Warm ribena is a god send during the cold autumnal months and is also helping get me through 18/6 fasting, however I don’t know enough about if it would be triggering my insulin/blood sugars and if that will take me out of the fat burning state.
the sugar free version has 10 calories per cup. It would be similar to drinking Diet Coke during your fast. The nutrasweet/aspartame/sucralose whatever they use would set off my insulin and ruins my fasting. I stick to clack coffee and water.
YMMV.