If by "need", you mean "need to survive", you could survive your entire life just by drinking human milk. By not eating plants, you'd be improving animal welfar

If by "need", you mean "need to survive", you could survive your entire life just by drinking human milk. By not eating plants, you'd be improving animal welfare even more, since agriculture kills many more animals than grazing-based animal husbandry (think all the pests that are eliminated: the rodents, insects, etc.)