Dear Dr. Myrtle,
My lover is a 27-year-old man who has a very low sex drive. He tells me that this has not always been the case. He is an athlete, training for marathons and eating an extremely low-fat diet. I realize that one does not always have the libido of an 18-year-old, but I’m wondering if his exercise and diet regime may be playing a part. If so, other than becoming a couch potato, is there anything we can do?
Before answering your question directly, it’s important to make a distinction between the states of "desire", "arousal", and "erectile function." In other words, does he want to be intimate (desire)? Does his body physiologically respond when he wants to be intimate (signs of arousal such as pupil dilation, flushing, etc.)? If he desires to be intimate by penetrating with his penis, does his arousal lead to an enlarged and stiffened penis (penile engorgement and erection)? It’s important to know whether, for instance, that his penis can become erect when he wants to masturbate, or when he wakes up in the morning--that the body parts "work", so to speak.
If this is not the case, then your question would have much more to do with erectile function than libido. Some men with erectile dysfunction also have problems with libido, because if they experience erectile dysfunction, they fear that it will happen again, which can lead to a cycle of avoidance.
So, if lack of desire (aka libido) for sexual intimacy is really what you two are facing, do both of you perceive his low libido as a problem, or does only one of you? What was his libido like as an 18-year-old? What does he think about his libido? If he also perceives his libido as being low, does he want it to be different?
It is possible that this level of marathon training and reduced caloric (specifically fat) intake could lead to a decreased libido in some people. His body may be reacting to his regimen saying something like "Why are you starving me?" It turns out that people who exercise a lot (or are anorexic, or are really stressed, or malnourished) can have a hormone (in this case, growth hormone) come in and do what’s called "counter-regulation" of their blood sugar levels. In other words, when your lover is burning his calories and not replacing them, his body is trying to maintain his blood sugar levels any way it can--such as releasing more growth hormone, which helps to restore blood sugar levels. Unfortunately for your sex life, increased levels of growth hormone may be decreasing his libido, among other things.
Other possible medical causes of decreased libido are androgen (testosterone) deficiency, medication side effects, and psychological disturbances. It’s probably worth it to have a medical evaluation at some point, whether with a general physician or an exercise physiologist. From what you said, if his exercise regimen is extreme enough to be triggering this response, he may be better off consulting someone about whether this regimen is safe for him in the long run (pun intended). Good luck.