The Dark Mystery of Black Olives, Ferrous Gluconate and Gluten

The Dark Mystery of Black Olives, Ferrous Gluconate and Gluten

mediterranean meal_black olives


As you may suspect, I spend a fair portion of my time researching and investigating ingredients.

This serves several purposes; one is finding hidden gluten in foods where it may not appear obvious, especially to those new to gluten-free living.

Of course, I’ve “found” hidden gluten in some pretty unthinkable places, like my coffee cup. You can read about my last gluten oops with flavored coffee if you’re interested.

We simply can’t be too careful, can we?

That applies to scouring labels, of course, but also to choosing reliable sources of information.

This is particularly true when it comes to information about our health and the foods we eat (or avoid).

I pride myself on serving you in this area by providing my unique “Smart Nutrition Backed by Science”.

This recent reader comment on the website reminded me how important accurate information is:

“Black olives purchased in a can or jar contain gluten.  The black that you see in the olive is an injected color to make them more “black”.  The injected color has gluten in it!

I didn’t approve the comment. I was afraid someone would skim the page, see the info and take it as fact.

It is false. Black olives do NOT contain gluten.

I suspect the source of this reader’s confusion is ferrous gluconate. It’s an ingredient in most black olives.

GLUconate. Like GLUten, right?


Ferrous gluconate is water soluble iron salt of gluconic acid. Gluconic acid occurs naturally in some fruits and vegetables, wine and honey.

Ferrous indicates iron. Gluconate indicates the sugar acid, as in glucose. Not gluten.

It’s an iron supplement, folks. You can get a prescription for the little black tablets from your doctor if your iron levels need a boost. (There are other forms of iron, so not every iron supplement is a gluconate, but some are.)

In addition to knowing this from a science perspective, I know it first hand; I have a genetic issue that prevents my body from storing iron. I have been on daily lifetime iron therapy for years. Learn more about Iron Deficiency Anemia here.

Of course, like many compounds, there is more than one application for ferrous gluconate.

It is used in the olive industry as a color stabilizer in black olives. It is not injected into the olives. It’s simply added to the brine in which olives are fermented.

The iron (ferrous gluconate) reacts with natural tannins in the olive skin, helping hold the rich black color and keeping it uniform.

Without it, black olives would appear chocolate brown with some color variation.

So, you got that, right?

Ferrous gluconate is not gluten. Black olives are gluten-free.

Thank goodness. Now, you can go back to enjoying those earthy little nuggets of gluten-free joy! ;)

Of course, I can’t leave it at that.

My sweet reader who asserts black olives contain gluten also says she “found out the hard way” with digestive upset and symptoms similar to having ingested gluten.

It wasn’t gluten, but science can explain what it was!

You see, according to the National Institutes of Health (NIH), digestive side effects are common with ferrous gluconate. They include heartburn, upset stomach, stomach pain, nausea, vomiting, constipation and/or diarrhea.

Keep in mind these side effects usually occur when ferrous gluconate is used as an iron supplement, not when consumed via black olives.

It is possible, however, to have an iron allergy. Signs of an allergic reaction to ferrous gluconate include hives, difficulty breathing, and facial, throat or mouth swelling. This constitutes a medical emergency and should be taken very seriously.

There’s one other thing to keep in mind when it comes to consuming ferrous gluconate.

It is derived from corn or potato acid. (Perhaps one of these was the source of my dear reader’s issues with black olives?)

For those with corn allergy or who wish to avoid corn or potato products and derivatives, no fretting!

Lindsay Olives produces ferrous gluconate free black olives (which appear a dark chocolate brown, but are no less delish!) as part of the Lindsay Naturals line. They also state on their website that all their olives are gluten-free.

(No, this is not an olive commercial. They don’t even know I’m telling you about it yet.)

There are other brands to look for, too, the next time you’re browsing the olive aisle. ;)

One last note on olives and ingredients. Olives are casein-free (dairy-free), too. The lactic acid on the ingredients list makes some people pause and wonder. It is not dairy-derived.

I’ve done the research, and shared it in my article on probiotics in June’s issue of “Food Solutions”. If you missed that one, you missed a great issue!

Alright, Honey Bunch. I think that clears up the dark mystery of black olives and gluten.

Of course, this piece did raise another question… Is ferrous gluconate derived from corn GMO-free? 

Well, since about 87% of all corn in the USA is GMO, there is a very good chance it is not. And that is where we leave it until next week when we talk about GMO products and where we draw the line. Don’t miss it!!

In comments below, let me know which ingredients are you uncertain about in foods you’d like to eat, but are afraid they might contain gluten.

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As someone with celiac disease and multiple food allergies, Gigi understands how food can harm or heal. Fully restoring her own health with diet alone after a 25-year health struggle, Gigi now uses her own experiences and the skills she gained as a former neuroscience researcher to share practical, easy-to-understand strategies, science-backed nutrition information, immediately useful tips and recipes to make gluten-free living liberating and positive for everyone!
  • Carolyn Gifreda

    Great article! Thank you so much for such detailed information about olives! :)

  • JillW

    Whew! Glad to hear that about olives, since we enjoy them often. And I’m fascinated by the iron allergy info. We like Lindsay olives here. :)

  • Morghan

    Hola Gigi,
    since you say that about 87% of all corn in the USA is GMO ……………. how can you say a definite statement that is in not in ferrous gluconate?
    Thank you, Morghan

    • Gluten Free Gigi

      I did not say a definite statement that it is not in ferrous gluconate, Morghan. I said, in fact, it raises the question, “Is it?”.

      Maybe read that part of the post again… ;)

      The implication is that it may very well be… but I made no definite statement either way, on purpose. There is more to come, as I said in the article.


  • Anna Popescu

    Thanks for this clarification, Gigi. I make my own kalamata olive tapenade and add black olives to it so it is not so salty.

    I know I’m not alone here in saying that I so appreciate how much time you take to research issues and share your finding with us. :-)


  • Rosemary Erickson

    thank goodness! I loooove black olives ;)

  • Corrina Quader

    Thank you for posting this! Its interesting where we find any food! My son is allergic to potatoes (of all things)……you’d never think of potatoes in olives, now would we! Let the diligence prevail :)

  • Evon Burk

    Thanks for the info Gigi! My quesiton may appear to show ignorance on my part….if so, well….perhaps I am. I’m wondering about Turkey Bacon, Turkey Weiners, Turkey Deli Meat.
    Would you consider these gluten free – or are they a no no? I’ve eaten the Turkey Bacon but thing is, if I ingest gluten, I have no obvious sign of doing so. That’s good that I don’t get sick(er) but bad that I may be ingesting gluten and have no clue. So, if you get the time, would appreciate you to address these items for me. Many thanks for ALL you do!

    • Gluten Free Gigi


      Not ignorant at all! My goodness, we are ALL at the beginning at some point, right?

      I think not asking would be ignorant, but not knowing, well, that’s just part of our learning.

      All of us, even me, are still learning every day.

      Now, turkey products you’re speaking of here, well, flip over and read the label first for signs of obvious gluten (wheat). But my best advice is to recommend products like Applegate Farms, Boar’s Head, Dietz & Watson and Pedersen’s for those products. They produce all natural, preservative free, gluten free meats, including turkey products. Applegate is by far my favorite!

      Hope this helps,


  • Gloria Case

    THANK YOU HONEY BUNCH, for the great info that black olives are gluten free. i looove black olives, even the juice. probably shouldn’t drink the juice because of the salt content.. don’t do it very often. i’m 91 now & still crave black olives as i did when i was pregnant many yrs ago.

    • Gluten Free Gigi

      Hey, sweet Gloria! :)

      I always love hearing from you, my 91-years-young darling.

      I like the juice, too. I know the sodium content is sky high, so I abstain, but I’m an olive lover to the gluten-free core!!

      And you’re so welcome. It is always my absolute pleasure to share info with my Gluten Free Gigi family!

      Enjoy your weekend!!


  • http://n/a Dorothy

    Hi Gigi;
    Enjoy your notes every day…so very informative. Isn’t there a zine you send out called “Food Solutions”? I thought I got it one time and would like to know if I could get it again.

    Keep up the great work…I learn so many helpful tips to pass on to my family/friends.

    • Gluten Free Gigi

      Yes, Dorothy.

      Food Solutions was a mail only hard copy magazine on gluten free living; however, beginning with our July issue, it will be a digital eZine, available to download from my Info Boutique on the website.

      I announced just today this news via the Daily Gluten Free Fix email. If you’re not signed up, perhaps you’d like to so you don’t miss the date of our Shop opening online. ;)


  • Barb

    Interesting article, thank you. I once heard (where, I don’t recall) that black olives are soaked in lye. Please tell me that that isn’t true! Thanks, again.

    • Gluten Free Gigi

      Actually, Barb, lye was never used to cure black olives as far as I know, but dating back as long as 2000 years, maybe longer, green olives were ly cured and some still are. It is not as horrid as you are thinking. Modern olives that use a lye cure include the French Lucques, Italian Cerignola and Spanish manzanilla. I love them all! ;)


  • Alan

    Dear Gigi,
    I wonder if you could find out if Nutritional Yeast has any gluten in it for me, your article was very interesting my preference is for green olives and the bigger the better.
    Alan from the UK

    • Gluten Free Gigi

      Hi, Alan!

      Nutritional yeast doesn’t have gluten. Here in the US we have gluten-free GMO-free varieties, as I suspect you do, too.

      I love green olives, too!

      Glad you like the article… happy weekend!


  • Anita


    Thank you for clearing that up. I had a horrible reaction to black olives and the dietary consultant told me about the “dye” in them. I swelled just like I would if I had eaten wheat. I am neither allergic to corn or potatoes. Maybe it was the iron in them. I cannot take vitamins of any sort so I guess that would explain my reaction. I had eaten them all along and had no problems until the gluten issue came about. I also…so they tell me, have IBS. Just between you and me only…I think this Dr could not explain some other issues and labeled it IBS. I have no other explanation as to why these things happen to me. There are fruits I cannot eat and I cannot eat lettuce in a salad style meal. Makes me swell really bad. I have certified gluten free salad dressing and raw vegetables. I am so confused! I gain weigh on this gluten free diet. I don’t eat much. I don’t even eat bread very much beca

  • Anita

    My computer messed up on the above comment. I just want to tell you thank you for all you do! Love you Gigi!


    • Gluten Free Gigi

      No worries, at all, Anita!

      I think it may be the iron that got to you; however, from the bit in previous comment about IBS, etc., I think there are several “issues” going on with you.

      I can tell your issues are not totally resolved in terms of digestive health.

      Keep following along and as you have questions, please ask here or via email (

      I’m here to help, it is my pleasure!!


  • Evelyn Scofield

    I know this isn’t about olives but it made me think, I’m looking for a sweet pickle relish that’s gluten-free?

    • glutenfreegigi

      Most should be gluten-free. Read labels carefully, of course.

  • MamaCassi

    LOVED THIS. Thank you. i’ve missed olives, (since many are in vinegar and i react) and thought of adding black olives to my gluten-free pizza tonight but was a little worried. This was wonderful and very helpful!

    • glutenfreegigi

      Great news! Yes, there is cause for much concern when you really cannot have gluten and hear rumors about certain foods having/not having gluten in them. That is why the Smart Nutrition Backed by Science approach is best – facts! :)

      Enjoy that gf pizza next time, WITH the black olives. Also, I recommend Lindsay naturals – have yo u seen these? They have NO additives, and they are not “black” because of this – they are a deep gray/brownish color but are real black olives. They just don’t have the color enhancer, which is great to me!