Dear Dr. Roach: I have a sister who is in her mid 60s and has a very low weight. She cooks a lot but doesn’t eat much. Is there something to suggest to make her gain some weight? She weighs 80 pounds and is 5 feet, 2 inches tall. By contrast, I’m 5 feet, 4 inches tall and weigh 150 pounds. I am worried that if she gets sick, she will not have anything to live off of. — P.L.W.

Answer: With all the recent press about GLP-1 medicines helping people to lose weight, there has been much discussion about the health risks of being obese. However, there are health risks of being underweight as well. In fact, the health risk of being as underweight as your sister is (a BMI level of 14) is about the same as the overall health risk of a person with a BMI of 50. (For her, this would equate to about 300 pounds.)

I don’t know why she isn’t gaining weight, but there are many possibilities. Weight loss due to psychiatric diseases like major depression and anorexia nervosa is common and underdiagnosed. Some diseases keep people from absorbing nutrients, especially celiac or Crohn’s disease. People can lose calories through their urine (such as diabetics), while others use up their calories too fast (due to high thyroid levels, advanced cancer, etc.). There are many other less-common possibilities.

One critically important issue is to be sure of how much your sister is eating. It sounds like she isn’t eating much, which makes some issues more likely than others. While there are appetite stimulants, I don’t use these unless I feel like I understand what is causing the problem. A careful evaluation by a general physician is called for.

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.