Not sure why the urine was brown.
However, for healthy individuals, spiking insulin is completely fine and even for someone who is diabetic, their body will utilize the sugar consumed around a workout much better than they typically handle sugar. A diabetic might want to start with a smaller amount of sugar and gradually work up and resistance training improves their blood glucose levels.
I personally start to drink my workout drink before the training session. Maybe give that a try to see if you use the dextrose more effectively.
James 1:16-17 ESV
Do not be deceived, my beloved brothers. Every good gift and every perfect gift is from above, coming down from the Father of lights
With God's help...Mens sana in corpore sano