The absorbability of calcium from spinach was
compared with the absorbability of Ca from milk in 13 healthy
adults. Absorption was higher from milk in every case, with the mean
absorption from milk averaging 27.6% and from spinach, 5.1%. These
results conclusively establish that spinach Ca is much less
readily available than milk Ca.
The presence of lactose
(milk sugar), lactase (the enzyme that breaks down lactose), and the
acidic amino acids, lysine and arginine, are all factors that improve
calcium absorption. On the other hand, decreased absorption is associated
with diets high in fiber. Foods such as whole grains and spinach are high
in phytates and oxalates, compounds known to bind with calcium and reduce
absorption.
The amount of calcium absorbed in the intestine depends on habitual
calcium intake. When intake is low, active transcellular
calcium transport in the duodenum is upregulated and a larger
proportion of calcium is absorbed by the active process than by
the passive paracellular process that prevails in the jejunum
and ileum. Bioavailability of the calcium source
digestibility
and solubilizationplays
a role under conditions of low calcium intake but is relatively
unimportant when calcium intakes are high (e.g. >800 mg/d in
people). Vitamin D intake is a second factor, as active calcium
transport is directly and proportionally dependent on the
presence in the intestinal cell of calbindin D9k,
the biosynthesis of which is totally vitamin D dependent.
Passive absorption in jejunum and ileum is the major absorptive
process when calcium intake is adequate or high. Passive calcium
absorption is a complicated function of solubility in the distal
small intestine, the length of sojourn of the chyme in a given
intestinal segment, and the rate of paracellular diffusion from
lumen to lymph and blood. Calcium that reaches the large intestine
undergoes absorption there by both active and passive processes.
Probably no more than 10% of total calcium absorption takes place
in the large intestine, whether calcium intake is low or high.
Calcium absorption by the large bowel can assume nutritional
importance under conditions of significant small bowel
resection.
