so, i created a food which heals you and the entity hitted by it, but i want the food to lose durability everytime you hit someone. heres my event

@SubscribeEvent
public static void onDamage(AttackEntityEvent event){
    if(event.getEntityLiving().getHeldItemMainhand().getItem() == RegistryHandler.HEAL_SOUP.get()){
        if(event.getTarget().isAlive()){
            LivingEntity target = (LivingEntity) event.getTarget();

                PlayerEntity player = event.getPlayer();
                target.addPotionEffect(new EffectInstance(Effects.REGENERATION, 15*20, 2));

                if (event.getPlayer().getEntityWorld().isRemote){

                    String msg = TextFormatting.BLUE + "The mob is healing!";
                    player.sendMessage(new StringTextComponent(msg), player.getUniqueID());
                }
        }
    }

add defaultMaxDamage(*theMaxUsesYouWant*) to your item properties when constructing the item, so you give it a finite number of uses before it breaks (keep in mind your food won't be stackable). Then you need to use ItemStack#damageItem to decrease its life when you hit an entity.