"Tinned tuna could wreck your guts as it has up to 100 times more zinc than is safe," is the headline from The Sun. The report was prompted by a laboratory experiment that aimed to test whether the levels of zinc found in the linings of some food containers were leaking into the contents and causing digestion problems.
However, it appears there was an error in the researchers' calculations that meant the levels of zinc should actually have been well within guideline recommendations.
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.
The cell experiments showed reduced cell function after exposure to 996mg zinc, but this is unlikely to accurately represent what would happen after cell exposure to 2.1mg of zinc.
Zinc is an essential mineral that helps with many bodily functions, including making new cells and healing wounds. High doses can reduce the amount of copper the body can absorb, which could lead to anaemia and weakening of the bones.
Where did the story come from?
The study was carried out by researchers from Binghamton University and the Department of Agriculture, both in the US. It was funded by the US National Institutes of Health, the Mexican National Council of Science and Technology, and the Virginia Tech National Center for Earth and Environmental Nanotechnology Infrastructure. The study was published in the peer-reviewed journal Food and Function.
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.
The Mail Online also linked zinc exposure to conditions and symptoms such as multiple sclerosis, seizures, fever, vomiting and fainting, none of which were tested in this study because it was not carried out in humans. The Mail Online reported that the study was performed in a laboratory, but The Sun did not mention this.
What kind of research was this?
This was a laboratory study that aimed to measure the level of zinc in different canned foods and see whether these concentrations had any effect on human cells present in the stomach lining. Specifically, it investigated whether the zinc levels affected the cells' ability to absorb different nutrients.
While this type of research can provide preliminary data, it does not necessarily translate to what would happen in the human body. It's also not possible to predict long-term outcomes or link this data to medical conditions.
What did the research involve?
The researchers took samples of 4 canned foods, which they freeze-dried and ground into a powder before measuring the level of zinc in each sample. The foods were asparagus, chicken, tuna and sweetcorn. They tried to calculate from the levels of zinc found in the samples how much zinc would be consumed in a typical portion of each food.
Next, the researchers performed a variety of experiments on Caco-2 and HT29-MTX cells. These cells come from human tissue taken from the colon and are commonly used in laboratory experiments to represent the small intestine.
The cells were exposed to the doses of zinc the researchers had calculated to be in a typical portion of tuna plus a typical portion of asparagus to see if it affected the cells' ability to absorb iron, glucose and fatty acids.
What were the basic results?
The researchers reported that the levels of zinc in a portion of tuna and a portion of asparagus amounted to 100 times the recommended daily intake. However, it appears they miscalculated this figure.
The initial measurement found that there was 0.02687mg of zinc per gram of tinned tuna after it had been dehydrated. A typical 112g portion of tuna weighs 47g when dehydrated, which would mean there would be 1.27mg of zinc per portion.
For asparagus, the concentration was 0.06665mg of zinc per gram after dehydration. A typical 126g portion of tinned asparagus weighs 12.2g when dehydrated, which equates to 0.81mg of zinc per portion.
We therefore calculated there would be 2.1mg of zinc in a meal containing a portion of tuna and a portion of asparagus. The researchers, for some reason, reported a figure of 996mg.
They found that exposing the cells to 996mg zinc led to:
- a 75% decrease in iron absorption and a 30% decrease in glucose absorption
- a reduction of the surface area of the cells, so there was less surface area available to absorb nutrients
How did the researchers interpret the results?
The researchers said the ingestion of zinc oxide particles in food could alter intestinal function in laboratory models of the human small intestine, adding that this study highlighted the importance of assessing the safety of food products that could contain zinc particles.
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.
There was also the major limitation that cells may not respond in the same way in humans as they do in a laboratory. We are certainly not able to link consumption of these canned foods to specific medical conditions.
Finally, it's also worth noting that the Food Standard Agency's UK Total Diet Study, carried out in 2000, assessed the levels of several metal elements (including zinc) found in foods and supplements, and whether they posed a risk to human health. It concluded that dietary levels of zinc posed no such risk. More information on this study can be found on the FSA website.