The athlete and former free running world champion made the comment during an interview with The Times.
Shieff also revealed that he has received angry feedback since telling people he eat animals - including people telling him to kill himself, and saying they would 'spit on him' if they saw him in the street.
'Killing an animal'
Shieff told The Times he will eat the 'occasional' steak, saying: "I think a cow, grass-fed, could be the most vegan product in terms of food because one animal could sustain someone for three months."
He then added: "I do feel like the next stage for me is to kill an animal myself. I’ve got to really face this."
No longer vegan
Shieff initially admitted he had been eating eggs and fish last November in a YouTube video - but said he wanted to recommit to a vegan lifestyle.
In January of this year, he confirmed he had returned to a fully omnivorous diet, blaming veganism for a host of health issues.
Some commentators have blamed some of Shieff's more extreme activities - including a 35-day unsupervised water fast and drinking his own urine for any medical problems. Shieff denies this, saying he did these things in a bid to improve his health.
Vegan diet is healthy at all stages of life
It should be noted that major dietary organizations agree that a vegan diet can be safe for all stages of life, with the American Academy of Nutrition and Dietetics taking the position that 'appropriately planned vegetarian, including vegan, diets are healthful, nutritionally adequate, and may provide health benefits for the prevention and treatment of certain diseases'.
"These diets are appropriate for all stages of the life cycle, including pregnancy, lactation, infancy, childhood, adolescence, older adulthood, and for athletes. Plant-based diets are more environmentally sustainable than diets rich in animal products because they use fewer natural resources and are associated with much less environmental damage," it adds.
"Vegetarians and vegans are at reduced risk of certain health conditions, including ischemic heart disease, type 2 diabetes, hypertension, certain types of cancer, and obesity."