New research suggests that maintaining adequate serum vitamin D levels can ensure ideal parathyroid hormone (PTH) values without the need for calcium intake above 800 mg/day, according to a new report in the November 9 issue of JAMA.¹ These findings provide further evidence of the importance of vitamin D to optimal bone health.

"Our study indicates that for the normal population, 800 mg of calcium per day is enough if their vitamin D status is good, either by supplements of 400 to 600 U per day or by rich exposure to sunlight," corresponding author Gunnar Sigurdsson, MD, PhD, chief of the division of endocrinology and metabolism in the department of internal medicine at Landspítali-University Hospital in Reykjavik, Iceland, tells CIAOMed. "Vitamin D sufficiency has such calcium-sparing effects that higher calcium intake than 800 mg is unnecessary."

The Icelandic investigators note that while researchers generally recognize the importance of preventing unnecessary increases in serum PTH to ensure bone health, they also say that there is less confirmatory evidence of the exact levels considered detrimental. "Secondary hyperparathyroidism is considered to play a significant role in the pathogenesis of age-related bone loss," the researchers write, "[and] it is well recognized that serum PTH is the principal systemic determinant of bone remodeling."

In the new cross-sectional study, 2310 healthy Icelandic adults were divided equally by age group (30–45 years, 50–65 years, or 70–85 years). Participants were administered a semiquantitative food frequency questionnaire to assess vitamin D and calcium intake. They were then further divided into groups according to calcium intake (<800 mg/day, 800–1200 mg/d, and >1200 mg/day) and serum 25-hydroxyvitamin D level (<10 ng/mL, 10–18 ng/mL, and >18 ng/mL). A total of 944 participants completed the dietary questionnaire.

After adjusting for relevant factors, serum intact PTH was found to be lowest in the group with a serum 25-hydroxyvitamin D level of >18 ng/mL, but highest in the group with a serum 25-hydroxyvitamin D level of <10 ng/mL, the study showed. 

The trial also found that at the low serum 25-hydroxyvitamin D level (<10 ng/mL), calcium intake of <800 mg/day versus calcium intake of >1200 mg/day was significantly associated with higher serum PTH. At a calcium intake of >1200 mg/day, there was a significant difference between the lowest and highest vitamin D groups, the study showed.

Serum 25-hydroxyvitamin D is the generally accepted indicator of vitamin D status, but there is no universal consensus regarding which serum values constitute sufficiency. An inverse relationship between serum 25-hydroxyvitamin D and serum PTH is well established, and an insufficiency of vitamin D or calcium is generally associated with an increase in PTH. However, the study authors point out that to their knowledge the relative importance of each nutrient had not previously been addressed.

"The main effect of maintaining a normal calcium balance by keeping PTH levels normal in the blood is to prevent long-term bone loss, which occurs when raised PTH stimulates the osteoclasts to release calcium from bone to maintain a normal calcium level in the blood for normal cell function in various tissues," Dr. Sigurdsson, also a professor of medicine at the University of Iceland, explains. "This occurs very gradually over a long period of time—many years if PTH is high," he says, adding that it is not surprising to see no effect from administering extra vitamin D and calcium over approximately 2 years to a population not grossly deficient in either.
 
Extra supplementation increases risk of side effects

"Calcium absorption from the gut is largely vitamin-D dependent, so if we make sure that we meet our vitamin D requirements, then we absorb our calcium better and therefore we need less calcium in the diet and don't have to worry about constipation due to extra calcium tablets," Dr. Sigurdsson tells CIAOMed. "There is no point in advocating excessive intake of calcium (>800 mg/day) or vitamin D intake above 800 U a day or so for [people in the normal population]. Such recommendations are useless and may have disadvantages such as extra cost and constipation."

Individuals on certain medications still need more calcium

"Our study was mainly on a normal population, as we excluded those individuals receiving drugs that might interfere with normal calcium metabolism such prednisolone, bisphosphonate (Fosamax® or Actonel®), or Forteoâ„¢," Dr. Sigurdsson says. "Those individuals need extra calcium supplements."

Reference

1. Steingrimsdottir L, Gunnarsson O, Indridason OS, et al. Relationship between serum parathyroid hormone levels, vitamin D sufficiency, and calcium intake. JAMA. 2005;294:2336-2341.