That study reporting worrisome levels of zinc in tuna? It’s being retracted

Recently, a rash of news outlets posted concerns that canned tuna and other products may contain potentially dangerous levels of zinc. They were all wrong.

News outlets such as The Daily Mail and The Sun reported findings from a recent study, which showed that canned foods such as tuna may contain 100 times the daily limit of zinc — raising concerns about how such huge doses of the mineral could be causing digestion problems. The last author of the study told Retraction Watch the paper is going to be retracted, because the authors made a fundamental error calculating the amount of zinc present in canned foods.

Last week, the UK’s biggest health website NHS Choices posted a critique of the paper, in which they recalculated the levels of zinc present in canned foods:

The researchers measured the level of zinc in samples of canned tuna, asparagus, chicken and sweetcorn, and calculated there would be 996mg of zinc in a meal containing typical portions of tuna and of asparagus. They then exposed cells from the human small intestine to this level of zinc.

However, we calculated this meal should have contained 2.1mg of zinc, not 996mg. The recommended daily allowance is about 9.5mg a day for men and 7mg for women, so this would be within the limit.

Last author of the paper, Gretchen Mahler of Binghamton University, told us:

[The news stories about the study] brought the attention of writers and scientists and we heard back from a lot of people very quickly that there was a mistake. We responded to everyone and never tried to cover up the mistake or anything. We let the publisher know as soon as we figured out that there was an error. We had asked to publish a correction, but the publisher decided to retract, probably based on all of the press. We are sorry, and I am embarrassed that there was such a stupid mistake in our calculations, but in the end it is good that we know and that people are checking facts before they publish. Again, we tried to do the right thing as soon as possible and there was no scientific misconduct, it was an honest mistake.

ZnO nanoparticles affect intestinal function in an in vitro model” was flagged this week by the journal Food & Function with an Expression of Concern, which notes the data are “not reliable.”

Mahler explained the nature of the mistake:

We are interested in nanoparticles that are used in food and food packaging. We know that [zinc oxide, ZnO] nanoparticles are sometimes used in food packaging, but couldn’t find any reports on the amount of ZnO [nanoparticles] that are potentially ingested. We needed an estimate to conduct out studies, so that is why we measured the zinc in naturally low-zinc canned foods and converted that amount to ZnO nanoparticles. The calculations that converted the concentration of zinc in our small samples to a serving size that would be ingested had a unit error. The error made the amount of potential zinc ingested two orders of magnitude higher than it should be if the calculations had been correct.

We asked Mahler about the news stories that had covered the study — and an an accompanying press release — which reported worrisome levels of zinc in canned foods:

Yes, those news stories are incorrect. Again, if the calculations were correct they would have shown that the amount of zinc in a serving of tuna is within the [recommended daily allowance]. The goal of our work was not to test cans for zinc, it was to study realistic doses of ZnO [nanoparticles] on gut function. We only tested three cans from two different brands, which is not a comprehensive study.

According to the NHS Choices critique:

The Sun and the Mail Online reported the results of this research quite dramatically, both stating that tuna could be wreaking havoc with people’s digestive systems, but neither noticed that the calculations appear inaccurate. However, to be fair, you would probably need to spend several hours analysing the researchers’ methods to spot the inaccuracy – and most journalists have neither the time nor the resources to do this…It’s difficult to form any conclusions from this study because it was based on zinc levels that were vastly higher than would normally be consumed in tinned food.

Mahler added:

Overall, I think that the data shows that ZnO [nanoparticles] do not have a large effect on gut function. We didn’t see overt toxicity, like cell death, even at the highest doses studied. The more subtle effects that we did see, like changes in glucose absorption, were present at the low dose, however. This is interesting, I think. Again, I stand behind the rest of the data in the study and I think it is interesting and worthy of publication. We will rewrite the article and resubmit it. I hope it will be published.

In 2015, the middle author of the paper, Elad Tako, retracted another paper — about iron levels in maize — after the authors realized some aspects of it were incorrect.

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5 thoughts on “That study reporting worrisome levels of zinc in tuna? It’s being retracted”

  1. How the heck could a finding of almost 1 gram of zinc per serving not raised the alarm in anyone at that lab. Don’t they have any appreciation of what a gram of any common salt looks like on a balance pan? Indeed it would be more than 1 gm of ZnO. Where did they think this zinc was coming from anyway?

  2. What is the point of an “Expression of concern” when the expression contains no information? The journal would be better placed to simply link to RetractionWatch.

    Regarding journalists not having time to carefully check methods and calculations when reporting on papers published in reputable, peer reviewed and therefore completely accepted by all including skilled scientists – at least until it is marked with EoC or retraction, rather indicates a problem with treating some journal articles with undue belief in results posted. Even “predatory” journals sometimes publish good stuff, why assume “peer reviewed” journals ONLY publish good stuff?

  3. As Agesilaus notes, it’s rather…surprising…that none of the authors questioned the finding of (nearly) a full gram of zinc in a meal of canned food. At those levels you would expect to see significant reporting of zinc toxicity in the canned-food-eating population. On general principles I would suspect trouble, and want to sanity test my ICP-MS result by doing something straightforward like spiking a gram of ZnO into a can of tuna.

    Where were the reviewers in this process? Leaving aside spiking tests and other possible experiments, I suspect that part of the problem is that we’re too deferential and reluctant to ask our fellow scientists to “show their work”. If an undergrad (or sometimes a new grad) student in my lab tells me that they need 500 mg of antibody in 10 mL of buffer to do their Western blot, I get them to walk me through their calculations step by step until they find their error. It’s difficult to find a tactful way to ask a senior scientist to do the same.

    As a final thought, there’s an ethical question here. If I discover evidence of serious metal contamination in a widely-consumed food product, what are my moral (and , for that matter, legal) obligations to report that information? In this instance, I feel like “Use the information to do at least a few months of cell biology experiments; demonstrate in an in vitro model that these doses have toxic effects; write a paper that will spend another couple of months in peer review; and let the governmental food inspection authorities find out about the problem in The Daily Mail” may not be the optimal handling.

    Similarly, do journal editors – and peer reviewers – have an ethical obligation to act when presented with a manuscript that deals with significant unreported public health risk?

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