Friday, August 30, 2013

Should I take calcium? Part II

In Part I of this series of posts, I discussed the science behind calcium intake.  In short, what goes out via intestinal loss (at least 150 mg/day or more) and kidney excretion (at least 100 mg/day or more)(1) must be replaced in order to maintain a steady level of calcium in the extracellular fluid (ECF) including blood.  If calcium out is greater than calcium in, the parathyroid glands release a hormone which facilitates breakdown of bone mineral matrix freeing calcium to enter the ECF. Thus, bone-based calcium is not only an integral part of your skeletal strength but also a readily available calcium reserve in case you're running short.

Clinical research scientists who regularly conduct studies on every sort of medical dilemma have compiled an enormous amount of data on the benefits of calcium intake. Starting from an evolutionary perspective--my personal favorite--let's first consider our ancestors from 10,000 years ago when life was quite different but the human genome was not. 

Radiologists and anthropologists from Emory University have determined that our ancestors--both human and primate--ate a lot of high-calcium insects and high-calcium plant food.(2)  These experts estimate that our Stone-Age predecessors took in at least twice as much calcium, mostly from plant based sources, along with more fiber, micronutrients, and protein than we consume with our modern diet. They ate virtually no grains which are not only a poor source of calcium but certain varieties including wheat contain phytates, a compound which binds minerals and decreases their absorption. X-rays of our foreparents' fossilized skeletons confirm that the outer layers of their bones were nearly 20% thicker than ours.  Strong Cro-Magnon structure was the result of a whole lot of outdoor hunting and gathering plus lots of calcium.  The authors of this review conclude that the best-for-modern-bone plan would include a return to the "nutritional pattern for which we have been genetically programmed by evolution."

There is an enormous amount written about the effects of calcium supplementation, with or without vitamin D, in a modern population that avoids the sun and doesn't snack on praying mantises.  The evidence is clear that calcium intake can promote a positive calcium balance--more in than out--which in turn reduces the rate of bone loss and may result in an increase in bone density.  What is not clear is whether or not this has a positive effect on fracture risk. Some experts point out that many of these studies were too short in duration to fully evaluate the long-term benefits of better bone. One study out of France(3) featuring 3,000+ old ladies with added calcium, however, demonstrated decreased fracture risk in just 18 months of follow-up! Half the subjects in this investigation received 1200 mg of calcium plus 800 units of D each day and ended up 43% less likely to have broken their hips by study's end compared with their colleagues who took look-alike placebos.

The US Preventive Services Task Force spent a lot of time reviewing this mountain of medical evidence.  They considered "meta-analyses" that pooled data from multiple studies and concluded that there was not enough evidence to support a recommendation for the use of supplements, and declared that smaller doses (less than 1,000 mg/day calcium and 400 units D) made no dent in fracture risk at all.  Because extra calcium intake can increase the incidence of kidney stones in susceptible persons, the USPSTF graded such dosing as Grade D, i.e. don't do it!, a 'living' on-line textbook for doctors, also considered the data, last updating the section on calcium supplements on August 28, 2013.  The reviewers agreed that the fracture data was variable, but were particularly impressed with the data from the Women's Health Initiative, the same trial that created headline news about negative health outcomes from the long-term use of HRT.  Over 36,000 women were assigned to take 1,000 mg/day of calcium citrate with 400 units of vitamin D or placebo pills.  Those who were most compliant with the regimen, taking at least 80% of the supplements over 7 years of follow-up, had a nearly 30% decreased risk of hip fracture.  Overall, compliant or not, the calcium/D group had a 12% decrease in fractures.

Uptodate's wrapped up their discussion after considering the latest data as of just two days ago by stating "Based upon the meta-analyses discussed above, we recommend 1200 mg of calcium (total of diet and supplement) and 800 int. units of vitamin D daily for most postmenopausal women with osteoporosis."  And, spoiler alert, they took into account the effect of supplemental calcium on the risk of cardiovascular disease.

More on that in Part III.
1) Houillier, P et al.  "What serum calcium can tell us and what it can't".  Nephrol. Dial. Transplant. 21 (1): 29-32.
(2)Eaton, SB and Nelson, DA.  "Calcium in Evolutionary Perspective." Check it out for the calcium content of grasshoppers and moths!
(3)Chapuy, MC et al.  Vitamin D3 and calcium to prevent hip fractures in the elderly women.  N Engl J Med 1992 Dec 3;327(23):1637-42.

No comments: