One reason—if not the main reason—we diet and exercise is that we want to look good to the opposite sex (or maybe the same sex).

And, of course, one reason—if not the main reason—we want to do that is to be attractive to our (real or imagined) sexual partner.

Now, what does any of this have to do with health, you wonder? The answer is, plenty. A healthy sex life improves your overall quality of life. It improves your immune system because it significantly relieves stress. Good physical exercise burns calories, and it improves your mood by pumping endorphins into your bloodstream that make you feel good. It also plays a key role in keeping couples together, so the benefits of sex are innumerable.

But once you get on into your forties, you might find your sex drive shifting into a lower gear. This diminished or lack of sex drive is more common in women than it is in men. Even men with erectile dysfunction usually have a normal sex drive. While libido problems can be either physical or psychological, the root causes tend to be the same in both sexes. Alcoholism is the main physical factor responsible for a decreased libido; another is drug abuse, of cocaine, for example.

Obesity and anemia are other potential physical problems. And there are certain tumors of the pituitary gland that increase the hormone prolactin, which lowers the libido. Some prescribed medications, especially antidepressants, lower the level of the hormone testosterone, which is needed by both sexes to maintain an adequate sex drive. Psychological factors influencing libido include depression, stress, and confusions about sexual orientation.

Anyone with a lack of sexual desire should first try to take these factors out of the equation. So if you’re drinking excessively, overweight, depressed, or taking medications, these issues need to be dealt with to resolve a flagging libido. Counseling can help with the psychological problems of sexual hang-ups, depression, or stress.

There is no magic remedy for the loss of sexual libido. Though testosterone has been identified as a key hormone that improves sexual appetite in women, doctors who have been giving women testosterone supplements for the past 30 years have found that it has little effect on their libido, while it sometimes causes facial hair growth, a deepened voice, and an enlargement of the clitoris.

I have no doubt that one day there will be a libido pill for women and men, as I’m sure the drug companies are hard at work on this potentially lucrative solution.

There are a number of other sexual problems that women may experience at any age. One is dyspareunia, or painful sexual intercourse. Any part of the genitals can cause pain during sex, including the skin around the vagina. Vaginal infections, like yeast infections or viral infections, are a common cause, and the pain can be felt when either a tampon or penis is inserted into the vagina. It can also occur from just sitting or wearing pants. To treat dyspareunia, physicians may recommend hormone creams, dilators to help stretch the vagina, Kegel exercises, or, in rare cases, antidepressants.

Another potential cause of dyspareunia is vaginismus, an involuntary contraction of the vaginal muscles that may prevent insertion of the penis during intercourse. The diagnosis of vaginismus is usually problematic because it’s often difficult to separate the physical pain with the emotional anxiety of experiencing that pain; in other words, just the fear of the pain can cause vaginismus.

Any woman complaining of these symptoms should be taken seriously. A doctor must conduct a physical examination to eliminate the possibility of such physical causes as infections, fibroids, or anatomical deformities of the uterus, ovaries, or vagina. Even vaginal dryness can cause painful sex. A decrease in estrogen at menopause can cause the vaginal walls to become dry, creating a discomfort or pain during intercourse.

If there are no treatable physical conditions, it’s important to discuss the woman’s feelings as well as the physical situations that lead to this type of discomfort. Some women have a very positive attitude toward sex; other women have had negative sexual experiences that play a significant role in their fears and negative feelings about sex.

Some women may have a history of sexual abuse, rape, or trauma, for instance; these things need to be identified in a very delicate way. Treatment of vaginismus usually involves practicing relaxation techniques and doing Kegel exercises to relax the vaginal muscles. At home, one exercise that may prove beneficial is to have your partner gradually insert a dilator into your vagina. This must be done at a pace with which you feel comfortable until the pain and discomfort are overcome. Partner, doctor, and patient all have to be in sync for this type of therapy to be successful.

Many women experience discomfort or pain at the time of their period. This pain is caused by contractions of the muscle of the uterus during menstruation that occur due to the release of the prostaglandins, which are hormones that are produced in the lining of the uterus. For most women these menstrual contractions are neither severe nor disabling. But some women experience significant menstrual pains called dysmenorrhea.

Women suffering from dysmenorrhea should exercise, get plenty of sleep, and avoid stress. Over-the-counter painkillers can minimize the amount of prostaglandins released, and they usually help reduce the pain. If the painkillers are not effective, your doctor will have to look for other things that are causing the pain. And ultrasound is sometimes used in such cases to make sure you don’t have any other medical conditions, like pelvic inflammatory disease, endometriosis, or fibroids.

Click here to check out Dr. Manny's book The Check List (Harper Collins, 2007), from which this article was excerpted.

Dr. Manny Alvarez is the managing editor of health news at FOXNews.com, and is a regular medical contributor on the FOX News Channel. He is chairman of the Department of Obstetrics and Gynecology and Reproductive Science at Hackensack University Medical Center in New Jersey. Additionally, Alvarez is Adjunct Professor of Obstetrics and Gynecology at New York University School of Medicine in New York City.

Dr. Manny Alvarez serves as Fox News Channel's senior managing health editor. He also serves as chairman of the department of obstetrics/gynecology and reproductive science at Hackensack University Medical Center in New Jersey. Click here for more information on Dr. Manny's work with Hackensack University Medical Center. Visit AskDrManny.com for more.