Water, water,...What?

Aug 02 2010 Published by under [Medicine&Pharma]

So in between titillating tweets Sunday, I spent some quality time with the August issue of Allure magazine. On page 80 I hit a review of a new book by skin-care guru Howard Murad, MD, The Water Secret: The Cellular Breakthrough to Look and Feel 10 Years Younger. The 4" column (Byline: Elizabeth Siegel) focused on some "myths" that Murad debunks in the book.

The last "myth" caught my attention:

Me? Cause bloat?

Salty foods cause bloat.

As a nephrologist who teaches salt and water pathophysiology, I understand how salt can lead to water retention and edema. The answer really surprised me:

When too much of the fluid in our cells leaks out into the connective tissue, "that's what causes bloat," says Murad. Strengthen your cell walls to keep water inside them, and you may even lose weight.

Sigh. My work is never done.

Bloat, puffiness, swelling; all occur because there is extra fluid in the extracellular space, primarily the connective tissue elements of the skin. That statement I will gladly verify. However, most of the time the source of the extra fluid is NOT merely a shift of fluid from the intracellular space.

Body fluid compartments and distribution

Normally body fluids can be divided between the intracellular and extracellular spaces. These fluids differ in their chemical composition because pumps on the membrane of every cell in the body move potassium (K) inside the cell in exchange for sodium (Na). Intracellular fluid has lots of K, while extracellular fluid has lots of Na.

These pumps work constantly to maintain this difference. Any extra intake of salt (NaCl) remains within the extracellular space because the cells make that happen.

Most of the time salty foods make us thirsty. Fast-food places bundle sodas with the fries for this reason! Water taken in with the Na will remain with it in the extracellular space; this is why a salty meal can make you bloated with short-term weight gain.

Osmolar balance

Well, what if you did eat a salt load without taking in fluid? Could it still cause bloat?

Well, yes!

Cells are bags of fluid surrounded by membranes. These membranes have transporters that let stuff in and out. Almost every membrane in the body is freely permeable to water.

Water moves back and forth across these membranes to maintain osmolar balance. Osmolality is the total number of molecules in fluid. It does not matter what the molecule might be; our cells just want the amount of molecules relative to water to be the same everywhere in the body, even if the molecule is K inside the cell and Na outside.

If we take in a load of Na without extra fluid, those pumps in our cell membranes make sure that Na stays in the extracellular compartment, outside of our cells. Since that means there are more molecules relative to water outside of the cells than in them, water will move from in the cells to the outside area, producing puffiness (what we doctor-types call edema). So salty foods can cause that shift in fluid that leads to puffiness.

Can we stop fluid movement?

Now we have an easy question to answer: HELL NO!!!

First, cells in our bodies DO NOT HAVE WALLS. They have membranes that freely allow water to flow through them because this osmolar balance is DAMN IMPORTANT for body function! I have no idea how one could possibly go about "strengthening" these membranes or changing their water permeability.

And I really have no clue how any of this would make you lose weight.

The bottom line

Salty foods will make you swell. If you are prone to edema lay off the NaCl.

Salt shaker image courtesy of PhotoXpress.

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4 responses so far

  • Glfadkt says:

    Despite his misunderstanding (or oversimplification) of the basics of body fluid homeostasis, I gasped in amazement when I came upon Dr. Murad's use of the term "cell wall" in the context of human physiology. Anyone who has taken an introductory biology course should recognize that one of the distinctions between plant cells and animal cells is the presence of a cell wall vs. cell membrane. Jeeezzzz..... This is such a basic fact that the use of "cell wall" in the context of human physiology immediately removed from my mind any credibility that the author might have regarding human physiology.

  • physioprof says:

    It does not matter what the molecule might be; our cells just want the amount of molecules relative to water to be the same everywhere in the body, even if the molecule is K inside the cell and Na outside.

    I know the italicized bit is just a colloquialism, but it is still a bit misleading. Usually when one says that a biological system "wants" something, it implies that the system is actively working towards that end. But in this case, the cell doesn't have a choice, because water always partitions passively across cell membranes. One could imagine hypothetical mechanisms by which cells could actively transport water across membranes, and if such mechanisms existed, then it would make sense to say that cells "want" to achieve the water partitioning that results.

    They have membranes that freely allow water to flow through them because this osmolar balance is DAMN IMPORTANT for body function! I have no idea how one could possibly go about “strengthening” these membranes or changing their water permeability.

    Actually, cell membranes do not "freely" allow water to flow through them. Water flows across membranes through aqueous channel proteins called aquaporins, and the rate of water flow across a cell membrane is a function of the density of aquaporin molecules in that membrane. Peter Agre won the Nobel Prize for his lab's discovery of aquaporins.

    Cells can thus adjust the permeability of their membranes to water by regulating the expression and trafficking of aquaporins. However, the permeability of the cell membrane to water doesn't alter the fact that at equilibrium water partitions such that osmolality is the same on each side of the membrane. All it does is alter the rate at which water crosses the membrane when the system is away from equilibrium.

    • whizbang says:

      Indeed, what Comrade PhysioProf states is true; however, the systems behave as if they work the way I explained it (I have to tell a lot of layfolks about complicated stuff, and I often ignore scientific purity).
      Most cells in the body express aquaporins in a manner that allows free passage of water through the membrane. Some exceptions occur; the collecting duct of the kidney depends on controlled expression (under the control of antidiuretic hormone) to vary water excretion to ultimately return osmolality to its normal value.
      The bottom line is that dysregulating osmotic balance in the body in an attempt to "prevent bloat" sounds like a really bad idea. And I'm not letting sodium off the hook for its role in expanding volume and promoting edema.

  • Kausik Datta says:

    Scientopia is already living up to its claim to fame... There is a nice lesson in cellular physiology right there... WTG, CPP!

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