Having low levels of vitamin D in your blood affects your body's ability to absorb calcium. Eventually, this may lead to weak, brittle bones, increasing your risk of breakage. Very few foods supply adequate vitamin D, so your doctor may suggest taking a supplement to get your vitamin D levels back up. There are a few things that affect your body's ability to absorb vitamin D properly. If your vitamin D serum levels continue to stay low, talk with your doctor about adjusting your supplements or dosage.
Absorption
Vitamin D is a fat-soluble vitamin, meaning it absorbs with the help of fat. Taking a vitamin D supplement requires you to have it with some kind of food containing fat in order to be absorbed. If you take your supplement on an empty stomach, your body may not be able to fully absorb it. Vitamin D is unique in that your body can synthesize it when your skin is exposed to direct sunlight, but you need some from your diet or supplements as well. Your body cannot use vitamin D until it activates; it is inactive until it undergoes biochemical processes. Your liver activates vitamin D by turning it into 25-hydroxyvitamin D or calcidiol, which is the most common form of vitamin D in your body, says the Linus Pauling Institute.
How Much Do You Need?
Your supplement may be measured in micrograms (mcg) or international units (IU). Knowing measurements is important for you to determine how much you need each day. As a healthy adult, you need 15 mcg, or 600 IU, daily, according to the Linus Pauling Institute. Since your risk of bone loss increases with age, this amount jumps to 20 mcg, or 800 IU, after age 70. Most vitamin supplements contain vitamin D3, or cholecalciferol, and provide anywhere from 400 to 2,000 IU of the vitamin per capsule. Your daily multivitamin may contain as much as 400 IU as well. Space out your supplements into two to three equal doses throughout the day. This may help your body break down and absorb the vitamin, elevating vitamin D levels in your blood.
Normal Serum Levels
Your doctor can check your vitamin D serum levels by running a 25-hydroxy vitamin D test. This routine blood test measures the major form of vitamin D in your blood. Normally, your levels should fall between 30 and 74 ng/mL, reports MedlinePlus. Having inadequate intake or problems with absorption can affect your vitamin D levels.
Factors Affecting Absorption
Having a fat malabsorption disorder, such as cystic fibrosis or cholestatic liver disease, may affect your ability to properly absorb vitamin D, since it needs fat in order to metabolize. Irritable bowel syndrome or Crohn's disease may push the vitamin through your intestine before it gets a chance for your body to utilize it. One study published in the "European Journal of Endocrinology" in 2011 showed that children and adolescents who have high percentages of body fat have increased chances of low vitamin D levels. If you are overweight, your body may not be able to sustain normal levels of vitamin D. Your age may also affect your vitamin D levels, since your body slows down with age, affecting your ability to absorb your vitamin D supplements.
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