section
37.3
Nonessentiai Trace Elements
891
o f histones and m agnesium to the exposed phosphate
groups.
In
green plants, chlorophyll— a m agnesium -
porphyrin com plex sim ilar to hem e— is vital for photo-
synthesis.
A normal 70-kg adult contains about 25 g o f m agne-
sium (1 1 -1 8 m m ol/kg w et w eight). B on e contains 60-
65% (about 45 m m ol/kg w et w eight) o f the b od y’s M g2+,
com plexed w ith phosphate. O f the other tissues, liver and
m uscle contain the highest concentrations o f M g2+, ap-
proxim ately 7 -8 m m ol/kg w et w eight. O nly 1% o f total
body m agnesium is in extracellular fluid. T he skeletal and
extracellular fluid M g2+ p ools probably exchange freely
w ith each other but not w ith the intracellular p ool, w hich
rem ains stable even w hen there are large fluctuations in
the level o f serum M g2+. Thus, the plasm a concentration
does not accurately reflect total body stores o f m agnesium .
Plasm a contains 1 .6 -2 .3 m g/dL (0 .7 -1 .0 m m ol/L ) o f
m agnesium , 20-25% o f w hich is protein bound. A s with
Ca2+, the unbound (ionized) portion o f plasm a m agne-
sium is the b iologically active fraction. Intracellular M g2+
concentration is about 10 m m ol/L , m ostly bound to or-
ganelles. T he intracellular concentration o f free M g2+ is
0 .0 2 5 -0 .5 m m ol/L .
The R D A for M g2+ is 350 m g (14.5 m m ol) for m en
and 300 m g (12.5 m m ol) for w om en. D uring pregnancy
and lactation, 4 5 0 m g (18.5 m m ol/d) is needed to m ain-
tain positive M g2+ balance. L eafy green vegetables are
a good source because o f the chlorophyll they contain.
W hole grains, dried beans, peas, cocoa, nuts, soybeans,
and som e seafoods also are good sources o f M g2+. M ost
dietary m agnesium is absorbed in the sm all intestine, but
sm all am ounts are also absorbed in the colon. A lthough the
fractional absorption o f dietary m agnesium is fairly con -
stant and independent o f intake, it ranges betw een 25%
and 65% am ong individuals. V itam in D and its m etabo-
lites are not important for its absorption. C alcium affects
m agnesium absorption, perhaps by com peting w ith it for
uptake sites. H ow ever, the occurrence o f a rare genetic dis-
ease in w hich absorption o f M g2+ is selectively decreased
suggests that transport for M g2+ is independent o f that
for Ca2+.
The kidney appears to be the m ain organ responsible
for m aintaining plasm a M g2+ concentration w ithin nor-
m al lim its. M ost o f the serum M g2+ that is filtered at the
glom erulus is reabsorbed, and on ly about 3-5% is excreted
in the urine. Urinary excretion varies w ith plasm a M g2+
concentration. PTH enhances the tubular reabsorption o f
M g2+, w hereas aldosterone decreases it.
A bove, it w as noted that hyperm agnesem ia suppresses
PTH secretion w hereas hypom agnesem ia, if not too se-
vere, stim ulates PTH secretion or inhibits it at very low
plasm a M g2+ concentrations. The suppressive effect o f
hypom agnesem ia
on
PTH
secretion
occurs
even
in
hypocalcem ia.
T he
h ypocalcem ia
that
accom panies
marked hypom agnesem ia can be corrected by m agnesium
repletion. Both hypom agnesem ia and hypocalcem ia cause
tetany. A t plasm a M g2+ concentrations o f 20 m g/dL , anes-
thesia and paralysis o f peripheral neurom uscular activity
occur; they can be reversed by intravenous adm inistration
o f calcium .
H yperm agnesem ia occurs in acute or chronic renal fail-
ure, in hem odialysis, and in w om en receiving m agnesium
sulfate for treatment o f preeclam psia. The clinical m ani-
festations resem ble the effects o f curare. A t serum M g2+
levels o f 2 .5 -5 .0 m m ol/L , cardiac conduction is affected,
and at concentrations above 12.5 m m ol/L , cardiac arrest
occurs in diastole. H ypom agnesem ia can occur in steat-
orrhea, alcoholism , diabetic k etoacidosis, and m any other
disorders. Tetany usually occurs at serum M g2+ con cen -
trations below 1 m m ol/L .
37.3 Nonessential Trace Elements
The trace elem ents include all inorganic elem ents found
in living system s at very low concentration (picogram s to
m icrogram s per gram o f cells or w et tissue). S om e trace
elem ents are essential for norm al health and developm ent;
others are toxic; and still others have not been show n to
be either. C hem ically sim ilar elem ents m ay com pete for
intestinal or cellular uptake or for m etal binding sites on
m etalloenzym es and proteins. Interactions m ay also be
com plem entary or synergistic.
M any nonessential trace elem ents are found in the body.
D epending on the local environm ent, at least 43 elem ents
are norm ally incorporated into developing teeth; another
25 elem ents are seen less frequently. T he rest, notably the
heavy m etals, have never been detected in teeth. M any
trace elem ents, particularly the heavy m etals, are con sid -
ered w hen testing for m etal poisoning. M any plants con -
centrate essential and nonessential elem ents from soil and
water, including alum inum (several sp ecies o f subtropi-
cal plants), selenium (m any plants), strontium (m esquite
beans), and lithium (w olfberries, used by N ative A m eri-
cans in the southw estern U nited States for jam ). Ingestion
o f these plants can cause toxicity for the elem ent involved.
Cadmium
Cadm ium , a w aste product from several industrial pro-
cesses, is the cause o f
itai-itai
(“ouch-ouch”) disease,
w hich is characterized by osteom alacia with painful frac-
tures o f affected, bones and by nephropathy w ith an excre-
tory pattern sim ilar to that seen in F anconi’s syndrom e. In
