Dear Doctor: There is an ongoing debate in my house about fluid/water intake to keep hydrated. Doesn't it make sense to drink water primarily when you feel thirsty? If not, how much do you need?

Dear Reader: Many of my patients ask how much water they should drink per day, and I give different estimations based upon their individual kidney and heart function. But I feel somewhat at a loss on what to tell them in a broad sense.

Current medical belief holds that an individual should drink eight 8-ounce glasses of water per day for a daily total of a half-gallon, or roughly 2 liters. Although thirst might logically seem to be a good indicator, some people are less likely to get thirsty due to illness, age or simply because they've grown accustomed to drinking little water. So it's better to keep an eye on total intake.

But the science on that intake is based on very few studies.

One 2002 study asked 20,297 Seventh-day Adventists -- average age of 58 with no history of diabetes -- about daily fluid intake, then followed them for six years. Men who drank five or more glasses of water per day had a 64 percent reduction in fatal heart disease compared to men who drank two glasses or fewer. Women who drank five or more glasses of water per day had a 41 percent reduction compared to those who drank two glasses or fewer.

When the researchers assessed intake of other fluids (coffee, tea, milk, juice, soft drinks), they found an opposite correlation. Men who drank more than five glasses had a 46 percent increased risk of fatal heart disease compared to men who drank two glasses or fewer. In women, this increase was a dramatic 247 percent.

A 2010 study from the Netherlands asked 120,852 people about their fluid intake, then followed them for 10 years. Here, neither increased water intake nor total fluid intake was correlated with a risk of fatal heart attacks or with strokes.

Then there was a 2012 study of 465 stroke patients who were followed for 17 months. It found a 27 percent decreased risk of stroke, heart attack or death among people who drank 2 or more liters of total fluid per day versus those who drank less.

As for older adults, a 2017 study assessed fluid intake among 1,055 Australian women over the age of 70, then followed them for 10 years. No association was found between death rates and either water intake or total intake of fluids.

Notably, in various other studies, higher water intake has been linked to a lower rate of kidney disease. Because kidney disease has been associated with higher rates of cardiovascular disease, this connection may be why there is less death from heart disease among people with higher fluid intake. This may be more applicable to an older population.

Of course, the amount of water you need to drink depends on losses of water from sweating or from illness and from fluid obtained via food (soups, vegetables, fruits and so on). Although the data are hardly ironclad, it seems to me that five glasses of water per day is healthy.

(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 feeling a bit down lately, and I read that even one hour a week of exercise could reduce depression. How much exercise is optimal to reduce the likelihood of depression?

Dear Reader: We're very sorry to hear that you're struggling, but we are glad that you're open to the idea of physical exercise as a way to improve your mood. In addition to therapy and medication, a combination that offers relief to millions of Americans who suffer from depression or anxiety, studies continue to show that regular exercise can have a beneficial effect on mood and emotional well-being.

The idea of a sound body leading to a sound mind dates back to at least the ancient Greeks. In labs and think tanks throughout the world, scientists have been exploring the connection between exercise and mood for decades. Here in the United States, where health and fitness fads sweep through popular culture like clockwork, plain old exercise remains a constant among the more exotic (goat yoga) and outlandish (sauna suits) approaches that come and -- thankfully -- go.

The latest confirmation of the existence of the mind-body connection comes from a group of scientists who took a new look at data collected from more than 33,000 adults living in Norway. The study subjects, none of whom had mental health or physical issues, were followed for 11 years. During that time, researchers monitored the onset of depression or anxiety among members of the group. They also tracked physical activity.

Their finding that regular exercise had a positive effect on mood wasn't a big surprise. What was startling was just how little physical activity it took for the study participants to experience the mental health benefits.

According to the analysis of the 11 years of data, participants who exercised as little as one hour per week had a 44 percent lower risk of developing depression during the term of the study than the those who remained sedentary. On top of that, exercise intensity appeared not to play a role: Mild and moderate exercise was just as effective as strenuous workouts.

And while increased intensity and duration of physical activity do yield physical benefits, researchers concluded that when it came to protection against depression, mental health benefits leveled off beyond two hours of activity per week. The other interesting outcome of this second look at the data was that while exercise did offer a degree of protection against depression, it had no effect on anxiety.

If your next question is why just one hour of exercise a week can be beneficial to mental health, the authors of the study admit they don't know. But with such intriguing findings in an area of study that's important to millions of people worldwide, you can bet that follow-up research is already in the works.

In the meantime, the good news is that the data say we can benefit from as little as 10 minutes of exercise per day. That's a walk around the block, a couple of laps in the pool or a few flights of stairs at work instead of the elevator.

We hope you'll give it a try and, after a few months, will let us know how you're doing.

Dear Doctor: A friend of mine was diagnosed with, and treated for, anal cancer about two years ago. She never told me the location of her cancer until recently because she thought that "anus" and "anal" were shameful words. She also let the symptoms drag on for a year or so because she thought they were due to hemorrhoids. What are the symptoms, how is this type of cancer treated, and is it on the increase?

Dear Reader: In our everyday life, we focus on our usual responsibilities: our family, our job, paying our mortgage or rent, maintaining friendships, worrying about politics, etc. This complex existence often doesn't afford us time to deal with nagging symptoms. When those symptoms occur in a part of the body that people don't like to consider, like the anus, the likelihood of a delay in diagnosis increases.

Also, anal cancer is relatively uncommon, meaning people are unlikely to suspect the disease. Each year, it's diagnosed in approximately 8,200 people in the United States, leading to about 1,100 deaths; only about 0.2 percent of the population will be diagnosed with it over their lifetime. But to answer your last question: Even though the numbers are small, yes, the rate of anal cancer is increasing. Compared to diagnoses 40 years ago, the rate of anal cancer has increased three times in men and 1.7 times in women. As with cervical cancer, infection with the human papillomavirus (HPV) increases the risk, as does homosexuality (for men), anal sex and a history of anal warts.

As for symptoms, anal bleeding occurs in 45 percent of patients but, because hemorrhoids can similarly cause bleeding, people often can ignore this sign. Anal pain or a sensation of fullness in the rectum occurs in about 30 percent of patients, and 20 percent of patients have no symptoms at all.

In the 1960s, treatment consisted of the removal of the anus and the surrounding tissues, leading to a permanent colostomy, that is, a rerouting of the colon through an opening in the abdomen. Three percent of patients died from the procedure alone.

Today, treatment consists largely of radiation and chemotherapy. The radiation therapy is generally given in 25 treatments over a period of five weeks. The chemotherapy -- typically, 5-fluorouracil and Mitomycin (or cisplatin) -- is often given on the first and fifth weeks of radiation treatment. This combined approach has decreased not only the death rate from anal cancer, but also the need for a colostomy. Today, only 14 to 35 percent of patients need a colostomy after five years. Survival rates depend on the tumor size at diagnosis. The five-year survival rate ranges from 86 percent to 42 percent, depending on the severity of the disease.

Radiation, however, has side effects, potentially leading to more frequent bowel movements, flatulence and rectal incontinence -- as well as potential impotence in men and vaginal damage in women. Newer forms of radiation therapy using computer-controlled techniques and 3-D imaging can reduce some of these side effects.

It's a testament to your friendship that your friend was finally able to share her diagnosis with you. By seeking to learn more about anal cancer, you're not only showing concern, but also helping raise awareness.

(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.)