SEATTLE—Millions of Americans, particularly post-menopausal women, take daily calcium supplements in the hope of reducing risk of osteoporosis and consequent fractures. But many fail to obtain the expected benefits owing to a lack of understanding of calcium metabolism, said Michael Murray, ND, at the annual meeting of the American Association of Naturopathic Physicians.
There is a lot of hype about the health benefits of calcium, and plenty of controversy over the “best” form of this essential mineral. Better knowledge of how calcium is absorbed, metabolized, and incorporated into various tissues will lead to more reasonable expectations about what calcium can and cannot do.
According to Dr. Murray, calcium deficiency is a very real and common health problem. In general, both men and women need about 1,000 mg of dietary calcium daily from the ages of 19 to 50, and 1,200 after age 50. By best current estimates, the average daily intake among US adults is only 450–550 mg. Roughly 67% of women between 18 and 30 years old, and 75% of those over 35 fail to get even close to the recommended daily allowances (RDAs), which are very conservative recommendations.
Calcium is the main mineral component in bones and teeth, but it also plays a critical role in muscle contraction, cardiac repolarization, and intracellular signaling. If used properly, supplemental calcium can provide many benefits, not only in bone strength, but also in improved cardiovascular function and digestive health. But like most things in nature, calcium is not a magic bullet, and it does not work alone.
Complex Regulation
Intracellular calcium is present in a free state as Ca++ ions, and also in a number of bound forms. In healthy individuals, albumin represents 40% of total intracellular calcium, while calcium phosphate and calcium citrate together comprise another 10% of total calcium, Dr. Murray explained.
Regulation of intracellular calcium levels is a complex process, reflecting dynamic relationships between the endocrine, skeletal, digestive and renal systems. The process is governed by 3 factors: parathyroid hormone (PTH), calcitonin, and vitamin D.
PTH is secreted by the parathyroid in response to low intracellular calcium levels, and stimulates osteoclastic activity. PTH also stimulates the renal conversion of 25-OH-vitamin D to its active form, and this plays a critical role in active transport of calcium through the intestinal wall. The net result is a release of calcium from bone, an increase in GI calcium absorption, and an overall increase in circulating calcium levels.
Calcitonin, secreted by the thyroid in response to elevated intracellular calcium levels, inhibits osteoclasts, slowing calcium release from bone. It also stimulates an increase in bone resorption of calcium and reduces urinary calcium excretion.
Vitamin D stimulates osteoblasts and increases gastrointestinal absorption of dietary calcium.
Generally speaking, when circulating calcium levels are low, there is an increase in PTH and a decrease in calcitonin. When calcium levels are high, there is a decline in PTH and an increase in calcitonin. Dr. Murray stressed that, “bone is dynamic living tissue, requiring much more than calcium to be healthy.”
Vitamin D Mediates Absorption
It is important to realize that calcium supplementation alone will do little to improve bone mineral density. There are 15 clinical studies involving over 1,800 subjects, looking at the relationship between calcium supplementation and calcium deposition in bone. Overall, the data indicate that while supplements do raise serum levels, they have little effect on BMD. At best, supplements in isolation raise BMD by only 1%–2%, with no measurable effect on fracture rates.
Add vitamin D into the mix, and it is a whole different story, Dr. Murray said. “Passive absorption of calcium through the gut is not nearly as important as active, vitamin D-mediated absorption. With the vitamin-D mediated process, you also get greater bone deposition.” He contended that calcium supplement claims of “easy absorption” are largely meaningless, unless there’s vitamin D in the formula. Without adequate vitamin D, no form of calcium is well-absorbed.
There are 25 different trials showing that when combined with vitamin D, calcium supplements can improve BMD significantly and reduce hip and vertebral fractures. In fact, the data suggest that if you had to choose between the two, you’re likely to get better bone effects from vitamin D alone than from calcium supplements alone. Vitamin D resulted in an increased rate of discontinuing osteoporosis medication in comparison to control (Papadimitropoulos E, et al. Endocr Rev 2002; 23(4): 560–569).
No “Best” Form
The supplements aisles of the nation’s health food stores overflow with calcium products claiming to be the “Most Absorbable,” “Most Bioavailable,” or “Best for reducing osteoporosis risk.”
Dr. Murray stressed that, “there is no single “best” form of calcium for everyone.
“You really need to look at cost and ease of use.” He generally recommends a combination of calcium citrate and calcium carbonate, taken on an empty stomach with vitamin D. This is based on a 1985 study suggesting that these give the best overall absorption (Nicar MJ, Pak CY. J Clin Endocrinol Metab 1985; 61(2): 391–393).
However, there is also some evidence that calcium triphosphate may be a better choice for some people, as indicated by a recent Mayo Clinic study (Heaney RP. Mayo Clin Proc 2004; 79(1): 91–97). Phosphate is required for proper utilization of calcium in bone, and repeated exposure to high doses of calcium tends to deplete phosphate stores. Use of the triphosphate form makes good sense for post-menopausal women, since between 10% and 15% of all women in this age bracket are markedly phosphate deficient.
It is true, however, that different forms of supplemental calcium contain different levels of elemental calcium. As a general guideline, calcium carbonate contains about 40% elemental calcium, meaning that it takes a daily dose of roughly 2,500 mg to obtain the recommended 1,000 mg of elemental calcium per day. Calcium citrate or malate are about 20% elemental calcium, meaning that one needs on the order of 5,000 mg per day to give the recommended 1000 mg. Calcium triphosphate is more or less equivalent to calcium carbonate at approximately 38% elemental calcium (see chart).
Whatever form of calcium a patient chooses, it is important that he or she also take magnesium. “Supplement according to the ratios you naturally find in the body,” said Dr. Murray. “A 2 to 1 calcium to magnesium ratio is generally a good way to go, but if you are using magnesium as a therapy for cardiovascular conditions, you can bump that up a bit.”
Significant Heart, GI Benefits
Beyond its importance in maintaining bone density, optimal calcium levels also improve lipid profiles and reduce blood pressure, reported Dr. Murray. In a study of 223 post-menopausal women, supplementation with 1,000 mg per day of calcium citrate lowered LDL by a mean of 6%, and raised HDL by 7% (Reid IR, et al. Am J Med 2002; 112(5): 343–347). “The 16% increase in the HDL to LDL ratio would be predicted to reduce incidence of cardiovascular events by 20%–30%.”
There is also evidence that calcium supplementation can reduce risk of premalignant colon polyps. The Calcium Polyp Prevention Study, a 4-year double-blind trial, showed a 30% reduced risk of polyp recurrence in patients who already had polyps and took calcium daily (Grau MV, et al. J Natl Cancer Inst 2003; 95(23): 1765–1771). However, this was only true in those who had adequate vitamin D levels, again underscoring the importance of this vitamin in optimizing calcium’s effects.
Got Fractures?
One of the biggest calcium controversies is over the role of milk and dairy products as a source for this mineral. “There is certainly a lot of hype around this,” Dr. Murray said. “The dairy industry has definitely capitalized on it.” A dairy industry sponsored study comparing dairy to calcium supplements showed that the dairy sources seemed to be more effective in raising calcium levels than the supplement tablets (Zemel MB. Am J Clin Nutr 2004; 79(5): 907S–912S).
But Dr. Murray quickly pointed out that raising calcium levels is not the same thing as slowing osteoporosis or reducing fracture risk. In this regard, dairy products don’t seem quite so golden. Data from the Nurses’ Health Study showed that women who drank two or more glasses of milk per day had a 45% greater relative risk of hip fractures compared to women who drank one glass or less (Feskanich D, et al. Am J Publ Health 1997; 87(6): 992–997). “The dairy industry has done a great job convincing everyone that they need more milk. But the epidemiology doesn’t seem to support that.”
If patients like dairy and are able to digest it adequately, there is no problem. But there is no strong body of data to support widespread increases in dairy intake for adults simply based on an osteoporosis prevention rationale. Equivalent benefits can be obtained through use of good calcium and vitamin D supplements.
