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RIVERA
ond part of the problem is to relate Sr/calcium levels in the diet to
9Sr/calcium levels in bone. This paper is concerned principally with
this second part.
A relatively simple model relating diet and bone %Sr/calcium
ratios is described by the equation
CaiX, = Ca Xa _- f,Can_- 7 Xn_1 + f,Can_;,KnZy
+ (Cay— Can-1) KnZn
where Ca, = skeletal
calcium of an individual at the end of yearn
Xn = Sr/calcium
ratio
of
the
skeleton
at the end of yearn
f, =fraction of skeletal calcium exchanged during the year
K_
= bone/diet observed ratio
Z, = °’Sr/calcium ratio of the diet
This model assumes that the Sr content of the skeleton at the end of
a year is equal to that of the previous year minus that lost by resorption plus that gained by replacement and that gained by accretion
during the year. The metabolic parameters that must be known to
apply
the model are therefore the net calcium accretion rate, the
bone/diet
sr/calcium
discrimination factor, and the bone turnover
rate.
Mitchell et al.! have estimated the net calcium accretion rate for
every age from birth to 20 years, after which skeletal growth ceases
and the net calcium accretion rate becomes zero. These estimates are
the best available for the net calcium accretion rate. An attempt to
estimate the bone/diet *°Sr/calcium discrimination factor and the bone
turnover rate was made by considering the results of diet measurements” and bone-survey results in New York, Chicago, and San Francisco for the last two years.? The method used was to apply Eq. 1 to
relate yearly changes in the °Sr concentrations inbone at specific ages
to observed changes in the diet *Sr/calcium levels with different val-
ues of discrimination factors and turnover rates.
Where a sufficient number of bone samples were available, the
ranges of values for the discrimination factor and the turnover rate
which gave good fits to the data were small. For adults the best dis-
crimination factor was 0.25, which is in agreement with other studies, .
and the best turnover rate was about 3% per year. For infants (0 to
1 year of age) the discrimination factor calculated was about 0.35, and
the average turnover rate was about 75% per year. These estimates are
in rough agreement with those of Bryant and Loutit‘ and Beninson et
al,
For individuals between 2 and 20 years of age, however, there
were relatively few bone samples, and the estimated discrimination
factors and turnover rates were therefore more uncertain. From the