Ask Surly Amy: Prenatal Vitamins and Isotonicity

Surly Amy,

I’m thinking about pregnancy, so my nutritionist suggested I start on prenatal vitamins. Well, that part I’m not skeptical about. But then she went on to say I should buy powdered vitamins that you mix with water, because then that mixture is “isotonic with your body’s internal pressure” and so “you absorb 100% of the nutrients.” This set off my Bullshitometer, big time. But I wanted to make sure, since prenatal nutrition is a big deal for any responsible wannabe mom. Thanks!


Dear Marissa,

Nutritionist: Although technically a person with a master’s degree in nutrition can be called a nutritionist the mass majority of people who call themselves a nutritionist do not have a degree and many states do not require a degree or certification of any kind to use this title.

Dietitian: Requires a degree. A registered dietitian must be credentialed by the American Dietetic Association and to get this credential one needs at least a four-year Bachelor’s degree in nutrition and dietetics (or graduate degree), and must complete an internship and pass a national registration exam from the American Dietetic Association.

Take that as you will. A retired athlete or a part time chef could get a certificate that says they are a nutritionist. I could go to “Hollywood Upstairs Herbal School” and get a certificate from Mack the yoga guy that says I am a nutritionist. This is not to insinuate that all nutritionists are uneducated or uniformed, many are indeed quite credible. This is just a warning that a healthy dose of skepticism is good when that title is tossed around.

Now that we have that out of the way, here is a message about prenatal vitamins in general:

Prescription prenatal vitamins here in the USA are regulated by the FDA and have to have what they say they have on the label. They also have to do what they say they will do. Over the counter supplements are not regulated. Vitamins handed or recommended to you by a nutritionist who claim they are “isotonic with your body’s internal pressure” and “you absorb 100% of the nutrients” is code for: I do not have a degree in biology, medicine or pharmacology and I know less about these topics than that wacky Skepchick-artist they call Surly Amy.

Now on to the term isotonic.

I’m not a dietitian, doctor, biologist or a pharmacist so I contacted my super-smart twitter friend, PharmacistScott, aka: Scott Gavura to get the lowdown on what isotonic actually means.

Scott Gavura is a pharmacist and skeptic who blogs at Science-Based Medicine, Science-Based Pharmacy, and Skeptic North.

This was Scott’s response to my inquiry:

Hi Amy,

Tonicity refers to the number of dissolved elements or molecules in a solution. For safety and comfort, many liquid drug products must be “isotonic” with our bodies when administered. Isotonicity means that both the solution and our body fluid has a similar amount of dissolved particles in them. Injectables, eye drops, and nasal sprays are all products that would be very irritating on our tissues if they were not isotonic.

Drug products and supplements we take orally, however, do not need to be isotonic. That’s because anything we consume is deposited into a bath of stomach acids and digestive juices, immediately changing the initial tonicity. So an “isotonic” vitamin won’t be absorbed any better than any other vitamin – liquid, tablet or capsule. What’s further there’s no evidence to suggest that liquid vitamins in general are superior to tablets, though they may be preferable (and worth the extra cost) to people who want a vitamin supplement but don’t like tablets or capsules.

Here’s a definition of tonicity. Bottom line is whatever you eat is mixed in with your stomach acid. It doesn’t matter if it’s in a liquid state when you consume it. It must dissolve to be absorbed. And there is no evidence that an “isotonic” vitamin product is superior.

Another issue that you should be aware of is that isotonic is a term that seems to be being pushed by a multi level marketing company. I am not going to link to them because they do not deserve the traffic. But the lack of evidence for the product combined with plenty of evidence for the traditional vitamin tablets would encourage me to get and to recommend regular prenatal vitamins prescribed or recommended by an MD.

Good luck with your planned pregnancy. I hope this helped!

Got a question you would like some Surly-Skepchick advice on? Send it in! We won’t publish your real name, unless you want us to and creative pseudonyms get bonus points! Just use the contact link on the top left of the page.

Amy Roth

Amy Davis Roth (aka Surly Amy) is a multimedia, science-loving artist who resides in Los Angeles, California. She makes Surly-Ramics and is currently in love with pottery. Daily maker of art and leader of Mad Art Lab. Support her on Patreon. Tip Jar is here.

Related Articles


  1. Awesome response, Amy. Knocked it out of the park!

    LawnBoy: that Dara O’Briain bit was playing through my head the whole time I was reading the question. Brilliant.

  2. Pure ancedata but several friends have mentioned that they experienced less stomach upset with liquid vitamins than with the pills or capsules. If you are still taking the pills after you become pregnant this could help in the first trimester.

  3. I never heard the isotonic argument for liquid vits, and I’m sure it’s wrong based on what you provide here. I have heard that some of the things that are in vitamin tablets are not aborbed as effeciently (or as completely, more to the point) in tablet form as they would be in liquid form, and I never knew if that was woo or real.

    There could be something to it. Various vitamins or minerals are passed into our system at various stages in the digestive process and with various different things happening to them. It’s complicated. EG. some forms of B12 are affected by enzymes in the stomach (released by the enzymes) so if they are in a form that somehow protected them from that activity they would not be as effective. (I’m not saying that happens …. just pointing out the factors involved). Most of the time, though, I would think a tablet or capsule would become liquid while in the stomach. So you are taking a liquid vitamin no matter what you do, in a sense.

    However, it is true that a big piece of pharmacology is in delivery. It is not true, for example, that the hormones in a birth control pill can just be put in a capsule and ingested …… they would be digested to a level of ineffectiveness. They have to be packaged (chemically) with buffers to get them into the blood stream without being digested for their energy.

    My skeptical skepticism spidey sense tells me that if delivery of stuff in general (drugs, etc.) to the blood stream (or wherever) is sometimes tricky and requires proper packaging, than it is probably not true that all vitamins and mineral can be willy nilly shoved into the face any way you like and the results will always be the same. Maybe with many, but I doubt with all. But I can find no immediately available information on that.

    I consulted the Skeptical Search Engine (which polls this blog as well as Science Based Medicine):

    The term “Liquid vitamin” got me nothing which tell us it hasn’t been talked about much in the science based med or skeptical blogosphere. Vitamin absorption did not yield useful results, but I did find some quackery about how some enzyme supplements can help absorption of vitamins. (Not true):

    Checking the internet for “vitamin absorption” comes up with lots of claims on sites that sell liquid vitamins.

    “Liquid vitamins are a great way to increase absorpion into the blood stream and produce maximum efficacy”

    “Up to 98% absorbed” (but not stated as compared to non-liquid!)

    “A liquid vitamin with a higher absorbency than tablets and capsules. ”

    “Up to 700% More Absorbable” (than what, not stated)

    So it’s definitely out there as a claim, though more generally than as “isotonic” effects.

    The real place for liquid vitimans, of course, IF you have a reason to use them at all, is for little kids or people who can’t swallow pills.

  4. I’d like to add something. This whole “isotonic” business is woowoo on so many more levels.

    1) An isotonic solution basically means a solution that does not pull or push water into or out of a cell. Solutes that can cross the membrane do not affect the tonicity of that solution. This is distinct from osmolarity or osmolality. Basically since this is about delivering a nutrient, any nutrient that can pass that membrane doesn’t affect the tonicity of that solution.

    2) Isotonic to the human body is 290 mosm/L. So even if isotonic vitamin solutions did somehow give some absorption advantage, the likelihood of actually getting an isotonic vitamin solution by mixing powder in water is pretty damn low. It would require precise measurement, a knowledge of the molecular weight of the compounds involved, and the way they dissociate in water.

    3)And if that weren’t enough, a lot of the vitamins are fat soluble, not water soluble, so they aren’t any kind of “tonic”.

    Basically, it is beyond woowoo.

  5. Unfortunately the isotonic nonsense has been used in all kinds of marketing for many years. In the UK and Ireland, we have had isotonic sports drinks since the 90s (here is one of the most popular drinks:, supposed to help athletes perform better (over water, fruit juice or any other drink) because they are “in balance with your body fluids”. Besides the fact that the toniciy of the drink alters the moment it is mixed with acids, enzymes and whatever else happens to be in your stomach when you drink it (e.g. anything you may have ingested within the previous 3-4 hours), how does one determine the tonicity of any athletes body fluids after he or she has been sprinting, swimming, cycling, rowing….. and consequently losing a few litres of sweat with its various minerals and organic compounds? Now, if they were intended for intravenous, ocular, rectal or nasal application, maybe isotonic sports drinks makers would be less guilty of bullplop marketing… But I guess then they wouldn’t quite sell so much, either.

  6. Based on my own experiences of being pregnant, I can think of one evidence-based reason to choose liquid vitamins over tablets, and that is that a prenatal vitamin tablet is about the size of a golf ball – not something you want to try swallowing when your gag reflex is already operating on overdrive due to morning sickness.

  7. Hi all – Marissa, the OP, here. For what it’s worth, my nutritionist is a Registered Dietitian and Licensed Dietitian, as well as a Certified Diabetes Educator, which is why I see her – got a busted pancreas.

    Anyway, many thanks to Amy for hunting this down. I will happily get regular vitamins and not worry I’m cheating my future babies.

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Back to top button