Dear Doctor: A new study said that working out too much could ruin your sex life. I like to be in shape -- but not at that high a price. What's the deal?

Dear Reader: Exercise is unequivocally good for men's libido. Compared with sedentary individuals, men who exercise regularly have higher testosterone levels, greater libido and greater fertility. But extreme exercise is not without risks. Hormones are complex, and although the body is adept at regulating them through feedback mechanisms -- a finely tuned system known as the hormonal axis -- that feedback can be disrupted. In women, chronic endurance exercise training can lead to disruption of the hormonal axis, which can affect their menstrual cycle and fertility. It can similarly disrupt men's hormonal axis, leading to testosterone levels at the lower end of normal. The question is: What degree of exercise leads to this disruption, and does the disruption lead to decreased libido?

The study you mentioned, published earlier this year in Medicine and Science in Sports and Exercise, attempted to answer this question -- at least among men. Male subjects filled out online surveys about their level of exercise and their level of libido. Data were gathered from participants who were healthy, free of hormonal disorders, not taking prescription medications, older than 18 and involved in sporting activities. The researchers' primary focus was those men involved in organizations associated with endurance running, biking or triathlons.

Of the men who filled out online surveys, 1,077 men met the overall criteria, with respondents answering questions about the total number of hours exercised per week and the level of exercise intensity. The researchers also separated the men into two groups: low libido and normal-to-high libido. Notably, there were considerably fewer men in the low-libido group: only 60 compared to 1,017 in the normal-to-high libido group.

In men reporting normal-to-high libido, 33.1 percent exercised 4 to 6 hours per week; 35 percent exercised 7 to 10 hours per week; and 22.2 percent exercised more than 10 hours per week. Conversely, in the low-libido group, 10 percent exercised 4 to 6 hours per week; 23.3 percent exercised 7 to 10 hours per week; and 65 percent exercised more than 10 hours per week.

Researchers also assessed level of intensity and how many years the participant had been exercising at that intensity. In the low-libido group, there were three times as many intense/prolonged exercisers than there were moderate intensity/duration exercisers.

So, yes, this study shows that males who exercised at a high intensity for many years had a lower libido than those with a more moderate track record. But keep two things in mind. One, the low-libido group consisted of only 60 people, so the power of the data is not strong. Two, 22.2 percent of men in the normal-to-high libido group exercised more than 10 hours a week; 65 percent in the low-libido group exercised that much. Unless you exercise more than 10 hours per week, every week, I wouldn't be concerned.

That said, if you're finding that your libido is low and you're exercising more than 10 hours per week, you might want to reconsider your priorities.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: Is there anything I can do to reduce my risk of prostate cancer? No one in my family has had cancer, but I just turned 55 and want to do whatever I can to stay healthy.

Dear Reader: While there is no single approach to prevent prostate cancer, research has shown that certain lifestyle changes may reduce your risk of developing the disease. Considering that these behaviors also promote good health and well-being, you won't go wrong by adopting any or all of them.

Let's start with some facts about the disease.

Prostate cancer is the second-most common cancer in men, right behind skin cancer. According to the American Cancer Society, one in seven men will get a diagnosis of prostate cancer during his lifetime. However, that's not as dire as it sounds. Prostate cancers grow slowly and are slow to spread. As a result, survival rates of prostate cancer are high. The five-year survival rate is close to 100 percent. The 15-year survival rate is 95 percent.

The major risk factors for prostate cancer are age, race, family history, a diet high in red meat and animal fat, and tobacco use. While the first three can't be changed, they can alert you to be more vigilant.

About 70 percent of new cases of prostate cancer occur in men 65 and older. Men of African-American descent are at higher risk of developing the disease, and their cancers can be more aggressive. If you fall into those categories, and if cancer runs in your family, be sure to always include these facts in your medical history.

So how do you reduce your risk? Limit or skip red meat, dairy and animal fats. Opt for lean proteins like chicken, fish and turkey instead. Focus on healthy fats from plant-based sources like olive oil and nuts.

We're sure you've heard it before, but that's because it's true: leafy greens, whole grains and fresh fruits and vegetables provide a wide range of vitamins, minerals and fiber. Cooked tomatoes (which contain lycopene), cruciferous vegetables like broccoli and cauliflower, soy products and green tea are also recommended.

Studies show that being obese or overweight is linked to higher rates of many kinds of cancer, including prostate cancer. Regular physical activity, which not only burns calories and fat but also builds muscle mass, results in a lower -- and healthier -- body mass index.

Do you use tobacco products? As physicians, we must ask you to please stop. Tobacco plays a role in a daunting array of diseases and conditions, including prostate cancer. We know that quitting can be difficult and have given our own patients a hand in leaving tobacco behind. Don't be afraid to ask for help from your physician.

Cancers that are caught early are more easily treated. That makes screening important. Thanks to new understanding about slow-growing prostate cancers, many physicians will now recommend active surveillance in older men with slow-growing tumors.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: I've heard that cellphones increase the risk of brain tumors -- and also that they don't. Which is true?

Dear Reader: First, let's look at brain tumors in general. They are classified as either benign or malignant, meaning cancerous. The most common type of benign brain tumor is a meningioma, which arises from the meninges, the outer lining of the brain and spinal cord. The most common type of malignant brain tumor is a glioma, which arises from glial cells that line the individual nerve cells.

Ionizing radiation from nuclear accidents or explosions, radiation therapy, or even too-frequent dental X-rays can increase the risk of brain tumors. This type of radiation can lead to mutations within the DNA, increasing the risk of tumor cells.

Cellphones and cordless phones, on the other hand, emit radiofrequency energy, a form of non-ionizing radiation. This type of radiation has little potential to cause DNA damage. From that perspective, because cellphones produce non-ionizing radiation, their risk of cancer should be pretty low. Now let's check out the data.

A 2015 Swedish study in the Journal Oncology Reports looked at meningiomas diagnosed between 1997 and 2009. People with meningiomas filled out questionnaires asking about their use of cellphones, home cordless phones and analog phones (phones attached to a wire). The authors also asked whether the patients had placed the phone toward one ear or the other, or used both ears equally.

The authors found no statistical difference between mobile phone use and cordless phone use when compared to analog phones. However, those with the greatest amount of cordless and cellphone use had a higher rate of meningiomas compared to those with the lowest amount of cordless and cellphone use.

The interesting aspect of this is that those who frequently used analog phones also had more meningiomas than those who used analog phones less frequently. So this research carries the possibility of recall bias, in which someone with a disease may overly represent a possible risk factor.

A 2017 analysis of many studies, published in the journal Neurological Sciences, found no overall association between cellphone use and brain tumors. Rather, the authors found that many studies on the topic were methodologically flawed.

The authors did note that the higher-quality studies showed a correlation between cellphone use and brain tumors. The authors also concluded that in the higher-quality studies, long-term use of mobile phones was linked to an increase in the risk of brain tumors. Long-term use was defined as greater than 10 years, or more than 1,640 hours.

Another 2017 analysis, published in the International Journal of Occupational Medicine and Environmental Health, reached a similar conclusion.

It's difficult to make judgments based on this data. There does appear to be some evidence that more than 10 years of cellphone use may increase the risk of brain tumors. Yet the potential for recall bias means the data aren't irrefutable. Nonetheless, I would recommend being conscious of the amount of time you spend with a cellphone or a cordless phone against your head. Luckily, we can now communicate through a Bluetooth device, reducing whatever risk there may be.

One encouraging note? Rates of both benign and malignant brain tumors decreased in the United States between 2009 and 2013.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)