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700 calories is crazy low. In that heavy a deficit, your body can absolutely start eating muscle… including cardiac muscle. This is almost eating disorder crazy. You didn’t gain all the weight overnight… trying to lose it all overnight is seriously dangerous.
My tip would be: once you reach your goal weight, you must bring your calories back up slowly. I made the mistake where I let myself go a bit. Having lost 14 KGS in 2 months, over another 6 month long period, I gained all around 15 kgs (over half of which was muscle, so that was the only silver lining). I have again lost around 12 KGS, and must lose another 15. My build is far more muscular than my first major cut, so I look much better.
This is starvation territory and unsustainable long-term. What are your plans for when you hit your goal weight? Because if you then adjust your calories to a more healthy amount, you’ll just gain all the weight back (and probably more since you’ve also knackered your metabolism in the process).