Many of you dieted down in time for the recent beach-season, and no doubt many of you noticed some sort of lowered sex drive during the diet. Dieting over time can lead to a lack of energy, loss of motivation. Spending too long in diet mode can also lead to muscle loss, both directly due to the body's energy requirements, but also indirectly due to diminished training performance in the gym. These side effects are difficult to avoid entirely but can be lessened significantly with calorie cycling and proper supplementation. Unfortunately, these are not the only complications that come with dieting down.
Science has pinpointed one of the main contributors to all of these issues occurring in response to a diet. This is, of course, the hormone leptin. In fact, a study shows that declining leptin concentrations also lead to reductions in thyroid hormone and the catecholamines including norepinephrine. And leptin has its fingers on the remote controls of many other hormones and enzymes. When you restrict calories, the cell-sensing units, detect the shift in energy flux and inform the brain accordingly. The brain can then take action by reducing leptin levels. The net effect is the metabolic slow-down we all suffer trough when dieting.
Calorie restriction and testosterone
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. 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. 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, meaningless LH release meaningless 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.
What with libido?
Long-term calorie restriction while you diet can also lead to a sharp decline in libido in both men and women, and can also lead to a cease of menstruation in females. The reason for this is one of survival - the brain does not want its body using precious energy on reproduction when there is no certainty that there is enough energy available to keep itself alive.
This is calculated based on what scientists have dubbed energy "overheads". When the overheads become too great, the brain begins to shut them down like a business cutting its losses. Likewise, this explains why your metabolism slows in response to dieting, making getting leaner progressively more difficult the more ripped you become. In the past, I have discussed the effect dieting can have on testosterone levels.
In short, it causes levels to drop, but not to the degree that many people would believe based on the downturn in their libido. But it does drop, and this is clearly a concern during a cutting phase.
The research discovered that androgens reduce the sensitivity of GnRH neurons to glucose, whereas estrogen does not. In other words, less estrogen and more testosterone mean the neurons don't detect the changes in energy. If the glucosensing apparatus of the neurons can't work so effectively it simply assumes that all is well, so doesn't diminish the release of GnRH.