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Rapid response to:


Effects of calcium supplementation on bone density in healthy children: meta-analysis of randomised controlled trials

BMJ 2006; 333 doi: (Published 12 October 2006) Cite this as: BMJ 2006;333:775

Rapid Response:

Effect size at the end of supplementation period in terms of g/cm2

To the Editor:

            In the meta-analysis on role of
calcium supplementation in children
, Winzenberg et al (1) used standardised mean differences (SMD) to summarize their results and to base their
conclusions.  Although the use of SMD is recognized as a valid approach in summarizing mean
differences across trials in the Cochrane Review methodology (1), its primary
purpose is for comparing variables with different units and measurement scales
of different length
(2). The SMD is calculated by dividing the group
differences by the standard deviation. 
This converts a variable which has units to a unitless
score.  In other words, a variable which
once had clinical meaning becomes clinically meaningless.

            In Winzenberg et al.’s meta-analysis, all
measurements of bone mineral density (BMD)
were reported as grams per square centimetre (mg/cm2).
 Under these circumstances, we believe
that the use of SMDs is unnecessary.  An alternate approach is to summarize the
treatment effects as absolute differences. 
We have re-constructed Table 2 from
the meta-analysis by calculating the effect size at the end of supplementation
period in terms of g/cm2, the usual units of measurement for BMD.  We hope that our re-constructed Table will
help clinicians better-appraise the magnitude of effect size for this

            In regards to interpreting the results from the Table,
all bone sites show consistent increase in BMD at the end of a median calcium
supplementation period of one year.  We
disagree with
et al.’s
that the observed relative increase in upper limb body BMD is not clinically important.
 Not only is this result statistically
significant, but a yearly 0.007 g/cm2 increase (or a 1.8% relative
increase) in BMD is a clinically meaningful change.  If this increase continued throughout
childhood, it would likely translate to a substantial gain in bone strength.

            We are concerned that the results of
Winzenberg et al.’s meta-analysis could be
construed to imply calcium is not important in childhood, even though the
meta-analysis focused on the role of calcium supplementation and did not
address calcium requirements. This interpretation of the results was promoted
by the accompanying Editorial
It was written by a member of the Physicians Committee for Responsible
Medicine, a group that promotes vegan diets devoid of dairy products.  This thinking is at odds with the American
Association of Clinical Endocrinologists
(4), the National Institutes of Health
Consensus Development Panel on Osteoporosis Prevention, Diagnosis and Therapy
(5), the Institute of Medicine (6), and the Scientific Advisory Council of
Osteoporosis Canada
These groups recommend adequate intakes of calcium and vitamin D,
combined with weight bearing physical activity, throughout childhood to promote
the attainment of an optimal peak bone mass. 
It is likely that calcium intake is a necessary but not sufficient
condition for the development of a strong skeleton, as physical activity and
calcium both play key roles in the attainment of a high peak bone mass

            Until we are absolutely certain
about what the minimum and optimum combinations of calcium, vitamin D, foods
from plant sources and physical activity are required to achieve a bone mass
that will sustain the bones of individuals through their older ages without
fragility fractures, it seems prudent to continue to follow the consensus-based
recommendations for intakes of calcium and vitamin D.


Tanis R. Fenton, PhD Candidate, RD
Department of Community Health Sciences
University of Calgary

Eliasziw, PhD

Department of Community Health Sciences
University of Calgary
Calgary AB, Canada
David A. Hanley, MD, FRCPC
Departments of Medicine, Oncology and Community Health Sciences
Division of Endocrinology and Metabolism
University of Calgary



Effects of calcium supplementation on BMD at
different sites (median treatment of one year)

Bone Site

Number of


Number of


Mean BMD



Mean BMD



Mean BMD




Increase in

BMD (%)

P-value for




Femoral neck








Lumbar spine








Upper limb













   1.   Winzenberg
T, Shaw K, Fryer J, Jones G. Effects of calcium supplementation on bone density
in healthy children: meta-analysis of randomised controlled trials. BMJ 2006; 333:775.

   2.   Cohen
J. Statistical Power Analysis for the Behavioral
Sciences. New York: Academic Press; 1977.

   3.   LanouAJ. Bone
health in children.
BMJ 2006; 333:763-4.

   4.   Hodgson
SF, Watts NB, Bilezikian JP, Clarke BL, Gray TK, Harris DW et al. American
Association of Clinical Endocrinologists medical guidelines for clinical
practice for the prevention and treatment of postmenopausal osteoporosis: 2001
edition, with selected updates for 2003. EndocrPract 2003; 9:544-64.

   5.    Osteoporosis prevention, diagnosis, and
therapy. JAMA 2001; 285:785-95.

   6.   Institute
of Medicine (IOM). Dietary Reference Intakes for calcium, phosphorus,
magnesium, vitamin D and fluoride. The National Academies
Press; 1997.

   7.   Brown
JP, JosseRG. 2002 clinical
practice guidelines for the diagnosis and management of osteoporosis in Canada.
CMAJ 2002; 167:S1-34.

   8.   Courteix D, Jaffre C, Lespessailles
E, Benhamou L. Cumulative effects of calcium
supplementation and physical activity on bone accretion in premenarchal
children: a double-blind randomised placebo-controlled trial. Int J Sports Med 2005; 26:332-8.



Competing interests:
Until 2006, DAH served on a research grant review panel for the Dairy Farmers of Canada, and received an honorarium for this activity.

Competing interests: No competing interests

20 April 2007
Tanis R Fenton
Professional Practice Leader, Clinical Nutrition
Michael Eliasziw, David A. Hanley
Alberta Children's Hospital, T3B 6A8