Dear Doctor: Our uncle, who is 44, recently got diagnosed with a blood clot in his leg, and it was a medical emergency. Our mom (she's his older sister) is a nurse and says that because he's so tall -- 6-foot-4 -- his risk is higher. Is that really true? I thought that it depended more on weight than height.

Dear Reader: Kudos to your mother, who is up on the latest research. According to a study published last year in the American Heart Association's journal Circulation: Cardiovascular Genetics, scientists in Sweden have identified a potential relationship between someone's height and their risk of developing a blood clot.

In the study, men under 5 feet 3 inches were found to be 65 percent less likely than men taller than 6 feet 2 inches to develop a venous blood clot. In women, the likelihood of a blood clot was 69 percent lower in individuals shorter than 5 feet 1 inch than in those who were 6 feet or taller. For the taller men in the study, height was linked to blood clots in the lungs, legs and other locations. For women, being tall increased the risk of blood clots only in the legs.

These conclusions are drawn from an analysis of medical data collected from -- and this is an interesting twist -- more than 2.5 million adult siblings in a national registry. The study period ranged from 30 to 40 years, and siblings were used to help account for any potential genetic factors in the results. None of the subjects had venous blood clots at the start of the study period.

The ability of blood to clot, or coagulate, is crucial to our survival. When an injury occurs, factors present in our blood and plasma work together to form the clot, which slows or stops the flow of blood at the site. As the injury heals, the clot dissolves. However, blood clots can form within blood vessels absent of injury or trauma, and they can persist. The danger is that they can restrict blood flow.

Blood clots can also separate from their point of origin and travel through the heart and to lungs, where they become life-threatening. According to statistics from the Centers for Disease Control and Prevention, between 60,000 and 100,000 deaths per year are attributable to blood clots.

You're correct that being overweight or obese is a risk factor in developing a blood clot. So are smoking, pregnancy, being immobile for long stretches of time, having high blood pressure or high cholesterol, and family history, to name just a few. Although you can't stop being tall, you can take steps to reduce blood-clot risk.

The first three -- lose weight, exercise regularly and stop smoking -- not only lower blood clot risk, they contribute to health and well-being in general. Air travel, desk work, bed rest and medical recovery can mean sitting still for hours at a time. Mitigate this by walking, stretching and flexing your leg muscles at regular intervals. And if you do find yourself seated for a long while, avoid crossing your legs, which can impede blood flow.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10880 Wilshire Blvd., Suite 1450, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: I read that microbes can affect how our body deals with immunotherapy. A friend of mine is about to start immunotherapy for lung cancer. Is there anything she can do to increase the chances that her gut will help her body beat this cancer?

Dear Reader: Your question sends us into the brave new world of precision medicine, which is rapidly transforming how we approach cancer treatment. Unlike chemotherapy, which targets all of the fast-dividing cells within the body no matter their origin, or radiation, which kills cancer cells by damaging the DNA in all of the cells in its path whether they are healthy or cancer cells, immunotherapy approaches cancer quite precisely, and at the cellular level. The point isn't so much to destroy cancer cells as to disrupt them.

For instance (and very broadly -- it's far more complex than our space here allows), researchers have designed antibodies that disable cancer cells by targeting specific sites within those cells. They have also created chemo- and radiolabeled antibodies, which deliver microdoses of powerful anti-cancer drugs and radiation to cancer at the cellular level. In an approach known as "adoptive T cell transfer," a patient's own immune cells are collected, modified to enhance cancer-fighting properties, and then re-infused.

Now, new research suggests that this targeted approach can be bolstered by beefing up a patient's own gut microbiome, which is the vast and varied collection of microbial communities that live within each of us. This is important because, although immunotherapy is brilliant in theory, in practice the results thus far have been mixed.

That's because the immune system is so alert to intruders of any kind that the presence of the immunotherapy drugs themselves can set it off. Side effects of immunotherapy can include rashes, fever, headache, weakness, elevated liver enzymes, low blood cell counts, breathing issues, diarrhea and vomiting. In some cases, adverse reactions to immunotherapy can be severe enough to be life-threatening. In exploring avenues to help a patient's body tolerate immunotherapy, researchers looked to the gut microbiome.

According to a pair of studies featured in the journal Science, the patients who responded best to treatment with a certain class of immunotherapy drugs were those with the more diverse and robust microbiomes.

One study, done by researchers in Texas, focused on patients with melanoma. The other, conducted in France, included patients who had undergone a course of antibiotics to deal with lung, bladder and kidney cancer. The French researchers found that patients who had undergone a course of antibiotics to deal with problems like a urinary tract infection had the poorest response to immunotherapy. The Texas researchers are now planning to check their results with a clinical trial. The research has reportedly caught the interest of several biotech companies, which are also doing clinical research into the matter.

Meanwhile, your friend can take steps to improve her own microbiome. These include adding fermented foods and beverages to her diet, eating high-fiber fresh fruits and vegetables to provide plenty of nutrients for her microbial community, and steering clear of artificial sweeteners and processed foods. Finally, we believe it would be wise for her to enlist her health care team in this endeavor.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10880 Wilshire Blvd., Suite 1450, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: Does eating for one's blood type really work? For my own blood type, I am supposed to avoid foods like avocados, bananas, strawberries and many other foods that I would consider healthy choices. I certainly don't want to feel guilty for eating foods that this plan suggests avoiding. What is your take on this?

Dear Reader: You're referring to an eating plan that was popularized by a diet book published in the mid-1990s. The author's premise was that what you eat, as well as the way in which you exercise, should be based on your own particular blood type. For instance, individuals with Type A blood are advised to become vegetarians, substituting plant-based proteins for meat, which the diet refers to as "toxic." The plan also recommends they seek out gentle and calming exercise, such as yoga and tai chi. For those with Type O blood, by contrast, the dietary emphasis is on meats, with advice to limit grains and dairy. Type O individuals are urged to take up vigorous exercise like running, contact sports and martial arts. Those with Types B and AB blood also have personalized diet and exercise guidelines. Each of the eating plans, which also includes vitamins and supplements, is quite specific and even restrictive. The outcome of following the diet, according to claims made by the author, is improved health and a lower risk of disease.

The argument for this dietary approach hinges in part on the differences in blood group antigens, which are markers that are present on the membranes of red blood cells. These antigens will set off an immune response when they encounter foreign antigens. For example, someone with Type A blood cannot receive a transfusion of Type B blood because the antigens in the Type B blood will set off alarms and cause the immune system to go on the attack. Someone with Type AB blood can receive either Type A or B blood but, due to the blood antigens, can safely donate only to someone else with Type AB.

It's true that blood can reveal a lot about a person's overall health. Physicians rely on blood tests to learn a patient's glucose and cholesterol levels, hormone levels, how well the liver, kidneys and vascular system are functioning, whether the body is fighting some type of infection, and whether the signs of certain cancers are present. In addition, it is now generally accepted that specific blood types are associated with a higher risk of certain diseases and conditions, including pancreatic cancer, deep vein blood clots and heart attack associated with coronary artery disease. But whether blood antigens dictate food choices and exercise methods in the ways that this diet lays out remains up for debate. At this time, rigorous studies in peer-reviewed journals are lacking.

Diet is a highly personal choice, and as we all know from the ever-shifting content (and lately, the shape) of the so-called food pyramid, it's not an exact science. In our opinion, as long as you're eating a healthful and balanced diet, you have nothing to feel guilty about.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10880 Wilshire Blvd., Suite 1450, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)