I havnt measured but it doesnt feel unsustainable. I weigh about 172 right now and ultimately I'd like to walk around at 160 pounds. I could be ignorant here but eventually if I continue this diet my body will get to a weight where my caloric intake is enough to maintain my weight. I havnt seen any decrease in performance mentally or physically so I wouldn't call it unsustainable. I went on a 15 mile walk this weekend and felt fine.