If we are going to understand how hBAT functions in the body physiology and use its calorie-burning ability to treat illnesses of metabolism such as diabetes, we will need to approach the challenge using several different methods. One key reason is that BAT is not easy to find in the body, and the cells that make it up are different depending on where they are and what the body needs. Brown fat is composed of at least two known types of distinct cells, termed constitutive /brown and inducible /beige; and there are also white fat cells in varying proportions. Given that there is currently no known way to estimate the amount of brown fat in the body from a simple blood test, our lab group has successfully relied on collecting small amounts of fat from the site of surgery in patients who are already undergoing clinically indicated procedures. Information obtained from these fat tissues has addressed questions related to (a) how brown fat cells develop from immature stem cells; (b) what genes are active; and (c) how many calories the brown fat can burn. These earlier studies have also provided the preliminary cells necessary for establishing long-lasting human brown and white cells that can be grown indefinitely in the laboratory. Despite this progress, much remains unknown about the functional differences between brown and beige fat cells and how these could affect metabolism in humans of different age, sex, and genetic backgrounds, as well as the changes seen in the brown fat cells in different illnesses. The goal of this study is to address these and related questions.