Although veganism tends to split opinion among sports nutritionists and the general public alike, Smalling is pleased with the results so far.
"My wife is vegan and has been for a few years," the England international told The Telegraph. "She's often tried to persuade me. She cooks a lot at home and half the time I was eating vegan anyway.
"After a while I started to cut out red meat, because red meat causes quite a lot of inflammation and I had a lot of tendinitis in my knee, which many footballers get.
"Red meat causes a lot of that inflammation and when I was cutting that down, the tendinitis started to go. It used to be a problem in terms of warm-ups, because getting going was a bit of a nightmare. My tendinitis has got better and better.
"Now I've gone fully vegan and normally the start of pre-season can be quite a tough time, but I haven't felt it at all. At first, I was half and half, so I was cutting out red meat but still eating other meat. But then the missus throws a lot of documentaries at you and it opens your eyes a little bit in terms of different industries and that persuaded me to go full vegan.
"Since then I've played a lot of games. Normally the second day after a game would be the worst. I would feel very tired, but I felt like I was recovering very quickly. In terms of the vegan diet there were a lot of factors that helped with that and I was able to keep banging out games without being too sore."
Under Jose Mourinho, the 28-year-old featured 46 times last term as Man United finished second in the Premier League.