J. Elem. s. 277–288 DOI: 10.5601/jelem.2014.19.1.436
FROM DAIRY PRODUCTS
AND ITS RELEASE FROM FOOD
BY IN VITRO DIGESTION
Food products are an exclusive source of nutrients necessary for the development and
proper functioning of human organism. The key issue in nutrition is the bioavailability of
nutrients rather than the supply of their adequate amounts in a diet. Calcium is the basic
structural element of bones and teeth. It is also an element with a wide range of physiologi-
cal functions, e.g. it is responsible for the right excitability of muscular and nervous system,
normal heart function, hormonal secretion, blood clotting, cell membrane permeability and
activation of various enzymes. It is universally acknowledged that the main source of calcium
for a human organism is milk and dairy products, owing to their rich content and high availa-
bility of calcium. However, certain stages in food processing technologies may depress the high
bioavailability of dietary nutrients.
In this paper, long-term research findings about the bioavailability of calcium in milk
products have been summarized, in addition to the information about calcium uptake from
bread or other popular breakfast meals composed of milk products in combination with cereal
component. The popular opinion that milk and other dairy products are the best and most va-
luable source of highly available calcium has indeed been supported. However, an in vitro study
demonstrates that an addition of cereal to milk or yoghurt significantly diminishes the quantity
of calcium released during digestion, which could contribute to the development of calcium in-
sufficiency or augmentation of its symptoms.
milk, dairy products; bread, cereals, calcium deficiency.
prof. dr hab. Krystyna A. Skibniewska prof. zw., Chair of Foundations of Safety, University
of Warmia and Mazury , Heweliusza 10, 10-726 Olsztyn, Poland, e-mail: firstname.lastname@example.org
Pożywienie jest jedynym źródłem składników odżywczych niezbędnych do rozwoju i pra-
widłowego funkcjonowania organizmu człowieka. Kluczowym zagadnieniem w żywieniu jest
dostarczenie odpowiedniej ilości składnika odżywczego, w związku z czym duże znaczenie ma
biodostępność z diety. Wapń to podstawowy składnik budulcowy kości i zębów, również pierwia-
stek o bardzo szerokim znaczeniu fizjologicznym, odpowiada bowiem za prawidłową pobudliwość
układu mięśniowo-nerwowego, właściwą pracę serca, sekrecję hormonalną, krzepnięcie krwi,
przepuszczalność przez błonę komórkową oraz aktywację wielu enzymów. Powszechnie przyję-
te jest, że głównym źródłem wapnia dla organizmu człowieka jest mleko i nabiał zarówno ze
względu na dużą zawartość, jak i przyswajalność tego pierwiastka z produktów mleczarskich.
Zmiany w technologii przetwórstwa żywności są zagrożeniem dla wysokiej biodostępności skład-
ników odżywczych z diety.
W pracy podsumowano wyniki wieloletnich badań autorów nad biodostępnością wapnia z
produktów mleczarskich oraz uwalnianiem tego pierwiastka z chleba oraz posiłków śniadanio-
wych składających się z produktów mlecznych z dodatkiem zbożowych. Potwierdzono powszech-
ną opinię o mleku i nabiale jako najcenniejszych źródłach łatwo przyswajalnego wapnia. Jednak
w badaniach in vitro wykazano, że dodatek produktów zbożowych do mleka lub jogurtu znacz-
nie zmniejsza ilość wapnia uwalnianego w procesie trawienia, co może wywoływać lub nasilać
objawy niedoboru wapnia u człowieka.
mleko, nabiał, chleb, produkty zbożowe, niedobór wapnia.
Calcium is the most abundant macroelement in a human body (Pecken-
Paugh 2011, Miciński et al 2012, 2013b). Out of the average 1200 g of the el-
ement detected in an adult organism, 99% is stored in bones and teeth, and
the remaining 1% can be found in body fluids and soft tissues. Calcium has
a very broad range of physiological functions, of which the most important
- calcium is the basic structural element (as calcium phosphate-hy-
droxyapatite) of bones and teeth, necessary for their proper develop-
- it is responsible for the regulation of the nervous and muscular sys-
tem (e.g. it participates in the systolic and diastolic contractions of the
- calcium is cne of the main factors in a cascade of blood coagulation
process (transformation of prothrombin into thrombin);
- it acts as activator of some biocatalyzers (enzymes), e.g. lipase, ATP-ase.
- it diminishes the passage of serum through capillaries and is used to
reduce amounts of allergic exudates;
- calcium mediates secretion of insulin and calcitonin from the cells.
Symptoms of calcium deficiency in humans may be discussed on two
levels: deficiency resulting from an insufficient dietary supply or as hypoc-
alcaemia manifested by decreased Ca concentration in blood, most often de-
termined by the occurrence of hormonal disorders (e.g. parathormone – PTH
deficiency), insufficient supply, synthesis and/or absorption disorders of vita-
min D, resulting in a lower level of 1,25-(OH)2D3 (1,25dihydroxycholecalcyph-
erol). Calcium deficiency may result in the growth disorders in children and
improper bones calcification (rachitis), leading to deformations and reduced
mechanical resistance of bones. Elderly people, especially women after men-
opause, are exposed to bone softening (osteomalacia) and loss of bone mass,
which is a risk factor in osteoporosis, which may lead to pathological bone
fractures. Most prevalent are lumbar spine fractures, femur neck fractures,
wrist and forearm fractures.
Calcium homeostasis means an organism is able of maintain a constant
concentration of Ca in blood serum within the range of 2.3-2.7 mmol dm-3,
accepted as physiological norm. Calcium homeostasis is achieved by increas-
ing or decreasing the intestinal Ca absorption, its excretion with urine and
faeces and utilization of reserves in bone tissue. The latter coincides with
an initiation of hormonal regulatory mechanisms. A decrease in the blood
calcium concentration is accompanied by excretion of parathormon from
parathyroids; the hormone is responsible for conversion of vitamin D to its
active, di-hydroxylic form. As a result, the rate of calcium absorption from
the gastrointestinal tract increases. PTH and 1,25-(OH)2D3 simultaneously,
influence the Ca release from bones and inhibit the excretion of this element
in urine through the process of Ca re-absorption in kidneys. Calcitonin,
secreted by the thyroid gland, is a PTH antagonist, which means that it
acts opposite to PTH and decreases calcium concentrations in blood serum
(Miciński et al 2013a).
Minerals, including calcium, occur in food as inorganic and organic
compounds, often in water solution, which results in the varied Ca release
and absorption. Bioavailability (biological availability, bioassimilation) is
understood as a degree to which a nutrient is converted by the human orga-
nism into the form ready to be absorbed and utilized in metabolic processes
and/or for storage (Fairweather-tait 1995, Jackson 1996). Bioavailability of
nutrients is determined through basic measurements: apparent absorption
(A) defined as a difference between amounts ingested and excreted with
faeces, equal absorbed amount, and apparent retention (R), understood
as a difference between amounts absorbed and excreted with urine, equal
amount of nutrient retained within the organism i.e. utilized in metabolism
or stored. The bioavailability parameters is expressed in mg(µg) day-1 or in
percentage units in comparison to ingested amount. The so-called indices of
specific bioavailability are used for some bioelements (iron, selenium, zinc);
a specific bioavailability index defines the portion of a mineral bound into
biologically active compounds (e.g. activity of glutathione peroxidase for se-
lenium or level of ferritin in blood serum as an indicator of iron store in the
Many factors influence bioavailability of minerals in human organisms.
Generally, they can be divided into two groups: exogenous, directly connec-
ted with food, and endogenous, related to the organism. Among the food-re-
lated factors, the most important ones are: properties of a given mineral,
type and amount of compounds containing the mineral, oxidation status,
solubility, presence of antagonistic (competitive) ions, presence of substances
facilitating or hindering absorption of the mineral. Endogenous factors are:
genetic conditions, age, sex, physiological status (e.g. pregnancy, breast-fe-
eding), mineral supply status (stock), emotional state, illnesses.
CALCIUM BIOAVAILABILITY FROM MILk AND DAIRY
Milk and dairy products are an excellent source of calcium (Ca) for a
number of reasons. This statement refers to total calcium content in this
group of food products, ratio of calcium to protein and of calcium to pho-
sphorous (Table 1). The highest Ca content was found in ripening cheeses
(from about 400 to 1400 mg 100 g-1 of the product). A lower calcium content
was determined in milk fermented beverages (103-170 mg 100 g-1) and fresh
cheeses (90-100 mg Ca 100 g-1). However, milk fermented beverages were
characterized by a nutritionally good ratio of calcium to phosphorous (1.4:1),
but the Ca : P ratio in fresh cheeses was low (0.4), similarly to the calcium
to protein ratio, which equalled ca 5 mg Ca in 1 g of protein.
It is worth emphasizing that consumption of 1 dm3 of milk supplies
100% of daily calcium requirement of adult person (1000 - 1200 mg per
day). kłobukowski at al. (1997ab) found a high calcium content, comparable
to that in ripening cheeses, in Ricotta-type non-ripening cheeses, where
it ranged from 750 mg in 100 g (cheese No 1) and 600 mg 100 g-1 (cheese
No 2). The study revealed a highly favourable Ca : P ratio of 1.85 and 1.55,
respectively. A higher ratio was found only for Parmesan (1.70) and Emen-
taler cheese (2.01) – Table 1.
The calcium to protein proportion (mg Ca in 1 g protein) in a diet ap-
peared to be a very helpful index, ready to be used for determination of the
influence of dietary protein content on calcium homeostasis and health sta-
tus of bones. It has been proved that high protein consumption negatively
influences calcium balance in an organism and increased calcium excretion
with urine when the Ca : protein index is below 20 (cashaM 2002, Zitter-
mann 2002). The value of this index is much higher for milk, milk fermented
beverages and ripening cheeses (Table 1), which proves that these dairy
products are an excellent Ca source. Low values of this index calculated for
fresh cheeses were due to the technological process which involved elimina-
tion of whey containing even 80% of the milk calcium. Milk souring causes
a release of insoluble calcium phosphate from casein micelles, which then
dissolves in milk acid, transforms into an ionic form and is transferred into
whey after separation of milk curd.
Bioavailability, determined in balance studies, is a very important crite-
rion measuring how a human organism utilizes minerals, including calcium.
The highest values (expressed as percentages) of calcium apparent ab-
sorption and retention were noted for fresh cheeses produced by both the
classical method (acidic coagulation) and with addition of the probiotic Lacto-
bacillus plantarum, prebiotics: 2.5% addition of inuline or maltodextrin
and synbiotics: Lactobacillus plantarum plus 2.5% addition of inuline or
Content of calcium, protein, phosphorus and ratios between these nutrients
in milk and dairy products
(g, mg 100 g-1 of product) Ratio**
Ca : protein
Ca : P
Milk 2% of fat 3.4 120 86 35.3 1.40
Natural yoghurt 2% of fat 4.3 170 122 39.5 1.39
Kefir 2% of fat 3.4 103 74 30.3 1.39
Buttermilk 2% of fat 3.4 110 80 32.4 1.38
Whey 0.8 68 40 85.0 1.70
Full-fat Brie cheese 19.8 600 380 30.3 1.58
Full-fat Camembert cheese 21.4 386 310 18.0 1.25
Full-fat Cheddar cheese 27.1 703 487 25.9 1.44
Full-fat Ementaler cheese 28.8 835 416 29.0 2.01
Parmesan cheese 41.5 1380 810 33.2 1.70
Gouda cheese 27.9 807 516 28.9 1.56
Tylżycki cheese 26.1 815 510 31.2 1.60
Acid-type fresh cheese 19.8 96 240 4.85 0.40
Fromage cheese 10.2 55 123 5.4 0.45
Homogenized cream cheese 17.7 88 216 5.0 0.41
Edamski processed cheese 13.5 367 578 27.2 0.63
** protein, calcium and phosphorous content after the Polish tables of food content (kunachowicz
et al. 2008);
** authors’ calculations
maltodextrin. In almost all cases these measures of bioavailability were
much above 80% (Table 2). Lower results (by about 20 percent units) of
A and R parameters were obtained for diets containing ripening cheeses
(Jeziorański and Tylżycki) produced by immersion in traditional (20% NaCl
solution) and in modified brine (1 : 1 20% NaCl and KCl solution).
The lowest (about 50-60%) values of A and R (Table 2) were found for
Ricotta-type non-ripening cheeses produced by the method of thermal-acidic
coagulation of milk protein and for whey and permeate, byproducts of fresh
cheese production. It should be stressed that all the values of apparent
absorption and retention presented in table 2 are very high, several-fold in
excess of the bioavailability of Ca from cereal products (usually <10%).
The highest calcium availability in milk and dairy products among all
foodstuffs results from both nutritional and non-nutritional factors. The nu-
tritional factors are connected with the age (higher intestinal absorption by
young people), sex, physiological state (pregnancy, breast-feeding), state of
organism, saturation with vitamin D, illnesses or medicine taking (e.g. di-
uretics). It has been proven that young, growing rats absorbed almost three
times more calcium from the intestine than mature ones (McErloy et al.
1991). Higher resorption of calcium from bones was observed in women in
early menopause due to a lower estrogen level (nordin et al. 2004). Deficien-
Values of calcium apparent absorption (A) and retention (R) coefficients
from diet containing dairy products
1. Jeziorański (NaCl)
2. Jeziorański (NaCl+KCl)
3. Tylżycki (NaCl)
4. Tylżycki (NaCl+KCl)
Ricotta-type unripened cheeses:
1. Ricotta-type cheese 1
2. Ricotta-type cheese 2
1. Acid-type fresh cheese
2. Acid-type fresh cheese with L. plantarum
3. Acid-type fresh cheese with L. plantarum+inulin
4. Acid-type fresh cheese with L. plantarum+maltodextrin
5. Acid-type fresh cheese with inulin
6. Acid-type fresh cheese with maltodextrin
Permeate 51.8 49.5
Whey 54.4 52.2
* Bioavailability indexes have been calculated utilizing published (kłobukowski et al. 1997a and b,
2004, 2009, ModzElEwska-kapituła et al. 2008) and unpublished data of kłobukowski et.al.
cy of vitamin D decreases calcium intestinal absorption and utilization in an
organism (lips 2012). An active person, spending much time outdoors and
consuming fish, eggs and fatty dairy products does not suffer from vitamin
D deficiency, because it is synthesized in the skin from 7-dehydrocholesterol
under the influence of ultra violet rays (Zittermann et al. 1998). The vita-
min D synthesis rate decreases in elderly people when the solar exposure is
The following nutritional factors affect the bioavailability of calcium
from diets containing dairy products: amount and form of calcium, content
of lactose, proteins and phosphorous. The beneficial effect of lactose on
calcium absorption has been demonstrated in studies on the influence of
calcium and milk components on values of Ca apparent absorption indices
(buchowski, miller 1991). Lactose facilitates Ca absorption by stimulating,
independently from vitamin D, the diffusive system from the calcium intes-
tinal transfer. In all probability, lactose cooperates with intestinal villi and
enhances the Ca penetrability. The resence of calcium bound to casein in the
form of colloidal di- and tripeptide (phosphopeptides) may be a barrier to its
absorption (tsuchita et al. 1995). soral-ŚMiEtana et al. (2013) proved that
acid whey concentrates produced by various membrane processes did not
differ in the calcium bioavailability coefficients determined for rat diets. On
the other hand, pantako et al. (1994) proved that whey proteins, especially
a-lactoalbumin, may favour calcium absorption by rats.
A diet typical for Poland and many other countries, also highly devel-
oped one does not supply enough calcium for a human organism. It has been
proved (kErstEttErEt al. 2003) that an insufficient supply of standard value
protein and dietary calcium (e.g. a diet without or with too little of milk and
dairy products) increases the risk of bone injuries and prolongs the rehabil-
itation time after fractures. Owing to a high content of easily bioavailable
calcium, milk and its products can provide staple human food with Ca,
preventing its deficiency which leads to osteoporosis. As a consequence, the
diet with sufficient supply of milk products promote proper development and
health of humans.
CALCIUM AVAILABILITY FROM BREAD AND CEREAL
ADDED TO DAIRY MEALS
Bioavailability can be assessed accurately on a model of living orga-
nisms. Experiments with humans are very rare because of restrictions and
high costs; most common are experiments on rats. It is easier and less
time-consuming to determine the rate of nutrient release during in vitro
simulated digestion in the human gastrointestinal tract. This way it is pos-
sible to assess the amount of a nutrient ready to be utilized by the organism
(skibniEwska et al. 2002).
Bread and other cereal products are very popular foodstuffs throughout
the world, and are often the main source of energy, nutrients, minerals and
vitamins. The content of minerals in bread strongly depends on type of flour:
white flour, made mostly from kernel endosperm, contains much less of mi-
nerals than darker types of flour, containing some share of germ and bran.
Brown bread, especially whole-meal one, contains also phytates and dietary
fiber, compounds with anti-nutritional activity which decrease the bioavaila-
bility mineral by forming non-digestible, insoluble compounds with minerals
(GrasEs et al. 2001). Figure 1 presents the content of calcium and its amo-
unt released in the in vitro digestion process from bread baked from white
flour and with addition of various amounts of bran. Brown bread contained
not much more of the element, but the amount of released Ca dramatically
decreased with an increase in the bran proportion.
While searching for a method to raise amounts of minerals released
during digestion, bread baked from dough subjected to prolonged fermen-
tation (no less than 5 h, up to 10-11 h) was examined (skibniEwska et al.
2004). The aim was to check whether prolonged fermentation would allow
phytase to decompose phytic acid and release minerals. The expected result,
Fig. 1. Content of calcium and its amount released in the process of digestion
from bread with various additions of bran
Content of calcium in Polish bread (kunachowicz et al. 2008)
Bread Ca content (mg 100 g-1)
Rye bread (white) 14
Rye bread (brown) 25
Rye bread (Litewski) 61
Rye bread with soya and sunflower seeds 41
Popular bread (wheat and rye) 17
Graham bread 22
Wheat bread 16
i.e. a high percentage of minerals released from bread produced with this
method, was not obtained (63% of Ca was released on average). Another
experiment was run to study the influence of various methods of dough
preparation on mineral release (skibniEwska et al. 2005). Calcium was
released in just 5% from whole-meal spelt flour, 16% from whole-meal
common wheat flour and 66% (in a range of 34 to 87%) from mixed wheat
and rye bread, the most popular type of wheat on the Polish market. skib-
niEwska et al (2002) studied the influence of baking additives (natural
leaven, starter cultures and complex dough improver) on the in vitro dige-
stibility of minerals from white and whole-meal bread. As in the previous
experiments, the share of bran had the strongest influence on the release
of minerals: 74-86% of Ca was released from white bread and 16-51% from
the whole-meal one (51% was released from bread baked with addition of
natural leaven only).
In order to elucidate the influence of intestinal bacteria on mineral re-
lease, an experiment with the addition of Bifidobacterium bifidum was per-
formed (nalePa et al. 2012). Amounts of minerals released during enzymatic
digestion varied depending on the element, bran content and presence of
bacteria. As in all the previous experiments, a higher bran content lowered
the release of elements. Addition of bacteria influenced mineral release in a
different manner, e.g. the release of calcium (together with iron) was redu-
ced in the presence of Bifidobacterium bifidum (bacteria probably utilized
some of the elements for their growth).
Bread, even whole-meal one, contains little calcium (Table 3) in compa-
rison to the daily recommended dose (about 1 000 mg depending on age, sex
and physiological status), which means that the low availability of calcium
from bread is not a problem. But the decreasing Ca availability from food
caused by anti-nutrients from cereal products is a cause of worries.
Fig. 2. Content of calcium and its amount released in the process of digestion
from breakfast meals
In an experiment carried out in 2005, breakfast meals composed of milk
alone or with addition of cereals were digested in vitro (Figure 2). Almost
all the Ca included in milk was released into the solution. Corn and barley
flakes, as well as semolina, decreased only slightly the amount of Ca deter-
mined in the solution after digestion, although brown bread and rolls captu-
red about half of the element. Rolls were a surprise because they were made
from white flour. They were bought in a shop, hence their exact composition
In 2012, a paper on calcium availability from pure yoghurt and yoghurt
with an addition of cereal products was published (skibniEwska et al. 2010).
The yoghurts contained less calcium than reported in the Polish tables of
food composition (kunachowicz et al. 2008). Moreover, a very small portion
of the element was released: on average 45%, in a range from 28.5 to 77.9%.
At this moment, the results raise a question for discussion: in view of the
of general deficiency of calcium in the human diet, should dieticians recom-
mend mixing milk and cereals?
The above information (high values of apparent absorption and reten-
tion indexes) explicitly confirms the opinion that milk and dairy products
are the most valuable source of highly available calcium among all the food
products is the human diet.
At present, the human bone system is exposed to three problems: a dec-
line in the milk and dairy product consumption causing a dramatic decline
in calcium intake, innovations in dairy processing technologies leading to a
decreased calcium concentration in food, and dialecticians recommending to
increase consumption of whole-meal cereals, which leads to a worse calcium
bioavailability. Because osteoporosis has grown to be a global health pro-
blem, monitoring calcium bioavailability from food is necessary.
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