Calcium and Osteoporosis
By Kenneth Noller, MD
Some time ago, i was talked into developing a lecture on calcium and osteoporosis, and I was asked to omit any reference to hormone replacement therapy (HRT) because obstetricians and gynecologists "already know all about that." I don’t know why I agreed to do the talk, but I am very thankful because I became familiar with a new topic and several journals that I had never previously perused.
What is Osteoporosis?
Although there are several different definitions, most experts would currently define osteoporosis as a bone mineral density (BMD) more than two standard deviations below the peak average for a young woman (approximate age, 20). Although many different instruments have been developed to measure bone density, the "gold standard" is currently a DEXA scan (dual energy x-ray absorptiometry). While simple x-rays can diagnose osteoporosis when it is severe, a routine film may vary bone density by as much as 30% simply by changing the method of film development. Additionally, up to 50% of bone mass must be lost before simple x-ray changes are noted.
Only a few years ago, single photon absorptiometry and dual photon absorptiometry were considered standard measurement techniques. However, neither was found to be accurate enough to establish a definite diagnosis in many cases. Little by little, the DEXA scan became the standard technique.
Risk Factors
Both men and women lose bone mass beginning from about age 25-30 and older. Unfortunately, at the time of menopause, women begin to lose bone mass more quickly, though the rate of decrease slowly lessens. At the age of 70 or older, women parallel the loss that males experience. At all ages, however, women will exhibit lower bone mineral densities than men. The two most important risk factors are advancing age and female gender.
Other risk factors include early age at menopause, a family history of osteoporosis, and a thin frame. Diet is extremely important. Low calcium diets or very large intakes of caffeine and alcohol are all associated with osteoporosis. Smoking and a sedentary lifestyle likewise increase a patient’s risk. Recently, great emphasis has been placed on the risk of osteoporosis among young girls and women who have exercise-induced amenorrhea. Indeed, the "female athlete triad" has been described, which includes disordered eating, amenorrhea, and osteoporosis.
Screening
Far too few physicians routinely obtain a height at the time of each general examination. In general, any adult who loses more than 1.5 inches in height probably has osteoporosis. Of course, non-traumatic vertebral column fractures or marked changes in posture are also hallmarks of this disease.
Some experts have suggested that women should receive DEXA scanning routinely at the time of menopause, but the test is expensive, and widespread screening has not yet been shown to be cost-effective. Rather, DEXA scanning should be reserved for use in those women who prefer not to use HRT. If the scan shows that the BMD is already low, that fact can be used to encourage the woman to begin HRT. On the other hand, if her BMD is normal (or even above normal), and she is reluctant to use HRT, a high-calcium diet may prevent the occurrence of osteoporosis.
Urinary N-telopeptide (NTx) testing is now widely available. This test measures a circulating product of bone breakdown. In general, a high NTx level indicates that bone is being resorbed, but it is not a good screening test. NTx should be reserved for following patients on active therapy for osteoporosis.
When HRT is Not Possible
Hormone replacement therapy, if used appropriately, can prevent osteoporosis in females until very late in life. However, there are some women who are not able to use the medication for some reason, or who refuse to do so. In such women, the use of a calcium-rich diet with vitamin D supplementation to aid absorption can also postpone the development of osteoporosis. Unfortunately, diet is not as effective as HRT, and even very compliant women may develop the disease.
A great deal (thousands and thousands of articles) has been written about the average daily requirement of calcium for men and women of various ages. Little by little, a consensus has developed that postmenopausal women on HRT should ingest 1000 mg of calcium per day, and similar women not on HRT should receive a minimum of 1500 mg per day. Unfortunately, the average adult female diet in the United States contains only about 500 mg of calcium per day. Dietary supplementation is very important. Although food alone can supply sufficient calcium, few adults are willing to consume the large quantities of (primarily) dairy products in order to ingest 1500 mg per day. Even the single best source of dietary calciumyogurtrequires at least four large servings to supply one day’s need.
Calcium tablets with vitamin D are readily available in virtually all pharmacies at relatively low cost. The use of common calcium-containing antacids is a good source of calcium, but these preparations lack vitamin D, which is necessary for maximum absorption. Occasionally, a patient will ingest too much calcium. Daily intakes exceeding 4 g paradoxically result in decreased absorption but may also cause the development of stones and renal damage.
Treatment of Osteoporosis
Several different treatments for osteoporosis are available. The use of sodium fluoride and parathyroid hormone increase bone formation. Unfortunately, sodium fluoride seems to result in the deposition of bone of poor quality, and the frequent side effects associated with its use make it an unpopular therapy at the present time. Calcium and vitamin D (usually considered preventive measures rather than treatment), estrogens, calcitonin, and bisphosphonates inhibit bone resorption.
Calcitonin represented a breakthrough in the treatment of osteoporosis when it was introduced several years ago. Unfortunately, it must be used intermittently, taken several times per week for a few months, and repeated on a yearly basis. Although originally available only as an injection, a nasal spray is now available. The response to calcitonin therapy is variable, the treatment is expensive, and side effects are common. Few clinicians consider it a useful drug.
The bisphosphonates represent a category of drugs that has become quite useful for the treatment of osteoporosis. Although they are poorly absorbed from the GI tract, the small amount of drug that is absorbed is rapidly deposited in bone, where it remains relatively indefinitely. Etidronate disodium was the first commercially available drug and inhibits dissolution of bone. Unfortunately, it should be taken only two weeks each calendar quarter with calcium supplementation during the remaining 11 weeks.
More recently, alendronate has been introduced. This drug localizes in the area of bone under osteoclasts and inhibits their activity. The area is rapidly covered with new bone. Unfortunately, the drug has been associated with esophageal ulceration if not taken in the prescribed manner. It is most important that anyone ingesting the drug do so while standing and drink at least one full glass of water. The patient should remain standing for at least 30 minutes and should eat nothing during this time.
In the future, new and less problematic bisphosphonates will likely be introduced. Also, investigation into the use of prostaglandins, parathyroid hormone, and certain cytokines suggest that there may be a role for the use of these drugs in the treatment of osteoporosis in the future.
References
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Which of the following techniques is the most accurate for the diagnosis of osteoporosis?
a. X-ray of the lumbar spine
b. Single photon absorptiometry (SPA)
c. Dual photon absorptiometry (DPA)
d. Dual energy x-ray absorptiometry (DEXA)
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