Can you tell me what could be causing my low vitamin D level? Do you believe that this is the issue, or are the low levels caused by something else?

DEAR READER: Vitamin D is a fat-soluble vitamin that is essential for promoting calcium absorption, maintaining adequate serum calcium and phosphate levels, bone growth and remodeling, reduction of inflammation, and neuromuscular and immune function.

If your levels are low, you are at risk of developing weak, brittle and/or deformed bones. This could also affect your immune system, making you more susceptible to infection, allergens and more. Your low vitamin D level may indeed be the cause of your allergies, which are causing your cold and flu-like symptoms; however, this does not explain why you have a low D level to begin with.

In healthy people, vitamin D deficiency can typically be avoided by adequate sun exposure and a well-balanced diet. Because there are very few foods that naturally contain D, fortified cereals and dairy products are the best way to achieve sufficient dietary amounts.

Older adults, breastfed infants and those with limited sun exposure, dark skin or fat malabsorption are at increased risk of developing deficiency. Obese individuals may have difficulty absorbing vitamin D from sun exposure because the subcutaneous fat essentially blocks part of the D from entering the bloodstream. Those who have undergone gastric bypass may also have difficulty because most forms of this surgery bypass the upper small intestine where D is absorbed.

Symptoms of vitamin D deficiency typically go unnoticed. The most common consequence is weakened bones, also known as rickets in children, osteomalacia in pre-menopausal women and men, and osteoporosis is post-menopausal women. There may be an increased risk of developing cancer, especially that of the digestive tract. There is also research suggesting deficiency may play a role in the prevention and treatment of diabetes types 1 and 2, hypertension, glucose intolerance, multiple sclerosis and more. However, because these studies have not been randomized clinical trials, the implications will continue to be debated until definitive proof is found.

According to the Food and Nutrition Board (FNB) at the Institute of Medicine of the National Academies, adequate intakes (AIs) for healthy people between birth and age 50 should be 200 IU of vitamin D daily. Those aged 51 and over should double that. Recently, the American Academy of Pediatrics issued its own recommended intakes that exceed those of the FNB. This led to an expert committee formation by the FNB, which decided that recommended intakes be re-evaluated. To the best of my knowledge, the decision of this committee should be made public later in 2010. Tolerable upper-intake levels (ULs) for those between birth and 12 months is 1,000 IUs; age 1 and older is 2,000 IUs daily. This, too, was challenged and is currently under review.

I suggest that you speak to your physician to request that the underlying cause be found. It may be as simple as not getting enough sun exposure and may be helped by exposure without sunblock for 15 or 20 minutes each day. Longer unprotected exposure is not recommended because of the increased risk of developing skin cancer.

If the cause cannot be found, follow your physician’s advice regarding supplementation and including more vitamin D-enriched foods into your diet.

You should also be watchful for signs of excess intake. Most commonly, these symptoms are nonspecific and include nausea, weakness, constipation, weight loss, vomiting and poor appetite. If healthy people continually take levels above the UL, it may result in elevated blood levels of calcium, which may cause confusion and other mental-status changes, as well as heart-rhythm abnormalities. There is also an increased risk of developing kidney stones, especially when used in conjunction with calcium; however, because you are deficient and under the care of a physician, these are not likely to result.

To provide related information, I am sending you a copy of my Health Report “Vitamins and Minerals.” Other readers who would like a copy should send a self-addressed stamped No. 10 envelope and a $2 check or money order to Newsletter, P.O. Box 167, Wickliffe, OH 44092. Be sure to mention the title. Order forms are also available for printing on my Web site at www.AskDrGottMD.com.