The part of the brain called the hypothalamus affects body weight by controlling how the body stores and uses energy, says the Pituitary Network Association. Damage to the hypothalamus can cause weight gain and obesity that can't be controlled with exercise or dieting.
As of 2015, experiments with rats show that damage to the ventromedial hypothalamus causes weight gain even when researchers reduce the rats' caloric intake, according to the Pituitary Network Association. This is because damage to the ventromedial hypothalamus affects the rats' metabolism and causes their bodies to store energy instead of burning it. Researchers have also noted that the ventromedial hypothalamus influences the production of insulin, which determines how much blood sugar is converted into fat.
Parts of the brain create new cells even in adults, and recent research indicates that the hypothalamus may be such a region, says Johns Hopkins Medicine. Experiments with mice show that a consistently high-fat diet from a young age results in four times the normal growth rate of new cells in the hypothalamus of adults. When researchers eliminated these new cells, the mice gained less weight and created less fat. Scientists speculate that the formation of new cells in the hypothalamus may have evolved as a way to stimulate the appetite in the presence of an abundant food source.