Hard to answer without knowing specifics.When you say work out regularly does that mean you run 2, 5, 10 miles per day 5 days a week? Are you doing weight training as well? How often are you fasting? How many days per week, how strict is your fast?
You have to be careful with food intake and exercise. I would make sure you are taking a multivitamin regardless.
I have not had a regular fast but I can get really addicted to exercise so I have had to watch my food intake. When I am really into a work out routine (I used to run2-5 miles 5-6 days a week, lift weights 3 days a week and do martial arts 3 days a week) I tend to eat healthier but not enough. I’d talk to a nutritionist or your doctor about it so they can give you a specific dietary plan for you. Remember that you have to have a certain amount to protein, fat and carbs each day. (your nervous system uses carbs for fuel, so don’t skip those, just choose good ones).
Good luck