The prime minister made the comments during an interview with BBC Breakfast.
Presenter Dan Walker asked Johnson about his 2020 resolution to lose weight - and asked if he would be ditching meat and dairy products from his diet.
"No, I'm not, no I'm not. I had thought of it - but it requires so much concentration," Johnson replied.
"I do take my hat off to vegans who can handle it, who can manage to avoid non-vegan products...You can't eat cheese can you? You can't eat cheese if you're a vegan? It's just a crime against...cheese lovers."
Last year, it was reported that the Conservative politician told a fellow MP that he was 'toying with going vegan', saying 'I'm trying it out'.
He reportedly added that he lost 12lb in two weeks after ditching meat, eggs, and dairy.
However, even if Johnson did try out the plant-based diet, it is unlikely to be persuaded by the ethical argument, as someone who has previously voiced support for bloodsports - including fox hunting and bullfighting - in the past.