Core Insights - The article discusses the dual role of Neuropeptide Y (NPY) in regulating appetite and energy expenditure, revealing its complex functions in both the brain and peripheral fat tissues [7][12][16]. Group 1: NPY's Role in Appetite Regulation - NPY is known to stimulate appetite in the brain, but its absence does not significantly affect food intake in mice [11]. - Mutations in the NPY gene are associated with higher Body Mass Index (BMI) in humans, indicating a more complex relationship between NPY and weight regulation beyond just appetite stimulation [11][17]. Group 2: NPY's Role in Energy Expenditure - In peripheral fat tissues, NPY helps maintain the thermogenic capacity of brown adipose tissue (BAT) and promotes energy expenditure [7][12]. - Research shows that when NPY is knocked out in the sympathetic nerves of mice, their brown fat turns "white," leading to reduced thermogenic ability and increased susceptibility to weight gain, even without changes in food intake or activity levels [14][16]. Group 3: Mechanisms of NPY Action - Approximately 40% of sympathetic neurons in white adipose tissue express NPY, indicating its significant presence in fat tissue regulation [12][14]. - NPY promotes the proliferation of perivascular cells, which can differentiate into thermogenic fat cells, thus playing a crucial role in fat tissue regulation [14][16].