Minerals and Trace Elements 197estimated to meet the requirement of 50% of the
individuals in a life-stage and sex group). This esti-
mate was set to 265 and 350 mg/day for adult women
and men, respectively. This value is similar to the
mean magnesium intake reported for women and
men in the USA (228 and 323 mg/day). Collectively,
these data suggest that most Americans are not con-
suming enough magnesium in their diet and this also
appears to be the case for several European popula-
tions. However, while the public health relevance of
this observation is currently being debated, the fact
that there is not a universally accepted reliable mag-
nesium status assessment tool makes it diffi cult to
determine the actual consequence of this apparent
low intake.
For those who want to increase their magnesium
intake, a number of high magnesium foods and
dietary practices will lead to adequate intake. Foods
with a high magnesium content include whole grains,
legumes, green leafy vegetables, and tofu; meat, fruits,
and dairy products have an intermediate magnesium
content (Table 9.4). The poorest sources of magne-
sium are refi ned foods. Although high levels of
calcium, phosphate, or fi ber may lead to reduced bio-
availability of magnesium, differences in bioavailabil-
ity of magnesium from various food sources does not
appear to be a signifi cant barrier to achieving ade-
quate magnesium status. Thus, the current recom-
mendations for a healthy diet based on the food
pyramid are consistent with the goals of reaching the
US RDA for magnesium.Micronutrient interactions
As mentioned above, phosphorus as phosphate, espe-
cially in phytate, may decrease intestinal magnesium
absorption. In general, calcium intake in the usual
dietary range does not affect magnesium absorption,
but calcium intakes in excess of 2.6 g have been
reported to reduce magnesium balance. Magnesium
intake in the usual dietary range does not appear to
alter calcium balance.9.4 Phosphorus
Phosphorus is never found free in nature, but is
widely distributed in combination with minerals.
Phosphate rock, which contains the mineral apatite,
an impure tricalcium phosphate, is an important
source of the element. Phosphorus is most commonly
found in nature in its pentavalent form in combina-
tion with oxygen as phosphate (PO 43 −). Phosphorus
(as phosphate) is an essential constituent of all known
protoplasm and is uniform across most plant and
animal tissues. A practical consequence is that, as
organisms consume other organisms lower in the
food chain (whether animal or plant), they automati-
cally obtain their phosphorus.Absorption, transport, and
tissue distribution
Phosphorus makes up about 0.65–1.1% of the adult
body (~600 g). In the adult body 85% of phosphorus
is in bone and the remaining 15% is distributed in
soft tissues. Total phosphorus concentration in whole
blood is 13 mmol/l, most of which is in the phospho-
lipids of erythrocytes and plasma lipoproteins, with
approximately 1 mmol/l present as inorganic phos-
phate. This inorganic component, while constituting
only a minute percentage of body phosphorus
(<0.1%), is of critical importance. In adults, this com-
ponent makes up about 15 mmol in total and is
located mainly in the blood and ECF. It is into the
inorganic compartment that phosphate is inserted on
absorption from the diet and resorption from bone,
and from this compartment that most urinaryTable 9.4 Magnesium content of some common foods
Food source Description
Mg content
(mg/100 g)Beef Lean (from six different cuts) 20
Lamb Lean (from six different cuts) 24
Pork Lean (from three different cuts) 22
Chicken Raw, meat only 25
Cod, plaice,
whiting
Raw 22–28Eggs Chicken, whole, raw 12
Cheese Soft and hard varieties 8–45
Pulses Raw 17–250
Wheat fl our Whole fl our 120
Wheat fl our White fl our 20–31
Milk Cow’s (3.9, 1.6 and 0.1% fat) 11–12
Yoghurt Whole milk 19
Tofu Soyabean, steamed 23–59
Green leafy
vegetables
Raw 8–34Rice Raw, white, polished 32
Potatoes Raw 14–17
Data from Holland et al. (1995). Reproduced with permission from
HMSO.