@zand is correct. I was given a stark choice by an endocrinologist when in my early twenties after embarking on a diet similar to the Newcastle Diet and managing to lose about 7lbs in as many weeks but make myself v ill in that my metabolism had gone into survival mode and was shutting my body down to preserve energy. He told me that I was one of his few patients that could honestly blame their metabolism for their size and weight probs and I could either eat healthily and remain as I was or be slim and dead - guess which option I chose!