From Scientific American:
When you've spent the weekend splurging on greasy fast foods, your bathroom scale isn't alone in reeling from the impact. Your brain does, too. New research shows just how saturated fat tricks us into eating more and elucidates the evolutionary basis for the propensity for poundage in developed nations. Our brain physiology, it seems, is glaringly out-of-date in the modern world.
Researchers have long known that the hormones leptin and insulin play key roles in appetite and food intake. In healthy people leptin, which is secreted by fat tissue, acts as a molecular measuring tape for our waistlines, quashing feelings of hunger. Insulin spikes when the pancreas gets a whiff of the blood sugar increase after a meal; once the brain detects the spike, it knows to tamp down the desire for food.