The lowering of leptin in response to calorie restriction does not appear to have any direct control over testosterone levels, but it can have an effect on the overall tone of the brain. This can indirectly influence testosterone levels. For instance, in a previous article discussing Neuropeptide Y (NPY) I alluded to one mechanism by which dieting can lower testosterone.
To understand it requires a quick digression. Simply, part of the brain (the hypothalamus) releases a hormone called gonadotropin-releasing hormone (GnRH). GnRH then stimulates another part of the brain (the pituitary) to release luteinizing hormone (LH). LH then travels down to the testes where it activates cells known as Leydig cells. It is these cells that manufacture and release testosterone under the command of LH. The diagram here from the Triazole™ write-up should help make this clear. Now back to NPY.
NPY is a hunger-stimulating hormone. Your body releases in during periods of low energy, such as unexpected long durations between meals (you know how this feels) and also more often during dieting when you are restricting calories or burning more through heightened activity. During these periods of low energy, NPY increases levels of a hormone called corticotrophin-releasing hormone (CRH), a hormone that ultimately stimulates cortisol release. CRH feeds back on GnRH neurons, slowing their rate, leading to reductions in their output. The net effect, of course, is less GnRH release, meaning less LH release meaning less testosterone creation. Bad times. Incidentally, this is also one of the reasons why periods of heavy stresses (from work or financial worries etc.) are typically accompanied by a lack of sex drive.
New research is suggesting another way that the body detects changes in energy balance for controlling testosterone, and it is a much more direct pathway.