There is a danger in telling God how spiritual you are. God desires to see a humble heart. The people in Isaiah’s time were told that outwardly they appeared to be just and following God. They were doing the right “things” and wanted God to recognize that and to answer their prayers for what they wanted. But God saw their hearts-when they declared a fast they carried out their own pursuits and worked all of their laborers and by the end of the day they were fighting and quarrelling.
God said that fasting is not a day of penance, being bowed down in sackcloth and ashes (all outward showings of what they were doing). It should be a day of righting wrongs: releasing those held unjustly, setting free the oppressed, sharing their bread with the hungry, sheltering the oppressed and homeless and clothing the naked. The Lord said when we do those things then our light shines brightly and our wounds are healed. We are vindicated and the Lord in our right guard and then when we call for help He answers and says “Here I am!”
Fasting isn’t about how much you can do without but how much you can serve.