Low Vitamin D Levels in Adults

By Daniel L. Hurley, MD, FACE

Where does vitamin D come from?

Vitamin D has two basic forms. Vitamin D2 is found in vegetables and supplements. Vitamin D3 is mainly formed from your skin being exposed to ultraviolet B (UVB) radiation with sunlight or tanning beds. It also comes from foods such as fatty fish and dairy foods with added vitamin D, and from vitamins containing D3. Both vitamin D2 and D3 do not become active until they pass through the liver. Calcitriol [KAL-si-TRYE-ole] is then produced in the kidney and is the most active form of vitamin D in the body.

Why is vitamin D important?

Vitamin D is needed to build healthy bones and to help prevent bone loss. Vitamin D helps calcium get from the intestines to the bone. Calcium is a mineral needed to make new bone and replace new or aging bone. Not getting enough calcium can lead to bone loss, or less dense bones. This is called “osteopenia” [os-tee-oh-PEEN-ee-ah] if the condition is mild, or “osteoporosis” [os-tee-oh-puh-RO-sis] if the bone is much less dense than normal. Vitamin D is also needed to harden new bone, just like wet cement becomes concrete. Thus, vitamin D keeps bone from being soft, thin, or osteoporotic [os-tee-oh-puh-ROT-ik] by giving the bone adequate calcium.

Taking vitamin D can also make muscles stronger in people who don’t have enough of the vitamin, and help reduce the number of falls by about 50%. This is important because most broken bones in patients with bone loss occur after a fall.

How common is a low vitamin D level?

Having too little vitamin D is more common than once thought. Up to 50% of adults who see their doctor have it. Not enough adults have optimal vitamin D levels, which should be over 25 ng/mL (nanograms per milliliter). Up to 70% of adult whites and 95% of adult African Americans in the US have a vitamin D level under 30 ng/mL. Having a level of 10 ng/mL or lower of vitamin D is considered very low. People in the northern United States and in less sunny climates are likely to have low vitamin D levels.

Who is at risk for low vitamin D levels?

It is now well known that many people have vitamin D levels that are too low for good health. Vitamin D is made mainly when skin is exposed to UVB from the sun. But many factors limit the sun exposure to the skin. Clothing, sunscreen, time of day, season, regions with less sun, low altitudes, skin pigment, and age all affect the skin’s ability to produce vitamin D. Not many foods contain vitamin D, and vitamin D added to foods is not well regulated. In addition, even eating foods with added vitamin D may not give you enough of the vitamin for optimum health. The two most common causes of low vitamin D levels are lack of sun and not enough intake of vitamin D by mouth. Other causes of low vitamin D levels include the body’s inability to absorb enough vitamin D (intestines made shorter by surgery, gastric bypass, and intestinal diseases), chronic kidney disease, and liver failure (Table 1).

Who should be tested for low vitamin D levels?

Because it is expensive, checking blood for vitamin D levels is not recommended for everyone. But if you’re at risk for low vitamin D levels (see Table 1), you should get tested.

The best form of vitamin D in the blood to check is that produced by the liver – 25 hydroxycholecalciferol [hye-DROK-see-ko-leh-kal-SIF-er-ol]. A healthy range is anywhere between 25 and 80 ng/mL. But even levels that are somewhat low can cause bone loss and osteoporosis. Table 2 lists vitamin D levels found in health and disease states.

How Can We Prevent and Treat Low Vitamin D Levels?

It is safe and does not cost much to add vitamin D to your diet. However, many people with low vitamin D levels don’t know they have the condition or don’t get the right treatment. You can get vitamin D through what you eat. However, except for fatty fish, vitamin D in most foods, including dairy products with added vitamin D (“fortified”), is low to none. For instance, one cup of fortified milk provides 300 mg calcium and 100 international units (IU) of vitamin D. Drinking four cups of milk (or a dairy equivalent) provides a total of 1200 mg calcium but only 400 IU of vitamin D. The current Food and Nutrition Board guidelines for adequate intake (AI) of 400-600 IU of vitamin D can be met by diet and a daily multivitamin. However, this may still not be enough for many people. New recommendations by the Institute of Medicine were published in late 2010. Reading food labels can help you figure out how much vitamin D you’re getting, but the only way to be sure how much you’re getting is to get your blood tested.

Which vitamin D is best, and how much is enough?

Vitamin D2 and vitamin D3 are safe dietary supplements. Which one is better is not clear, but both seem to be good choices as long as an “optimal” vitamin D blood level is reached. Both can be taken once daily from 400 IU to up to 2000 IU or in larger doses available by prescription only (up to 50,000 IU) taken less often such as once a week or even once a month. Both vitamin D2 and D3 are best absorbed in the gut if taken with a meal containing some fat.

There are many ways to take vitamin D if your blood levels are low. A common mistake in treatment is to stop taking vitamin D when the blood level reaches the “optimal” level. No matter what dose you start with, and if you haven’t changed your sun exposure levels or diet, a total intake of at least 1000 to 2000 IU daily will likely be needed on a regular basis to be healthy.

People with malabsorption often need larger amounts of vitamin D. For example, patients after gastric bypass may require 50,000 IU of vitamin D2 or D3 from once weekly to once daily. In people with extreme malabsorption, UVB from sunlight or tanning beds can help. Obtaining a blood level is the only way to be sure vitamin D intake is adequate for any given person.

Dr. Daniel L. Hurley is a consultant in the Department of Medicine and Division of Endocrinology, Diabetes, Metabolism, and Nutrition at Mayo Clinic. Dr. Hurley received his Internal Medicine and Endocrinology training at Mayo Graduate School of Medicine. He was awarded the Randall G. Sprague Award for Outstanding Achievement as an Endocrine fellow, the Department of Medicine Teacher of the Year Award, and the Henry S. Plummer Distinguished Physician Award for the Department of Medicine. His clinical interests include metabolic bone disease, nutritional health, and mentoring endocrine fellows and staff physicians new to Mayo Foundation.