Letters
Milk intake and bone mineral acquisition in adolescent girls
Increases in bone density may be result of micronutrients in
additional cereal Results in two groups are not so differentAdding milk to adolescent diet may not be best means of
preventing osteoporosisAuthors' replyIncreases in bone density may be result of micronutrients in additional cereal
EDITOR Most cereals available in the United Kingdom are supplemented with a
variety of vitamins and minerals. Furthermore, there is evidence in the
literature that these micronutrients are important to bone
health,2 and the effect observed may therefore reflect in
part the additional micronutrient intake from this source. Indeed, the
intake of phosphorus, magnesium, zinc, and vitamins B-1 and B-2 were
higher in the milk supplemented group. This may not be solely
attributable to the milk.
Details of how the milk was consumed are not given in the paper, but
the data should be available from the interim food diaries recording
non-weighed food intake over four days that subjects completed on five
interim occasions. It would be interesting and important for these
additional data to be presented to avoid possible misinterpretation of
the results.
Results in two groups are not so different
EDITOR From the data presented, we calculate that at the end of the study the
mean total bone mineral content was still greater in the control group
than the milk group (1845 g v 1835 g), and the mean
total bone mineral density was virtually identical in both groups
(0.980 g/cm2 milk v
0.981 g/cm2 control).
Could the results not be explained on the basis of a quirk in the
randomisation and the fact that the milk treated group were simply
biologically "catching up" (or regressing to the mean)?
Adding milk to adolescent diet may not be best means of
preventing osteoporosis
EDITOR Other studies, however, have shown that calcium excretion rates
increase with increasing protein intake,
2 3
generally
resulting in negative calcium balance, although this seems to depend on
the source of protein.4 Thus an increase in protein intake
might be expected to limit, rather than enhance, bone mineral
acquisition. In suggesting that an increase in milk consumption could
make an important contribution to reducing rates of osteoporotic
fracture the authors overlook the lessons of ecological studies, which
link a diet high in animal protein to an increased incidence of hip
fracture in women.5
Adding milk to the adolescent diet may be an effective way of
increasing calcium intake, but it may not be the best means of
preventing osteoporosis in later life. Future studies should compare
the effects of different methods of calcium supplementation, including
mineral supplements and non-dairy sources of calcium, on bone mineral
acquisition.
Authors' reply
EDITOR We fully appreciate that nutrients other than calcium are essential for
bone health and alluded to this in the paper. We performed principal
components analysis in an attempt to ascertain the effects of
individual nutrient combinations on bone mineral accretion. The models
generated for increments in total body bone mineral content and total
body bone mineral density consistently gave equal weighting to calcium,
protein, energy, and phosphorus. No other micronutrients were
significant in the model.
Griffiths and Francis commented on the lower total body bone mineral
content and total body bone mineral density in the girls randomised to
receive milk at baseline and wondered whether the effect we attributed
to the milk could have been accounted for by "catch up" growth. If
the milk group had less mature skeletons this could have accounted for
the difference. The groups did, however, not differ in maturity as
assessed by Tanner stage, menarchal status, and oestradiol
concentration.
Longitudinal studies of bone density (or any measurement) are subject
to statistical artefacts.1 Firstly, there may be
regression towards the mean. We used analysis of covariance, and the
milk effect was still significant after adjusting for baseline bone
mass. Secondly, there may be a common variable effect.2 To
avoid these related problems, we compared the slope of the regression
lines of bone mass against time, and these differed significantly (for
example, gain per day in bone mineral content was 770 mg/day for milk
group and 690 mg/day for controls). We also used the mean of all four
time points as denominator when calculating rates of gain, rather than
just the baseline measurement of bone mass, as recommended by
Oldham.2 Thirdly, there may be a "horse racing"effect.
This effect was first described for the natural history of chronic
bronchitis3 and has subsequently been applied to bone
density studies.4 In our study, this would mean that the
girls with the lowest bone mineral content at baseline would have the
lowest increment in bone mineral content and so would represent a
conservative bias. We believe that the horse racing effect is the most
likely source of bias and would have resulted in an underestimate of
the effects of the milk supplement.
Cadogan et al reported the effect of increased milk intake
on bone mineral acquisition in English schoolgirls.1 The
results of their study have important implications for childhood
dietary policy. We are concerned, however, that the results may be
confounded by associated changes in dietary intake. We noted comments
made by a participant in the study on a BBC news item that she consumed
her additional milk with extra cereal.
Gordon Ferns
Centre for Nutrition and Food Safety, School of Biological
Sciences, University of Surrey, Guildford GU2 5XH.
Bryan Starkey
The Royal Surrey County Hospital, Guildford GU2 5XX
We read with interest the already widely quoted trial by Cadogan
et al of the effect of milk supplementation on bone mineralisation in
adolescent girls.
1 2
R M Francis
Cadogan et al show that supplementing the diets of
adolescent girls with dairy milk over 18 months produces a significant
increase in bone mineral acquisition.1 During the study
the intervention group recorded a 52% increase in mean daily calcium
intake, compared with a slight decrease in the control group. The
intervention group also recorded a significant increase in mean protein
intake from 59.1 to 70.7 g/day. The authors suggest that this may
partly account for their findings.
57 Sharland Close, Wantage, Oxfordshire OX12 0AF
New et al asked whether a change in cereal intake could account
for the beneficial effect of a milk supplement in adolescent girls.
Cereal consumption at baseline was 3.4% (median) of dietary energy
consumption, and this did not change in either group after 18 months.
Division of Clinical Sciences, University of Sheffield,
Northern General Hospital, Sheffield S5 7AU
Joanna Cadogan
Department of Food and Nutrition, Purdue University, West
Lafayette, IN 47907-1264, USA
Nicola Bright
Trent Cancer Registry, Weston Park Hospital, Sheffield S10 2SJ
Margo E Barker
Division of Clinical Sciences, University of Sheffield,
Northern General Hospital, Sheffield S5 7AU
© BMJ 1998
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Related Article
- Milk intake and bone mineral acquisition in adolescent girls: randomised, controlled intervention trial
- Joanna Cadogan, Richard Eastell, Nicola Jones, and Margo E Barker
BMJ 1997 315: 1255-1260.[Abstract] [Full Text]
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