It's because it tastes, looks and smells pretty gross.

There are two schools of thought when it comes to canned tuna – some hail it as their go-to protein source for snacks and lunches on the run, others are put off by talk of high levels of mercury or rumours that the protein in tuna is less "bio-available" than other forms of protein.

Mercury exists in the environment including our oceans, and all fish will contain low levels of methylmercury. Predatory fish, such as shark, perch, swordfish, marlin and catfish, live longer and grow bigger so therefore absorb more mercury.

High mercury exposure can damage the nervous system, and is particularly dangerous for unborn babies as their brains develop. Studies have shown that babies born to mothers in communities that eat a lot of high-mercury fish have subtle developmental delays.

But given canned tuna is generally made from smaller tuna fish that are caught within their first year of life, Food Standards Australia and New Zealand say eating a can of tuna per week is perfectly fine, even for pregnant women.

"Canned tuna is likely to have less mercury than fresh tuna steak because it's made with younger fish," Simone Austin, spokesperson for the Dietitian's Association of Australia, told Coach.

But Austin suggests tuna eaters read labels carefully – particularly when it comes to quantities of omega 3 fatty acids, which are important for our heart health.

"People like to have tuna for omega 3 fatty acids but the levels from brand to brand can be quite different – some have 100mg while others have 400mg," Austin says.

"Don't forget other canned fish like sardines and salmon tend to have a lot more omega 3s."



A sandwich can partially disguise canned tuna's vile taste

Flavoured tunas can also have excess sugar and salt that you might want to be wary of.

"I saw one with sweet chilli had about 10g of sugar in it, but the ones in oil have basically none," Austin says.

"Some brands don't use extra virgin olive oil, so it can be a good idea to drain the oil out and add your own extra virgin olive oil, which has the most antioxidants."

Tuna in spring water tends to have less flavour, but Austin says it can be a good option for cooking.

RELATED: Go fish: Which varieties pack the most nutrient punch

As for suggestions that protein in tuna isn't as bio-available, Austin says there is no evidence this is correct.

In fact, there is 24g of protein in every 100g serve.

"Fish is a very good source of protein – it's a complete animal protein, which means it's got all the amino acids and is very bio-available," she says.

"I like people to remember to keep variety in their diets. You might have tuna a couple of days a week, but you also should look for protein from eggs or a cottage cheese or fresh chicken or sardines or prawns so you get a wide range of nutrients."

RELATED: Fish oil can turn you into a fat-burning machine

Tuna used to get slammed for being an environmental sucker, but Greenpeace have given Fish 4 Eva, Safcol, Coles and Aldi brands a pretty good tick, given most of them have moved to 'pole and line caught' tuna, and tend to be phasing out overfished species, such as yellowfin tuna.

Moral of the story is that tuna is a good source of protein, it's cheap and the fact it comes in a can means it's easy to store for a quick snack or salad topping.

But let's be honest, none of this information takes into account the main reason not to eat canned tuna: it's often grey and kind of gross. [Like eating rancid cat food — Ed.]

So we'll leave it up to you whether you eat it or not.