable practical application: Oh yeah, I qualify for this. I can't post about it here, but if you know me (or meet me), ask for a description of my qualifying research.



I know what a tadpole is: I do!


Badges that I'm still working on:


Nyah nyah nyah nyah nyah, I've got a TV gig: Does a TV news appearance get at least some points for this badge?



Arts and crafts: I've got lots of scientific photography, but still haven't sewn anything scientific.



Destroyer of quackery: I don't do it as much as I should, though I do attack creationism whenever my students bring it up, and am quite proud of steering a woman away from quackery that a fellow blood-donor was trying to push on her.



Will gladly kick sexual harasser's ass: Happy to do it, but haven't had too much opportunity (though maybe that's a sign I don't deserve the badge ...).

(via BoingBoing)

Mouse party

I love mice, so this is just great: an animation by the University of Utah Genetic Science Learning Center that shows a simplified version of what various drugs do to mouse brains. They call it mouse party. Enjoy. (via PZ)

Teaching links - reading a meniscus

Every now and then I come across a website that has some tidbit of information, or an image, that is just right for a lecture or lab I'm working on. Often this information isn't anything earth-shattering, and thus I don't post it. However, since the information is useful, I think I'm going to change my policies and start posting links to this type of information. The posts won't be long, and probably won't be exciting, but hopefully will be useful to others 'in the trenches.'

Today I'll start the series with a link to Wikipedia's meniscus page; it's got a great (GNU FDL licensed) figure that shows how to read menisci. It's useful for those first-day-of-lab lectures on how to use graduated cylinders¹.

¹ Hey, I told you it wouldn't be exciting.

Death by water intake - a look at osmosis and water balance

Many of you have probably already heard about Jennifer Strange, the woman who recently died while attempting to win a free Wii in a water-drinking contest. If you haven't, Orac has two posts that provide lots of background. Death from excessive water intake is not unheard of; for example, long-distance runners can die if they drink too much water in an attempt to stay hydrated (see here). Given water's relatively innocuous reputation, I thought it would be useful to look at a bit of the physiology behind what happens when a person drinks too much water.

Since much of the physiology of water balance depends on osmosis, I'm going to start by reviewing osmosis before getting into the physiology. If you're familiar with osmosis, feel free to skip the osmosis section (quiz to see if you can skip the osmosis section: which direction will water flow if a cell containing a 300 mOsm solution is placed in a beaker with a 150 mOsm solution?¹)

An introduction to osmosis

A key concept to understand when looking at water intake (and water balance) in organisms is osmosis. Osmosis is the tendency of water to move so that it ends up in equal concentrations on both sides of a semipermeable membrane. As an example, if you have a cell full of saltwater that's surrounded with pure water on the outside, the concentration of water inside the cell will be lower than that of the water outside (the salt inside the cell can be thought of as diluting the water), and thus water will move by osmosis from outside the cell to inside the cell. If you were to flip the concentrations (put pure water inside a cell that's surrounded with salty water), water would move the opposite direction (water would leave the cell). While osmosis may sound complicated, it's nothing more than the diffusion of water across a semipermeable membrane.

We can measure the likelihood that water will move into a solution via osmosis; we call this a solution's osmolarity (or osmotic pressure; units are mOsm). The osmolarity of a pure water solution is defined as having an osmolarity of 0 mOsm; the osmolarity of a solution is increased by adding solutes (salt, amino acids, glucose, or whatever else you want) to it. The higher the osmolarity of a solution, the more likely water is to osmose into it. So, to return to the salt and pure water example above, the pure water solution outside the cell has an osmolarity of 0 mOsm, while the salt solution inside the cell has a higher osmolarity (let's call it 100 mOsm, but at the least we know it's greater than 0), and thus water will move from the lower to the higher osmolarity solution (water will move into the cell).

We use osmosis in our bodies for many things. Osmotic gradients lead to water flowing into our bodies through tiny gaps between the cells of our small intestines (our guts are regulated so that the osmolarity of the gut contents is lower than that of the surrounding cells and interstitial spaces); this bulk flow of water brings along with it digested nutrients, and this is one of the mechanisms of nutrient absorption in our gut. Our kidneys use osmotic gradients to concentrate (or dilute) urine as it's being produced; to make concentrated urine the kidneys pass "pre urine" through an area of high osmotic pressure (high salt and urea concentration), thus causing water to osmose out of the urine (making the urine more concentrated).

Most regions of our body have the same osmolarity (~290 mOsm), and thus osmosis doesn't normally cause a net movement of water into or out of them. However, if we change the osmolarity of one component of the body (say, the blood plasma), then water will start moving from one region of the body to another. So, for instance, if you drank a lot of water, that water will be absorbed into your blood, and your blood plasma's osmolarity will be lowered (as the extra water dilutes the solutes). Once your blood osmolarity drops (say from 290 mOsm to 280 mOsm), water would start moving by osmosis from your blood into the other tissues of your body (as water would move from the lower osmolarity region in the blood plasma to the higher osmolarity region in your body tissues).

The physiology of drinking too much water

Now that we've gotten the general idea of osmosis and water movement, let's take a look at what happens when a person drinks too much water. Before I get into too much detail, however, I want to mention that my chemistry and physiology here will be filled with simplifying assumptions², and that I'm not a doctor and thus nothing I say should be taken as medical advice.

Jennifer Strange is reported to have consumed approximately 2 gallons (~7.5 liters) of water during the contest (data from Orac's post and this article). To put that volume into context, we need to look at how much water is in the human body.

Rough physiological formulas indicate that about 60% of our body mass is water (Berne et al. 1998). About 2/3 of that water is contained inside our cells, and is called intracellular fluid (Berne et al. 1998). The remaining 1/3 is contained outside our cells, and is called extracellular fluid (Berne et al. 1998). About 1/4 of the water contained outside of our cells is blood plasma; the rest is contained in the interstitial spaces around our cells (Berne et al. 1998). So, if we assume that Jennifer Strange was 165 pounds (the average weight of adult females in the US³; I have no idea what her weight was), we get the following amounts of water:

• Body mass: 165 lb (~75 kg)
• Total body water content: 45 L (75 kg * 0.6 * 1 L/kg)
• Total intracellular fluid: 30 L (45 L * 2/3)
• Total extracellular fluid (including blood plasma): 15 L (45 L * 1/3)
• Total blood plasma: 3.75 L (15 L * 1/4)

While those numbers are clearly just approximations, we can now see how large a volume of water 2 gallons (7.5 L) is: it's double the volume of water in the average female's blood plasma, and half the total water content of the average female's extracellular fluid. Adding that much water to a solution can clearly alter its osmolarity (by diluting the ions in solution), and thus will have a strong effect on osmotic movement of water in the body.

Of course we have kidneys, and one of their functions is to excrete excess water. So, a person could safely drink 2 gallons of water, as long as they could excrete those 2 gallons as quickly as they absorbed them. Unfortunately, the kidneys are limited in how fast they can excrete water; Orac cites data that the kidneys of a healthy adult can excrete a maximum of about 1 L of water per hour.

So, to fully understand what happens when a person drinks a large volume of water, we must look at this as a dynamic process. Water is being ingested at a specific rate, is then absorbed into the body (first stop: the blood plasma), is circulated around the body (where it is exposed to the various tissues of the body), and is then excreted from the body by the kidneys.

Based on the newspaper reports, contestants were given 0.25 L of water every 12 minutes, which is about 1.25 L per hour⁴. However, at some point in the contest the ingestion rate was increased, as contestants are reported as being given a "larger bottle." I don't have specific information on the duration of the contest, but to simplify things lets assume that contestants drank 1.25 L of water each hour for the first two hours, and then drank the remainder of the water in the next 2 hours (4 hours total seems like a good estimate for the duration of a radio show contest). Here's the net balance:

• Intake:
• First 2 hours: 2.5 L
  
• Last 2 hours: 5 L
• Total intake: 7.5 L
  
• Kidney filtration:
• First 2 hours: 2 L
• Last 2 hours: 2 L
• Total excretion: 4 L
  
• Net balance:
• Gain of 3.5 L of water

Note that this assumes both that all the water ingested was absorbed immediately (an incorrect assumption, as osmosis of water from the gut into the body will take time), and that all of the kidney filtration occurred immediately (again an incorrect assumption, especially as the contestants were prevented from urinating during the contest). However, lets hope that the delayed absorption of water in the gut is canceled out by the delayed (and possibly inhibited) filtration of water by the kidneys, and keep things simple by ignoring both.

The osmolarity of blood plasma is determined largely by the concentration of sodium in the plasma (typically 145 mmEq/L, which leads to a net blood plasma osmolarity of ~290mOsm; Berne et al. 1998). Given a starting blood plasma volume of 3.75 L (with 145 mmEq/L Na), and a final volume of 7.25 L (3.75 L starting + 3.5 L gain), the sodium concentration of the blood at the end of the contest (assuming no input of sodium from other body stores) would drop to 75 mmEq/L (3.75 L * 145 mmEq/L Na * 1/(7.25 L)). Orac specifies that a sudden drop in blood plasma sodium concentration from normal levels to below 120 mmEq is often fatal, so this drop in sodium concentration would be fatal.

As the sodium concentration of the blood plasma drops, the blood plasma's osmolarity will also drop (75 mmEq/L Na would lead to a blood plasma osmolarity of ~150 mOsm). This drop in plasma osmolarity would create an osmotic gradient between the blood plasma and extracellular fluid (the extracellular fluid would be 290 mOsm initially, as it is generally isotonic to the blood plasma under normal conditions). Thus, once some water is absorbed into the plasma, it will osmose from the blood plasma to the extracellular fluid.

So, let's assume that the extra 3.5 L of water isn't all stored in the blood plasma, but is also moved to the extracellular fluid that's in the interstitial spaces. In this case we start with 15L of extracellular fluid with a sodium concentration of 145 mmEq/L (extracellular fluid has about the same sodium concentration as blood plasma), and end with 18.5 L of extracellular fluid (15 L + 3.5 L gain). This scenario leads to a final extracellular fluid sodium concentration of 118 mmEq/L (~236 mOsm), again low enough to lead to death.

But what is actually causing death? While I'm not an expert here (and haven't been able to find a good reference for this quickly), one of the problems that the body runs into in this situation is that those 3.5 liters of water have to go somewhere. As we've seen above, that somewhere will initially be the blood plasma and extracellular fluid, which will be followed by movement of water into the intracellular fluid pool (i.e., inside cells, which are ~290 mOsm to begin with, and will thus begin absorbing water as soon as the osmolarity of the extracellular fluid drops). All of this water movement means that tissues throughout the body are going to be gaining water, and when tissues gain water they swell. This swelling can be tolerated to some extent in many tissues (e.g., your leg muscles), but can lead to extremely negative effects when it occurs in tissues that have only a limited ability to expand, such as your brain (which is mostly surrounded by bone). Swelling in the brain increases pressure on the tissues of the brain, which can lead to many problems, including reduced transport of nutrients from the capillaries in the brain to the cells of the brain⁵. Many of the symptoms of hyperhydration/hyponatremia are neurologic (fatigue, headache, loss of alertness, cognitive impairment; see here and here), and thus it seems likely that swelling in the brain is at least a contributing factor to why people die after drinking too much water⁶.

To end with a little taxonomic diversity, plants can actually tolerate this type of osmotic situation very easily (you water your plants with tapwater, right?) The difference is that plant cells are surrounded by a rigid cell wall, whereas animal cells have just a wimpy little plasma membrane (plant cells have a plasma membrane too). This cell wall restricts the ability of plant cells to expand (it's like a little suit of armor), and thus once the plant cell is full of water (turgid), the cell wall exerts a force (a pressure) that counteracts osmosis and prevents water from entering the cell. Thus, plant cells can have a higher osmolarity than their environment (e.g., be immersed in gallons of pure water), but not risk death due to excessive water intake⁷.


Reference

Berne, R. M., M. N. Levy, B. M. Koeppen, and B. A. Stanton. 1998. Physiology: 4th edition. Mosby, St. Louis.


¹ Water will flow into the cell.
² In addition to the assumptions stated in the rest of the article, I'm assuming (among other things) that all consumed water was absorbed into the body (it could have been vomited out, remained in the gut, or excreted with fecal material), that the water ingested was pure water (it wasn't; fresh water typically has an osmolarity around 70 mOsm), and that non-excretory sources of water loss (breathing, sweating, crying, etc.) were minimal. I'm also entirely ignoring the lymph system, which transports excess interstitial fluid back to the blood.
³ Data from the Wikipedia and this article.
⁴ Based on this quote "participants were given two minutes to drink an 8-ounce [~0.25 L] bottle of water and then given another bottle to drink after a 10-minute break," and then this quote "Sherrod said she managed to drink eight, eight-ounce bottles but became nauseated after drinking half of a larger bottle." (both quotes from here)
⁵ Nutrient delivery from capillaries to the surrounding tissues requires a pressure difference between the inside of the capillaries and the interstitial fluid surrounding them (the fluid in the capillaries is under higher pressure thanks to the heart; this higher pressure forces nutrient-filled fluid out of the capillaries). Increased pressure in the brain would negate this pressure difference, and could thus reduce (or eliminate) nutrient delivery.
⁶ Note, however, that I am not certain of this. Other factors could also play a role; for example, it seems at least feasible that the changes in ion concentrations might alter nerve and muscle resting membrane potentials enough to cause problems.
⁷ So why do terrestrial plants die from overwatering, you ask? It's actually because plant roots need oxygen to survive (they're an oxidatively metabolizing tissue; they can't do photosynthesis to generate oxygen or carbohydrates because it's rather dark in the soil), and when soil is saturated with water the roots can't obtain enough oxygen. Thus, overwatered plants functionally die by drowning.

[Updated to correct the name to Jennifer Strange.]
[Update 2, July 2007: The Georgetown Medical Center has a detailed article on water requirements during exercise here.]