The Bioavailability of Dietary Calcium
Le´on Gue´guen, MsScAgr, and Alain Pointillart, DVM, PhD
Laboratoire de Nutrition et Se´curite´ Alimentaire, Institut National de la Recherche Agronomique, Jouy-en-Josas, FRANCE
Key words: calcium, bioavailability, humans, milk, absorption, bone
This update focuses on the bioavailability of dietary calcium for humans. Fundamentals of calcium
metabolism, intestinal absorption, urinary excretion and balance are recalled. Dietary factors, especially lactose
and other milk components, influencing calcium bioavailability at intestinal and renal levels are reviewed. A
critical examination of all the methods used for evaluating calcium bioavailability is made. This includes in vitro
assays, classical and isotopic balances, urinary excretion, isotope labeling in the urine, plasma and bones, long
term evaluation of bone mineralization and the use of biological bone markers. Importance and advantages of
animal models are discussed. The state of the art in the comparative bioavailability of calcium in foods is detailed
including a comparison of sources of calcium (dairy products and calcium salts) in human studies and in some
animal studies, casein phosphopeptides, proteins, lactose and lactase and their relation with calcium bioavail-
ability (in humans and rats). An update on the consumption of dairy products and bone mass is presented.
Emphasis on peculiarities and advantages of calcium in milk and dairy products is given.
Key teaching points:
• Milk provides large amounts of calcium and phosphorus and components such as lactose and casein phosphopeptides which may
enhance calcium absorption and mineral retention.
• Using a variety of methods, no one has shown that the calcium in milk is more efficiently absorbed that most calcium salts.
• Intestinal absorption does not necessarily reflect the bioavailability of calcium to the whole organism because calcium must be
retained and used in bone formation and mineralization.
• Three sources of calcium, milk, calcium carbonate and calcium citromalate, have been extensively studied. They all ensure the
efficient absorption of calcium and also ensure, over the long term, that calcium is retained and used for bone mineralization.
• There is, as yet, no evidence showing that the calcium from mineral water is as effective.
• Many direct or indirect methods may be used to evaluate calcium bioavailability. The values obtained depend on the method; thus,
conclusions or comparisons must be drawn with care.
Both scientists and the general public are becoming increas-
ingly aware of the importance of dietary calcium. This is
largely due to the many research studies that have demonstrated
links between dietary calcium intake and diseases such as
osteoporosis, arterial hypertension and colon cancer. These
diseases have many causes, but the scientific community now
recognizes that dietary calcium helps prevent them.
The average recommended dietary allowance (RDA) or
adequate intake (AI) of calcium is about 900 mg per day (800
to 1000 mg, depending on the country) for adults, rising to
1200 mg/day for adolescents and the elderly. These RDAs are
safety levels designed to provide adults with maximum protec-
tion against a negative calcium balance and, hence, against
bone loss. But they are also set so as to ensure that adolescents
produce the maximum amount of bone that is genetically
possible and, hence, remain above the fracture threshold when
they become older.
A recent review of calcium consumption in France 
determined the percentage of each sector of the population that
consumed less than two thirds of the RDA, the critical thresh-
old for defining groups at risk. These groups included about
20% to 25% of men aged 18 to 65, 30% of women aged 18 to
50, 50% of adolescent girls and men aged over 65, and 75% of
women over the age of 55. Elderly women living in institutions
had particularly low calcium intakes.
About 70% of dietary calcium comes from milk and dairy
products, mainly cheese in adults. Only a few green vege-
tables and dried fruits are good sources of calcium (16% of
Address reprint requests to: Alain Pointillart, DVM, PhD, Laboratoire de Nutrition et Se´curite´ Alimentaire, I.N.R.A., 78352 Jouy-en-Josas Cedex, FRANCE. E-mail
Journal of the American College of Nutrition, Vol. 19, No. 2, 119S–136S (2000)
Published by the American College of Nutrition
intake), and drinking water, including mineral water, pro-
vides 6% to 7%.
There is no doubt that milk provides large amounts of
calcium. While there is also no question of the nutritional
effectiveness of the calcium provided by milk, there is still
some debate as to whether this source of calcium is biologically
better than other sources, such as calcium salts, certain vege-
tables or mineral waters. We have therefore included recent
publications in which the calcium provided by dairy products is
compared to calcium from these other sources. While our
coverage is more extensive than that of many other reviews,
there has been so much work published on this topic that we
cannot claim to have cited all data. For general aspects of
calcium metabolism and factors influencing bioavailability, we
have drawn extensively on our earlier reviews of calcium
availability [2,3] and complementary data may be found in
other reviews [4–6].
The review focuses particularity on the bioavailability of
calcium from milk and dairy products.
DEFINITIONS AND WAYS OF
Absorbability, or the availability of calcium for absorption
by the intestines, is often used as a synonym for bioavailability.
It is, however, no more than the first step towards bioavailabil-
ity. Calcium must be soluble in the acid medium of the stomach
before it can be absorbed. Good solubility in water is an
advantage but is not absolutely necessary. The intestinal ab-
sorption values measured in humans and animals are not al-
ways equivalent to, and are generally lower than, calcium
absorbability. The potential absorbability of calcium depends
on the food, whereas absorption depends also on the absorptive
capacity of the intestines, which is affected by physiological
factors such as calcium reserves, hormonal regulation or pre-
vious dietary calcium supply. The potential absorbability is
thus the absorption under the most favorable physiological
Bioavailability depends on absorbability and the incorpora-
tion of absorbed calcium into bone. Hence, it also depends on
the urinary excretion and fecal loss of endogenous calcium. As
for intestinal absorption, physiological factors, particularly hor-
mones, play a major role in the incorporation of calcium into
bone. However, certain types of food increase the likelihood
that absorbed calcium will be incorporated into bone, whereas
others result in calcium being mainly excreted in the urine. The
effects of small changes in the diet on the net calcium balance
have been emphasized by several studies. Thus, certain anions,
such as sulfate and chloride, organic ligands (chelators) and
excess protein or sodium all increase the loss of calcium in the
urine and, thus, hinder its incorporation into bone. Conversely,
the incorporation of absorbed calcium into bone is stimulated
by phosphorus, but excess phosphorus may also cause unde-
sirable ectopic calcification (outside of the bone). The bioavail-
ability of calcium may therefore be defined as the fraction of
dietary calcium that is potentially absorbable by the intestine
and can be used for physiological functions, particularly bone
mineralization, or to limit bone loss.
Absorbability and bioavailability may be absolute or rela-
tive. Unless defining dietary needs by the factorial method,
relative values are sufficient for determining the fraction ab-
sorbed in comparison with different sources of calcium. The
values are then expressed relative to a reference source.
The values measured may be mean values or discrete val-
ues. Mean values are recorded for a whole diet or a single
source of calcium studied over a period of weeks or months
after adaptation. Discrete values are for a single meal or a
single oral calcium load. They correspond less to normal di-
etary conditions than mean values and do not take into account
the large variations that occur over time.
Calcium must be in a soluble form, generally ionized
), at least in the upper small intestine or bound to a
soluble organic molecule before it can cross the wall of the
intestine. Absorption is the result of two processes, active
transport across cells, mainly in the duodenum and the upper
jejunum, and passive diffusion, which occurs throughout the
small intestine, but mainly in the ileum  and very little in the
large intestine .
Active Transport. The active transport system for calcium
is saturable and regulated by dietary intake and the needs of the
body. It involves three stages: entry across the brush border of
the enterocyte, diffusion across the cytoplasm and secretion
across the basolateral membrane into the extracellular liquid
Calcium enters the cell via a positive electrochemical gra-
dient because the calcium concentration in the cytoplasm is
very low. It crosses the membrane via calcium channels and via
membrane-binding transport proteins (calmodulin and mem-
brane calcium-binding proteins). It may be stored transiently in
organelles like the Golgi apparatus, endoplasmic reticulum
(ER) or mitochondria, but then crosses the cytoplasm attached
to a calcium binding protein (CaBP or calbindin-D 9K), which
is the rate limiting factor in active calcium transport. Calcium
may travel bound to the protein if the CaBP remains in the
cytoplasm or via membrane-bound vesicles if the CaBP is
incorporated into the lysosomes . It is extruded from the cell
against an electrochemical gradient by two routes. A small
fraction leaves by exchanging 3 Na
, but most
leaves via a calcium pump, a Ca-ATPase activated by calcium,
CaBP and calmodulin.
The Bioavailability of Dietary Calcium
120S VOL. 19, NO. 2
Vitamin D influences several steps in this active transport.
The active metabolite is 1,25 dihydroxycholecalciferol
or calcitriol), which is produced by two hy-
droxylations of vitamin D, one in the liver (at position 25) and
the other in the kidney (at position 1). These reactions occur
whether vitamin D3 comes from the diet or from UV irradiation
of 7-dehydrocholesterol in the skin. The most striking effect of
calcitriol is its control of the expression of the gene encoding
CaBP, causing the synthesis of the protein, thereby regulating
the migration of calcium across intestinal cells. Calcitriol also
has a “liponomic” action, increasing membrane permeability
and activating the Ca-ATPase [9–11]. Calcitriol behaves like a
hormone. Its renal production is regulated by parathyroid hor-
mone (PTH), the secretion of which is, in turn, stimulated by a
fall in plasma calcium concentration, which may itself stimu-
late calcitriol synthesis. The PTH-calcitriol system is also in-
volved in bone resorption and increases the reabsorption of
calcium by the renal tubule. This hormone system therefore
controls all the calcium that enters the extracellular pool of
exchangeable calcium and ensures that the plasma calcium
concentration varies little from 100 mg/L.
The rate of saturable, physiologically regulated active ab-
sorption is negatively correlated with dietary calcium intake.
Newborn babies lack this active process, and old animals (most
studies have been done on rats) have calcitriol receptors, but
they are less abundant than in younger animals and the renal
1-alpha-hydroxylase is less active; this is also the case in
elderly people .
Supplementing the diet with vitamin D is not always al-
lowed (it is forbidden in France), so most vitamin D comes
from UV irradiation of the skin. However, the recommended
daily dietary intake of vitamin D for adults is about 400 IU (10
Some of the membrane and cytosolic proteins involved in
calcium transport are not vitamin D-dependent. One such pro-
tein is calmodulin, and others may be dietary proteins like alpha
lactalbumin, which may act like calmodulin . Apart from
vitamin D deficiency, these are the only dietary means of
affecting this highly regulated physiological route of calcium
Passive Diffusion. Passive absorption down an electro-
chemical gradient occurs via intercellular junctions or spaces. It
involves the mass movement of water and major solutes such as
sodium and glucose. It is not saturable and therefore increases
with dietary intake, provided that the calcium in the intestines
is in an absorbable form. It is independent of vitamin D and age
All components of the diet that make calcium soluble or
keep it in solution within the ileum should stimulate passive
diffusion. Several molecules do this, particularly milk proteins
like the phosphopeptides derived from casein [13,14] and
amino acids like L-lysine and L-arginine, which form soluble
chelates with calcium . Lactose and other carbohydrates,
which are gradually absorbed, also have an effect but the
mechanism involved is still a matter of controversy. It is now
generally agreed that lactose, at least in high doses, increases
the passive absorption of calcium in the absence of vitamin D
and, consequently, decreases intestinal CaBP concentration and
active transport of calcium .
All molecules that increase the osmolarity of the liquid in
the ileum are likely to stimulate the passive diffusion of cal-
cium , whereas certain amino acids act on the intercellular
space causing contraction of the cytoskeleton .
Other dietary factors make calcium irreversibly insoluble at
near-neutral pH values, by converting it into forms such as
phosphates, oxalates, phytates and soaps, which prevent pas-
sive absorption in the ileum. A variety of dietary factors have
been shown to affect the passive diffusion of calcium, and this
is a promising area of research aimed at producing the “extra”
absorption that is generally desired.
Excretion, Retention and Balance of
Most retained calcium is stored in the skeleton (99% of the
body’s calcium), depending on its needs. The main factors
affecting the efficiency of calcium storage in bone are not
dietary; they are physiological, related to growth, pregnancy
and lactation, for example. The deposition and resorption of
bone are regulated by several hormones (e.g., PTH, calcitonin,
calcitriol and estrogens), the actions of which are outside the
scope of this review. Excess absorbed calcium that cannot be
stored in bone is excreted in urine, feces and sweat. The
calcium balance in adult humans is zero, so all absorbed cal-
cium is excreted by these routes, possibly after being incorpo-
rated into and then released from bone.
Almost all the calcium reabsorbed by the intestinal tract
comes from secretions like the bile, and the endogenous cal-
cium excreted in feces is the fraction that is not reabsorbed.
The urinary loss results from glomerular filtration (about
10g Ca per day) and tubular reabsorption, which retrieves over
98% of the filtered load . Like intestinal absorption and
bone exchange, the renal calcium flux is regulated by hor-
mones, tubular reabsorption being particularly tightly regulated
The amounts of calcium in human urine are much larger
than those in the urine of other animals. Changes in the amount
of calcium excreted in the urine may therefore have a major
impact on calcium balance . Certain dietary factors can
influence the tubular reabsorption of calcium (see below), and
these must be carefully noted when evaluating calcium bio-
Fig. 1 shows the main pathways of calcium in adult humans.
Human adults lose approximately 0.3% of their bone mass each
year; this means that their calcium balance is negative and they
lose about 10 mg of calcium each day. This loss of bone mass
may be ten times greater in post-menopausal women.
The ultimate goal of all hormonal regulation of intestinal
The Bioavailability of Dietary Calcium
JOURNAL OF THE AMERICAN COLLEGE OF NUTRITION 121S
absorption, bone resorption and renal tubular reabsorption of
calcium is to keep the plasma calcium concentration constant,
particularly the 50% of calcium in the ionic form. PTH and
calcitriol are the most important hormones in calcium ho-
meostasis. This complex control mechanism also regulates
extracellular calcium, of which there are about 900 mg in the
human body. Extracellular fluid (ECF) contains about 10
; the concentration of calcium ions in the cytosol is more
than a 1000 times lower .
DIETARY FACTORS INFLUENCING
This review covers only exogenous factors associated with
the diet. Other endogenous factors like age, physiological con-
dition and hormonal regulation have been discussed earlier
[4–6,17] and are not discussed here. The main cause of
changes in the rate of absorption and retention of calcium is
clearly dietary intake, and there is an inverse relationship
between intake and utilization. These changes have little to do
with potential bioavailability, which is not controlled by hor-
mones and does not reflect the absorptive capacity of the
intestines or the retentive capacity of bone.
Dietary Factors Influencing Intestinal Absorption
Some components of the diet, such as the phytates found in
bran and most cereals and seeds, oxalates in spinach, rhubarb,
walnuts and sorrel, and tannins (tea), can form insoluble com-
plexes with calcium, thereby reducing its absorbability. This
only seems to affect calcium balance if the diet is unbalanced,
high-fiber strict vegetarian diets lacking dairy products (calci-
um), for example. This must be taken into account when
comparing dairy products with soybean-based products, which
are generally phytate-rich. The apparently negative influence of
fiber on calcium absorption is mainly due to the phytates that
are frequently associated with dietary fiber. Other plant com-
pounds, lightly methoxylated pectins for example, strongly
inhibit the absorption of calcium and other minerals .
Fibers themselves (cellulose, hemicelluloses, lignins and non-
cellulose polysaccharides) seem to have no direct effect on
calcium absorbability. Some indigestible carbohydrates and
hard-to-digest oligosaccharides have been shown to increase
calcium absorption in the distal intestine by enhancing bacterial
fermentation, thereby lowering the pH . The effect of fibers
and phytates has been examined in several reviews [e.g., 4,20].
A relative excess of phosphate has been thought to increase the
fecal excretion of calcium. However, contrary to this widely
held view, excess P does not reduce calcium absorption, at least
if calcium intake is adequate. All Western-type meals have a
Ca/P ratio well below 1, which favors the precipitation of
calcium. This does not, however, prevent the normal absorption
of calcium. Furthermore, the calcium in calcium phosphate is
as well absorbed as the calcium in other inorganic salts,
whether eaten with or without lactose .
Lipids, especially milk fats, are thought by some to form
insoluble soaps with calcium, reducing its bioavailability.
However, although this chemical reaction is possible, it does
not, in practice, interfere with calcium absorption . The
dietary soaps are dissociated at the low pH of the stomach and
cannot reform until they reach the ileum, which is beyond the
main area of calcium absorption. Fecal soaps are formed from
free long-chain saturated fatty acids and unabsorbed calcium.
The saturated fatty acids in milk and cheese can displace
calcium from phosphates in the ileum, forming less soluble
soaps which are excreted, but this has no effect on the absorp-
tion of ingested calcium .
Other constituents of food, particularly components of milk,
are thought to favor the intestinal absorption of calcium and to
keep it in a soluble form until it reaches the distal intestine,
where it can be absorbed by unsaturable routes that are inde-
pendent of vitamin D. The best known are lactose, proteins and
Many in vivo and in vitro studies on proteins and phos-
phopeptides have demonstrated a positive effect of these mol-
ecules on calcium absorption. Phosphopeptides, derived from
the enzymatic hydrolysis of caseins in particular, have been
shown to sequester calcium and other cations, protecting them
from potentially precipitating anions like phosphates in the
intestine [13,14,23]. Phosphopeptides therefore help to keep
calcium in solution until it reaches the distal intestine, thereby
facilitating its absorption by passive diffusion.
Whey proteins, such as alpha lactalbumin and beta lacto-
globulin, also bind calcium. Alpha lactalbumin binds calcium
very tightly, making it a true binding protein, like calmodulin.
However, despite the sometimes spectacular effects of these
proteins and peptides on the solubility of calcium in the intes-
tines in vitro, they have a much less dramatic effect on calcium
absorption and retention in vivo .
The beneficial effect of lactose on the absorption of calcium
and other cations has been more intensively studied than the
effects of any other components of milk since it was demon-
strated in rats by Bergheim in 1926 . Interest increased
Fig. 1. The main pathways of calcium in adult humans.
The Bioavailability of Dietary Calcium
122S VOL. 19, NO. 2
following the studies of French [25–27] and American [28–32]
groups in the 1950s on the “lactose effect”. The scientific
debate on this issue is well described in the review by Miller
, which is very well documented, but still incomplete.
It was first thought by the group of Fournier that lactose and
other “structural” sugars acted directly on bone like precursors
of bone proteins. This notion was replaced by theories of an
intestinal action [7,28,29]. It is now clear that lactose, like other
slowly absorbed sugars, must be at the site of absorption ,
that it prolongs the passive, vitamin D-independent absorption
of calcium in the ileum [5,33] and that the effects of this action
may be spectacular (doubling absorption) if a high dose of
lactose (15% to 30% of the diet) is given.
Several theories have stressed the importance of keeping
calcium soluble in the distal part of the intestines by forming
soluble chelates  or by competition with inhibitors, such as
phosphates. Fournier et al.  studied the effect of competi-
tion between lactose and phosphate on calcium absorption:
lactose, like any other sugar that can be phosphorylated, ac-
cepts a phosphate group in a reaction catalyzed by alkaline
phosphatase, thereby reducing the inhibition by phosphates
within the lumen of the intestines. These authors therefore
provided an explanation of why lactose in milk has little effect:
the lactose and phosphate in milk have opposing effects.
It has been shown, however, that lactose does not act by
increasing the concentration of soluble calcium in the lumen or
by increasing the solubility of calcium phosphate in vitro .
There is also no cotransport of lactose and calcium . The
American group supported the idea that lactose acts on the
intestinal mucosa to increase its permeability. All high osmo-
larity solutions double or triple the passive diffusion of cal-
cium, probably by increasing the space of the intercellular
junctions. This simple explanation may account for the effect of
high doses of lactose . Other studies [7,35] have shown that
lactose and other sugars increase the absorption of calcium in
the jejunum proportionally to their effects on water and sodium
The reduced bone resorption leads to the inhibition of bone
turnover  in rats fed a lactose-enriched diet. This is caused
by a large increase in intestinal calcium absorption . How-
ever, the rat is not the appropriate model in which to study
human bone remodeling (see below).
The effect of lactose has been clearly demonstrated in many
experiments in vitro and in short- and long-term trials in rats,
but its significance for human nutrition is much less clear
[5,21]. Paradoxically, lactose, at least at physiological concen-
trations, does not seem to significantly affect the absorption
of calcium from milk [36,37]. Only very high doses of lactose
(50 g/day) have a net effect [7,38]. Calcium from yogurt, in
which lactose is partially hydrolyzed, or from cheese, which
contains no lactose, is absorbed as efficiently as that from milk
Thus, lactose, at the concentrations normally found in milk,
seems to have no significant effect on calcium absorption in
healthy adults on a normal diet . However, any effect of
lactose on passive absorption may be masked by active trans-
port, which is generally sufficient if the dietary intake of
calcium is moderate and there is no lack of vitamin D. Lactose
may be more important if calcium intake is high, especially in
babies and the elderly, in whom solubility is a limiting factor
and passive absorption is the predominant route . Lactase
deficiency does not prevent the calcium in milk from being
well absorbed [36,41,42]. According to the excellent review by
Scrimshaw and Murray  on lactose intolerance, which is
prevalent in most of the world, with the notable exception of
people originating from western and central Europe, even alac-
tasic subjects can tolerate 250 g of milk per day and, thus,
benefit from its calcium.
Meals have a major effect on the absorption of insoluble
calcium supplements like calcium carbonate. Calcium carbon-
ate is better absorbed when given as part of a meal than when
it is given without food, particularly in fasting subjects. This
has been clearly shown in humans  and in pigs  and is
likely due to the calcium’s being dissolved by the gastric juices
and to slower gastric emptying.
Dietary Factors Influencing the Excretion of
Calcium in Urine
Contrarily to the simultaneous intake of phosphorus, which
can be confused with the meal effect (all common foods are
rich in phosphorus), and certain constituents that raise the pH
(bicarbonate, potassium salts), all the other dietary factors that
have an effect at the kidney level increase the urinary loss of
calcium generally by reducing tubular reabsorption .
Phosphorus may have a direct effect by increasing the
reabsorption of calcium in the distal part of the nephron or an
indirect effect by stimulating PTH secretion or by enhancing
the uptake of absorbed calcium into bone . The simulta-
neous absorption of calcium and phosphorus increases the
uptake of calcium by bone, thereby decreasing its loss in
Excess protein generally leads to an increase in the amount
of calcium lost in the urine, which may be masked by the
opposing effect of excess P (from dietary components rich in
both protein and P). This is especially true for proteins with
high contents of sulfur-containing amino acids (cysteine, me-
thionine), the breakdown of which releases sulfur oxidized as
sulfate, causing moderate acidosis and increasing the excretion
of calcium in the urine [48,49]. Sulfate ions also bind calcium,
preventing its tubular reabsorption [46,50–52] and even its
incorporation into bone . It is therefore not surprising that
an excess of protein rich in sulfur amino acids or other sources
of sulfate (certain mineral waters) causes more calcium to be
lost in the urine than other foods, such as those eaten as part of
a vegetarian diet or with bicarbonates [49,54,55].
Chronic metabolic acidosis due to excessive intakes of
sulfate and chloride anions leads to higher losses of calcium in
The Bioavailability of Dietary Calcium
JOURNAL OF THE AMERICAN COLLEGE OF NUTRITION 123S
the urine. The alkalosis resulting from ingestion of bicarbonate
or potassium citrate has the opposite effect .
It has long been known that the renal clearance of calcium
is linked to that of sodium. As almost all ingested sodium is
excreted in the urine, this effect is particularly sensitive and several
groups have developed equations describing it [46,56–60]. Ac-
cording to these equations, every extra two grams of dietary
sodium increases urinary calcium excretion by an average of 30 to
Clearly, dietary factors affecting the amount of calcium lost
in the urine have a major influence on calcium balance and may
even be more important than those that influence the intestinal
availability of calcium . This is why the inevitable loss of
calcium in the urine (accounting for a large part of the main-
tenance requirement) is greater for Western-type diets that are
high in unfavorable factors such as animal protein, sulfates,
sodium, coffee, tea and alcohol, than for other diets with lower
levels of consumption of these factors.
METHODS FOR MEASURING
In vitro Tests
Solubility in a slightly acidic medium is necessary, but not
sufficient for bioavailability. The first step in the absorption of
certain insoluble calcium supplements, given as tablets, is their
disintegration and dissolution in the stomach. The solubility of
tablets is investigated using a kinetic test (USP) of
dissolution in acetic acid (vinegar) in the US. Other primary
tests of absorbability use dialysis, ultrafiltration and various
membrane techniques, particularly the isolated intestinal loop,
pieces of mucosa or cell layers (caco-2 cells). None of these
methods takes into account the whole range of nutritional,
physiological and ecological factors that influence absorption,
and none provides results directly comparable with those ob-
tained in vivo using whole animals.
Classical Balance Studies
This is the only method that provides true, absolute data on
absorbability and bioavailability. It also gives mean values,
although these are only valid for the period tested, and the test
period must be at least one week after starting the diet (but
several weeks are often needed). The balance method provides
data for apparent (intake—fecal) absorption and net retention
(intake—fecal—urinary) and the corresponding coefficients.
Isotope dilution studies, using a stable or radioactive isotope of
calcium, injected at the start of the evaluation, give the fecal
loss of endogenous calcium and, hence, true absorption (in-
Balance studies are slow, labor-intensive and expensive.
The validity of the results obtained depends on how accurately
the intake and output parameters are estimated. Even under the
most rigorous experimental conditions (animals in metabolic
cages) the inevitable small errors in assessment of intake (mea-
sured by excess) and fecal and urinary losses (measured by
defect) always lead to an overestimation of the amounts re-
tained. This overestimation may be very large when retention is
low, as is always the case for adults.
Balance studies are essential for estimating the dietary
needs of growing animals by the factorial method, but they are
of little use for studies on human adults. Even under the most
rigorous conditions (several days in a metabolic unit), the
measurements are poorly reliable. Consequently, too much of
the work published on human adults (normally in negative
balance or equilibrium) has indicated that the individuals tested
had a positive daily calcium balance as high as 200 to 300 mg
calcium, which is most unlikely.
Fortunately, it is not necessary to carry out absolute balance
studies or obtain absorption and retention coefficients for the
comparison of several dietary sources of calcium. This can be
achieved with values given relative to a reference source. This
method only provides the bioavailability of calcium for an
average diet over the test period for human subjects and cannot
be used to compare two sources of calcium. Calcium sources
can only be compared if the calcium is given as a single load,
a test meal, giving the bioavailability of calcium at that time
point only. One method used for human subjects [61,62] in-
volves a preliminary intestinal lavage with isotonic solution
followed by the test meal and, then, 12 hours later, a second
intestinal lavage to collect the unabsorbed fecal residue. This
rather drastic and unphysiological method has been used to
show that there is little difference in the bioavailabilities of
soluble and poorly soluble calcium salts .
Isotope Balance Methods
As in classical balance studies, all feces and urine must be
collected over a period of several days, but the balance is
calculated only on the tracer isotope in the source of calcium
being studied. The intake is known accurately because it is a
single dose of radioactive (
Ca) or stable (
Ca) isotope given in a test meal. The absorption and
retention coefficients obtained are regarded as being represen-
tative of all the calcium in the labeled source.
Unlike the classical balance, the isotope test measures only
the instantaneous bioavailability of a single dose taken as part
of a meal. There is generally no period of adaptation, and
variations over time are not taken into account, even though the
coefficient of variation between meals and between days is
probably over 10% . The results obtained depend greatly on
the experimental protocol, particularly the timing of the oper-
ations, such as whether the isotope is given to the fasting
subject before, with or after the meal.
One of the main problems with assessments involving iso-
tope tracers (see below) is the labeling technique itself. Ideally,
an intrinsic marker should be used; for example, calcium in
The Bioavailability of Dietary Calcium
124S VOL. 19, NO. 2
milk can be labeled by giving the cow several injections of
Ca, whereas plant calcium can be labeled by adding the
isotope to the fertilizer. Most labeling is extrinsic, however;
this means that the food to be studied is mixed with the isotope,
, for example. This assumes that there is a perfect
exchange between the calcium in the foodstuff and the added
isotope. Most dairy products seem to come rapidly to equilib-
rium , as do many other foodstuffs , but this is not true
for certain plant products that contain insoluble calcium salts
such as phytates and oxalates . The bioavailability of cal-
cium in these foods may therefore be considerably overesti-
Urinary Excretion of an Oral Calcium Load
This is one of the methods most frequently used to compare
sources of calcium in human studies. Unlike some animals
(e.g., rats and pigs), which lose little or less calcium via the
urine, humans excrete large amounts of calcium in urine. The
increase in the amount of calcium lost after a calcium load is
given to a fasting subject (about 500 mg Ca) may be thought of
as reflecting the effectiveness of calcium absorption. However,
the results reflect instant absorbability and also depend on
several dietary factors that affect the loss of calcium in urine,
by reducing it (phosphorus) or increasing it (sodium, high-
sulfur protein, sulfate, certain carbohydrates).
This test is simple and fast. A urinary response can be
obtained three to four hours after ingesting the test meal, and
the urinary calcium data (relative to urinary creatinine) can be
used to compare different sources of calcium [68–70]. Varia-
tions on this test use test meals labeled with stable isotopes.
Measuring Isotopes Labeled in the Blood, Urine
This may involve a single label for the rapid comparison
(two to four hours after a single oral load) of labeled sources of
calcium by measuring (sometimes with kinetic studies) radio-
activity or stable isotope enrichment in the blood, urine or bone
(used particularly for animals). An estimate of true relative
absorption may also be obtained by measuring the area under
the plasma isotope concentration curve. The direct measure-
ment of the radioactivity taken up by a representative bone is
possible using the
Ca isotope (a gamma ray emitter).
The most commonly used method at present is a double-
label method in which a test meal labeled with one Ca isotope
is ingested and a second Ca isotope is injected intravenously.
The behavior of this second isotope reflects, in principle, 100%
absorption. The isotope concentrations are measured later two
to four hours in the blood, 24–36 hours in the urine). The ratio
of the two isotopes (ingested and injected) is assumed to be
equal to the fractional absorption (between 0 and 1) of the test
calcium. Several sources of calcium can be compared rapidly
and absolute true absorbability determined over several days
without collecting feces. In addition, as these assays are rela-
tively short in duration, they can be repeated on the same
subjects after allowing for a “decontamination” interval.
This double radioisotope labeling technique has been
widely used in animals and in humans . A rapid method in
which one radioisotope of calcium is injected, followed by a
second injection of the same isotope 2 hours later has been
devised by Chanard et al.,  and used routinely by Wynckel
et al. . Stable isotopes are now used in double-label studies
in humans . The validity of several variations of this
method, differing in the type of blood sample or urine sample
used and in the method of calculation, has recently been ana-
Several accurate mass spectrometry methods for measuring
the enrichment of stable isotopes of calcium are now available
. The validity of bioavailability assessments based on these
techniques depends, however, on several factors, including the
quality of labeling of the test load, the representative nature of
the samples and variations in the physiological and nutritional
status of the experimental subjects.
Long-Term Evaluation of Bone Mineralization
Measuring bone parameters after prolonged treatment (sev-
eral weeks for growing animals) is undoubtedly the most reli-
able way of estimating the long-term effects of qualitatively
and quantitatively different calcium intakes. The mineral con-
tent, mineral density, breaking strength and morphometric pa-
rameters of a representative bone can be measured for experi-
mental animals once they have been killed. The best methods
currently available for measuring bone mass in several parts of
the human skeleton are double X-ray absorptiometry
(DEXA) or quantitative computed tomography (QCT) for
These bone criteria are generally sensitive enough for com-
paring sources of calcium, provided that the subjects are young,
and reactive, with large calcium requirements and that they are
assayed over a sufficiently long period. A single calcium intake
concentration can be used for several sources, but it is better to
use several intake concentrations for each source. This provides
bone responses that vary with the intake. The slope-ratio of the
curves for each source gives the bioavailability. This method
gives very good results because it eliminates the large effect of
small changes in calcium intake.
It is easier to interpret these data if the basal diet is low in
calcium, because then almost all the calcium ingested comes
from the test source, provided that all the other dietary factors
affecting calcium absorption, such as proteins, phosphorus,
phytates and sodium, remain the same.
Measuring of Biological Markers in the Blood
The concentration of PTH in the plasma falls when there is
a small transient increase in plasma calcium concentration (or
The Bioavailability of Dietary Calcium
JOURNAL OF THE AMERICAN COLLEGE OF NUTRITION 125S
) due to the intestinal absorption of an oral calcium
load. This transient decrease is, however, proportional to the
efficiency of absorption. This test is easy to perform in short-
term comparative studies on human subjects, but it does not
take into account further urinary loss of calcium, hence its
retention by bone.
Some factors in the blood or urine vary with the degree of
bone accretion or resorption. They can therefore be used in
comparative tests to measure the effect of various amounts of
absorbed calcium. The loss of hydroxyproline in urine is an
indicator of bone resorption. It is now used in complement with
or has been replaced by assays of more specific bone markers,
collagen “cross-links”, pyridinoline or, better still, deoxypyr-
SELECTION OF ANIMAL MODELS
In vitro tests can be used to detect factors likely to alter the
intestinal absorbability of calcium, but they are not really of use
for quantifying bioavailability and cannot replace in vivo trials
on animals. Experiments in humans are, of course, ultimately
required, but it is still necessary, for many reasons, to carry out
Selecting a Species
The main species used are rats, pigs, guinea pigs and pri-
mates. The dietary behavior of the animal must be taken into
account, including the type of diet and frequency of meals, for
example. Pigs are omnivores that eat rapidly two or three meals
per day. This similarity to human behavior makes them an ideal
model. Rats and guinea pigs eat grain and are continuous
nibblers or gnawers without well defined meals.
The physiological characteristics of the rat also make it an
unsuitable model. Its intestine presents high levels of phytase
activity enabling it to hydrolyze phytates in food and to absorb
calcium down as far as the large intestine. Neither pigs nor
humans are able to do this, at least not to the same extent. The
main problem with guinea pigs, rabbits and, to a lesser extent
rats, is that they are coprophagous, a circumstance which
makes interpretation of true absorption results complicated.
Rats are poorer animal models than pigs for studies on bone
metabolism because their skeletons are continuously growing
and never reach a bone remodeling stage paralleling that of
human adults. In pigs, closure of epiphyseal cartilage occurs at
the age of two to four years . There is, however, no
evidence that this difference, which may be important when
studying factors affecting osteoporosis , has any effect on
the absorptive capacity of the intestine.
Pigs and rats lose very little calcium in the urine, whereas
humans and guinea pigs have very high urinary calcium levels.
This factor limits the suitability of pigs for use in studies on the
factors that may influence urinary calcium levels in humans.
The lack of renal excretion of the excess absorbed calcium is
probably offset in pigs by greater elimination via the endoge-
nous fecal route.
Interest and Advantages of Animal Experiments
There is no doubt that it is preferable to carry out experi-
ments on animals than on humans for ethical, material and
financial reasons. Clearly it is much more feasible to work with
young growing animals, whose calcium metabolism is very
active, than to attempt such studies on children. Such experi-
ments are important because the coefficients of calcium ab-
sorption and retention often depend more on the physiological
condition of the subject than on the nature of the calcium
ingested. Comparative studies on the bioavailability of several
sources of calcium must therefore make use of subjects that are
physiologically capable of retaining the ingested calcium.
Several technical manipulations are possible, such as the
insertion of intestinal cannulae or catheters for repeated blood
sampling. Radioisotopes are less expensive to use than stable
isotopes, and they are also easier to measure accurately. Long-
term trials involving extended periods in metabolic cages can
be used to accurately measure ingested and excreted amounts;
such trials are far from easy in humans. It is also possible to
take organ samples from animals killed at a specific stage,
which is of particular value for representative bone samples for
a range of chemical, biological, morphometric and histological
tests and for mechanical tests of breaking strength.
The power of the statistical tests that can be used is much
greater with animal models. It is easy to set up very uniform
groups with most of the animal species used (except, perhaps,
primates), with very little variation between individuals and
parameters like breed, strain, gender, age, weight, physiological
state and dietary history all the same. Dietary components may
be altered as required and the amounts consumed and excreted
are accurately known.
It is thus possible to detect small but statistically significant
differences between groups of ten individuals for animals,
whereas dozens or even hundreds of individuals per group
would be required if the experiments were done on humans. For
example, we know that the average urinary loss of calcium in
a human adult is 150⫾50 mg Ca/day (coefficient of varia-
tion⫽30%). Therefore, an increase of 15 mg per day in re-
sponse to a dietary factor (e.g., sulfate) can only be statistically
significant if the trial includes at least 100 subjects per group in
a long term cross-over trial or many more subjects per group if
it is a short-term trial with two groups. In contrast, this type of
small effect is readily demonstrated in animals and has a very
considerable long-term physiological consequence. Among
other examples, the recent experiment done by Couzy et al.
 shows the limits of human experiments. They concluded
that sulfates in mineral water had no effect on urinary calcium
loss, relative to milk, because the observed 14% increase was at
the limit of statistical significance. In fact, because of the large
The Bioavailability of Dietary Calcium
126S VOL. 19, NO. 2
variation between individuals that is inevitable in this type of
study, such a difference between two groups containing only
nine adult subjects cannot be significant. However, the in-
creases in urinary sulfate (⫹35%) and magnesium (⫹18%)
were significant at the 5% level.
Only animal experiments can be used to show the statistical
significance of small changes, and their demonstration in ani-
mals indicates that they may also exist in humans.
OF CALCIUM IN FOODS: REVIEW
Comparison of Sources of Calcium
Human Studies. Many trials have been carried out over the
past 15 years to compare calcium in milk with several other
sources of calcium, such as salts, mineral waters and plant
products. Almost 20 of the studies on bioavailability were
carried out on men or women, using a variety of methods (true
or apparent absorption, urinary calcium). None of the studies
showed that the calcium in milk was more efficiently used than
any calcium salt. Carbonate, gluconolactate, citromalate
(CCM), chloride, lactate, acetate and citrate were tested
[40,44,62,79–88]. The calcium from mineral water, bicarbonate
or sulfate, was not found to be any better for absorption
[73,74,89,90]. The findings were similar for several milk de-
rivatives (yogurts, cheeses, chocolate milk, acidified milk)
[40,70,90,91]. The calcium in milk and dairy products is much
better absorbed than the calcium in spinach or watercress, as
these plants have high oxalate contents [64,84,92–96]. Studies
in humans, comparing the absorption of calcium from milk
with that of CCM, suggest that calcium availability from CCM
is higher [84,87], even than that from calcium carbonate
[33,44,84,87]. A study carried out on women showed that the
fractional absorption of calcium from cabbage was better than
that of calcium from milk .
Studies on Rats and Pigs. There have been about 15
studies performed on rats over the past 15 years. They show a
similar pattern, but many also include measurements of bone
retention of labeled calcium [27,39,65,99,101–103] and tested
a wider spectrum of minerals and milk products as sources of
calcium that would be possible in studies on humans. Thus,
studies in rats show that the calcium in whey is as efficiently
absorbed and utilized for bone mineralization as that bound to
casein [104,105] and that there is little difference between dairy
products in general (milk, acidified milk, yogurt, skim milk,
cream cheese, hard cheeses) [27,65,100,106]. Two studies in
rats found differences between the “calcium value” of yogurt
and milk [102,107], but their findings are contradictory. Studies
in humans have shown that calcium absorption from these two
sources is similar [40,41,91].
Long term studies in growing pigs [108,109] or ovariecto-
mized mini-pigs  have provided no evidence that calcium
is better absorbed from milk and milk products (casein phos-
phopeptides, skim milk or yogurt) than from mineral salts
). However, bone mineralization (evaluated by
breaking strength) is better in animals fed yogurt as a calcium
source than in those that obtain their calcium mainly from
As in humans, most trials in rats have found no difference
between the use of Ca from yogurt and that from other milk or
mineral sources [27,40,41,65,91,100]. However, adding yogurt
to the diet improves the fractional absorption of calcium .
Calcium in cheese is as efficiently used as the calcium in milk
or carbonate [40,65,70,91,106]. A study on growing rats by
Dupuis et al.  found that calcium is initially better retained
from milk products than from calcium carbonate, but that this
difference is later lost. Calcium from plants (apart from cab-
bage and some other crucifers), particularly that from cereals, is
generally less well absorbed than the calcium from milk [112–
114]. Phytates (present in large amounts in wheat bran and in
soybean-based products) reduce the absorption of calcium from
calcium carbonate  and from milk . A study on rats
using goat milk products found that the calcium from goat’s
cheese is less well retained than that from milk . Only one
study in rats found that increasing the dietary calcium intake
with calcium sulfate leads to an increase, in four weeks, in bone
mineral content. The same calcium intake from milk provides
similar (ash as % dry matter) or higher levels of mineralization
(Caasa%ofbone dry matter) than that provided by calcium
sulfate . The bioavailability of calcium from milk was
estimated to be 113% that of calcium from calcium sulfate in
To summarize. The mean apparent calcium absorption
(% intake) from all collected data concerning calcium salts
from human studies varied from 23% to 37%, excluding phos-
phates, because of the too large range of variation and the
paucity of data. The following averages have been calculated
from 3 to 8 references (citrate, citromalate, chloride, lactate,
gluconate or a mixture of lactate and gluconate) to 12 to 14
references (carbonate, milk): carbonate, from 26.4 (fasting) to
29 (meal), citromalate from 32 (fasting) to 37 (meal), citrate
23.5 (fasting), lactate⫹gluconate 24.5 (fasting), chloride 30.6
(fasting), milk 32.4, cheese 32.8, mineral waters 32.3, oxalate-
rich products (calcium oxalate, spinach, watercress) 13.2.
These values are to be considered with care because they result
from trials that compare different diets, ages and many other
parameters. Furthermore, as suggested above, some calcium
sources have been well investigated and some not.
Calcium Absorption versus Bone Retention
Intestinal absorption does not necessarily reflect the bio-
availability to the whole organism because calcium must be
retained and used in bone formation and mineralization. Phos-
phorus must also be present for the production of hydroxyap-
atite (a complex tricalcium phosphate). The dissociation of
The Bioavailability of Dietary Calcium
JOURNAL OF THE AMERICAN COLLEGE OF NUTRITION 127S
calcium intake from that of phosphorus (if, for example, the
calcium source is not ingested with the meal and/or this source
contains no P), may restrict bone mineralization. This has been
known for some time and was recently confirmed in growing
pigs, which are extremely sensitive to dietary mineral supplies
Three sources of calcium, calcium carbonate, CCM and
milk, have been extensively studied. They all ensure the effi-
cient absorption of calcium and also, over a long term (one to
four years), that calcium is retained and used for bone miner-
alization. This was reported by Prince et al.  in a study on
menopausal women, in whom calcium supplements, given as
tablets or as milk, reduce the bone loss measured over
a two year period. Similarly, Smith et al.  studied 169
women aged from 35 to 65 years who were given calcium
carbonate supplements (or a placebo) for four years. The cal-
cium carbonate supplements reduced bone loss around meno-
pause (bone mineralization study) at 12 sites. A group of
adolescents was given a calcium supplement (one g/day, from
900 mL milk or calcium carbonate tablets), and it was found
that bone density, determined 10, 18 and 24 months later, was
higher in those given calcium than in those given the placebo
and that the milk and carbonate sources were equally effective
. Recently  it was reported that calcium-enriched
foods significantly increased bone mass accrual in prepubertal
girls, with a preferential effect in the appendicular skeleton and
greater benefit at lower spontaneous calcium intake. Lastly,
postmenopausal bone loss was reduced at the main sites of
spongy bone (but not of cortical bone) by supplementing cal-
cium intake with calcium carbonate or CCM in a two year
study by Dawson-Hughes et al. . CCM was found to be
the most effective. Other salts have been used in long-term
studies (tricalcium phosphate, glucono-lactate plus calcium car-
bonate) and shown to reduce bone loss or the incidence of hip
Such longitudinal clinical studies have yet to be done using
mineral water as the source of calcium. Hence, there is, as yet,
no evidence showing that calcium from mineral water is sim-
ilarly effective. While several human studies indicate that cal-
cium from these sources is as well absorbed as that from milk
or calcium carbonate, the effect of prolonged mineral water
consumption on bone mineralization is not yet clear. Only one
study, that of Cepollaro et al.,  reported a positive effect
of consuming a high-calcium bicarbonate water on the bone
density of 45 menopausal women, after 13 months of this form
of supplementation. The control group (who drank a low-
calcium water) was given no calcium supplement (calcium
intake: supplemented, 1500 mg/day; non-supplemented, 949
mg/day). Apart from this trial, there have only been very
short-term studies (a few days) for high-calcium mineral wa-
ters, and such studies are too short to test for any bone effect
. Careful interpretation is therefore required; the efficient
absorption of calcium from these high-sulfate, high-bicarbonate
waters, similar to that of calcium from milk or carbonate, does
not necessarily show that this calcium is as well retained by
bone. A recent preliminary report  showed that giving a
calcium supplement in the form of calcium-rich water to post-
menopausal women for two months reduced bone resorption
(determined by the excretion of markers of bone resorption),
but that the effect was much less marked with high-sulfate
water than with high-bicarbonate water. It is well known that
the urinary loss of calcium is lower with alkalogenic diets, rich
in vegetables and fruits or bicarbonates [54,127]. The problem
of urinary loss of calcium with these calcium sulfate sources
remains to be determined over longer periods.
Our recent studies in growing pigs have shown greater bone
mineralization (measured as ash, density and breaking strength
of various bones) in pigs fed a “milk” diet (70% of the calcium
intake as powdered skim milk) than in pigs fed a “sulfate” diet
(50% of total Ca intake as CaSO
and 33% as CaCO
“carbonate” diet (80% of intake). The sulfate and carbonate
diets gave similar levels of mineralization. All the diets had the
same energy, protein and calcium contents (Pointillart and
Gue´guen, unpublished results).
Casein Phosphopeptides, Proteins and
The positive effects of milk casein phosphopeptides (CPP)
on the absorption of calcium have been shown mainly with in
vitro studies of calcium transfer (ligated intestinal loops or
everted sacs) in rats [128–133] and in a few in vivo studies in
rats in which calcium absorption and bone retention were
measured [134–137]. The CPP were compared to soybean
protein extracts, egg white , wheat gluten or gelatin 
or fibrin . A study on isolated chicken intestinal loops also
showed that CPP increased calcium transfer . A diet in
which 50% of calcium and about 33% of P are provided by
CPP has no effect on calcium absorption or bone retention in
pigs . Feeding of casein, a potential substrate to the
release of CPP, to growing miniature pigs improved femur
mineralization as compared to whey protein. This observation
was true when vitamin D deficient diets were given but not
when adequate vitamin D supply was provided .
Studies on unweaned babies show that those fed soybean-
based formula have 25% less bone mineralization (from den-
sitometry measurements) than those fed milk-based formula
[138,139]. Conversely, in vivo studies on ovariectomized rats
showed that these animals lost less bone if fed a diet containing
soybean protein extract than if fed a milk-based diet [140, 141].
The authors interpreted this as being due to the phytoestrogens
in soybean. It is difficult to extrapolate these results to humans,
given that Tsuchita et al.  clearly showed less bone loss
following ovariectomy in rats fed CPP than in rats given Ca and
P as pure minerals.
Partridge  showed greater calcium absorption in very
young pigs fed milk than in those fed an isocalcium diet
containing soybean meal. Similar results were obtained in pigs
The Bioavailability of Dietary Calcium
128S VOL. 19, NO. 2
by Matsui et al. . The opposite pattern is later observed
(soyabean⬎milk) in pigs aged four months, and there is no
difference in older animals (soya⫽milk).
The positive estrogen-mimetic effect on bone has only been
observed in ovariectomized rats, and soybean products have a
high phytate content which may reduce calcium absorption, as
has been clearly demonstrated, including in women [116,145].
Lastly, several in vivo studies have shown that calcium in diets
with various soyabean and CPP contents is similarly absorbed
[rat: 104,146,147; pig:108]. A clinical study on unweaned
babies up to six months old compared bone density at various
stages of development, and found no difference between moth-
er’s milk and formulas based on soybean or on cow’s milk
. In contrast, the amount of animal protein consumed by
women was found to be strongly correlated with the incidence
of hip fracture in a retrospective epidemiological study carried
out by Abelow et al. .
It has been shown in many studies that the greater the
amount of dietary protein, the higher the urinary calcium level,
regardless of whether the protein is casein or soybean protein
[in rats: 147; in man: reviewed by Abelow et al., 149]. Thus,
high levels of protein consumption lead to a negative calcium
balance. Reducing the milk protein content of the diet reduces
urinary calcium loss in man . Calcium supplementation in
the form of milk increases the amount of sulfate in urine
because milk has a high content of sulfur-containing amino
acids , and some studies have implicated these amino acids
in the hypercalciuria and negative calcium balance associated
with diets containing too much animal protein [51,55,147,151–
153]. A horizontal study carried out in China on women who
had consumed a variety of diets (with and without animal
protein, plus or minus milk) indicated a greater correlation
between urinary calcium and the consumption of animal pro-
teins not derived from milk . However, things are not that
simple. A study performed by Allen et al.  on humans
with controlled diets and for whom the dietary protein was
tripled from 12 g N/day to 36 g N/day by adding soya extract
clearly showed an increased urinary calcium loss (1.5-fold)
which changed the calcium balance from ⫺37 mg/day for 12 g
N/day to ⫺137 mg/day on 36 g N/day, despite high calcium
intakes (1400 mg/day) and the similar absorptions. This prob-
lem of the effect of excess protein on bone has been recently
In Conclusion. While the proteins in milk or milk products
may have beneficial effects on bone mineralization, this is not
always so. In contrast, the positive effects of soya on calcium
retention have only been demonstrated in one rather special
system, ovariectomized rats, while it has been clearly shown
that the phytates in soya can reduce calcium absorption in
humans. Both milk and CPP have a favorable effect on calcium
absorption. The high phosphorus content of milk may offset the
hypercalciuria induced by protein , although the intakes of
both calcium and phosphorus provided by the milk help pro-
mote bone mineralization.
The hypercalciuric effects of high-protein diets, particularly
those containing animal proteins, are well known. However,
human studies on the nature of these proteins, plant/animal,
milk/non-milk proteins, and their long term influence on cal-
cium balance or bone metabolism are still necessary before we
can come to any conclusion about whether plant proteins are
advantageous or not. Thus, particularly strict vegetarian diets
that contain no milk products may present risks to bone min-
eralization  because they do not provide an adequate
calcium intake, without recourse to supplements provided by
mineral calcium tablets .
Lactose, Lactase and Calcium Bioavailability
In Rats. Lactose is reputed to stimulate calcium absorption
and most of the experimental evidence for this has been ob-
tained in rats [21,25,26,29,39,158]. These in vivo studies pro-
vide direct evidence that it acts on the intestines and on bone.
There is also indirect, in vitro, evidence obtained from studies
on isolated intestinal loops [31,159] in which lactose was
compared to another sugar or the absence of lactose .
Other studies on isolated gut loops in situ have, however,
shown that 30% lactose can reduce the absorption of calcium
chloride . There is also other indirect experimental evi-
dence. For example, in vivo studies have compared the calcium
absorbed from normal milk and from milk in which the lactose
had been hydrolyzed . Others have shown that lactose,
unlike sucrose, reduces the effects of a lack of vitamin D on
bone . Lastly, adding lactose to cheddar cheese was found to
give better calcium absorption than with cheese alone .
In Humans. The effect of lactose is perhaps less clear cut
in man because it is complicated by the problem of lactose
intolerance and thus of a lactase deficiency [see review by Scrim-
shaw and Murray: 43]. Griessen et al.  found that lactose
increased the fractional absorption of calcium in lactase deficient
(LD) patients, but most studies have shown a reverse effect
[38,160–162] or no effect . A group of five studies showed
that the presence of lactose, or its addition, stimulated calcium
absorption in lactose-tolerant subjects [35,38,161,163,164], but
three other studies demonstrated no effect [37,165,166].
There is no real proof that hypolactasic patients absorb
calcium less well than others. At least one study  found that
the absorption of calcium from a standard diet by lactase-
deficient patients was better than that of lactose-tolerant pa-
tients, another found that it was poorer , while still others
have reported that the basal absorptions were similar
[36,38,41,111], even when there was milk or yogurt in the diet
. Yogurt can increase calcium absorption in both LD and
non-LD subjects  compared to a CaCl
Normal mother’s milk results in better absorption of cal-
cium by unweaned babies than when the lactose is removed,
and adding lactase to mother’s milk can increase calcium
absorption . Lactose seems to have an even greater effect
The Bioavailability of Dietary Calcium
JOURNAL OF THE AMERICAN COLLEGE OF NUTRITION 129S
on calcium absorption when absorption is basically poor
A recent clinical study  on children about 10 years old
found no difference between the bone densities of lactose-
intolerant and paired (height and weight) controls, but they did
find that there was a strong correlation (r⫽0.9) between bone
density and calcium intake in the lactose-intolerant children.
Several epidemiological studies have also shown that lactose-
intolerant subjects consume less calcium (from dairy products),
which may predispose them to osteoporosis [168,169]. This
link is not always found, as indicated by the lack of a difference
between the bone densities of female twins, one of whom was
hypolactasic and the other lactose tolerant (in response to an
oral load) . The answer may lie in the total amount of Ca
consumed, i.e. from dairy and non-dairy foods.
In conclusion. Several studies have shown that lactose has
a positive effect on calcium utilization, but there is some
uncertainty, at least in LD people in whom lactose can reduce
absorption. It is possible that this effect is only temporary, as
suggested by certain studies on the changes in calcium absorp-
tion after a meal . The literature contains many contradic-
tions concerning the reduced absorption in LD subjects. It is
quite probable that the non-consumption of dairy products by
these subjects tends to reduce their calcium intake, but that
could be offset by more efficient absorption, provided that they
still have the capacity to adapt; it is far from clear that the
elderly have such a capacity.
Influence of Dairy Product Consumption on
All of the 14 clinical or epidemiological studies published
over the past decade [118,171–181], except one , have
shown that the consumption of dairy products in childhood and
adolescence has a positive effect on bone mineralization later in
life, as assessed by bone density measured at several sites in
adults. They therefore confirm the classic findings of Matkovic
et al. . This effect on subsequent bone density is reduced
or lost when milk is consumed between the ages of 20 and 30
[172,173,179]. Several studies have found that a dairy product
supplement increased bone density in adolescents [174,177] or
reduced bone loss in post-menopausal women . Lastly,
osteoporotic women were found to have consumed less dairy
product than healthy controls when they were children and
adolescents [172,175]. A recent review  done on children
found that consumption of extra calcium increased their bone
density 1% to 5% or even 10% when the source of calcium was
dairy products. This was recently confirmed with a double-
bind, placebo-controlled trial in prepubertal girls . The
question remains on whether such effects persist after six to 36
months of intervention.
Only one study, that by Prince et al. , has compared the
effects of calcium supplements, given as 1-g/day mineral tab-
lets or dairy products, for two years on women at least 10 years
after menopause. Both types of supplements have similar ef-
fects: they reduce bone loss from several sites in the hip, but not
from the lumbar vertebrae. The findings of a number of recent
meta-analyses of data from horizontal studies looking for a link
between calcium intake and bone loss have arrived at contra-
dictory conclusions. However, prospective studies on the ef-
fects of calcium supplements have generally shown that it has
a positive impact on bone loss [185,186]. According to Nordin
, some contradictory conclusions could be due to errors in
the dietary data. Nordin analyzed 19 trials, three using dairy
products. This analysis clearly showed that calcium supplemen-
tation reduced bone loss from 1.26% per year in controls to
0.12% in those receiving calcium supplements (p⬍0.005).
These are data for the bone densities of 1300 postmenopausal
women, measured at 11 bone sites, including cortical and/or
trabecular bone. The difference between the annual percent
bone loss between supplemented and unsupplemented women
varied from ⫹0.28 (spine) to ⫹4.1 (femoral diaphysis). Lastly,
Lyritis et al.  found a correlation between the consump-
tion of dairy products by young adult humans and their bone
We can therefore say that a greater calcium intake, particularly
of milk products, during the period of peak bone formation has a
positive effect on bone density of adults and undoubtedly reduces
the risk of osteoporosis. But only intervention studies have
shown that calcium supplementation has a beneficial effect on
bone loss, while the results of horizontal epidemiological stud-
ies are more controversial.
PECULIARITIES AND ADVANTAGES
OF THE CALCIUM IN MILK AND
It is well worth remembering that milk and milk products
are by far the main source of calcium in our diet . Cow’s
milk contains an average of 1.20 g calcium per liter, 20% of
which is bound to casein as an insoluble organic colloid and the
remaining 80% in mineral form (45% in the tricalcium phos-
phate of the phospho-caseinate, which is also insoluble and
colloidal, and 35% soluble, including 12% as ionized calcium)
. The organic or mineral calcium bound to casein is
readily released during digestion, and there is general agree-
ment that its potential bioavailability is high. Most solubility
studies use milk calcium as a reference standard. The calcium
in spinach, which is present as an insoluble oxalate, is taken as
the extreme example of poor bioavailability. However, except
for newborns fed on mother’s milk (calves drinking cow’s
milk) which can absorb almost all the ingested calcium, the
percent of milk calcium absorbed seldom exceeds 40% under
normal dietary conditions.
The calcium in cheeses is readily available, despite the fact
that cheese often contains large amounts of saturated long chain
The Bioavailability of Dietary Calcium
130S VOL. 19, NO. 2
fatty acids and no lactose . Tests on rats fed cheddar cheese
Ca showed that the calcium was as well ab-
sorbed as was that from milk and that absorption was not
influenced by the maturation time .
There is therefore no difference in the availability of cal-
cium from milk and most of the best mineral or organic sources
of calcium which are often used as medicines or dietary sup-
plements and whose coefficient of absorption is about 30% to
40%. Only a few organic forms, like citrate-malate, can provide
slightly better calcium availability .
Nevertheless, the calcium in milk differs in several inter-
esting features from the calcium in other foodstuffs or supple-
ments. These can be important when it is necessary to ensure
high absorption of calcium under unfavorable physiological
conditions . Because it is bound to peptides and proteins,
milk calcium is more likely to remain in solution when the pH
is unfavorable, such as in achlorhydria. Milk calcium may be
absorbed in the absence of vitamin D, under the influence of
lactose in the distal small intestine via the paracellular route.
Thus milk can provide calcium with “ensured absorbability”
which is generally insensitive to external factors, except for
inhibitors, such as oxalic acid. Dairy products do not contain
anything likely to inhibit the intestinal absorption of calcium,
like phytates, oxalates, uronic acids or the polyphenols of
certain plant foods. The hypercalciuric effect of sulfates from
milk proteins is offset by the hypocalciuric effect of phospho-
rus . The endogenous sulfates produced by the breakdown
of sulfur-containing amino acids produces a SO
/Ca ratio of
0.6, while this ratio is 2.6 in some high-sulfate, high-calcium
Lastly, it should be remembered that milk and dairy prod-
ucts are not only excellent sources of calcium, but also provide
an almost complete diet whose consumption provides a “meal
effect” . This fosters the absorption of calcium and pro-
vides a simultaneous intake of phosphorus that is essential for
bone deposition. These advantages cannot be provided by any
other source of calcium, such as calcium supplements or Ca-
As milk provides calcium with “protected absorbability,”
“prolonged absorption” and “extended bone deposition,” milk
is the most suitable dietary constituent that meets the high
calcium intake required by postmenopausal women and the
elderly. This is especially important because, according to
some workers , and for still unknown reasons, the inhibi-
tion of bone remodeling that generally occurs in response to a
high calcium intake is less marked when calcium is supplied by
milk products. Further studies are now needed to identify a
possible specific effect of milk products on bone, although this
beneficial effect could be simply due to different rates of
calcium absorption, with slower gastric emptying and a pro-
longed passive diffusion that ensures an extended supply of
calcium to the bone.
The authors are indebted to ARILAIT-Recherches (Paris)
for constructive discussion and financial support. We also
thank Owen Parkes, Colette Colin and Marie-Claire Kopka for
their help in editing the text.
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