Postmenopausal women rightfully worry about osteoporosis; they are at increased risk. No one wants the curved spine that my mother had or brittle bones that result in fractures. Vitamin D is essential for strong bones and works together with calcium to mineralize or harden bones. A new study in mice found that vitamin D only benefits bones, if enough dietary calcium is absorbed from the intestine. When not enough calcium is absorbed from food, vitamin D becomes the bad guy and stops adding calcium to bone, making them brittle.
Benefits of vitamin D
In a large 18-year-long study, postmenopausal women (72,337 participants) with the highest intake of vitamin D had 37 percent fewer hip fractures. Besides making strong bones and teeth, vitamin D is also associated with:
- Reduction in autoimmune diseases such as type 1 diabetes, MS (multiple sclerosis), and rheumatoid arthritis
- Lower incidence of type 2 diabetes
- Lower incidence of heart disease
- Lower incidence of cancer
- Increased muscle strength
- Greater ability to fight colds and infections
How vitamin D works
You get vitamin D when the sun hits your skin (but not through glass), or as an additive in foods (foods naturally don’t contain enough vitamin D), or as a supplement. Vitamin D, obtained in these ways needs to be activated inside your body. Activated vitamin D helps the intestine to absorb calcium from food and controls the blood level of calcium in your body. Bone, the intestine, and other tissues in your body are controlled by vitamin D, because they have vitamin D receptors that respond to vitamin D.
Too much vitamin D
Brittle bones caused by vitamin D supplements have only been documented when an extremely high dose of vitamin D was taken. For example, in one study women took 500,000 I.U. of vitamin D all at once just once a year.
In the new study, when researchers inactivated the intestinal vitamin D receptors in mice, the mice suffered osteopenia (low bone density). The intestinal vitamin D receptors help absorb calcium from food. Although most of the intestinal calcium absorption was prevented, there never was a drop in blood calcium concentration. The results showed that vitamin D maintained blood calcium concentration in spite of poor calcium absorption by helping to remove calcium from bone instead of adding to it.
Implications
Although vitamin D is essential for maintaining bone health, lack of calcium intake in food or intestinal malabsorption problems could uncover the dark side of vitamin D and cause osteoporosis and fractures. Many foods besides dairy products contain calcium, even vegetables, and if you are a premenopausal woman, eating a healthy diet, you will not need to worry about overdosing on vitamin D unless you go to extremes. But if you are a postmenopausal woman, you may want to balance your vitamin D intake with a sufficient calcium intake. Anyone stressed by disease, intestinal disorders, or on a restricted diet should also use caution when supplementing with vitamin D. If you get your vitamin D from sunlight, you will not overdose, but too much sun can give you skin cancer.
Sources
Lieben, L. et al. Normocalcemia is maintained in mice under conditions of calcium malaborption by vitamin D-induced inhibition of bone mineralization. Journal of Clinical Investigation (2012) doi: 10.1172/ JCI145890
American Journal of Clinical Nutrition
University of Maryland Medical Center: Vitamin D
The Nutrition Source, Vitamin D and Health
