A Low Vitamin D Level Could Mean Increased Osteoporosis Risk
When blood tests show persistent low vitamin D levels despite adequate exposure to the sun... genetics or a medical condition may be the source of the problem.
The most common causes of low vitamin D levels are the following:
- Inadequate exposure to direct sunlight (rather than through glass) especially for dark skinned people and people over 60 years old
- Living north of the 42nd parallel in the Northern Hemisphere (north of Boston) or South of the 42nd parallel in the Southern Hemisphere-where the sunlight is too weak to produce vitamin D in the winter months
- Excessive use of sun protector of SPF of 8 or higher
- Obesity, which causes vitamin D to be sequestered in the fat rather than circulating in the blood
- Very low levels of magnesium
- Liver or kidney disease
- Use of anti-convulsive drugs
- Excessive use of alcohol
- Crohn's disease and inflammatory bowel disease
But I was recently intrigued by a friend in Arizona who is beautifully brown from working outdoors and suffered from none of the above conditions-but still had a persistent low vitamin D level on her blood tests. There are two other possibilities that we are still exploring.
Some people have a genetic predisposition that prevents adequate absorption of Vitamin D from food or sunlight.
Researchers at the Garvan Institute of Medical Research in Sydney Australia have discovered a gene that affects Vitamin D's action on bone and calcium metabolism. People with the BB version of the Vitamin D Receptor [VDR] gene have the lowest bone density and those with Bb version have the next lowest. Persons with the bb version of Vitamin D receptor gene have the best bone density. This may suggest why genetic factors have long been suspected as a risk factor for osteoporosis.
Other specialists challenge these finding, however. SH Ralston of the Department of Medicine and Therapeutics, University of Aberdeen, UK argues that while twin and family studies suggest that up to 85% of the variance in bone mineral density is genetically determined-the relationship between VDR polymorphisms and bone density is inconsistent, and poorly reproducible in different populations.
Low vitamin D levels are very common in patients with hyperparathyroidism.
Hyperparathyroidism is associated with high calcium in the blood caused by a parathyroid tumor. The parathyroid tumor makes a hormone (PTH) which takes calcium out of the bones and puts it into the blood. The body doesn't want the calcium to be high, so it will try to get rid of the calcium through the urine.
The body may also limit the amount of vitamin D in the body because it wants to shut down calcium absorption from the intestines as a protective measure. It does this by limiting the amount of Vitamin D in the body so you can't absorb so much calcium from your food.
If your PTH and calcium are high and your Vitamin D is low (or even normal) your health practitioner should check for a parathyroid tumor. Once a parathyroid tumor is removed, the vitamin D level will likely correct itself within a month.