Most diet conscious people prefer sugar free soft drinks over regular ones. Although they may provide fewer calories, but in reality they lead to weight gain in the long run. Eating natural sweetener activates food reward pathways in your brain, but artificial sweeteners don't fully activate these pathways. This can leave you feeling unsatisfied and leads to an increased appetite, even if you have already eaten enough calories for the day. The intensely sweet flavour of artificial sweeteners may also trigger cravings for sugary food, leading to excess calorie intake.
Apart from weight problems, Aspartame, which is usually added to diet colas can cause many health problems including headache, anxiety, heart palpitations, depression and neurological problems. So next time you feel like gulping down liquids, give preference to natural drinks like coconut water, buttermilk, lime water etc.