Agriculture Reference
In-Depth Information
the phosphorus in blood plasma is in organic form as phospholipids in lipoproteins.
The remaining 30% is inorganic, primarily as phosphate ions (HPO
4
2−
, H
2
PO
4
1−
, or
PO
4
3−
) or associated with minerals such as calcium, magnesium, or sodium.
Phosphorus has many functions in the body, including mineralization of bone,
energy storage and transfer in metabolic reactions, formation of nucleic acids (DNA
and RNA), maintenance of acid-base balance, and as a structural component (in
phospholipids) of cellular membranes.
Animal products (meat, poultry, fish, eggs, milk, and cheese) are food sources of
high phosphorus bioavailability. Legumes, nuts, and grains also have considerable
phosphorus but much may be bound in phytate (~80% in corn, wheat, and rice) and
poorly available. Genetic modification of corn to incorporate a phytase of bacterial
origin has shown promise for improving phosphorus availability. Cola drinks con-
tain phosphoric acid and may contribute substantially to the phosphorus intake of
habitual consumers. Recommended dietary allowances range from 460 mg per day
for 1- to 3-year-old children to 1250 mg per day for teenagers and young pregnant or
lactating women.
m
a g in e is i u m
Magnesium constitutes about 0.05% of the weight of adult humans, with about
55-60% in bone, 40-45% in soft tissues, and about 1% in extracellular fluids.
Absorption occurs primarily in the distal jejunum and ileum and involves a carrier-
mediated, active transport system at low magnesium intakes and simple diffusion at
higher intakes. Absorption may be enhanced by calcitriol, lactose, or fructose and
may be inhibited by phytate, nonfermentable fiber, and excessive calcium, phospho-
rus, or unabsorbed fatty acids. Efflux out of the enterocyte into the blood involves
a sodium and energy-dependent carrier and possibly a calcium-dependent carrier.
In blood plasma, about 50-55% of magnesium is present as free Mg
2+
, about 33%
is bound to protein (mostly to albumin), and about 13% is complexed with citrate,
phosphate, sulfate, or other anions.
About 70% of the magnesium in bone is associated with calcium and phosphorus
in the crystal lattice, but the approximately 30% found on bone surfaces is thought
to serve as an exchangeable reserve that assists in maintaining normal plasma mag-
nesium levels. This element participates in over 300 enzyme reactions, either as a
structural cofactor or as an enzyme activator. A very high proportion of intracellular
magnesium is associated with ATP or ADP and the enzyme systems in which they
participate. Thus, magnesium is important at numerous metabolic steps in energy
production; synthesis of DNA, RNA, and protein; contractility of cardiac and smooth
muscle; hydroxylation of vitamin D in the liver; regulation of ion movement across
cell membranes; and many other reactions.
Foods high in magnesium include nuts, legumes, whole-grain breads and cere-
als, green leafy vegetables (chlorophyll contains magnesium), chocolate, coffee,
tea, certain spices, and seafood. Supplements include a variety of magnesium salts,
such as magnesium sulfate, chloride, acetate, lactate, citrate, gluconate, or oxide.
Recommended dietary allowances range from 80 mg per day for 1- to 3-year-old
children to 420 mg per day for adult men.
