DEAR DR. ROACH: My wife and I are 74 and in good health. Recently, I started reading about vitamin D and its effect on the immune system. We both had a vitamin D blood test for the first time ever, despite having almost yearly blood tests for the past 40 years. My level was 32 nanograms per milliliter, and hers was 22. She is now taking 50,000 IU of vitamin D weekly on doctor’s orders. Her blood level is considered insufficient, and mine is barely over the optimal level, although the “optimal” extends to 100 ng. We have both taken multivitamins for years. How did we get so low, and are our are immune systems compromised? What levels should we be trying to achieve? — E.

ANSWER: Vitamin D has many effects on different tissues of the body, but the data showing definitive benefit to treatment mostly comes from its effect on bone.

Most of the body’s immune cells have the receptor for vitamin D, but evidence that treatment of low vitamin D improves immune function is scant. Randomized trials have examined the effect of vitamin D on upper respiratory infections and tuberculosis, but the benefit was limited to people with VERY low vitamin D levels (less than 10 ng/mL). Studies are ongoing to evaluate the effect of vitamin D supplementation on infection in general, but since very low levels should be treated anyway from the standpoint of bones, there is currently no indication to treat low vitamin D solely in order to prevent or treat infection.

The target level of vitamin D remains controversial. Most experts recommend levels between 20 and 50, which is usually achievable with vitamin D3 doses of 800-2,000 IU daily (I prefer daily to weekly dosing).

DEAR DR. ROACH: This 83-year-old male has lost three contemporaries recently to lung diseases: pneumonia, COPD and cancer. Now, the public has been thoroughly educated on the hazards to lung health, such as cigarette smoking, coal mining and asbestos handling. And diet and exercise are promoted for such things as cardiovascular, gastrointestinal and musculoskeletal health, but I’ve never seen comparable advice for lung health. Are there any exercises to perform, substances to inhale or foods to eat that will promote my lungs’ longevity? — R.J.B.

ANSWER: Humans have enormous lungs — if spread out, the surface area of a lung is roughly the size of a tennis court! In a healthy person, lung function rarely, if ever, limits performance. Over time, lung function slowly decreases, and a reasonable goal would be to slow that decrease. Mostly, that means avoiding factors that damage the lung: These are cigarette smoke; other lung irritants such as cooking smoke; indoor and outdoor air pollution; the occupational hazards you mention; and radon in the home. Get your house checked if you live in an area where this is likely; find out where at tinyurl.com/CDC-radon.

There are some proactive steps you can take. One is to reduce your likelihood of infection. Getting your flu shot yearly and your pneumonia vaccines when recommended will help. Serious infection can cause permanent loss of lung function. Diet, especially fruits, have been correlated to improved lung function. Regular, moderate exercise has likewise been shown to slow lung function loss and it certainly helps muscles — including the muscles needed to breathe. It also allows cells to get better at extracting oxygen from the blood.

* * *

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 send mail to 628 Virginia Dr., Orlando, FL 32803.