New finding fractures calcium consensus; other research poses prostate risk from high-dose calcium
by Craig Weatherby
Vital Choices readers may recall that our Halloween 2005 edition featured the results of research regarding vitamin D's critical role in preventing bone fractures among mature women (To read it, click here).
Blood levels of vitamin D levels are low in at least one in five women who live in North America and Northern Europe, and most post-menopausal women also get too little dietary calcium. But the former deficiency may be more important than the latter, according to new research results.
First, let's look at the interaction between three key players in bone strength.
Vitamin D, calcium, and PTH: a tightly intertwined bone-health trio
The body uses parathyroid hormone (PTH) to ensure adequate blood levels of calcium, because this mineral is vital to many metabolic functions. PTH is a critical factor in bone strength because high blood levels prompt release of calcium and phosphorus from bone. Secretion of PTH from the thyroid gland is prompted by low levels of calcium in the blood.
When blood levels of calcium are low, the thyroid gland secretes more PTH, whereas high calcium levels inhibit release of PTH.
However, high blood levels of vitamin D also inhibit secretion of PTH, thereby preventing loss of calcium from bones.
How much vitamin D is enough? Some researchers peg the lower limit of “normal” for blood levels of vitamin D (25-hydroxyvitamin D) at nine nano grams per milliliter (ng/mL), while others place it at 20 ng/mL. The leading vitamin D researchers consider a level of 20 ng/mL too low, and nine ng/mL dangerously low, because parathyroid hormone levels rise as vitamin D levels fall below 20 to 25 ng/mL
Therefore, low vitamin D levels prompt loss of calcium from bones, and raise the risks of osteoporosis and fractures.
This is one reason why most experts say the RDA for vitamin D (400 IU/day) is too low and should be increased to 600 to 800 IU/day or even higher.
The Icelandic investigation: vitamin D found more critical than calcium
The Journal of the American Medical Association recently published the results of a study by an Icelandic research team that set out to determine the relative importance of vitamin D and calcium in maintaining bone strength. The intriguing findings appeared in JAMA's November 9, 2005 edition.
The Icelandic team used blood levels of PTH as a measure of likely long term bone health. Again, high levels are considered bad for bones, while low levels are desirable.
Their findings were derived by analyzing nutrient intake data from 944 healthy Icelandic adults recruited between 2001 and 2003. The participants were divided into groups according to calcium intake—less than 800 mg/day, 800-1,200 mg/day, and greater than1,200 mg/day—and blood levels of vitamin D: i.e., less than 10 ng/mL, 10-18 ng/mL, and greater than 18 ng/mL.
The researchers found that blood PTH levels were lowest (i.e., desirably low) in the group with a vitamin D level of more than 18 ng/mL, but were highest (i.e., undesirably high) in the group with a vitamin D level of less than 10 ng/mL.
But the researchers also noticed there was still a significant difference in PTH between the lowest and highest vitamin D groups that in people with a high calcium intake of more than 1,200 mg per day, which is the recommended daily allowance for men and women aged 51 or older.
As the researchers said, “Our results suggest that vitamin D sufficiency can ensure ideal serum [blood] PTH values [concentrations] even when the calcium intake level is less than 800 mg/day, while high calcium intake (greater than 1200 mg/day) is not sufficient to maintain ideal serum PTH, as long as vitamin D status is insufficient."
They went on to conclude that “…the association between vitamin D status, calcium intake, and the interaction between these two with serum PTH levels is a strong indication of the relative importance of these nutrients.”
In other words, vitamin D appears to exert a calcium-sparing effect in our bones, and as long as vitamin D intake is adequate, it may be unnecessary to consume more than 800 mg of calcium per day in order to maintain healthy bones.
How much vitamin D is enough?
As we've seen, the lower limit of “normal” for blood levels of vitamin D is nine to 20 ng/mL, but PTH levels rise and start removing calcium from bones as vitamin D levels fall below 20 to 25 ng/mL
This is why most experts say the RDA for vitamin D (400 IU/day) is too low and should be increased to 600 to 800 IU/day or even higher.
Noted vitamin D researcher Michael Holick, M.D. of Boston University believes that people with little sunlight exposure—a category that includes most Americans, who work indoors—should take 400-1000 units per day, and that adults older than 70 should take 800-1000 units per day, because their skin is less able to make vitamin D from sunlight.
Sun and seafood: get your vitamin D
There are two sources of vitamin D: sunlight and foods. Judging by the results of our own lab tests versus the results of USDA tests, wild sockeye salmon may contain more vitamin D than any other whole food (687 IU per 3.5 ounce serving), followed, among our fish selection, by albacore tuna (544 IU), silver salmon (430 IU), king salmon (236 IU), sardines (222 IU), halibut (276) and sablefish (182).
The radiation that creates vitamin D in the skin is the same wavelength that causes sunburn, so consistent application of sunscreen can cut vitamin D production. At northern latitudes, there is not enough radiation to convert vitamin D, especially during the winter. And as we've noted, the skin of people aged 70 or older does not create as much vitamin D in response to sunlight exposure.
High-dose calcium may promote prostate cancer
Men who consume a lot of calcium may have a higher risk of developing prostate cancer, according to a new analysis.
Scientists from the National Cancer Institute presented their findings at a National Institutes of Health (NIH) conference in October of 2005, and again at a November meeting of the American Association for Cancer Research.
The findings were based on an analysis of data from 1,270 cases of prostate cancer identified in the famous Alpha-Tocopherol, Beta-Carotene Cancer Prevention (ATBC) Study, which involved 17 years of follow-up research on the participants' health status.
The NCI team's analysis revealed that men who consumed more than 2,000 mg of calcium per day nearly doubled their risk of developing prostate cancer.
They concluded that the risk of prostate cancer in the men was clearly linked to calcium intake, as opposed to dairy intake. While men who ate a lot of dairy products showed an increased risk of prostate cancer, that link disappeared after controlling for calcium content.
The only exception was cream, which was linked to a significantly higher prostate cancer risk. But no other dairy products were linked to an increased risk, nor were vitamin D or phosphorous.
As lead researcher Panagiota Mitrou, Ph.D. said: “These results might explain the positive association [with increased prostate cancer risk] seen with dairy products in our previous studies. Further research should focus on how dietary calcium could affect prostate cancer.”
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