What if we told you that your drinking water isn’t necessarily just water?
Turns out, there’s a lot more in your tap water than just plain old H2O, as the Flint water crisis
showed a few years ago. But even if you do have clean drinking water, that H2O isn’t quite as
straightforward as you might think.
That’s because of a particular isotope of hydrogen called deuterium, and the effects that
deuterium water has on the body. Here, we’re taking a closer look at light water versus heavy
water and why it’s time to make the switch to deuterium depleted water.
The Three Isotopes of Hydrogen
But first, let’s talk about some basic chemistry.
While you might think a hydrogen molecule is just a hydrogen molecule, it’s not quite that
simple. There are three naturally occurring isotopes of hydrogen:
Note the phrase naturally occurring . Scientists have synthesized other nuclei of hydrogen in
laboratories, but these isotopes are highly unstable and do not occur outside of a laboratory
Protium is by far the most common isotope of hydrogen, though it’s not often referred to by its
formal chemical name. Deuterium and tritium do occur naturally, but they are far less common in
nature than protium.
What’s in an Isotope?
What’s so special about these three isotopes?
At an atomic level, the difference between a protium atom versus a deuterium atom versus a
tritium atom is quite simple. Protium (the most common isotope of hydrogen, usually referred to
as simply hydrogen) has one proton and no neutrons, which is where it gets its name.
Deuterium, derived from the Greek word deteuros for “second”, has one proton and one
neutron. Last but not least is tritium, which has one proton and two neutrons.
What’s the big deal about one little neutron?
The addition of that one little neutron makes deuterium more massive (i.e. atomic mass) than
protium, which is why it’s sometimes called heavy hydrogen. This is also where heavy water
gets its name, and how you can distinguish between heavy versus light water.
Light Water vs. Heavy Water
Wait, there’s light water and there’s heavy water?
Yep. And no, we’re not talking about the weight of the water itself.
What is Heavy Water?
When you think of water, most of the time you’re thinking of good old fashioned H2O made up
of protium atoms bonded with oxygen.
Heavy water, on the other hand, is made of deuterium oxide (D2O) or deuterium protium oxide
(DHO). Deuterium oxide is a stable, non-radioactive isotope of water composed of two
deuterium atoms bonded to an oxygen atom. Deuterium protium oxide, on the other hand, is
made of an oxygen atom bonded to both a deuterium atom and a protium atom.
What’s the Difference Between Light Water and Heavy Water?
That one little neutron makes a huge difference in the water you’re dealing with.
The added neutron means that deuterium is more massive than protium. And while that might
not seem like a big deal when we’re talking about one little atom, it means that deuterium oxide
atoms are innately more massive than regular H2O. In fact, deuterium is about twice as heavy
as protium, which means D2O atoms are about twice as heavy as H2O.
When you get a bucket of those molecules together, you get heavy water. Trust us, you’d be
able to tell the difference–pure heavy water is denser and more viscous than regular water.
So, what is light water?
Light water is the exact inverse of heavy water. It’s water that has been treated to contain lower
than the usual number of deuterium atoms. It may contain some trace amounts of deuterium,
but not enough to change the properties of the water itself.
In non-science speak: if you had a glass of regular water and a glass of light water side-by-side,
you wouldn’t be able to tell the difference with the naked eye, or by feeling it. You would need a
How Abundant is Heavy Water?
The short answer? Not very.
The long answer? It depends.
Heavy water does occur naturally, but it’s much less common than light water. The natural
abundance of heavy water varies between water sources, but to put it in context, there’s only
one deuterium atom for every 6,400 atoms of hydrogen in the ocean. Or, in simpler terms,
99.98% of the hydrogen atoms in the ocean are not deuterium.
That said, deuterium does occur in natural concentrations in everyday drinking water.
Heavy water (i.e. water that is straight deuterium instead of protium) occurs the most in
laboratory settings and heavy water refineries. That’s because deuterium is primarily used as a
tracer in scientific research and thermonuclear reactions. In fact, heavy water is a critical
ingredient in nuclear reactors, since it eliminates the need to refine fissionable uranium.
So, to answer the original question: while there is some deuterium in your water, it’s not the
How Do I Know If I’m Drinking Heavy Water?
As a rule, the average person won’t stumble across heavy water (i.e. water that is mostly or
entirely composed of D2O instead of H2O). That’s because, to reiterate, pure heavy water is not
very common in nature, and much of the heavy water in the world is explicitly refined in
laboratories either for scientific research or for use in nuclear reactors.
You would know if you’re drinking heavy water basically as soon as you drank it. Again,
deuterium is more massive than protium, which means D2O is about twice as heavy as H2O. In
translation, heavy water is far more viscous and dense than regular water–you would be able to
spot it immediately.
Keep in mind, however, that pure heavy water is generally refined for laboratories and nuclear
reactors, so you’re unlikely to encounter it in your tap.
That said, you may run into deuterium in your own drinking water. While deuterium atoms are
more massive, they won’t change the density and viscosity of light water too much unless they
make up a significant percentage of the atoms in the water. In those cases, you wouldn’t know
there’s deuterium in your water without a chemical test.
What is Deuterium Poor Water?
Deuterium poor water, or deuterium depleted water, is exactly what the name implies.
To refresh, light water is water that has been treated to contain lower than the
naturally-occurring amount of deuterium atoms, though it may still have trace amounts of
deuterium. Like heavy water, light water is also refined for use in nuclear reactors and is actually
essential in allowing humans to generate electricity from nuclear energy.
Deuterium depleted water is water that has been treated to remove all or almost all trace
amounts of deuterium.
How is Deuterium Content Determined?
To understand how it works, it helps to understand how deuterium content is determined.
The amount of deuterium in a given water sample is typically determined using mass
spectrometry, which is an analytical tool for measuring the mass-to-charge ratio of one or more
molecules in a given sample. Mass spectrometry is typically used to identify unknown
substances via molecular weight determination, as the tool allows scientists to calculate the
exact molecular weight of a given sample.
We won’t drill you on the science, but the gist of it is that mass spectrometry allows scientists to
figure out what’s in a glass of water at the molecular level by checking the weight of the various
molecules in the sample. Keep in mind, deuterium is more massive than protium, which means
D2O is about twice as heavy as regular H2O.
Deuterium content is given in parts per million (ppm) - there’s 1 deuterium atom for 6400
When you get deuterium depleted water, you’ll see a rating marked in ppm. This tells you how
much deuterium is in the water per one million hydrogen atoms. The higher the ppm, the higher
the deuterium content of the water.
For context, the average ppm of drinking water is about 150 ppm (over 16 mmol/L) and more
than 10 mmol/L in living mammals. In translation, your drinking water has an average of 150
molecules of D2O for every one million hydrogen molecules.
How is Deuterium Depleted Water Made?
Deuterium depleted water is made by relying on the different physicochemical properties of
deuterium molecules compared to regular hydrogen atoms.
You’ll recall that deuterium has a neutron, while protium does not. This means that while D2O
molecules have similar properties to regular H2O, D2O molecules are about twice as massive
The method for separating deuterium from protium is highly energy intensive, which is again to
the credit of that extra neutron.
You see, deuterium’s mass means that it has a lower frequency of vibration and lower
zero-point energy compared to protium. Basically, it’s stable, it’s happy where it is, and it takes
quite a lot of energy to make deuterium break its bonds–quite a bit more than protium, anyway.
This translates to a slightly different boiling point for D2O versus H2O, which is important for
scientists because it means that the concentration of H2O in the vapor phase is higher than
D2O. To separate deuterium molecules from regular hydrogen, the treated water is evaporated
several times in various distillation columns. Since there’s more H2O in the vapor than D2O,
distillers can collect the vapor after several rounds of distillation.
Keep in mind that this really isn’t a process you can do at home. There’s a reason why heavy
water and light water distilleries exist–D2O has a slightly lower boiling point than regular H2O,
but not by much. Refining deuterium in or out of water is a precise process requiring specialized
equipment, which is why heavy water refineries are an expensive investment.
Why Drink Deuterium Depleted Water
If it’s that much trouble to separate deuterium from protium, why go to the trouble? Why buy
deuterium depleted water instead of drinking water from your tap?
The simple answer: because the human body doesn’t process deuterium that well.
Deuterium is still an isotope of hydrogen, so when your body encounters it, it treats deuterium
just like protium. It can swap in deuterium for almost any biochemical process where it would
normally use protium. The problem is that you don’t want your body to do that.
That’s because of the kinetic isotope effect , or KIE, a chemistry phenomenon associated with
isotopically substituted molecules exhibiting different reaction rates. Basically, while you can
expect deuterium and protium to behave more or less the same way, deuterium’s added mass
means that it has a lower frequency of vibration and lower zero-point energy.
The higher the frequency of vibration, the easier it is to break the bonds a given molecule forms.
Protium’s rate of vibration and zero-point energy are higher, which means the bonds are easier
to break. In basic terms, the more a molecule vibrates, the less energy required to make it
vibrate until the bonds break apart.
Because deuterium’s mass gives it a lower frequency of vibration and lower zero-point energy, it
takes a lot more energy to break the bonds that deuterium forms, which slows the rate of
For the human body, that’s bad news.
Basically, when your body substitutes deuterium for protium in any hydrogen bond (and trust us,
your body forms a lot of hydrogen bonds) your body will have to work significantly harder to
break those bonds once they form.
If your body has to break the bonds to release energy, for example, that means you have to
expend more energy to get the energy your body relies on. It’s a less efficient system all around.
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