If Christ son of Mary was God or son of God, why he had to keep a 40 days long fast?

Jesus was fully God and fully man. man fasts from time to time. that's not so hard to understand, at least not for me. also, Jesus was led by the Spirit to the wilderness and perhaps instructed to fast as far as i know. the Bible does not say that the Spirit asked this of Him, but it does tell us that it was the Spirit who led Him to the desert in the first place. third, Jesus was always obedient to the Father. when He lived His life on this earth as a man, He was without sin, and we can assume (at least i do) that it was in the Father's will for Jesus to fast... perhaps as example to us.