Dear Doctor: I'm 47 years old and was very happy to have three more years before my doctor started bugging me about a colonoscopy. But my husband says there are new guidelines, and I'm now actually two years late! Why the changes?

Dear Reader: Your husband is correct -- the American Cancer Society recently updated its screening guidelines for colorectal cancer. Rather than wait until age 50, it now recommends people at average risk for the disease should begin regular screenings at age 45. The organization cites the recent rise of colorectal cancer among young people as the impetus for the change. As of yet, neither the United States Preventive Services Task Force nor the American College of Gastroenterology has adopted the new guidelines.

According to a study published last year in the Journal of the National Cancer Institute, the proportion of rectal cancer diagnosed in adults younger than the age of 55 doubled from 1990 to 2013. That means nearly one-third of all diagnoses of rectal cancer now occur among people 55 and younger. Meanwhile, rates of rectal cancer for adults over the age of 55 are continuing their 40-year decline.

Researchers looking into this demographic shift are questioning whether the same lifestyle factors that appear to have a hand in the ongoing obesity epidemic are at work when it comes to the spike of colorectal cancers among younger adults. That is, a sedentary lifestyle, diets that are high in fat and low in fiber, an increase in the consumption of sugar and of highly processed foods, and the weight gain that accompanies these behaviors.

Although colorectal cancer is the fourth most-common cancer diagnosed among adults in the U.S., it is the second-leading cause of cancer death. That's frustrating because colon and rectal cancers are highly curable when caught early. The five-year relative survival rate for people with stage 1 rectal cancer is about 87 percent, according to statistics compiled by the American Cancer Society. For stage 1 colon cancer, that number jumps to 92 percent. Once colorectal cancers spread to other parts of the body, however, they become significantly harder to treat. Metastatic, or stage 4, colon cancers have a five-year relative survival rate of 11 percent. For stage 4 rectal cancers, it's 12 percent.

Catching these cancers early requires regular screening. That's because, in their early stages, colorectal cancers tend to be asymptomatic. Most begin as a growth in the inner surface of the colon or rectum. The majority of these growths, known as polyps, are not malignant. However, certain types of polyps, known as adenomas, have a higher risk of becoming cancers. And that's where the colonoscopy becomes important. Using a flexible tube with a tiny camera, a doctor can scan the interior of the colon to identify potential abnormalities. He or she may also collect tissue samples for biopsy and remove polyps. Stool-based lab tests and special CT scans are also effective.

We know that a colonoscopy can be daunting or even frightening, but the truth is it can save your life. The American College of Surgeons estimates that with regular screening, between 76 and 90 percent of colorectal cancers can be prevented.

(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: My husband's doctor prescribed finasteride to treat his enlarged prostate, and he's been taking it for years. Gradually, the side effects became noticeable, specifically erectile dysfunction. Shouldn't men be warned about this? Aren't there other options for an enlarged prostate?

Dear Reader: Erectile dysfunction is often a side effect of medications. Finasteride (trade name Proscar) is no different. It inhibits the 5-alpha-reductase enzyme that converts testosterone to the hormone dihydrotestosterone (DHT). DHT stimulates the prostate gland to enlarge, leads to male-pattern hair loss and, at the same time, spurs hair growth in almost every other part of the body.

Because of DHT's effect on the prostate gland, blocking the formation of DHT with the use of finasteride can help shrink the prostate. This is not an immediate effect and may take up to six months to cause substantial shrinkage. Similarly, finasteride is a good treatment for prostate cancer because it inhibits the stimulation of these cancers. For those who have hair loss, like myself, finasteride (at a lower dose) is used to block the male pattern hair baldness caused by DHT.

As you mentioned, one side effect of inhibiting DHT is erectile dysfunction. DHT appears to play a significant role in erection as seen from multiple animal studies, and the use of finasteride and its sister drug, dutasteride, have shown erectile dysfunction in rats. In studies in men, the rate of erectile dysfunction ranged from 7.7 to 15.8 percent. This is twice the rate than that seen with placebo.

Additionally, finasteride and dutasteride's inhibition of the enzyme 5-alpha-reductase leads to a decrease in other hormonally active chemicals. This may be the reason why some finasteride users show a decrease in libido. In studies, the number of patients reporting a loss of libido with finasteride ranged from 3.1 to 10 percent. This rate is less than two times of that seen with placebo. Lastly, finasteride has been linked to a small increase in the rate of ejaculatory problems, depression and anxiety. While the medication decreases the overall rate of low-grade prostate cancers, it has been linked to increased rate of high-grade prostate cancers.

There are other medications that decrease the symptoms of an enlarged prostate. Alpha-1 adrenergic blockers like tamsulosin, alfuzosin, terazosin and doxazosin are the first-line medications given for an enlarged prostate. They have a more immediate effect, unlike finasteride, which takes months to work, and they have been shown to improve symptoms better than finasteride.

These drugs are not without their side effects. They can lead to orthostatic hypotension, where the blood pressure drops significantly with standing, leading to lightheadedness and possibly passing out when standing quickly. Like finasteride, the alpha-1 blockers can lead to problems with ejaculation, but at a much higher rate (8 to 18 percent of men will have this as a side effect). A daily 5-milligram dose of Cialis has been shown to improve symptoms with an enlarged prostate, and this also may help erectile dysfunction.

If your husband is having problems with erections with finasteride, he should ask his doctor about alternatives.

(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 been an avid runner since high school, but I'm turning 65 this summer and my wife and my kids have been after me to switch to walking. I think I remember hearing about some study that said running actually makes you younger. If so, can you help with a recap? I need some good arguments for our next family gathering.

Dear Reader: Congratulations on your birthday, and for sticking to an exercise regimen over the decades. You haven't mentioned any injuries or other physical difficulties as a result of your running, so we'll assume you're simply dealing with the typical issues that come with aging.

We did a bit of research and found the study you're referring to. It was conducted by teams of researchers from the University of Colorado in Boulder and Humboldt State in Arcata, California, and was published in 2014 in the journal PLOS One. The takeaway of the study was that running may slow or even reverse certain aspects of the aging process in ways that walking for exercise does not.

Decades of research show that as people age, their "walking economy" declines. That is, despite expending increasingly more energy, the rate at which older adults walk grows progressively slower and less efficient. Since walking performance is tied to a variety of important health indicators in older adults, this slowdown has become a focus of research. In this particular study, scientists evaluated the walking performance of 30 women and men whose ages ranged from mid-60s to early 70s. Half regularly walked for at least 30 minutes at a time, three times per week. The other half also exercised three times a week for 30 minutes at a stretch, but at a brisker pace -- at least a jog.

After being evaluated on specialized equipment to measure energy output and oxygen usage, it turned out that the runners were the most efficient at walking. Even more surprising, their energy usage was comparable to that of a sedentary 20-year-old. The walking group, meanwhile, did not perform any better than people their own age who did not exercise. Bottom line, according to the researchers, is that the physiological effects of running slowed certain aspects of the aging process.

Lest any of you non-running readers decide to ditch exercise altogether (please don't!), it's important to note that walking still offers multiple important health benefits. These include improved cardiovascular health, improved mood, better weight control and a lower incidence of diabetes. What walking doesn't appear to do, according to this study anyway, is head off the age-associated dip in kinesthetic efficiency. Although the reason why isn't entirely clear, researchers suspect that running has a positive effect on muscle physiology and energy production, which take place at a cellular level.

One final note about running, which may make your wife and kids happy. Another study from the University of Iowa found that running at a slow pace for as little as five to 10 minutes at a time measurably reduced all-cause and cardiovascular mortality risk.

We hope this helps with your next dinner table conversation.

(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.)