Answer:
Because your body is used to eating foods that aren't healthy.
Explanation:
A sudden change in diet can surprise your body in a bad way. You could have odd bowel movements or gain mental problems like depression, or you could have head aches/fatigue. And your body isn't used to having the correct amount of nutrients all at one time. It'll take up to 1 to 2 months to see any changes (like weight gain or loss, or like how much energy you have during the day)
