No British train station or high street would be complete without a Greggs bakery. The merchants of mass-produced pastries are as quintessential as they come. And last year they won plaudits for turning vegan. On the back of their success, other fast food brands shortly followed suit.

In fact, Greggs’s vegan sausage rolls have been so successful, the company announced a “phenomenal year” for sales in 2019 driven in part by their new product and that they would share a £7m ($9.17m) bonus equally between staff.

Like the sausage roll, another staple of British fast food, fish and chips, has had a vegan make-over. Replacing cod or haddock with the plant-based tofu (sometimes also with a layer of crispy nori seaweed to create the effect of fish skin), vegans can now enjoy “tofish” with their chips.

Read more from The Vegan Factor on BBC Good Food

They might sound like a guilt-free alternative or a gateway for the vegan-curious, but vegan fast food alternatives are often worse for your health than the meat equivalent.

“Obviously the pros are that [veganism] is getting people to think about plant-based foods, but the con is that it makes us think that it is good for you when it can be equally or more unhealthy,” says Megan Rossi, a dietician at King’s College London and author of Eat Yourself Healthy.

“Tofu contains omega 3 but it is a type that is not as active as animal sources,” says Rossi. Plant-based sources contain a type of omega 3 called alpha-linolenic acid (ALA). For our bodies to be able to do anything particularly useful with ALA, we need to convert it into eicosapentaenoic acid (EPA) or docosahexaenoic acid (DHA). However, humans are not particularly well equipped to do this.