[jaggederest]

3 pounds a week is pretty unsustainable.

http://www.ncbi.nlm.nih.gov/pubmed/15615615

Edit: Since I got curious and followed a bunch of other studies, here's a cool calculator based on some of that research and the (admittedly flawed) NHANES data

https://www.supertracker.usda.gov/bwp/

[toomuchtodo]

It is sustainable when you're carrying excess fat. Your body will eventually reach an equilibrium. Each body is different, YMMV, etc.