Download DAILY DIETARY INTAKE OF SELENIUM IN EASTERN CROATIA

DAILY DIETARY INTAKE OF SELENIUM IN EASTERN CROATIA
Tomislav Klapec1, Milena L. Mandić1, Jerica Grgić2, Ljiljana Primorac1,
Marija Ikić3, Tomislav Lovrić4, Zdravko Grgić2 and Zoran Herceg4
1
Faculty of Food Technology, Osijek, Croatia, 2Institute of Public Health,
Osijek, Croatia,
3
Podravka Inc., Koprivnica and
4
Faculty of Food
Technology and Biotechnology, Zagreb, Croatia
ABSTRACT
Data on the selenium content in the soil and human milk in eastern Croatia
indicate a possibility of inadequate Se status of people in the area. In order
to determine the daily dietary intake of Se in eastern Croatia, a 7-day
duplicate diet study was performed. There were 41 participants (18-53 yr
old; 14 males and 27 females). Selenium measurement was carried out in a
HG-AAS. The average daily Se intake in the area is 27.3 µg. A significant
difference (p<0.05) exists between male (32.2 µg) and female (24.8 µg)
participants. Percentage of optimum Se intake shows an inverse association
with the increasing age of subjects (r=-0.43; p<0.01). Relationship between
Se intake and intakes of different foods (cereal products, milk and dairy
products, meat, fish etc.) was examined too. The suboptimal Se intake
(RDA is 55 µg/day for women and 70 µg/day for men) is a reflection of low
Se levels in the environment, so supplementation of fertilizers with Se
should be considered.
KEY WORDS
selenium, eastern Croatia, duplicate diet study, dietary intake of Se
INTRODUCTION
Selenium was recognized as an essential element for animals in the 1950s
[1], and final proof to its essentiality for humans was the discovery of its
preventive potency against Keshan disease [2; 3]. Further research has
revealed selenium's function in the antioxidant mechanisms of the body
through Se-dependent GPX [4], as well as in the metabolism of thyroid
hormones [5] and maintenance of male fertility through appropriate
selenoproteins [6]. Selenium also protects against toxicity of certain heavy
metals [7]. Many mechanisms of this element's positive role in the function
of the immune system [8], as well as the role in the etiology of cancer [9]
and CVD [10] have been proposed so far, with no definitive, generally
accepted explanations for its function at the moment. Optimum daily
dietary intake was determined as 40 µg/day per 60 kg of body mass [11].
The 1989 RDA for Se was established on the basis of this and is 55 µg/d for
women and 70 µg/d for men [12]. Scientific Committee for Food of the
European Union set the PRI (Population Reference Intake) for Se at 55
µg/d for both sexes [13]. Low selenium content found in agricultural soils
and occurrence of deficiency disorders in animals in eastern Croatia
pointed to a possibility of deficient intakes by humans as well [14; 15; 16].
Determined levels of Se in human milk correspond to suboptimal intakes by
mothers in the area [17]. Suboptimal selenium status has also been found in
neighboring countries [18; 19]. All this, together with the current awareness
of selenium's metabolic roles, urged us to determine the daily dietary intake
of Se in eastern Croatia.
SUBJECTS AND METHODS
Subjects
The daily dietary intake was determined using a duplicate diet study with
41 adult participants (18-53 yr old; 27 women and 14 men) over a 7-day
period (February 1996). The subjects were also asked to keep record of all
food and drink and their quantities using the forms we provided.
Samples
The collected duplicates of all food (only edible parts) and drink consumed
in a day were weighed and stored at -20°C. The frozen one-day samples by
a subject, collected over a week, were then joined together and thawed. The
thawed 7-day samples were homogenized in a blender (Stulz). The
homogenized samples were lyophilized (Bianchi).
Selenium determination
The lyophilized samples were prepared for Se determination by wet
digestion in a microwave oven (MDS-2000, CEM) with conc. HNO3 and
30% H2O2 using the method described in the operation manual. Prior to the
measurement of Se concentration, HCl was added to the samples to reach 6
N and the samples were heated in a water bath at 60°C for 30 min to reduce
Se (VI) to Se (IV) [20]. After the reduction, total Se as selenite was
transformed into H2Se in a hydride generator (MHS, Type 10) using 3%
NaBH4 in 1% NaOH. Atomic absorption was measured in a Perkin-Elmer
2380 spectrometer. Calibration was performed by the method of standard
additions. The technique was validated using milk powder (IAEA, AQCS)
and bovine liver (NIST, 1577b) as reference materials. For milk powder, the
found concentration was 35.2±6.1 µg/kg compared to the certified value of
33.9±7.2 µg/kg. For bovine liver we found 0.72±0.07 µg/g compared to
0.73±0.06 µg/g. Recovery of Se from a selenomethionine standard ranged
from 98-99%.
Data analysis
Interpretation of results using computer program Statistica 4.1 (Statsoft
Inc.) included data on individual daily intakes of Se and mean daily intakes
of foods from the following eight groups: meat and meat products, milk and
dairy products, fish, fruits, vegetables, eggs, cereals and cereal products
and water and beverages. Statistical significance of differences was tested
by ANOVA and relationship between variables was determined by
correlation analysis.
RESULTS AND DISCUSSION
Table 1 gives mean daily dietary intakes, as well as mean percentages of
optimum intakes for men and women respectively. Overall means for daily
intake and percentage of optimum intake are also given. The mean daily
dietary intake of 27.3 µg (which is 61.5% of the mean optimum daily intake
for the investigated sample of population, according to the RDA) is similar
to the ones found in New Zealand [21; 22], Finland [23], some parts of Italy
[24] and Belgium [25]. A number of ecological and epidemiological studies
have found a relationship between Se status and incidence of cancer [26; 27;
28; 29; 30; 31; 32] and CVD [33; 34; 35]. One would expect a higher
incidence of these diseases in New Zealand on account of the low dietary
intakes of Se by its inhabitants (25-30 µg/d). However, the incidences are
similar to the ones in the US and other countries with three to five times
higher Se intakes [22]. A few prospective studies carried out in Finland
associated low serum Se levels with a higher incidence of cardiovascular
diseases and cancer [28; 33]. This has led to government-approved
supplementation of fertilizers, which caused an increase of dietary intake of
Se from 20-30 µg/d to 80-100 µg/d [23]. However, survey of statistical data
on CVD and cancer mortality rates in Finland 15 yr before and 5 yr after
the supplementation showed no change whatsoever in the mortality rates.
The suboptimal daily intake we found in this study cannot be regarded
deficient unless there is proof to an impaired function of Se-containing
enzymes and/or to an occurrence of disorders that are curable by Se like
Keshan disease or muscular weakness in patients on long-term TPN [36].
Further research on Se status in the area should focus on establishing the
function of Se-containing enzymes since no cases of either Keshan disease
or TPN-syndrome have been reported in eastern Croatia. On the basis of
the average daily dietary intakes of Se in Keshan disease-affected and
nearby non-affected areas, it has been proposed that a minimum daily
requirement of Se is 16 g for adults [37].
We found a statistically significant difference between the first (18-30 yr)
and the third age group (41-53 yr) in the percentages of optimum Se intake
(p<0.05), which were 69%, 58% and 51% for 1., 2. and 3. age group
respectively. Correlating percentage of optimum Se intake to age, an
inverse association between the variables was noted (r=-0.43; p=0.005) (Fig.
1). This supports the theory of decreasing dietary Se intake as the main
reason for the observed decline in serum Se concentration with increasing
age, although most authors found significant drop in Se serum levels only
after 60, 70 or even 80 yr of age [38].
The difference in daily intakes between sexes was also significant: men: 32.2
µg/day; women 24.8 µg/day (p<0.05) (Table 1). Higher intakes by men are
implied considering their average greater body mass and consequently a
greater food intake. However, the lower food intake of women in our study
was not evenly distributed among food groups. A significantly lower daily
intake compared to men was only found for meat, cereals and cereal
products, and vegetables. The women's intake of meat, which is a good
source of Se, was almost half the amount by men (84 vs. 150 g; p<0.001).
The lower Se intakes reported in the literature still ensure an adequate Se
status of women. A great majority of workers reported equal serum Se
levels between sexes [38]. Nevertheless, comparing the mean percentages of
optimum intakes in our group, men showed a slightly better Se provision
than women (63% vs. 60% of the optimum values) (Table 1).
The daily intake of Se showed a good correlation with the mean daily intake
of fish in this group of subjects (r=0.63; p<0.001) (Fig. 2). Since the mean
daily intake of fish over the 7-day period was rather low (16 g), the good
correlation could be explained by a high concentration of Se in fish, which
is amongst foods richest in this microelement [39; 40]. Fish contributes
significantly to daily intake even if the intake is not very high [25; 41].
The correlation of daily intake of Se with the mean daily intake of meat is
much weaker (r=0.34; p<0.05) (Fig. 3). Possible reasons for this are a
relatively low mean daily intake of meat (106 g), and the low content of Se
in local soil as was determined previously [14]. This correlation surprises
because most authors found this group of foods to be one of best if not the
best source of Se [23; 42; 43].
Eggs represent a good source of Se according to the found coefficient of
correlation (r=0.46; p<0.005) (Fig. 4). Eggs efficiently concentrate Se and
make a significant fraction of dietary Se intake especially if this
microelement is deficient in soil [23; 43; 44].
Milk and dairy products' content of Se is heavily dependent on its
concentration in plant foods of cows. Comparing levels of Se in cow milk
from different countries, Alaejos and Romero found significant variations
dependent on geographical origin, pointing to the relevance of the local soil
content of Se [45]. The low Se content in the soil in eastern Croatia has
obviously effected low concentration of the element in milk and products,
which are weakly correlated to the daily intake of Se (r=0.26; p=0.11) (Fig.
5) in spite of a relatively high mean daily intake (244 g).
Animal products, in general, have higher levels of Se than plant foods,
making up the greatest share of the overall Se intake by humans. On the
basis of their ability to bioconcentrate this element, animal products have a
special significance for human nutrition in areas with low Se content in soil
[22; 23]. Low intake of foods of animal origin is one of the main etiologic
factors contributing to the occurrence of Keshan disease in Se-deficient
areas of China [46]. Although the recorded mean daily intake of animal
products in this study is not as low as by inhabitants of Keshan disease-
affected regions of China, it is lower than the values found in Belgium (470
vs. 380 g/d) [43]. The lower intakes of these foods undoubtedly affected the
intake of Se, but one must also take into consideration the found low level of
Se in the soil and forage, as well as the reports of Se deficiency disorders in
domestic animals in the area [14; 15; 16]. These findings prompted the
regulation of Se content in feeds in Croatia, defining only minimum values
for different categories of poultry feeds (0.15 mg/kg), suckling cattle and
piglets feeds (0.1 mg/kg) and other categories of pig feeds (0.3 mg/kg).
Available commercial feeds for laying hens, bullcalves and milking cows
contain 0.1 mg Se/kg. Although the regulated levels are only aimed at
preventing deficiency diseases, they can significantly raise the Se content in
animal products. It has been determined that eggs and internal organs of
animals like liver and kidney, have a much better capacity for accumulating
Se than muscle tissue [47]. The predominant form of Se in feeds is selenite
which can be oxidized by microorganisms in the gut of ruminants,
producing Se forms unavailable for absorption [48]. It has been found that
even levels of 0.5 mg/kg in cattle feeds do not raise significantly the Se
content in milk and meat. However, cattle feeds have a minor role in
supplying the animals with Se, because their diet mostly consists of grass,
hay and forages whose content of Se depends on its level in the soil.
Therefore, Se supplementation to fertilizers is substantially more efficient
in raising Se content in animal diets and consequently in animal products
for human consumption, as has been shown in Finland [23].
The daily intake of cereal products is well correlated to the daily intake of Se
(r=0.51; p= 0.001) (Fig. 6). Selenium concentration in cereals reflects Se content
in the soil. There is a wide spectrum of agricultural soil types in eastern Croatia
with wheat and corn as the main types of crops grown [49]. However, the most
frequent soil types are luvisols, eutric and dystric cambisols and chernozem.
Samples of typical and pseudogleyic luvisols from the western part of the region
have been analyzed for selenium and low levels ranging from 0.02 to 0.048 ppm
have been found [14]. The wheat samples grown in the area contained from less
than 0.01 to 0.018 ppm, while all the samples of corn had less than 0.01 ppm Se.
In comparison, the concentrations of Se in corn and rice in Keshan disease
affected areas in China were 0.007 and 0.005 ppm, respectively [50]. The
selenium level in cereals of 0.01 ppm and lower has also been found in Finland
prior to supplementation of fertilizers with Se [23]. In most other European
countries the selenium concentration is 0.02 to 0.05 ppm, while North American
cereals usually have 0.2 to 0.5 ppm Se. Considering the fact that most cereal and
bakery products in eastern Croatia come from local production, and assuming
that the found low concentration of Se exists in the agricultural soils throughout
the region, it becomes apparent that the found good association of intake of
cereals with the daily intake of Se is due to the high daily intake of foodstuffs
from this group (270 g). Cereal products are not rich in Se but it is their
irreplaceability on the menu that explains their substantial contributions to the
Se intake [7]. In most countries meat, fish and cereals together make 60-90% of
the daily dietary intake of Se [42; 43; 46].
Vegetables contribute weakly to the daily intake of Se according to the
coefficient of correlation (r=0.27; p=0.09) (Fig. 7). Most investigators found
the fraction of Se from vegetables in the daily intake of the element to be
below 10% [7; 43]. Some legumes are fairly rich in Se, making 28-32% of
the daily intake in northern Mexico [51] and rural Burundi [41], although
authors ascribe the good contribution primarily to a large consumption of
the foodstuff.
Fruits are poor sources of Se contributing less than 1% of the daily intake
of Se [41; 43]. A confirmation for this is the correlation coefficient for the
relationship between the intake of fruits and the mean daily intake of Se in
the investigated group (r=-0.06; p=0.72).
The intake of Se through consumption of water and beverages (r=0.10;
p=0.54) is dispensable. Similarly, the found contribution in Belgium was
1.5% [43].
CONCLUSIONS
Daily dietary intake of Se in eastern Croatia is inadequate. The mean intake
of 27.3 µg/day is 61.5% of the recommended optimal values.
A significant difference in intake was found between male and female
subjects, and an inverse relationship between age and Se intake.
The daily intake of Se is best correlated with the intake of fish, cereals, eggs
and meat. Milk and dairy products as well as vegetables contributed little,
while the contribution of fruits and water and beverages was dispensable in
the investigated group of subjects.
Although the found daily dietary intake does not meet the optimal
requirements there is yet no evidence that it affects health of the inhabitants
of eastern Croatia in an unfavourable fashion. Further research on the
relationship between such an intake and some biochemical parameters of Se
status, as well as on the occurrence of some Se status related diseases,
should be instrumental in establishing possible health implications of the
suboptimal intake of this essential microelement.
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Table 1 Mean daily dietary intakes of Se and mean % of
optimum intakes in the investigated group
MEAN DAILY
WOMEN
MEN
OVERALL
24.8 ± 7.3
32.2 ± 13.2
27.3 ± 10.2
60
63
61.5
INTAKE OF Se
(µg/day) ± SD
MEAN % OF
OPTIMUM INTAKE*
*
Optimum intake according to the RDA (1989) i.e. 40 µg/day per 60 kg of body mass.
The mean body masses were: overall: 66.6 kg; women: 62.0 kg; men: 76.7 kg.