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According to the National Osteoporosis Foundation
and the National Institutes of Health, adequate calcium intake
is
critical to good bone health. It has been asked if any other
nutrients are needed for healthy bones or to help calcium to
do its job. Although nutrients such as iron, sodium, oxalate
and magnesium may influence calcium's effectiveness, vitamin
D is the most important additional nutrient shown to enhance
calcium's ability to build and maintain bones.
The current recommended daily value for vitamin D is 400 I.U.
(or International Units). The Institute of Medicine also recommended
higher levels up to 600 IU in those over the age of 50(1)
. We get vitamin D through our diets and
by making it in our bodies in response to sunlight. However,
the housebound elderly are at increased risk for vitamin D deficiency
and dark pigmented children and adults. Some segments of the
general population in certain areas of the country who do not
get sufficient sunlight exposure during the winter months are
also at risk.
Milk in the United States is generally fortified with vitamin D, as are other
foods such as breakfast cereals, because it is known to enhance intestinal absorption
of calcium. Other good sources of vitamin D are fatty fish and cod liver oil,
but the most important source is simply sunlight. And unlike calcium, vitamin
D can be stored by the body, in the liver, for extended periods of time. It does
not have to be taken at the same time with calcium to provide its enhancing effect.
Vitamin D's Effect
on Calcium Absorption and Bone
Many studies on calcium's effects in building and protecting
bone and preventing fractures have shown a benefit from ensuring
adequate vitamin D(3,4). Vitamin D stimulates intestinal calcium
absorption and enhances the bone building process itself (5,
6).
One recent study showed that additional vitamin D increased
the short term absorption of calcium even in healthy individuals
with no apparent vitamin D deficiency(7). Vitamin D alone has been studied as a therapy for osteoporosis. However, the
best results are shown when osteoporotic patients are given both vitamin D and
calcium supplementation.
People At Risk
for Vitamin D Deficiency
The elderly clearly may benefit from supplemental vitamin D. In elderly individuals
unable or unwilling to obtain sufficient exposure to sunlight, vitamin D supplements
of 400 to 800 I.U. per day have shown some benefit. One study in France showed
that fracture risk was reduced in 3,279 elderly women living in nursing homes who were given 800 I.U. of vitamin D in
addition to 1,000 to 2,000 mg of calcium per day during an 18-month period(9).
In a review of the literature on calcium and vitamin D in Clinical Endocrinology,
Juliet Compston concludes that, "Overall, the results of these studies support
the use of vitamin D supplements to reduce the fracture rate in some elderly
populations(10).
For most people, obtaining adequate daily calcium is the primary
challenge. In the 1994 Report of the National Institutes of
Health Consensus Development Panel on Optimal Calcium Intake,
the principal recommendation is to increase calcium intake among
all groups of Americans(11).
References:
- (1) Dietary Reference Intakes for Calcium, Phosphorous,
Magnesium, Vitamin D, and Fluoride, Institute of Medicine
Food
and Nutrition Board, National Academy Press, 1997.
- (2) Third National Health and Nutrition Examination Survey
(NHANES III), National Center for Health Statistics,
1988-1991.
- (3) Menczel, J. et al., "Alfacalcidol (Alpha D,)
and Calcium in Osteoporosis, " Clinical Orthopaedics
and Related Research, 3 00:241-247, 1993.
- (4) Bilke, D.D., "Role of Vitamin-D, Its Metabolites,
and Analogs in the Management of Osteoporosis," Rheumatic
Disease Clinics of North America, 20:759-775, 1994.
- (5) The International Conference on Progress in Bone
and Mineral Research, "Vitamin D and Intestinal Calcium
Transport: Facts, Speculations and Hypotheses," American
Institute of Nutrition, 1995.
- (6) Johnson, J.A. and R. Kumar, "Vitamin D and
Renal Calcium Transport, " Current Opinion in Nephrology
and Hypertension, 3:424-429, 1994.
- (7) Mortensen, L. and P. Charles, "Bioavailability
of Calcium Supplements and the Effect of Vitamin D,"
American Journal of Clinical Nutrition, 63:354-357,
1996.
- (8) Bauwens, SF et al., "Pathogenesis'and Management
of Primary Osteoporosis," Clinical Pharmacology,
5:639-659, 1986,
- (9) Chapuy, M.C. et al., "Vitamin D3 and Calcium
to Prevent Hip Fractures in Elderly Women," New
England Journal of Medicine, 327 .-1637-1642, 1992.
- (10) Compston, JE, "The Role Vitamin D and Calcium
Supplementation in the Prevention of Osteoporotic Fractures
in the Elderly," Clinical
Endocrinology, 43:393-405, 1995.
- (11) "Optimal Calcium Intake," NIH Consensus
Development Panel on Optimal Calcium Intake, Journal
of the American Medical Association, 272:1942-1948,
1994.
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