DEAR DR. ROACH: A year ago, I weighed 175 pounds. My wife was quite concerned when I started to lose weight. I am generally in good shape.

I visited my local internist, and he ordered a series of blood tests, CT and MRI scans, and a brain scan, all of which were favorable. However, I continue to lose weight, and am now down to 155 and also quite concerned. I feel well, and eat three good meals each day, including dessert.

Have you run across this before? Can you give my doctor and me your advice? — W.M.K.

ANSWER: Unexplained weight loss is not an uncommon problem in clinical medicine. There are many causes, and I am sure your internist has looked for many of them.

There are way too many for me to list, but they come in five general categories. The first is not getting or absorbing enough calories. Malabsorption states, such as sprue or chronic pancreatitis, can keep your body from getting enough nutrition despite eating plenty. Diarrhea is almost universal in these cases.

The second is losing the calories you are taking in, and out-of-control diabetes mellitus is by far the most common cause. An elevated metabolic rate is third, and hyperthyroidism is the classic example. Chronic disease is the fourth, and cancer in particular has several ways it can cause weight loss. Chronic infections also sometimes show progressive weight loss. The fifth major category is psychiatric disease, especially depression, but anorexia nervosa as well.

Sometimes no cause can be found, and sometimes people gain back the weight.

However, it’s also the case that a condition reveals itself after months or even years, so you and your doctor need to diligently look for symptoms (that you notice) and signs (that he discovers on exam or by laboratory findings) that might indicate why you are losing weight. The fact that you are eating well and still losing weight suggests a hypermetabolic state, or calorie loss. I’d be willing to bet that your doctor looked at your thyroid, but you haven’t mentioned stool studies; fat in the stool is the simplest test for malabsorption.

DEAR DR. ROACH: With summer traveling upon us, what about food offered on the street in foreign nations? One would like to wash such things before eating, but even the water sometimes is unfit to drink! Would it be practical to buy bottled water to then wash off vendor food? Or, should tourists simply eat only food offered when they are back in their hotel, ship or such? — N.M.G.

ANSWER: Traveling in less-developed countries offers many worthwhile experiences, including trying new foods. No matter where you eat — hotel, ship, restaurant or street — there is always a risk of acquiring an unwelcome visitor to your gut, such as bacteria, virus or parasite. The Centers for Disease Control and Prevention recommends against street fare, but several studies have suggested that it isn’t any riskier than restaurant meals. If you choose to be adventurous and try street fare, some tips I can offer to reduce the risk from street fare would include:

—Eat food that is cooked in front of you, rather than precooked.

—Never eat anything uncooked that you don’t peel yourself, or rinse with your own bottled water. Keep your own hands clean with alcohol gels.

—Choose busy vendors, especially those catering to locals.

—Eat early in the day, not what has been sitting around.

The CDC gives additional advice at http://1.usa.gov/1gh22Bu.

Dr. Roach regrets that he is unable to answer individual letters, but will incorporate them in the column whenever possible. Readers may email questions to ToYourGoodHealth@med.cornell.edu or request an order form of available health newsletters at P.O. Box 536475, Orlando, FL 32853-6475. Health newsletters may be ordered from www.rbmamall.com.