When I did IP the first time ~2012 I continued lifting (Bodypump 2-3x/wk) and leisurely walked ~2mi 2-3x/wk (taking baby to the park). My coach advised me against "huffing, puffing" cardio for metabolic reasons, your body uses "quick energy" when you get revved up and because of ketosis, in lieu of carbs pulls muscle vs fat first as it's easier for the body to grab. With lifting I would make sure to have an extra packet before class. I was very successful and got back to running my first day of Phase 3 (when you get carbs back at breakfast) and was WAY faster than ever thanks to less weight (did a sub 2:00 half marathon time a few months later! I used to be proud of doing a sub 4:00 time...). IP is technically a "Protein Sparing Modified Fasting" (PSMF) diet, we are given enough protein to preserve current muscle, so if you want to add extra effort on top of that add in some more energy.
My second time on IP 2019 I continued to do my HIIT 2x/wk + Lifting 1x/wk (went with coworkers) - I wasn't as fast to lose, but I cheated more. I made great progress in the HIIT classes, although I think it was because I was progressively getting lighter with IP. I knew it wasn't recommended, but I loved the class and going with friends was a fun break at work. The HIIT classes did make me HUNGRY, which is probably why I cheated more and took longer to lose than the first time.
This third time (I almost want to just wipe out 2019 after pandemic) I keep telling myself I will lift/walk like the first time, but I need a group to motivate me!
I think if you want "max results" lifting and light cardio is okay - adding an extra packet close to lifting to ensure you are supporting muscle growth. If you want to do higher level cardio, maybe adding in a restricted BEFORE so you have that quick energy ready (although I was still HANGRY). My coach the first time was AMAZING, very knowledgeable and explained things, we'd have longer 30-45min appointments to talk things over - I feel like recently they are just there to sell you food and get you in/out in <15min.
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