Research has shown that men have significantly lower testosterone levels on a hypocaloric diet. In other words, staying in a state of caloric deficit for a long time could lead to low testosterone and all the related issues.
Caloric deficit means burning more calories than you get from your diet.
During caloric deficit, the body starts to look for additional sources of energy. Body fat is loaded with calories: a single gram of fat packs 9 calories while a gram of carbs or protein gives just 4 calories. As a result, the body considers fat as its main source of energy to support its metabolism.
That's why the body starts burning fat in a state of caloric deficit.
There are only two ways to reach caloric deficit: by eating fewer calories or by making your body use more calories. In both cases, the body won’t be getting all the energy it needs from your diet. As a result, it will have to slow down some processes.
Unfortunately, these processes include the production of testosterone, meaning your testosterone levels would eventually decrease. In the end, this results in decreased strength, sexual dysfunction, and low libido.
As if low testosterone levels weren't big enough of a problem, caloric deficit is always quite stressful too. This means the adrenal cortex would produce more of the stress hormone epinephrine, cortisol, and adrenaline.
Due to the action of these stress hormones, your testosterone levels will likely continue to drop. In this case, testosterone won't be able to properly carry out its most important roles in managing behavior. Yes, hormones drive some of the most common behavioral characteristics associated with being a man.
Another process that’s slowed down by a caloric deficit is the production of hormones in the thyroid gland. Since the thyroid hormones triiodothyronine (T3) and thyroxine (T4) aid in the production of testosterone, a drop in these hormones would lead to your testosterone levels falling even lower.
Of course, this does not mean that you should give up on losing weight or improving your physique. After all, men who have leaner physiques have significantly higher testosterone than males who have more body fat.
This just means that you’ll have to be smarter about achieving and maintaining a caloric deficit while, at the same time, making sure your testosterone levels do not suffer.