Will Diet Soda And Splenda Cause You To Gain Weight?

I’ve googled this a million times but still have trouble finding the raw answer to this question… I know diet soda has sodium and causes the body to retain water… but does the over all intake of both diet soda and splenda put on pure body fat??? i mean, they;re both calorie free, diet soda is sugar free and splenda is made from sugar, and isnt raw sugar… i’m simply asking this because i work out a lot, cardio 6 days a week and weight training 4 days a week..and eat very healthy…fruits, veggies, lean meats, whole grains, and drink a gallon of water a day…but in the morning i need a little splenda with my coffee…and sometime during the day, i feel deprived if i dont have a diet pepsi or two..but I don’t want to screw up my weight loss goals by drinking diet soda and using splenda…can somebody please help????