Fans of tuna will know it's a really great food (if you can get over the fishy fish smell, that is). Tuna works with pretty much anything and tastes awesome on its own - something few fish can claim they do. Tinned tuna, in particular, is a household staple that a lot of people can't do without. A tuna melt right now would be amazing.
When you're not mixing tuna in with worryingly high amounts of cheese and pasta, canned tuna is actually a great health food for losing weight and trying to stay healthy. One can is a great source of protein and essential nutrients such as omega-3 fatty acids, selenium and vitamin D. It turns out, however, that I (and a lot of other tuna fans) may have to slow our roll, because a new study reveals that canned tuna could be causing "leaky gut syndrome".
As it turns out, tinned tuna contains up to 100 times more zinc than it's safe to consume, which could wreak havoc on people's guts, as new research suggests. The mineral is commonly used to line the inside of cans due to its anti-microbial qualities, which help to prolong foods' shelf lives.
These new findings suggest zinc leaches into food and later becomes lodged in people's digestive systems, altering their abilities to absorb nutrients. This may also make their guts more permeable, allowing toxic substances to enter their bloodstreams, according to the researchers. Pretty gross for something so delicious.
Study author Professor Gretchen Mahler, from Binghamton University in New York, said: "An increase in intestinal permeability is not a good thing - it means that compounds that are not supposed to pass through into the bloodstream might be able to." This leads to the aforementioned "leaky gut syndrome."
Whilst not medically recognized, Professor Mahler claims disorders such as multiple sclerosis are caused by the immune system reacting to substances absorbed into the bloodstream via a porous bowel (a symptom of leaky gut syndrome). Excessive zinc intake has been linked to seizures, fever, vomiting and fainting.
The researchers analyzed cans of sweetcorn, tuna, asparagus and chicken. These foods were chosen as they were naturally low in zinc, and are normally packaged in tins lined with the mineral.
Results further suggest tuna tin linings and the fish at the center of such cans are contaminated with more than 5,000 parts per million (ppm) of zinc. The juices at the bottom of the can have around a third of the metal contamination as the food touching the tin.
The team went on to make a model of the human small intestine, and found that tiny fragments of zinc cause inflammation, which makes the gut more permeable and allows harmful chemicals to enter the bloodstream. The study also found the transport of iron and glucose falls by three quarters and almost a third, respectively, after zinc exposure.
Professor Mahler concluded: "we found zinc oxide nanoparticles at doses that are relevant to what you might normally eat in a meal or a day can change the way your intestine absorbs nutrients or your intestinal cell gene and protein expression."
"Nanoparticles tend to settle onto the cells representing the gastrointestinal tract and cause remodeling or loss of the microvilli, which are tiny projections on the surface of the intestinal absorptive cells that help to increase the surface area available for absorption."
The professor continues: "This loss of surface area tends to result in a decrease in nutrient absorption. Some of the nanoparticles also cause pro-inflammatory signalling at high doses and this can increase the permeability of the intestinal model."
She stresses the results are based on zinc's effect on cells grown in a laboratory, how this affects human health in the long term is still unclear. For now though, maybe just have a few fewer tuna melts.