Tinned tuna contains up to 100 times more zinc than is safe, which could wreck havoc on people’s guts, 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.
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.
Study author Professor Gretchen Mahler, from Binghamton University, 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.’
The proposed health condition ‘leaky gut syndrome’, which is not medically recognised, claims disorders such as multiple sclerosis are caused by the immune system reacting to substances absorbed into the bloodstream via a porous bowel.
Excessive zinc intake has been linked to seizures, fever, vomiting and fainting.
Tinned tuna contains up to 100 times more zinc than is safe, which could wreck havoc on people’s guts. The mineral is used to line cans due to its anti-microbial qualities (stock)
WHAT IS LEAKY GUT SYNDROME?
‘Leaky gut syndrome’ is a proposed condition that some doctors claim may be behind long-term disorders, such as chronic fatigue and multiple sclerosis.
Although it is not medically recognised, some believe it is caused by the immune system reacting to germs, toxins and other substances being absorbed into the bloodstream via a porous bowel.
There is little evidence supporting the theory that a porous bowel causes health problems or that ‘treatments’, such as herbal remedies, are effective.
Supporters of leaky gut syndrome being a diagnosis claim the intestine’s barrier prevents large molecules entering the bloodstream.
If leaky, this barrier becomes less effective and may lead to a host of health concerns.
A leaky gut barrier may be caused by alcohol or aspirin, which irritate the bowel lining but usually just cause mild inflammation.
Conditions such as Crohn’s, coeliac disease and sepsis may also increase intestine permeability.
Food allergies, asthma and autism are also claimed by some to cause the gut wall to become ‘leaky’.
Some believe probiotics, gluten-free foods and low-sugar diets can ‘cure’ leaky gut syndrome, however, evidence is limited.
Source: NHS Choices
Tuna is the most contaminated canned food
The researchers analysed cans of sweetcorn, tuna, asparagus and chicken.
These foods were chosen due to them being naturally low in zinc, as well as normally being packaged in tins lined with the mineral.
Results further suggest tuna tin linings and the fish at the centre of such cans are contaminated with more than 5,000 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.
Tinned chicken is the second most contaminated of those analysed, followed by asparagus, which has approximately two third the zinc levels of tuna, while sweetcorn has a third of the canned fishes’ content.
Zinc breaks down the gut, preventing nutrient absorption
A laboratory model of the human small intestine found tiny fragments of zinc cause inflammation, which makes the gut more permeable and allows harmful chemicals to enter the bloodstream.
The study, published in the journal Food & Function, also found the transport of iron and glucose falls by three quarters and almost a third, respectively, after zinc exposure.
Professor Mahler said: ‘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.
‘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.
Professor Mahler stresses, however, the results are based on zinc’s affect on cells grown in a laboratory, with long-term human health implications being unclear.
She added: ‘It is difficult to say what the long-term effects of nanoparticle ingestion are on human health, especially based on results from a cell-culture model.’
The researchers are investigating the impact on zinc on the guts of chickens.