Download THE SELENIUM CONTENT OF MEATS, SEAFOODS, AND

THE SELENIUM CONTENT OF MEATS, SEAFOODS,
AND VEGETABLES
by
XIAOYUN ZHANG, B.Med., M.Phm.
A THESIS
IN
FOOD AND NUTRITION
Submitted to the Graduate Faculty
of Texas Tech University in
Partial Fulfillment of
the Requirements for
the Degree of
MASTER OF SCIENCE
Approved
Accepted
Nay, 1993
r- go5
ACKNOWLEDGEMENTS
/3
fJcl'
4-93 ~
~-
J.+c-
,orq3
f/~ .03 I
am deeply indebted to my advisor, Dr. Julian E. Spallholz, for his patient
CJr.n, ;)- ~0' and helpful guidance during the research and preparation of this thesis. I thank
the other members of my committee, Dr.
Lee M. Boylan and Dr.
Leslie D.
Thompson for their helpful criticism. I would also like to thank the many faculty
and staff members at the Department of Education Nutrition Restaurant Hotel
Management who have helped me in these years. Finally, I would like to thank
my husband and my family for their support during this time .
..
11
1
l ) j 1jf1>r
TABLE OF CONTENTS
ACKNOWLEDGEMENTS
11
LIST OF TABLES
IV
LIST OF FIGURES
v
CHAPTER
I. INTRODUCTION
1
II. LITERATURE REVIEW
2.1 Biochemical functions of Se
2.2 Deficiency diseases of Se
2.3 Forms of Se
2.4 Bioavailability of Se in foods
2.5 RDA of Se
2.6 Distribution of Se
2. 7 Factors that affect the Se content of foods
3
3
5
I
8
9
9
10
III.
METHODS AND MATERIALS
3.1 Selection of food samples
3.2 Preparation of food samples
3.3 Analytical methods
18
18
18
19
IV.
RESULTS
4.1 Food Se concentration
4.2 Comparison of food Se content with other areas
4.3 Effect of drying methods on Se content
4.4 Effect of cooking methods on Se content
4.5 Contribution of 3 oz. of cooked meats to RDA for Se
22
22
22
23
24
24
DISCUSSION
29
V.
VI. CONCLUSIONS
33
REFERENCES
35
Ill
LIST OF TABLES
2.1 Bioavailability of selenium in foods
13
2.2 Summary of selenium content of foods in some areas
15
3.1 Contributions of selenium in the U.S.A. diet.
18
3.2 Procedure for selenium determination
19
4.1 Selenium content of seafood
21
4.2 Selenium content of ground beef and pork
21
4.3 Selenium content of poultry
22
4.4 Selenium content of vegetables
22
4.5 Selenium content of milk
22
4.6 Comparison of selenium content with other areas
24
4.7 Selenium content of raw, cooked and dried meats
25
4.8 Contribution of cooked meats to RDA for selenium
25
lV
LIST OF FIGURES
2.1 Functions of glutathione peroxidase, superoxide dismutase and
a-tocopherol in a cell.
12
2.2 Outline of selenium metabolism.
13
2.3 The regional distribution of forages and grain containing different
levels of selenium in the U.S.A.
14
CHAPTER I
INTRODUCTION
Selenium (Se) is a very important trace nutrient in the human body. The
Se-dependent enzymes, glutathione peroxidase (GSHPx) and
phospholipidhydroperoxide glutathione peroxidase (PLGSHPx), are antioxidant
enzymes which protect cells from oxidative damage. The major biochemical
effect of selenium is to maintain GSHPx activity and PLGSHPx activity. A
third Se-containing enzyme is type 1 iodothyronine 5'-deiodinase which catalyzes
1-thyroxin (T 4 ) to 3,3',5-triiodothyronine (T 3 ).
A dietary selenium deficiency can cause heart disease which was first
recognized in some areas of the People's Republic of China where the selenium
content of diets is extremely low. After much research, the Food and Nutrition
Board of the U.S. National Academy of Sciences set its 1989 Recommended
Dietary Allowance (RDA) for selenium at 70 and 55 J.1g/day for adult men and
women, respectively.
Dietary selenium in foods varies from different countries and also from
different areas of the same country mainly due to the concentration and
biological availability of selenium in the soil. The interest in dietary selenium
intake has increased because of selenium's important role in health. So the need
for the assessment and improvement of selenium food composition data from
different areas has also increased. A few studies of the selenium conteut in foods
1
2
from Maryland and Ohio have been reported but no information is available on
the selenium content of foods in Lubbock, Texas. In this study, data is provided
on the selenium content of some commonly consumed foods.
CHAPTER II
LITERATCRE REVIE\h/
Selenium (Se ), which is a group VI element under sulfur in the periodic
table, was first discovered by Berzelius in 1817. Since then, selenium has become
known as one of the most toxic elements ( cf. Shamberger [33]). In 1957, Schwarz
and Foltz first reported that dietary selenium was important in metabolism
because it could prevent liver necrosis in vitamin £-deficient rats (Schwarz and
Foltz, [31]). Since then, some other animal diseases have been found to respond
to selenium [24]. These diseases are white muscle disease in cattle, swine, and
sheep; liver necrosis in swine and rats; and growth depression in cattle, rats. and
sheep. Now, more than 40 species of animals are known to have a biological
requirement for selenium.
2.1 Biochemical functions of Se
There are three Se-dependent enzymes, glutathione peroxidase (GSHPx),
phospholipidhydroperoxide glutathione peroxidase (PLGSHPx) and type 1
iodothyronine 5'-deiodinase. In 1973, John Rotruck, a graduate student at the
University of Wisconsin, discovered that the enzyme glutathione peroxidase
(GSHPx) contained the element selenium [28]. Glutathione peroxidase is present
in cells and in plasma and contains four atoms of selenium which is an integral
part of the active site of glutathione peroxidase. Glutathione peroxidase can
3
4
reduce hydrogen peroxide (H2 0 2 ) to water and organic hydroperoxides (ROOH)
to an alcohol (ROH), as shown in Equation (2.1).
(2.1)
Normal cellular reactions and the metabolism of toxic substances can
produce hydrogen peroxide, hydroperoxides, superoxide, various free radicals
including hydroxy radical and possibly singlet oxygen. These free radicals can
initiate lipid peroxidation and cause membrane damage. Antioxidants in cells
can prevent lipid peroxidation by breaking free-radical chain reactions or
preventing the cellular accumulation of the free radicals. One of the important
groups of antioxidants are the antioxidant enzymes synthesized by cells which
includes glutathione peroxidase, catalase and superoxide dismutase (SOD). In a
typical animal cell, glutathione peroxidase reacts with peroxides and superoxide
dismutase reacts with superoxide in the cytosol and 0'-tocopherol scavenges free
radicals in the membranes (Figure 2.1 ).
Since glutathione peroxidase is a Se-dependent enzyme, selenium thus can
serve as a biological antioxidant. The enzymic activity of glutathione peroxidase
correlates well with the concentration of selenium in blood. Dietary Se-deficiency
causes glutathione peroxidase activity to decline in tissues. According to
Hafeman et al. [10], after four weeks of a Se-deficient diet, rat liver glutathione
peroxidase activity falls to undetectable levels. This research indicates that one
5
of major functions of selenium is to maintain GSHPx activity that protects cells
from oxidative damage.
Another antioxidant Se-containing enzyme, phospholipidhydroperoxide
glutathione peroxidase (PLGSHPx) discovered by Fulvio Ursini at the University
of Padua in 1985 [45], protects membranes. PLGSHPx is present in
phospholipids and contains one atom of selenium. PLGSHPx is similar to
glutathione peroxidase in that selenium is also an integral part of the active site
of PLGSHPx. PLGSHPx can protect liposomes and biomembranes from
peroxidative damage.
In 1990, Dietrich Behne in Germany found another selenoenzyme. type 1
iodothyronine 5 '-deiodinase [4). This Se-containing enzyme catalyzes the
deiodination of the prohormone 1-thyroxine (T 4 ) to the biologically active
thyroid hormone 3,3',5-triiodothyronine (T 3 ). Research has shown that in
Se-deficient rats, the T 3 production is decreased in the lin'r.
2.2 Deficiency diseases of Se
The first Se-deficiency disease in humans, named Keshan disease, was
discovered in Keshan area of the People's Republic of China in 1979, an area
where the selenium content in the soil is very low [48). Keshan disease affects the
heart muscle. Children less than 15 years of age are most susceptible. The
Keshan Disease Research Group of the Chinese Academy of Medical Sciences
has treated 36,603 children with 1.0 mg of sodium selenite per week. Only 21
6
cases occurred during the four years of investigation. However, there were 107
cases in 9,642 children who were not treated with sodium selenite (14]. The
mechanism by which Keshan disease develops is not totally clear. According to
Yang et al. [48], Se-dependent GSHPx has a stabilizing effect on erythrocyte
membranes and selenium may protect and stabilize membranes as PLGSHPx
and lessen their susceptibility to pathogenic factors in the em·ironment. For
example, in 1970 Shamberger found significantly lower death rate of heart
disease in the high selenium areas.
Another disease linked to Se-deficiency in China is Kashin-Beck disease [48].
It is an endemic osteoarthritis that occurs in children or young people. Oral
selenium supplementation seemed to prevent the disease [20].
A number of studies suggest that poor selenium status may be associated
with an increased risk of developing cancer [18]. There are about fifty dietary
experiments showing selenium to have anticancer actiYity in animals [34]. Some
research has observed a reduction in skin tumors when mice were treated with 7,
12- dimethylbenzanthracene (D~IBA) [27,32,35]. Other research has show11 that
certain amounts of selenium added to the diet or drinking water reduced the
incidence of liver tumors, colon tumors and breast tumors in animals [34].
According to many experiments using animal models, it could be concluded
that selenium may act as an anticarcinogenic agent when giYen either in the diet
or in drinking water, but this protective action is not universal. A proper
7
amount of selenium (1-5 ppm Se) given in food or drinking water may or may
not protect the organism against the effects of some carcinogens [14].
We already know that selenium is an essential part of the enzyme glutathione
peroxidase which is an antioxidant and can keep the organism free of peroxide
compounds. The function of selenium is to maintain glutathione peroxidase
activity. Since free radicals, such as 02 or OH·, may initiate the process of
carcinogenesis, selenium can increase glutathione peroxidase and reduce the
concentrations of free radicals and inhibit the initiation of the carcinogenic
process. Cancer patients are known to have lower serum activity of glutathione
peroxidase than healthy persons [14].
2.3 Forms of Se
Selenium occurs naturally in foods in organic compounds which are
selenomethionine, Se-methyl-selenomethionine, selenocystine, and selenocysteine
[25,29]. Tlwse dietary forms of selenium are well absorbed by monogastrics.
Absorbed selenium is transported through the body primarily bound with
plasma proteins.
There are two major forms of selenium in animal tissues, selenocysteine and
selenomethionine. Selenocysteine is derived from animals [11] and
selenomethionine from plants which cannot be synthesized in the body [25].
Selenomethionine is absorbed and transported via the same mechanisms as
methionine [4 7], but the method of selenocysteine absorption is unclear. \rhen
8
dietary selenomethionine is absorbed by the body, it combines with a variety of
proteins in tissues, but after dietary selenocysteine is absorbed, it can directly
follow selenium metabolism in the body or combine with glutathione peroxidase
(Figure 2.2).
As dietary selenium intake decreases, the tissue form of selenomethionine is
returned to the methionine pool and then transferred to the selenocysteine form.
After about one year of a low dietary selenium intake, selenium levels in blood
will be very low. Therefore, dietary selenium is a very important and only source
of selenium in the body.
2.4 Bioavailability of Se in foods
The determination of bioavailability of selenium in foods is as important as
the selenium content determination because it is not certain whether all the
different compounds that occur in foods can be metabolized to biologically
active selenium, even if they are well absorbed. Since the major function of
selenium is to maintain the activity of glutathione peroxidase, the best way to
determine the bioavailability of selenium is to measure glutathione peroxidase
activity in selenium-depleted animals. The bioavailability of selenium in some
foods has been estimated in the rat model. In rats the increase in liver
glutathione peroxidase activity is measured after feeding the foods to rats
previously depleted of selenium {Table 2.1 ). According to same studies. the
9
availability of selenium in mushrooms was very low. Wheat, Brazil nuts, and
beef kidney contained the most highly available selenium.
2.5 RDA of Se
In 1980, because few data about selenium in human nutrition were available,
the Food and Nutrition Board of the U.S. used the results of animal experiments
and proposed a safe and adequate daily dietary intake of selenium of 50 - 200 J.Lg
for adults [24]. Since more research and evidence showed the important roles of
selenium in health, the Food and Nutrition Board in 1989 set a Recommended
Dietary Allowance (RDA) for selenium of 70 and 55 J.Lg/ day for adult men and
women, respectively. For pregnant and lactating women the RDA for selenium is
65 and 75 J.Lg/ day, respectively. For infants age 0 - 0.5 years, the RDA is 10
J.Lg/day and to age one it is 15 J.Lg/day.
2.6 Distribution of Se
Selenium is widely distributed in the earth and can be found in soil, rocks,
water, air, plants, and animal and human tissues [12,26,43,46]. According to
Combs [6] the selenium content in soils varies in different areas and the amount
of selenium in food plants reflects the soil selenium. Jenkins [15] indicated that
the regional distribution of forages and grain containing different levels of
selenium in the United States and Canada (Figure 2.3). In adequate selenium
regions, about 80% of all forage and grain contained > 0.10 ppm selenium. In
10
intermediate regions, about 50% of all forage and grain contained
> 0.10 ppm
selenium. Regions considered low selenium have about 80% of all forage and
grain containing
< 0.10 ppm selenium.
2. 7 Factors that affect the Se content of foods
A number of factors influence the selenium concentration in foods.
1. different geographic locations or the natural selenium content of soils [44].
2. the soil concentration of biologically available plant selenium [44].
3. the treatment of soil by topdressing [22].
4. supplementation of animal feed [22].
5. the processing of food products [9)
6. cooking methods [13].
7. the protein content of the food [16).
Of all these factors, the most important factor is the geographic region of
production. Plants have no known selenium requirement. If no selenium is
available for uptake, the plant still thrives. However, animals have a
physiological requirement for selenium. If the selenium concentration in
feedstuffs is very low, the selenium level in these animal tissues are also very low.
Certain selenium compounds are easily destroyed, so the cooking methods may
affect the selenium content in foods. According to Higgs et al. [13), broiling
11
chops and steaks caused little change in the selenium content of the meat and
baking had little effect on the selenium level of chicken or fish.
Many studies of the selenium content of foods have been published. Table 2.2
is data summarized from a number of studies in the United States (21,37,30], in
England (42], in France (36], in New Zealand (41], and in Canada (7).
As we have already indicated, current research suggests that selenium is a
very important element in the human body and dietary selenium is a major
source. Since the selenium content of foods \'aries according to geographic
location and since interest in dietary selenium intake has increased, due to the
important role of selenium in health, there is an increased need for the
assessment and improvement of the selenium food composition data acquisition
from a wide variety of areas. There are some reports of the selenium content in
foods from the Maryland and Ohio, and also a report of selenium content in
canned foods or food products from Houston, Texas (16). Ko information is
available on the selenium content of foods in the Lubbock, vVest Texas area. In
fact, this provided part. of the motivation for the present study which provides
data on the selenium content of some commonly consumed foods in Lubbock.
Texas, and examines whether the selenium content is influenced during the
cooking or drying processes.
12
KEMBRANES
CYTOSOL
soluble enzymes
drug metabolism
mitochondrial membrane
and E. R enzymes
Se-GSHPx
H2 o2 ~
2GSH
2H 20
esse
USFA-~ (O)
2
(FA . )
(HO ·)
HO. ~ R-H
USFA - OOH
Se-eSHPx
ROOH
-~--=--o::::---~>
ROH
2GSH
Figure 2.1: Functions of glutathione peroxidase, superoxide dismutase and
a-tocopherol in a cell. ( ) = species scavenged or quenched by a-tocopherol
(modified from Sunde and Hoekstra, [19]).
13
TISSUE FORMS
DIETARY FORMS
(follow methionine
metabolism pathways)
SELENOMETHIONINE ~ METHIONINE :;:::: COMBINE
(derived from plants)
POOL
WITH
PROTEINS
!
SELENOCYSTEINE
(derived from animals)
:;::::
SELENIUM
METABOLISM
COMBINE
\VITH
GSHPx
!
EXCRETION
IN
URINE,
BREATH
Figure 2.2: Outline of selenium metabolism (modified from Levander and Burk.
[19])
Table 2.1: Bioavailability of selenium in foodsa.
Foods
Whole wheat bread 6
Brazil nutsc
Beef kidney, cookedd
Baltic herring, bakede
Wheat, boiled d
Shrimp, boilede
Crab, boilede
Tuna, canned, water-packedd
Oysters, boilede
Tuna, canned, oil-packedb
Mushroom, freeze-driedc
a. Levander, [18]
b. Alexander et al., [1]
c. Chansler et al., [5]
d. Douglass et al., [8]
e. Mutanen et al., [23]
Availability (%)
(selenite = 100)
142
117
97
90
83
73
58
57
38
32
4
14
~
L:.:.=;
LOW·A"AOXIMAHLY 10' OF ALL 'ORACl ANO CRAIN CONTAIN (O 10 "'M SH(,.IUM
c:J
D
1/ARIAIL£-A''ROXIMAT(L Y SO'.; CONTAIN
)o 10 ""' S(L(NIUM II,.ClVD£5 AlASII.AI
AOIOUATl-10 .. OF All '0RAGlS AND CRAIN CONTAIN )c IC ; , ... SHE,.IVM
IIHCLVDI S HAWAIII
Figure 2.3: The regional distribution of forages and grain containing different
levels of selenium in the U.S.A. (modified from Jenkins, [15])
15
Table 2.2: A summary of the selenium content of foods from a number of studies
in the United States and other countries.
Foods
Beef & Pork
beef,raw
beef,ckd
pork,raw
pork,ckd
Seafoods
tuna fish
cod,fillet
shrimp,raw
shrimp,ckd
salmon
Poultry
chicken,raw
chicken,ckd
turkey,raw
turkey,ckd
Vegetables
lettuce,raw
potatoes,raw
tomatoes,raw
Dairy prod.
milk, whole
milk,2% fat
milk,skim
ckd=cooked
MD
(1970)
Can.
(1972)
Se concentration (Jlg/ g)
Eng.
OH
MD
(1978) (1987) (1987)
0.195
0.127
0.030
0.239
0.296
0.140
0.428
0.588
0.116
0.210
0.068
0.068
0.131
0.720
0.531
0.530
0.640
0.600
0.160
0.200
0.180
0.34
0.250
0.008
0.023
<0.010
0.012
0.150
<0.010
0.060
0.080
0.100
0.010
0.005
0.005
0.048
I
0.863
0.220
0.260
0.330
0.350
Fr.
(1988)
0.531
0.178
N. Z.
(1990)
0.019
0.023
0.120
0.170
0.120
0.200
0.230
0.340
0.200
0.280
0.050
0.160
0.120
0.479
0.015
0.013
0.007
0.015
<0.001
0.006
<0.001
0.003
0.004
0.002
0.020
0.030
0.030
0.012
0.008
0.014
0.013
0.006
0.006
0.008
CHAPTER III
METHODS AND MATERIALS
3.1 Selection of food samples
Following the guidelines in Schubert et al. [30], a list of dietary selenium
contributors from foods was developed (see Table 3.1). The food selection
process was intended to include those frequently consumed foods that contribute
to the bulk of selenium in the diets of the United States population. These foods
are "core foods 1' for selenium which contribute about 41.39( of the selenium in
the U.S. diet.
From these "core foods," selections were made from commonly consumed
meats, seafood, poultry, dairy products, and vegetables. All these food samples
were purchased from a commercial packer or local supermarket; some of the
vegetables were also purchased from a road-side market.
3.2 Preparation of food samples
All meats, seafoods, and poultry were prepared as raw, cooked, freeze-dried
or air-dried meats. The vegetables were analyzed on a fresh. dried. or cooked
basis. We used ground beef, pork, chicken and turkey as samples. Fish was
deboned and shrimp was shelled and deheaded. Samples were homogenized in a
glass food blender. Meat, poultry and seafood samples were cooked in Teflon
pans at a med-high temperature until well-done. They also were dried in a
drying oven at 100°C for 24 hours or freeze-dried (Labconco freeze dryer). The
16
11
vegetables were peeled, outer leaves removed and dirt rinsed off using deionized
distilled water. They were dried in an oven at 100°C for 24 hours or boiled and
steamed for 10 minutes and then homogenized in a glass food blender.
3.3 Analytical methods
The selenium content was determined using the fluorometric technique of
Spallholz et al. [38]. This method involves:
( 1) preparation of samples and standards,
(2) digesting 0.10-0.50 g of samples in a molybdate sulfuric perchloric acid
mixture,
(3) masking potential mineral interference with 0.008 M disodium
ethylenediamine tetraacetate (EDTA). EDTA can chelate Ca, Zn, Fe, Mn
and Mg,
(4) adjusting the pH to 2 using either NH 4 0H or H 2 S04,
(5) complexing selenium with freshly prepared 3, 3'-diaminonaphthalene (DAN)
solution,
(6) adding cyclohexane,
(7) shaking each tube for 5 minutes and extracting the Se-DAN complex into
cyclohexane,
(8) samples were then analyzed in a spectrophotofluorometer with the
excitation at 363 nm and the emission at 525 nm.
18
(9) a standard curve was run with each set of analyses.
(10) the selenium content of the foods and standard solution were compared
with a National Bureau of Standards (NBS) liver standard 1577a containing
0. 71 ± 0.07 ppm selenium.
Table 3.1: Contributions of seleniwn in the U.S.A. dieta.
Foods
Mean(J.LgSe/g) % daily intake
Beef, cooked
17
0.26
Pork, cooked
0.35
9
Chicken, cooked
0.21
6
0.016
Whole milk
3
Tuna, canned
2
0.72
1.8
2% milk
0.026
Turkey, cooked
0.25
1
0.6
Shrimp, cooked
0.64
0.5
Skim milk
0.027
0.2
White potato
0.009
0.1
Lettuce, raw
0.015
0.1
Tomato, raw
0.007
a. Adapted from Schubert et al., [30)
19
Table 3.2: Procedure for selenium determination a.
Step
1
2
3
4
5
6
7
8
9
10
11
12
13
Operation
Prepare samples or standards
Add digestion mixture (2 ml)
Digestion
Dilute digest with 0.008 M EDTA
Add 2.1- 2.2 NH 4 0H (concentrated)
Adjust pH to 2 (NH 4 0H or H 2 S0 4 )
Prepare DAN solutionb
Add DAN solution ( 4 ml)
Incubate at 50°C in water bath
Cool tubes
Add cyclohexane ( 5 ml)
Extract by shaking
Measure fluorescence intensity:
excitation 363 nm; emission 525 run
14
Plot relative fluorescence against
ng Se/5 ml cydohexane
a. Spallholz et al., [38]
b. Prepare fresh; wash well with cyclohexane
Total
Volume (ml)
1
3
7
9.2
9.2
13.2
18.2
Approx.
Time (Min)
15
5
45
5
5
20
45
5
20
5
5
5
15
CHAPTER IV
RESULTS
4.1 Food Se concentration
The concentration of selenium of different foods is presented in the Tables 4.1
to 4.5. Foods with the highest average selenium concentrations (0.64 j.lg Se/g
wet weight) were the seafood group: tuna fish (1.09 J.lg/g), shrimp (0.58 J.lg/g)
and cod fillet (0.26 J.lg/g); followed by the beef and pork group: beef with 20%
fat (0.14 J.lg/g), beef with 15% fat (0.19 J.lg/g) and pork (0.26 J.lg/g). In poultry,
turkey (0.30 J.lg/g) had a higher selenium content than chicken (0.13 J.lg/g)
which was even higher than cod, pork and beef. Animal fat contained very low
selenium content. In the vegetable group, most vegetables were found to contain
less than 0.03 j.lg Se/g except mushrooms which contained 0.16 J.lg/g. According
to Allaway [2], a poisonous mushroom, Amanita, took up significant amounts of
selenium even when grown in soils containing low levels of selenium. Milk was
also low in selenium content. Skim milk (0.043 J.lg/ g) was higher than 2% low fat
milk (0.038 J.lg/ g) which was higher than whole homogenized milk (0.029 J.lg/ g).
The order of selenium content of all these foods was: tuna fish > shrimp >
turkey > cod fillet = pork > beef> mushrooms > chicken > milk > vegetables.
20
21
Table 4.1: Selenium content of seafood (J-Lg/g mean± SD).
Samples
Shrimp (no shell)
Raw
Cooked
Air-dried
Cod Filet
Raw
Cooked
Air-dried
Tuna Filet
Raw
Cooked
Freeze-dried
Number of Samples
Selenium
5
5
5
0.585±0.021
0.735±0.007
2.810±0.056
5
5
5
0.265±0.035
0.350±0.056
1.435±0.021
6
5
5
1.090±0.071
1.420±0.212
3.330±0.665
Table 4.2: Selenium content of ground beef and pork (J-Lg/ g mean ± SD ).
Samples
Ground Beef (15% fat)
Raw
Cooked
Air-dried
Ground Beef (20% fat)
Raw
Cooked
Freeze-dried
Air-dried
Fat
Ground Pork (extra lean)
Raw
Cooked
Air-dried
Number of Samples
Selenium
5
6
5
0.190±0.014
0.250±0.056
0.500±0.085
6
5
5
5
5
0.145±0.007
0.240±0.071
0.335±0.007
0.420±0.085
0.030±0.000
5
6
5
0.265±0.035
0.370±0.056
0. 735±0.120
22
Table 4.3: Selenium content of poultry (p,g/ g mean ± SD ).
Samples
Ground Chicken
Raw
Cooked
Freeze-dried
Air-dried
Fat
Ground Turkey
Raw
Cooked
Air-dried
Number of Samples
Selenium
5
6
6
5
5
0.130±0.028
0.275±0.049
0.385±0.021
0.365±0.007
0.015±0.007
5
5
5
0.300±0.056
0.445±0.021
1.169±0.269
Table 4.4: Selenium content of vegetables (p,g/ g mean ± SD ), n = 5.
Sample
Broccoli
Mushroom
Lettuce
Onion
Tomato
Potato
Fresh
0.032±0.003
0.165±0.035
0.014±0.002
0.012±0.001
0.002±0.001
0.019±0.001
Dry
0.180±0.020
2.050±0.230
0.210±0.020
0.054±0.008
0.022±0.007
0.067±0.006
Boil
0.205±0.007
1.335±0.130
Steam
0.255±0.007
Table 4.5: Selenium content of milk (p,g/ g mean±SD ), n = 5.
Samples
Skim milk
2% milk
Whole milk
Selenium
0.0430±0.001
0.0385±0.001
0.0295±0.001
23
4.2 Comparison of food Se content with other areas
Some areas of England, France, New Zealand and Canada have been known
as low selenium areas with low levels of selenium in soils and continuing up the
food chain resulting in low selenium status of its residents. The data listed in
Table 4.6 were reported to be from low selenium areas in these countries.
Maryland and Ohio are also considered as low selenium areas. In comparison
with England, France, New Zealand and Canada, the selenium content of beef,
pork and chicken in Lubbock was higher than that of meats in those countries.
But the selenium content in cod fillet from New Zealand and Canada was higher.
The selenium content in beef, pork and poultry from Lubbock was lower than
those from Maryland area, but the beef and pork contained higher content of
selenium than that from the Ohio area. Shrimp from these three areas contained
a higher and similar amounts of selenium. Vegetables were generally low for all
countries or states and did not differ greatly from the Lubbock values. The
selenium concentration in milk from Lubbock was similar to that from the
Maryland area which was higher than those from Ohio and other countries
except Canada. The whole milk in Canada contained 0.15 j.lg Se/ g which was
much higher than the other areas.
24
Table 4.6: Comparison of the selenium concentration in food samples in Lubbock,
Texas with those from other areas.
Foods
Beef
Pork
Chicken
Turkey
Shrimp
Cod
Tuna
Lettuce
Potato
Tomato
Whole milk
2% milk
Skim milk
a.
b.
c.
d.
e.
f.
g.
Lbka
0.190
0.265
0.130
0.300
0.585
0.265
1.090
0.014
0.019
0.002
0.029
0.038
0.043
Se concentration (J-Lg/ g, wet weight bases)
MDb OHC
Fr.e
Eng.d
N.Z?
0.220 0.068 0.030
0.060
0.019
0.330 0.131
0.140
0.080
0.120
0.200 0.178
0.050
0.120
0.340
0.530 0.531
0.230
0.100
0.200
0.340
0.720 0.531
0.120
0.015 0.015
<0.001 0.003
0.013
0.004
<0.010 0.006
0.007
<0.001 0.002
0.020 0.012 <0.010 0.013
0.006
0.030 0.008
0.030 0.014
0.006
0.008
Can. 9
0.127
0.296
0.160
0.863
0.008
0.023
0.150
This study, Lbk Lubbock
Schubert et al., (30], MD Maryland
Snook et al., (37), OH Ohio
Thorn et al., [41], Eng. England
Simon off, (36), Fr. France
Thomason and Robinson, [41), N.Z. New Zealand
Arthur, [3], Can. Cananda
4.3 Effect of drying methods on Se content
According to our study (Table 4. 7), the selenium concentration in foods
increased in the dried samples which may be due to loss of the moisture.
Selenium was not lost during drying. There were no real differences between
freeze-dried or air-dried samples of foods.
25
4.4 Effect of cooking methods on Se content
According to the results in Table 4. 7, all cooked food samples contained
higher selenium concentrations than did raw samples, but contained lower
selenium concentrations than dried food samples. One of the reasons may be
that part of the moisture and fat was lost during the cooking process.
Table 4.7: Selenium content of raw, cooked and dried meats, poultry and seafood
(J-Lg/ g, mean ± SD ), n=5.
Foods
Beef (20% fat)
Beef (15% fat)
Pork
Chicken
Turkey
Shrimp
Cod Fillet
Tuna Fish
Raw
0.145±0.007
0.190±0.014
0.265±0.035
0.130±0.028
0.300±0.056
0.585±0.021
0.265±0.035
1.090±0.071
Cooked
0.240±0.071
0.250±0.056
0.370±0.056
0.275±0.049
0.445±0.021
0. 735±0.007
0.350±0.056
1.420±0.212
Dried
0.420±0.085
0.500±0.085
0.735±0.120
0.365±0.007
1.160±0.269
2.810±0.056
1.435±0.021
3.330±0.665
Table 4.8: Contribution of 3 ounces of cooked meat, poultry and seafood to RDA
for selenium (3 oz. = 85.05g).
Food
Beef (15% fat)
Pork
Chicken
Turkey
Shrimp
Cod, fillet
Tuna
J-Lg/Se
16.2
22.5
11.1
25.5
49.8
22.5
92.7
% RDA Women
55 1-Lg/day
29.5
40.9
20.2
46.4
90.5
40.9
168.5
% RDA Men
70 J-lg/day
23.1
32.1
15.9
36.4
71.1
32.2
132.4
26
4.5 Contribution of 3 oz. of cooked meats to RDA for Se
In Table 4.8, we estimated how three ounces of each meat, poultry and
seafood would contribute to the percent of RDA of selenium for women and men
in Lubbock, Texas. Seafoods, meats and poultry, along with cereal grains, are
the major contributors to the RDA of selenium. Unfortunately, analytical
quantitation alone does not insure an adequate bioavailable intake of selenium
since the bioavailability of selenium in foods is thought to vary considerably.
CHAPTER V
DISCUSSION
According to Schubert et al. [30], there is a set of criteria specifically for
evaluating the quality of selenium data. There are five categories: number of
samples, analytical methods, sample handling, sampling plan, and analytical
quality control. Based on these criteria, at least five samples for each food were
assayed. We used a modification of one official fluorimetric method (Spallholz et
al., [38]) as the analytical method for selenium which has been used for long time
and is satisfactory in precision, accuracy and recovery of selenium. Samples were
selected from " core foods " except cereal grains for selenium (Schubert et al.
[30]). All selected foods are commonly consumed and contributed to the RDA for
selenium or significant sources of dietary selenium. Food samples were analyzt>d
in duplicate or were stored in plastic containers at -4°C until analysis, typically
within two weeks after procurement. The analyses were always performed in one
identical manner, in the same laboratory and using the same equipment.
From this study, the best sources of dietary selenium based on concentration
(on a wet method basis) were: fish and seafoods, beef and pork, and poultry.
These foods had moderate to high levels of selenium (
vegetables contained
> 0.13 f-Lg/ g). Milk and
< than 0.03 ILg/g selenium. Mushroom, one exception,
contained 0.165 f-Lg/g selenium.
_,
•)~
28
According to previous studies (Table 2.2) and this study, animal products,
like muscle meats, usually contain higher concentrations of selenium than
vegetables which have a very low selenium content. This is because selenium is
an essential trace element for animal species but not for plants. If animal tissue
selenium levels are too low, the animals will not survive. Therefore, they will try
to increase the retention of selenium at a low selenium intake to maintain
selenium concentrations in muscle and liver. Plant species as noted do not
require selenium for growth. They still thrive even if no soil selenium is available
for uptake. In contrast with the results stated in [6], our work suggests that
plants may be very low in selenium no matter how high or low the soil selenium
content.
Many factors influence the levels of selenium in foods. These include the
different geographic location, the soil concentration of biologically available
selenium, the treatment of soil, the supplementation of animal feeds, the
processing of food products, the cooking methods, and the protein content of the
food where much selenium is formed. In all studies on selenium in foods, data
for cooked foods is lacking. There is no separation of data on foods by cooking
methods and no specific conclusions on changes in selenium concentration as a
result of different cooking methods [30]. Table 4. 7 shows that selenium
concentrations in cooked meats, poultry and seafoods were higher than their raw
counterparts, which is probably due to lost moisture or part of the animal fat
29
during the cooking processes. This is very limited cooking method but it does
suggest that most selenium is retention in cooked foods.
Importation of foods from other geographical locations is another factor that
influences the selenium content of foods in any area. Lubbock, Texas, is in an
adequate-selenium area and Maryland is considered as low selenium region.
Table 4.6 shows that some foods purchased from the local Lubbock supermarket,
i.e., beef and pork, had a lower selenium content than those purchased from
Maryland. Therefore, food importation may cause wide variability in the
selenium content of some foods.
Selenium present in food appears to be associated with protein, so the
protein content of food affects the selenium concentration in the food. There is
no doubt that animal foods usually contain higher selenium content. Much of
the selenium in food appears to be selenocysteine in the animaL which is the
active form of selenium in glutathione peroxidase [40]. The major plant form of
dietary selenium is selenomethionine. A large proportion of the selenium in
wheat and about 60% of the selenium in soy beans has been identified as
selenomethionine [25,49], which may also be present in animals eating grains.
As discussed in the literature review, it is not certain whether the different
selenium compounds that occur in foods can be metabolized to biologically active
selenium. Therefore, the determination of bioavailability of selenium in foods is
also important. For example, seafoods usually contain higher concentrations of
30
selenium, but the bioavailability of selenium in tuna fish and certain other
seafoods like crab and oysters is reported to be relatively low, perhaps because
of heavy metals, particularly mercury, complexing with selenium [8].
CHAPTER VI
CONCLUSIONS
According to the literature and this experimental study, the following
conclusions may be made.
1. The best sources of selenium are: fish and other seafoods, beef, pork, and
poultry. The selenium level in these foods is typically higher than 0.13 J-Lg/ g.
The selenium concentration in milk and vegetables is usually very low ( < 0.03
J-Lg/ g ) no matter how high or low the selenium level for that area.
2. Animal products like muscle meats usually contain a higher selenium
content and vegetables contain a very low selenium content because animals
require dietary selenium but vegetables do not require soil selenium.
3. Selenium concentrations in cooked foods were usually higher than their
raw counterparts because the cooked foods lose part of their moisture and fat,
thus concentrating the minerals remaining. Therefore, the cooking or drying
methods in this study did not seem to affect selenium loss from food.
4. Lubbock, Texas, is in an adequate selenium area but some foods contained
selenium content lower than those of Maryland which is considered a low
selenium region. This is likely because of importation of foods from other
geographical locations, which is one of the factors that influence the selenium
content of foods.
31
32
5. It is important to determine the bioavailability of selenium in foods
because selenium analysis alone from foods cannot provide necessary information
on the biological requirement for the RDA for selenium.
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