What effect does dieting have on leptin levels?

Lowers level: If dieting is intended to lower fat content, then there should be expected drop in leptin levels as this hormone (cytokine) is produced by the fat cells (adipocytes).

Lowers leptin: Generally, dieting will decrease circulating levels of leptin. Without leptin activating receptors in the hypothalamus, the body will think that it is in a constant state of starvation due to the lack of sufficient energy stores. This will lead to an increase in levels of ghrelin causing a stimulation of hunger. Fasting in order to lose weight will cause ghrelin levels to remain high, feel hungry.