Agriculture Reference
In-Depth Information
Absorption of calcium from food requires that it be released in ionized form (Ca
2+
).
Solubilization is promoted by HCl in the stomach. Active absorption occurs princi-
pally in the duodenum and proximal jejunum and requires energy and a calcium-
binding protein called calbindin. This process is regulated by calcitriol, the active
form of vitamin D
3
. Calbindin not only facilitates absorption into the enterocyte but
also serves as a transport protein to deliver calcium to the basolateral membrane for
extrusion. As calcium is extruded into the blood plasma, magnesium moves into the
enterocyte in a process described as a calcium-magnesium pump. If calcium levels in
the intestinal lumen are high, absorption in the jejunum and ileum may proceed by
passive diffusion. Calcium in blood plasma is found in ionized form (~50%); bound
to proteins (~40%), mainly albumin and prealbumin; and complexed (~10%) with
sulfate, phosphate, or citrate.
Calcium concentrations, both within and outside cells, are tightly controlled by
parathormone (from the parathyroid), calcitonin (from thyroidal parafollicular C
cells), and cacitriol. Although most of the body's calcium is associated with integ-
rity of bone, calcium's nonosseous functions are extremely important and include
membrane permeability, nerve conduction, muscle contraction, blood clotting, and
enzyme regulation. Interactions with other nutrients, such as protein, phosphorus,
magnesium, sodium, potassium, iron, zinc, and boron, may variably affect absorp-
tion, metabolism, or excretion.
Good food sources include milk and dairy products, such as cheese and yogurt;
leafy vegetables, such as kale, turnip and mustard greens; sardines (with bones);
legumes and legume products, such as tofu; and calcium-fortified orange juice.
Calcium supplements include calcium carbonate, calcium acetate, calcium citrate,
calcium citrate-malate, calcium gluconate, calcium lactate, and calcium monophos-
phate. Estimated adequate calcium intakes range from 500 mg per day for 1- to
3-year-old children to 1300 mg per day for teenagers and young pregnant or lactating
women. Estimated adequate intakes for males or females 51 years of age or older are
1200 mg per day, assuming intakes early in life were adequate.
P
h o s P h o r u s
Phosphorus is the second most abundant mineral element in the body, with about
85% in the skeleton, 14% in soft tissues, and 1% in blood and other body fluids.
In foods and in the body, it is found in combination with inorganic elements or in
organic compounds. It is absorbed as inorganic phosphate; thus, organically bound
phosphorus in food must be hydrolyzed before absorption can occur. Alkaline phos-
phatase, a zinc-dependent enzyme, in the duodenum and jejunum frees phosphate
from many bound forms (although not from phytate), and phospholipase C, another
zinc-dependent enzyme, frees phosphate from phospholipids. Phosphorus absorption
occurs primarily in the duodenum and jejunum and appears to be a carrier-mediated,
sodium-dependent, active process modulated by calcitriol. At higher luminal phos-
phorus concentrations, some may be absorbed by passive diffusion. Excessive intakes
of calcium, magnesium, or aluminum may inhibit absorption. Phytase (a phosphate
esterase) produced by yeast or intestinal bacteria can liberate some of the phospho-
rus from phytate if it is not complexed with calcium, zinc, or iron. About 70% of
