Istanbul anon gonna hate on me for saying it again, but the Istanbul metro is an overcrowded rat tunnel with zero views whatsoever and no cash single-use token option either. Take a bus to the city instead. There's plenty of snacks to buy with any leftover cash. I ran out of money and was very disappointed to land hungry in Denver with nothing but an Uludağ water bottle.

Don't wait until the airport to buy something tasty. You're gonna get reamed. Bring it in through security. BTW there are three layers of security in IST - entrance security, secure area security and gate security. Give yourself lots of time.