137
Clinical Science (1980) 59, 137-142
The relation between bone loss and calcium balance in women
A. H O R S M A N , D . H. M A R S H A L L , B. E. C . N O R D I N , R. G. C R I L L Y
M. S I M P S O N
AND
MRC Mineral Metabolism Unit,The General Infirmay, Leeds, U.K.
(Received 1 7 December 1979; accepted I 4 April 1980)
Summary
1. We have examined the relation between the
rate of cortical bone loss and other measured
variables in 108 women, untreated and on
various therapies, observed for nearly 300 patient
years. The majority of the subjects were postmenopausal women with various degrees of
simple osteoporosis, but a few cases of hyperparathyroidism and other calcium disorders were
included.
2. Calcium balance, bone resorption rate,
urinary hydroxyproline and plasma alkaline
phosphatase were shown to be related to the rate
of bone loss; patients with the more negative
balance or the higher values of the other variables
had significantly higher rates of loss. The rate of
bone loss was independent of the rate of bone
formation.
Key words: bone loss, calcium balance, fractured femur, hyperparathyroidism, menopause,
morphometry, osteoporosis.
much larger series of women and have established
that there are a number of variables which reflect
the rate of bone loss.
Clinical material
The series comprises all those women, regardless
of diagnosis, in whom we have measured calcium
balance, bone formation and resorption rates,
urinary
hydroxyproline,
plasma
alkaline
phosphatase and sequential metacarpal bone loss
on the same therapeutic regimen. Most of the
women in the series (99 out of 108) were
postmenopausal, their mean age being 60.7 k
8.7 (1 SD) years. The mean interval between
menopause and the study was 16-0 f 9.3 years.
Table 1 shows the breakdown of the subjects by
diagnosis. Normal subjects, those with spinal
osteoporosis and those with fractured femur are
collectively described as group I, the remainder
(mostly cases of hyperparathyroidism) as group
11. For some subjects, observations were available in the untreated and treated state, or on more
TABLE1. Breakdown of cases according to presentation
Introduction
In previous publications (Nordin, Horsman,
Marshall, Simpson & Waterhouse, 1979a;
Nordin, Horsman, Crilly, Marshall & Simpson,
1980), we have reported the effects of various
therapies on metacarpal bone loss and on calcium
balance in postmenopausal women and have noted
that the effectiveness of therapy, judged by these
two independent methods, is reasonably consistent. We have now examined in more detail the
relation between bone loss, calcium balance and
the results of various other investigations in a
Correspondence: Dr A. Horsman, MRC Mineral
Metabolism Unit, The General Infirmary, Leeds LSl
3EX, West Yorkshire, U.K.
0143-5221/80/080137-06%01.50/1
Presentation
Group1
No. of
patients
42
Normal
Spinal osteoporosis
Fractured femur
29
12
Subtotal
Group I1
Hyperparathyroidism
Hypoparathyroidism
Thyrotoxicosis
Steatorrhoea
Hypopituitarism
Osteomalacia
Steroid osteoporosis
83
13
2
1
3
1
3
2
Subtotal
Total
25
108
8 1980 The Biochemical Society and the Medical Research Society
138
A . Horsman et al.
change in cortical area in that subject and the
projection on the horizontal axis being the period
over which that change developed.
In each Figure, the results for the subjects are
arranged in rank order according to the value of
another measured variable from the lowest value
on the extreme left to the highest on the right.
With this form of presentation, a dependence of
the rate of bone loss on the ranking variable
produces a curvature in the cusum line. If a
cusum is divided into two sections, the difference
in slope of the sections can be calculated and its
significance tested, by Student’s t-test, and the
dependence on the ranking variable established.
The choice of breakpoint dividing the sections is
based on a visual assessment of the data and is
Methods
such as to maximize the significance of the
Calcium balances were performed on a collagen- change in slope (Page, 1957). The slope of each
free diet at an intake of about 20 mmol/day with section is evaluated as the weighted mean slope of
a nonabsorbable polyethylene glycol marker as its component segments (Colquhoun, 1971)
described elsewhere (Nordin, Horsman & Aaron, where the weighting factor is the duration of
1976), the balance ( b )being expressed in mmol of observation; this slope is numerically equal to the
calcium/day. During the balance period, bone total change in mean metacarpal cortical area
formation rate (m), expressed in the same units, over the section divided by the total number of
was measured by a radioisotopic procedure patient years of observation in the section. The
involving an intravenous dose of 47Ca evaluations of the standard error of the slope of
(Burkinshaw, Marshall, Oxby, Spiers, Nordin & each section and the standard error of the
Young, 1969). The rate of bone resorption ( r difference in slopes, necessary for Student’s t-test,
mmol of calcium/day) was evaluated for each also involve the duration of observation as a
patient by subtracting the balance from the bone weighting factor.
formation rate (r = m - b). During the balance
period, plasma alkaline phosphatase activity
Results
(K.A. unitdl00 ml) was measured twice and
urinary hydroxyproline (mmol/day) was mea- The dependence of bone loss on calcium balance
sured over 7 days by standard Auto-Analyzer
is illustrated in Fig. 1, which shows the cusum
methods (Hodgkinson & Knowles, 1976). Bone
plot of the sequential changes in mean metaloss was assessed by sequential radiographic
carpal cortical area ranked by calcium balance. It
morphometry of the metacarpals (Horsman & is clear that as a group the subjects in most severe
Simpson, 1975) using a semi-automated measure- negative calcium balance (on the left) lost bone
ment system (Horsman, Simpson, Kirby & and that those in zero or positive calcium balance
Nordin, 1977) and expressed as the change in (on the right) did not, although this must, of
mean metacarpal cortical area (mm’) within each course, be a continuous progression. The most
subject’s observation period.
obvious change in slope occurred at a negative
balance of about 2-0 mmol/day and the slopes of
the two sections below and above this dividing
Presentation of results
point are given in Table 2. The difference between
In order to take into account the variability in the the slopes was highly significant ( P < 0.01).
duration of observation period between subjects, Table 2 also contains the number of observation
the results are presented as cusums (Ewan, periods on subjects in group I1 included in each
1963), the vertical axis in every Figure being the
section of each cusum. For the cusum ranked by
summated change in mean metacarpal cortical calcium balance, of the 6 1 observation periods in
area (mm2) and the horizontal axis being the the first section (b < -2.0 mmol/day), 11
summated period of observation (patient years).
involved subjects in group 11; of the 80 obserIn the cusum plots, each line segment (together
vation periods in the second section (b 2 -2.0
with its endpoint) represents the result obtained
mmol/day), 19 involved subjects in group 11.
during the observation period on one subject, the
The corresponding data ranked by bone
projection on the vertical axis being the measured
formation rate are shown in Fig. 2. Although
than one therapy, and the results presented below
are therefore based on a total of 141 observation
periods on 108 subjects. In 42 of the observation
periods the subjects were in the untreated state,
and in 99 of the observation periods they were on
one of a variety of treatments: calcium supplements (1.2 g/day), vitamin D (10 000-50 000
units/day), calcium and vitamin D together,
hormone therapy (usually ethinyloestradiol 25
yglday, 3 weeks out of 4, or norethisterone 5
mg/day where oestrogens were contra-indicated), lmhydroxyvitamin D (1-2 yglday) or a
combination of hormones with la-hydroxyvitamin D.
Calcium balance and bone loss
139
zero
balance
-3OJ
1
0
i
50
100
200
150
250
300
Patient years
FIG.1. Cumulative plot of the sequential changes in mean metacarpal cortical area ranked by
calcium balance. Each line segment represents the result for one observation period on one
patient. The observations are ordered according to the calcium balance measured during the
corresponding period, from most negative balances on the left to positive balances on the right.
TABLE2. Rates of change of cortical area in subjects divided into two groups according to the value of each measured
variable
Variable
Calcium balance (mmol/day)
Bone formation rate$
(mmol/day)
Resorption rate$ (mmol/day)
Urine hydroxyproIineS
(mmol/day)
Plasma alkaline phosphatasea
(K.A. units/100 ml)
Range
-8.42--2.11
-1.98-14.55
0-9.16
9.24-35.58
-2.81-12.98
13.W22.16
0 . 0 5 0 4 105
0.107-0.338
No. of
observation
periods.
Patient
years
Rate of change
of cortical areat
(mm'/year)
SE
61 (11)
80 (19)
124
154
234
49
236
47
92
185
114
136
-0.199
+0.013
-0.054
-0.257
-0.044
-0.325
+0.037
-0.139
+0.036
-0.172
0.063
0.047
0.042
<O.O'
0,118
(<O. 1)
0'042
0.115
0.053
0.052
<0.025
0'053
0,061
<0.025
1 .&6.1
6.2-15. I
P
N.S.
<0.05
* Figures in parentheses are the numbers of observation periods on subjects in group 11.
t The cusum slope, evaluated as the weighted mean rate of change of mean metacarpal cortical area. The weighting factor is the duration
of observationof each subject.
$ Not measured in two cases.
5 Not measured in 13 cases.
there was a suggestion that a more rapid bone
loss was associated with the higher formation
rates (above about 9.2 mmol/day), this
association was not significant (P < 0.1;Table
2). It was noted that of the 20 observation
periods in subjects with bone formation rates
above 9.2 mmol/day, nine involved subjects in
group 11.
The data are ranked by resorption rate in Fig.
3. A more rapid loss was associated with higher
resorption rates (above 13 mmol/day), the
change in cusum slope at this point being
significant (P < 0.025;Table 2). Eight of the 17
observation periods in the high range involved
subjects in group 11.
Fig. 4 shows the data ranked by mean daily
urine hydroxyproline during the balance period.
As a group, subjects with hydroxyproline excretion values below 0.107 mmol/day did not
lose bone (the slope of the cusum to this point is
positive and not significantly different from zero;
Table 2), whereas the subjects with higher values
lost bone at a significant rate (P < OeOl),which
was also significantly greater (P < 0.05) than the
rate in those with lower values (Table 2). Of the
29 observation periods on group I1 subjects, all
A . Horsman et al.
140
4
I
0
50
100
150
200
2 50
300
Patient years
FIG.2. Cumulative plot of the sequential changes in mean metacarpal cortical area ranked by
bone formation rate.
-30
i
I
0
50
100
150
2 00
250
300
Patient years
FIG.3. Cumulative plot of the sequential changes in mean metacarpal cortical area ranked by
resorption rate.
but five were included in the second section of the
cusum (urinary hydroxyproline 0.107 mmol/
day). However, in that section observation
periods on groilp I subjects formed the majority
,;9 out Of 1 3 ) .
Finally, the data ranked by plasma alkaline
phosphatase concentration are shown in Fig. 5. A
clear change in slope occurred at about 6 - 2 K.A.
unitsl100 ml. The group with plasma concentrations below this value lost no bone, the cusum
slope being positive and not significantly different
from zero, whereas the group with higher values
Calcium balance and bone loss
141
+I0
-20
t
50
0
100
150
200
250
.
300
Patient years
FIG. 4. Cumulative plot of the sequential changes in mean metacarpal cortical area ranked by
urinary hydroxyproline.
+lo1
I
A
unttdl00ml
6 ;".A.
0
-20
r
0
50
100
150
200
250
300
Patient years
FIG. 5. Cumulative plot of the sequential changes in mean metacarpal cortical area ranked by
plasma alkaline phosphatase activity.
lost bone at a significant rate (P < O-Ol), which
was also significantly greater (P < 0.025) than
the rate in the group with the lower values (Table
2). Fifteen of the 64 observation periods in the
high range involved subjects in group 11.
Discussion
These data confirm our previous preliminary
reports which indicated a reasonable correspon-
dence between calcium balance and rate of
metacarpal bone loss in postmenopausal women
on different therapeutic regimens (Nordin et al.,
1979a; Nordin, Horsman, Marshall, Hanes &
Jakeman, 1979b; Nordin el al., 1980). The
extended study described above involves a much
larger series and includes all the observations
currently available on women attending our Unit.
This correspondence suggests not only that
calcium balances, carefully performed, provide
142
A . Horsman el al.
useful information, if not in individuals at least in
groups, but also that bone loss from the metacarpals can be taken to represent the loss from
the skeleton as a whole. The overall implication is
that calcium balance studies, which by our
procedure take only 2 weeks, can be used to
judge the effectiveness of therapy in altering the
rate of bone loss in groups of patients. It is clear
that, as expected, the relationship between calcium balance and change in mean metacarpal
cortical area is a continuous one. Although we
have arbitrarily divided the data at a negative
balance of 2.0 mmol/day, bone loss is clearly
most rapid in those subjects with the most severe
negative balance; it is possible that there may
even be a gain of bone in patients in whom a
positive balance is observed, zero balance more
or less corresponding to zero bone loss.
Bone formation rate measured with radioactive calcium bears no significant relationship to
the rate of bone loss. There is little evidence from
our data that patients with low rates of bone
formation lose bone more rapidly than those with
high formation rates. There is, however, some
association between bone resorption rate and rate
of bone loss, albeit a weak one. It appears that
bone equilibrium can be maintained at almost
any bone formation rate, although when the
resorption rate exceeds about 13 mmollday it
almost invariably exceeds the bone formation
rate and is associated with bone loss. However,
these very high resorption rates are mainly
accounted for by cases of hyperparathyroidism.
Urinary hydroxyproline, although it correlates
highly with bone resorption (Nordin et al., 1976),
does seem to have better predictive value since
subjects excreting more than about 0.11 mmol of
hydroxyproline/day (on a collagen-free diet)
generally lose bone whereas those excreting less
generally do not.
After the calcium balance itself, the variable
with the highest predictive value for the rate of
bone loss is the plasma alkaline phosphatase.
This would not have been surprising if the
critical value of alkaline phosphatase had been at
the upper normal limit of about 12 K.A.
units/100 ml, but this did not turn out to be the
case; the critical value was about 6 K.A.
units/100 ml, which is approximately the value
which best separates age-matched pre- from
post-menopausal women (Crilly, Jones, Horsman
& Nordin, 1980). The data presented above show
that values below this level are associated with
bone equilibrium, whereas higher values are
associated with bone loss. Although the
association between plasma alkaline phosphatase
and rate of bone loss is not statistically as strong
as that between calcium balance and rate of loss,
measurement of plasma alkaline phosphatase
activity is of course very much simpler to
perform. Our data suggest that it might be
possible to use this simple measurement to
classify patients into two groups with potentially
low and high rates of loss.
We conclude that the long-term rate of bone
loss in groups of women is predictable from
short-term measurements of other variables,
although clearly in individual cases their value is
more limited.
Acknowledgments
The authors thank Wendy Jakeman, for her
sustained efforts on the balances, and H. B.
Bentley, Principal of the School of Radiography
at Leeds General Infirmary, who has supervised
the hand radiography for many years.
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