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Health benefits of soy beans and soy products: a review
ISSN 0378-5254 Tydskrif vir Gesinsekologie en Verbruikerswetenskappe, Vol 27: No 1, 1999
24
Health benefits of soy beans and
soy products: a review
Christina S Venter
OPSOMMING
Sojabone is volop, ekonomiese dieetbronne
van proteïen. Tans word gefokus op die
moontlike rol van sojabone in die voorkoming
en behandeling van sekere degeneratiewe Wes-
terse siektes soos hart- en bloedvatsiektes,
osteoporose en sekere tipes kanker. Heel soja-
bone bevat 40% proteïene, terwyl sojameel,
sojaproteïenkonsentraat en geïsoleerde soja-
proteïen respektiewelik 50%, 70% en 90% pro-
teïen bevat. Verskeie nie-nutriëntbestanddele
soos isoflavone, protease-inhibeerders, fi-
tiensuur, saponiene en fitosterole kom in soja-
bone voor en is moontlik vir sekere van die
voordelige gesondheidseienskappe van soja-
bone verantwoordelik. Die cholesterolverla-
gende effek van sojabone mag te danke wees
aan die aminosuursamestelling daarvan, en/of
verlaagde cholesterolabsorpsie of galsouther-
absorpsie, verhoogde laedigtheidlipoproteïen-
(LDL-) reseptoraktiwiteite, verlaagde lewercho-
lesterolsintese, anti-oksidantaktiwiteit of ver-
hoogde plasmatiroksienvlakke. Die nie-
nutriëntbestanddele van sojabone mag vir die
cholesterolverlagende, antikarsinogeniese en
beenversterkende effek verantwoordelik wees.
Meer navorsing is nodig om die meganismes
op te klaar waardeur sojabone en sojaprodukte
die gesondheid van die mens beïnvloed.
— Prof CS Venter
Department of Nutrition and Family Ecology, PU for CHE,
Potchefstroom
INTRODUCTION
Soy beans has been a food in China for thousands of
years. It is an abundant, economic source of protein.
No other nation has acquired the same taste for soy
beans as the Chinese and Japanese, but the bean
has become an important raw material for the interna-
tional food industry. Attention has recently focused on
the possible role of soy beans in the diet for the pre-
vention and treatment of degenerative Western dis-
eases (Anderson et al, 1995a; Knight & Eden, 1996;
Kurzer & Xu, 1997; Potter, 1998). Several studies
documented the hypocholesterolaemic effects of soy
beans (Anderson et al, 1995b, Potter, 1998), the anti-
carcinogenic effects of soy beans (Barnes et al,
1996), and the ability of soy beans to lower the risk of
osteoporosis (Adlercreutz & Mazur, 1997).
The purpose of this review is to describe the physio-
logical and biochemical effects of soy bean compo-
nents in the body, and their possible prevention of the
above diseases. Recommendations are submitted for
further research in this regard and on the amounts
that may safely be included in the human diet.
COMPOSITION OF SOY BEANS
Soy beans are classified as oil seeds, not as dry
beans. Table 1 contains the nutrient composition of
100 g of cooked dried haricot, kidney and soy beans
(Langenhoven et al, 1991), and Table 2 presents the
percentage contribution of the macronutrients in soy
and dry beans to the total energy content. Whole dry
soy beans contain about 40% protein (twice as much
as most other pulses) and up to 20% fat. Whole soy
beans are a good source of calcium, iron, zinc, phos-
phorus, magnesium, thiamin, riboflavin, niacin and
folacin.
It was recently recognised that the human diet con-
tains, in addition to essential macro and micronutri-
ents, a complex array of naturally occurring bioactive
nonnutrients called phytochemicals (plant-derived
compounds) that confer significant long-term health
benefits (Setchell, 1998). Among these phytochemi-
cals is the broad class of nonsteroidal oestrogens
called phytoestrogens that also behave as oestrogen
mimics. The major classes of phytoestrogens that are
of interest from a nutritional and health perspective,
are the lignans and the isoflavones. Soy beans con-
tain large amounts of the isoflavones diadzein, gen-
istein an glycitein (1-3 mg/g) and their acetyl and
malonyl conjugates (Song et al, 1998). Studies have
shown that concentration and composition vary in dif-
ferent soy beans or soy protein products (Murphy,
Health benefits of soy beans and soy products: a review
ISSN 0378-5254 Journal of Family Ecology and Consumer Sciences, Vol 27: No 1, 1999
25
1982) and that this variation is due to species differ-
ences (Franke et al, 1995), geographic and environ-
mental conditions (Eldridge & Kwolek, 1983), and the
extent of the industrial processing of soy beans
(Murphy, 1982; Coward et al, 1998). Table 3 illus-
trates the varied isoflavone concentration in a range
of soy products.
PROCESSING OF SOY BEANS
The processing of soy beans as described by Snyder
and Kwon (1987:74-78) may be summarised as fol-
lows:
Soy beans selected for processing are graded,
cleaned, dried to about 10% moisture content, and
TABLE 1: THE NUTRIENT COMPOSITION OF DRY AND SOY BEANS* COMPARED TO RECOM-
MENDED DIETARY ALLOWANCES**
Nutrient 100 g cooked beans RDA **
Haricot Kidney Soy
Moisture (%) 69,6 70,5 62,6
Energy (kJ) 413 423 706
Protein (g) 6,6 7,1 16,6 63,0
Fat (g) 0,5 0,3 9,0
Saturated fatty acids (g) - - 1,3
Monounsaturated fatty acids (g) - - 2,0
Polyunsaturated fatty acids (g) - - 5,1
Carbohydrate (g) 16,6 17,1 4,8
Dietary fibre (g) 7,4 5,1 1,6
NSP*** total (g) 8,3 6,7 2,9
Soluble NSP (g) 3,7 3,2 0,1****
Insoluble NSP (g) 4,6 3,5 2,0****
Calcium (mg) 65 19 102 800
Iron (mg) 2,5 1,7 5,1 10
Magnesium (mg) 45 33 86 350
Phosphorous (mg) 120 87 247 800
Potassium (mg) 320 400 515
Sodium (mg) 15 16 1
Zinc (mg) 1,0 1,0 1,2 15,0
Copper (mg) 0,14 0,16 0,41
Vitamins: Thiamin (mg) 0,11 0,14 0,16 1,5
Ribloflavin (mg) 0,06 0,07 0,29 1,7
Niacin (mg) 0,7 0,7 0,4 19,0
A (RE) - - 1 1000,0
E (mg α -TE) - - 0,35 10,0
Folic acid (µg) - - 54 200,0
* South African Food Tables (Langenhoven et al, 1991)
*** RDA: Recommended dietary allowances (Food and Nutrition Board, 1989)
*** NSP: Nonstarch polysaccharides (Englyst et al, 1988)
**** AOAC: Association of Analytical Chemists, In Slavin, 1991
Health benefits of soy beans and soy products: a review
ISSN 0378-5254 Tydskrif vir Gesinsekologie en Verbruikerswetenskappe, Vol 27: No 1, 1999
26
cracked to remove the hull. Soy bean hulls are proc-
essed to create fibre additives for breads, cereals,
snacks and livestock feed. After dehulling, the beans
are rolled into full-fat flakes that may be used in ani-
mal feed or processed into full-fat flour for various
commercial food uses. Flaking ruptures the oil cells in
the bean, improving the oil extraction process. The
next step is to extract the crude oil which is later re-
fined to produce cooking oil, margarine and shorten-
ing. Defatted soy flakes are used to produce animal
feed and form the basis of a variety of products for hu-
man consumption, including soy flour, soy concen-
trates and soy isolates. These products are used ex-
tensively in manufactured foods to help retain mois-
ture and to improve their shelf life, and they act as
emulsifiers and as substitutes for meat in food prod-
ucts. Soy flour is produced by grinding the defatted
flakes. The protein content of the flour is approxi-
mately 50%. Soy flour adds protein and improves the
crust colour and shelf life of baked goods. Soy iso-
lates are produced by a chemical process that with-
draws most of the protein from the defatted flake, re-
sulting in a product with about 90% protein content.
Soy isolates contain no fibre or carbohydrates. Iso-
lates are used in many dairy-like products, including
cheese, milk, nondairy frozen desserts, coffee whiten-
ers and meat products. Soy concentrates are pre-
pared by removing the soluble sugars from defatted
flakes. Soy concentrates contain about 70% protein
and retain most of the bean’s dietary fibre. The con-
centrates are used in protein drinks, as soup bases
and in gravies. Soy flour and soy protein concentrates
are used in meat products, primarily because of their
fat and water absorption properties. These products
are used in a texturised form as extenders in ground
meat products, in convenience foods, in pizza top-
pings, meat and fish spreads, and in poultry products
(Snyder & Kwon, 1987:74).
Texturised protein (textured vegetable protein (TVP) )
is produced by the thermoplastic extrusion of defatted
soy flour, soy concentrates or soy isolates, moistened
and mixed with a variety of additives (Wolf et al,
1981). During the extrusion process, small chunks are
produced which, when hydrated, have a chewy texture
and a meaty taste (Wolf et al, 1981).
TABLE 2: CONTRIBUTION OF MACRONUTRIENTS TO TOTAL ENERGY CONTENT OF DRY AND
SOY BEANS *
Percentage contribution of total energy
Nutrient Dietary goals
Haricot Kidney Soy
Total fat 4,6 2,7 48,4 <30
Saturated - - 7,0 <10
Monounsaturated - - 10,7 10
Polyunsaturated - - 27,5 10
Carbohydrate 68,7 68,7 11,6 ≥60
Protein 27,2 28,5 40,05 10-15
* Based on nutrient composition in Table 1
Beans
Food
Daidzein
(µg/g)
Genistein
(µg/g)
Glycitein
(µg/g)
Toasted soy flour*
Soy flour*
Isolated soy protein*
Textured vegetable protein*
Tofu**
Soy milk
*Coward et al, 1998s
**Song et al, 1998
1 343,4
829,7
789,3
919,7
133,1
1 772,0
1 509,5
834,4
1 258,0
1 092,1
169,0
3 804,0
242,5
142,9
114,2
98,4
20,9
327,0
TABLE 3: ISOFLAVONE CONCENTRATION IN SOY PRODUCTS
Health benefits of soy beans and soy products: a review
ISSN 0378-5254 Journal of Family Ecology and Consumer Sciences, Vol 27: No 1, 1999
27
CHOLESTEROL-LOWERING PROPERTIES OF
SOY
Potential cholesterol-lowering effects
The judicious substitution of soy for animal protein re-
duces saturated fat and cholesterol intakes, indirectly
resulting in a more favourable blood cholesterol level
and potentially reducing the risk of coronary heart dis-
ease.
The cholesterol-lowering effects of soy protein, com-
pared to animal protein, have been recognised in ani-
mals for more than 90 years (Ignatowsky, 1908 in
Anderson et al, 1995). A number of human studies
over the past 20 years have shown that the daily con-
sumption of 30 g to 60 g of soy protein contributes to
a decrease in total and LDL cholesterol of between
10% and 20% in individuals with elevated serum cho-
lesterol (Carroll, 1991). High-density lipoprotein
(HDL) cholesterol either remains unchanged or is in-
creased under these circumstances (Anderson et al,
1995b; Baum et al, 1998). In addition, a significant
10% reduction in triglycerides has been reported in
several studies, as noted in a meta-analysis by
Anderson et al (1995b). According to these studies
changes in lipid concentrations were independent of
changes in body weight and the dietary intake of total
fat, saturated fat and cholesterol.
As little as 30 g to 60 g of isolated soy protein in muf-
fins, breads, cookies and other commonly eaten bak-
ery items effectively lower raised cholesterol (Potter
et al, 1993). Replacing milk with a soy beverage has
been shown to decrease total serum cholesterol by
5% to 10% and low-density lipoprotein (LDL) choles-
terol by 10% to 20% within four weeks (Steele, 1992).
It is therefore suggested that very modest changes in
the diet to include soy products have a measurable
effect on the blood lipid levels.
Potential mechanism(s) of soy in lowering serum cho-
lesterol concentrations or the risk of coronary heart
disease
The mechanisms responsible for the effects of soy on
serum lipoproteins are not well known. Carroll (1991)
and Potter (1995) reviewed various hypotheses that
are presented in this section. These include the
amino acid composition of soy protein, an interruption
of the intestinal absorption of bile acids and dietary
cholesterol, direct effects on the hepatic metabolism
of cholesterol, alteration of the hormone concentration
involved in cholesterol metabolism, and the effects of
components such as isoflavones, fibre and saponins
in soy beans.
Studies on experimental animals have shown that the
dietary substitution of amino acids patterned after soy
protein produces significantly lower serum cholesterol
concentrations than amino acids patterned after ca-
sein. The extent of cholesterol lowering, however, is
not as great compared to the values in animals that
were fed intact soy protein (Huff et al 1977; Tasker &
Potter, 1993). According to Potter (1995), this indi-
cates that there may be another constituent associ-
ated with soy protein that is either lost or liberated dur-
ing hydrolysis of the protein, to be partly responsible
for the cholesterol-lowering effect.
Sugano and Koba (1993) also reported that an indi-
gestible fraction of soy protein lowers the serum and
liver cholesterol concentrations. In this study soy pro-
tein was digested with microbial proteases, and the
digestible and indigestible fractions were fed to rats.
The results indicated that progressive replacement of
casein with the indigestible fraction progressively low-
ered the serum and liver cholesterol. Faecal excretion
of both neutral and acidic sterols increased in animals
that were fed the indigestible fraction. However, when
the undigested fraction was treated with a methanol
extraction or was digested further, the cholesterol-
lowering effect diminished. There appears to be some
component in the indigestible fraction that is either lost
or altered upon methanol extraction or further diges-
tion.
Potter (1995) suggests that a component such as a
saponin or isoflavone or a peptide-peptide sequence
that alters the intestinal absorption of cholesterol and
bile acids may be a candidate. According to Oakenfull
(1981), only those plant fibres that contain saponins
bind bile acids in vitro. Potter et al (1979) suggest that
the hypocholesterolaemic action of whole soy protein
or protein hydrolysates is attributable to the presence
of saponins. In the studies with amino acids patterned
after the intact proteins (Nagata et al, 1982; Tanaka et
al, 1984), general decreases in serum cholesterol con-
centrations were noted - albeit less pronounced than
when intact soy protein was fed - without influencing
faecal bile excretion. Increased bile acid excretion on
soy diets was observed primarily in rabbits and rats
(Huff & Carroll, 1980; Nagata et al, 1982; Nagoaka et
al, 1997). Reports on other species, including hu-
mans, are less consistent (Fumagalli et al, 1982;
Grundy & Abrams, 1983). More recently, however,
Wong et al (1996) as quoted by Potter (1998), found
increases in the pool size of chenodeoxycholic acid in
humans who were given soy protein to eat, but nei-
ther the cholesterol absorption nor the cholic acid pool
size was affected.
In summary, it appears that when intact proteins are
fed, cholesterol-lowering on feeding soy protein may
be mediated by enhanced bile acid excretion in certain
species. However, when amino acids are fed, choles-
terol-lowering may be mediated by another mecha-
nism(s).
Studies in animal models (Huff & Carroll, 1980; Na-
gata et al, 1982) have shown that soy protein con-
sumption may directly influence the hepatic metabo-
lism of cholesterol by increasing the activity of 3-
hydroxy-3-methylglutaryl coenzyme A (HMG CoA) re-
ductase, thereby inhibiting hepatic cholesterol synthe-
sis. Lovati et al (1987) reported a sevenfold increase
in LDL receptor activity in humans, resulting in in-
creased clearance of cholesterol from the blood in pa-
Health benefits of soy beans and soy products: a review
ISSN 0378-5254 Tydskrif vir Gesinsekologie en Verbruikerswetenskappe, Vol 27: No 1, 1999
28
tients with raised serum cholesterol concentrations
who consumed a soy protein (Cholsoy) diet compared
to a standard low-lipid diet with animal protein.
These results were confirmed by other researchers,
as reported by Potter (1998). However, the hypothe-
sis of an activation of LDL receptors in liver cells is
still controversial and more extensive studies are
needed to ascertain the cholesterol-lowering mecha-
nism of soy beans.
It has been postulated that the consumption of soy
protein alters many hormones involved in lipid me-
tabolism (Forsythe, 1986; Scholz-Ahrens et al, 1990).
Scholz-Ahrens et al (1990) reported increases in
plasma total thyroxine, free thyroxine, and triiodothy-
ronine in minipigs fed soy protein compared to those
that were fed casein. Forsythe (1986) and Ham et al
(1993) reported decreased cholesterol concentrations
and increased plasma thyroxine in gerbils and hu-
mans respectively when soy protein was included in
the diet. However, Potter (1998) investigated this
phenomenon in several studies on animals and hu-
mans and did not find consistent results. In one study
involving gerbils, soy protein concentrate and soy pro-
tein isolate significantly reduced the total and LDL
cholesterol concentrations, but only soy protein iso-
late increased the thyroid hormone concentrations
(Potter et al, 1996).
As all these hormones are known to be involved in
cholesterol metabolism, it has been proposed that
variation in hormone secretion is responsible for the
cholesterol-lowering effect of soy protein. Especially
with regard to the thyroid hormones, the metabolic ef-
fects of hyperthyroidism are very similar to those ob-
served with soy protein feeding. That is, LDL receptor
activity increases, HMG CoA reductase activity in-
creases, bile acid excretion increases, and total and
LDL cholesterol decrease (Potter, 1995). Some ob-
servers, as discussed by Carroll (1991), suggest that
changes in the ratio of serum glucagon to insulin in
patients on a soy protein diet may be important. High
insulin:glukagon ratios are thought to be associated
with increased risk of cardiovascular disease because
of the stimulation of lipogenesis.
Isoflavones are known to have weak oestrogenic ac-
tivity in biologic systems. Therefore it is increasingly
popular to speculate that the mechanism by which
soy beans decrease serum cholesterol is via
“estrogenic” effects stimulated by the ingestion of
isoflavones (Potter, 1995; Anthony et al, 1996). It is
well known that mammalian oestrogens have a signifi-
cant impact on serum lipids, promoting decreases in
LDL and increases in HDL cholesterol. Evidence for
an effect of isoflavones on serum cholesterol concen-
trations has been demonstrated in rats, hamsters,
nonhuman primates and humans (Cassidy et al,
1995; Pelletier et al, 1995; Anthony et al, 1996; Balmir
et al, 1996; Clarkson et al, 1998). The three primate
studies reported by Anthony et al (1994, 1995a,
1995b) demonstrated that soy protein rich in isofla-
vones favourably affected serum lipids, and that soy
protein from which the oestrogens had been extracted
had a minimal effect. The authors concluded that soy
isoflavones may account for 60% to 70% of the hypo-
cholesterolemic effects of soy beans.
Cassidy et al (1995) reported that human consump-
tion of 45 mg of isoflavonoids per day significantly re-
duced the total and LDL cholesterol concentrations in
young females. Similar findings were reported by
Potter et al (1993) and Bakhit et al (1994). In con-
trast, Nestel et al (1997) reported no significant effect
on blood lipid concentrations of 45 mg of the isofla-
vone genistein, administered over a 5 to 10-week pe-
riod. However, a significant improvement in systemic
arterial elasticity was found in these women.
The effect of isoflavones on coronary vascular reac-
tivity in an atherosclerotic primate model was studied
by Honore et al (1997). They reported that the arter-
ies of females fed a low-isoflavone diet constricted in
response to acetylcholine, whereas the arteries of fe-
males who were fed a high-isoflavone diet dilated. In
a study involving male primates (Anthony et al, 1997),
the prevalence of atherosclerotic lesions was the low-
est in monkeys fed soy protein plus isoflavones, inter-
mediate in monkeys fed an alcohol-extracted soy pro-
tein low in isoflavones, and highest in monkeys fed a
mixture of casein and lactalbumin. Another mecha-
nism whereby soy beans may decrease the risk of
cardiovascular disease is to lower the susceptibility of
LDL cholesterol to oxidation (Lichtenstein, 1998).
Isoflavonoids have been reported to inhibit the oxida-
tive modification of LDL by macrophages (Kapiotis et
al, 1997), enhance the resistance of LDL to oxidation
(De Whalley et al, 1990; Kanazawa et al, 1995), and
exhibit antioxidant activities in an aqueous phase
(Ruiz-Larrea et al, 1997). Genistein inhibits bovine
aortic endothelial cell-mediated and human endothe-
lial cell-mediated LDL oxidation, and protects vascular
cells from damage by oxidised LDL (Kapiotis et al,
1997). Nestel et al (1997) did not observe the antioxi-
dative effect of genistein.
The soluble fibre content of soy beans is relatively low
(see Table 1). Yet some data demonstrate that soy
fibre is effective in lowering serum cholesterol in pa-
tients with raised cholesterol levels (Tsai et al, 1983;
Shorey et al, 1985; Lo et al, 1986). However, in two
studies that investigated whether different amounts of
soy protein with and without soy fibre were effective in
lowering serum lipids (Potter et al, 1993; Bakhit et al,
1994), no additive cholesterol-lowering effect of soy
cotyledon fibre could be demonstrated. According to
Potter et al (1993), this may indicate that the choles-
terol-lowering effect of soy protein consumption may
override the effects others observed with soy cotyle-
don fibre (Shorey et al, 1985; Lo et al, 1986). The
beneficial effects of the insoluble fibre components of
soy beans on bowel function (increased stool weight
and decreased gastrointestinal transit time) have
been demonstrated repeatedly, as discussed by
Slavin (1991) in a review article. The fibre component
of soy beans is therefore probably not responsible for
the cholesterol-lowering effect of soy beans. It is also
Health benefits of soy beans and soy products: a review
ISSN 0378-5254 Journal of Family Ecology and Consumer Sciences, Vol 27: No 1, 1999
29
likely that the cholesterol-lowering effect of soy is due
to a combination of components acting together, and
that the mechanism varies in different species. More
work is required to determine the mechanisms of in-
tact soy beans as well as its components in animals
and humans. A better understanding of the mecha-
nisms involved would help to optimise the use of die-
tary soy protein for the treatment of raised cholesterol
concentrations.
ANTICARCINOGENIC EFFECTS OF SOY BEANS
Potential anticarcinogenic effects
Evidence from epidemiological studies suggests, al-
though not entirely consistently, that soy bean-based
diets protect against cancer of the breast (Nagasawa,
1980; Wu et al, 1998), prostate (Severson, 1989;
Shimizu et al, 1991) and colon (Watanabe & Koessel,
1993). An epidemiological study carried out in Singa-
pore found an inverse relation between the consump-
tion of soy bean products and the risk of breast can-
cer in premenopausal women (Lee et al, 1991), but a
subsequent study of Chinese women failed to find a
similar association (Yuan et al, 1995). Further evi-
dence that soy may protect against breast cancer de-
velopment was provided by studies of rodent cancer
models in which dietary soy supplements inhibited
chemical and radiation-induced breast tumours (Troll
et al, 1980; Barnes et al, 1990; Constantinou et al,
1998), prostatic dysplasia (Mäkelä et al, 1991) and
colon cancer (Weed et al, 1985; Thiagarajan et al,
1998). Cell culture experiments have also shown that
soy bean constituents completely prevent or suppress
the induction of tumours in various organs (reviewed
by Herman et al, 1995). Epidemiological studies as
well as animal and cell culture experiments therefore
provide evidence that suggests that the intake of soy
beans lowers the risk of cancer.
Possible mechanisms in preventing cancer
A number of different compounds in soy beans may
be responsible for various types of anticarcinogenic
activity. These compounds include a protease inhibi-
tor (the Bowman-Birk inhibitor), a trypsin inhibitor,
isoflavones (genistein and diadzein), saponins, inosi-
tol hexaphosphate (phytic acid) and the sterol, -
sitosterol (reviewed by Kennedy, 1995). Examples of
different types of in vitro anticarcinogenic activity re-
ported for a variety of soy bean constituents are sum-
marised by Kennedy (1995). These constituents in-
clude the ability to prevent malignant transformation
(protease inhibitor), the ability to suppress promotion
(trypsin inhibitor), the inhibition of proliferate growth of
human breast cancer cell lines in culture (genistein),
and inhibition of the expression of an oncogenic virus
(saponins).
Twenty to 25% of the total protease inhibitor content
of soy bean protein is the Bowman-Birk inhibitor (BBI)
(Kennedy, 1995). BBI has shown the greatest sup-
pression of carcinogenesis in animal carcinogenesis
assays. St Clair et al (1990) observed that BBI can
completely prevent colon carcinogenesis (100% sup-
pression). It suppresses carcinogenesis in the liver by
71%, in the oral epithelium by 86%, and in the lung by
48%. The ability of BBI to suppress carcinogenesis in
the various systems that were studied far exceeds the
ability of other soy bean-derived compounds
(Kennedy, 1995). The anticarcinogenic activity of BBI
has been observed in many different tissues, in many
different cell types (including cells of epithelial and
connective tissue origin), with many different types of
carcinogenic agents, including ionising radiation used
in the mammary carcinogenesis studies reported by
Troll et al (1980) and chemical carcinogens used in
the studies reported by Barnes et al (1990), reviewed
by Kennedy (1993). The trypsin inhibitor inhibits the
growth of a variety of malignant cell types in vitro, but
this protease inhibitor does not have the ability to sup-
press oral carcinogenesis induced by 7,12-
dimethylbenzanthracene (DMBA) in hamsters
(Messadi et al, 1986).
The observation of Barnes et al (1990) that both raw
and autoclaved soy beans inhibited chemically in-
duced mammary cancer in rats, was important be-
cause the protease inhibitors in soy beans, which are
thought to be potent chemopreventive agents, are de-
stroyed by autoclaving (Messina & Messina, 1991).
The data of Barnes et al suggested that the isofla-
vones in soy beans were responsible for tumour inhi-
bition. In vitro genistein inhibits tyrosine protein
kinases, DNA topo-isomerases and S6 kinases
(Yamashita et al, 1990). The activity of these en-
zymes is enhanced in oncogene-transformed cells
(Yamashita et al, 1990). Isoflavones may conse-
quently have a role to play in preventing a wide range
of cancers. Several studies have looked specifically
at the oestrogenic/antioestrogenic effects of soy
beans (Wilcox et al, 1990; Cassidy et al, 1995).
These studies suggest that isoflavones possess both
antioestrogenic and oestrogenic activity, and in
premenopausal women soy consumption influences
hormonal patterns in a way that is potentially protec-
tive against breast cancer (Messina & Messina,
1991). Lamartiniere et al (1998) recently reported
that pharmacologic doses of genistein given to mature
rats enhance mammary gland differentiation, resulting
in a significantly less proliferative gland that is not as
susceptible to mammary cancer. These authors
speculate that breast cancer protection in Asian
women on traditional soy-containing diets is, in part,
derived from early exposure to genistein-containing
soy.
Three other compounds in soy beans have also been
shown to suppress carcinogenesis in animals, namely
saponins, phytic acid and -sitosterol. Saponins are
cytotoxic to sarcoma cells in culture (Huang et al,
1982, in Kennedy, 1995) and they inhibit the expres-
sion of an oncogenic virus (Tokuda, 1988). Phytic
acid was observed in one experiment on mice to sup-
press colon carcinogenesis by 25% (Shamsuddin et
al, 1989). In an experiment in which the soy bean
sterol ß-sitosterol was assayed for its ability to sup-
Health benefits of soy beans and soy products: a review
ISSN 0378-5254 Tydskrif vir Gesinsekologie en Verbruikerswetenskappe, Vol 27: No 1, 1999
30
press colon carcinogenesis, the sterol was able to re-
duce the total number of benign and malignant tu-
mours by 39% (Raicht et al, 1980).
As discussed above, there are a number of different
compounds in soy beans with various types of anti-
carcinogenic activity. For all of these compounds
there are likely to be toxic effects that have to be
studied along with their anticarcinogenic activities.
Human cancer prevention trials of the isoflavones and
of BBI have begun recently.
BONE-STRENGTHENING EFFECTS OF SOY
BEANS
There are indications that soy beans reduce the inci-
dence of postmenopausal osteoporosis. Genistein in
low doses maintained bone mass in ovariectomised
rat models (Anderson et al, 1995a; Arjmandi et al,
1996). Adlercreutz and Mazur (1997) reported some
effects of soy or isoflavonoid intake in patients with
menopausal symptoms such as hot flushes, vaginal
dryness and bone resorption, and discussed the low
incidence of menopausal problems in Japanese
women compared to Canadian, American and Finnish
women. The results of a study of the short-term ef-
fects of soy bean isoflavones on bone strength in
postmenopausal women indicated that a high-soy diet
increased bone mineral content and bone density in
the lumbar spine (Erdman, 1998). Potter et al (1998)
also reported a significant increase (2%) in both bone
mineral content and bone density in the lumbar spine
of postmenopausal women after six months on a diet
that included 40 g of protein per day from isolated
soy protein containing 2,25 mg isoflavones/g protein.
Their findings are of interest for two reasons:
Firstly, of all the skeletel sites they measured, the
spine is the area that is most sensitive to oestrogen
because of its higher trabecular bone content. The
spine is remodelled more rapidly than the hip which
contains a higher proportion of cortical bone (Ettinger
et al, 1992).
Secondly, although Potter et al (1998) had hypothe-
sised that a isoflavone-containing soy protein diet
would delay the decrease in bone density (compared
to the control diet), they actually found that there was
a slight increase in bone density and mineral content.
However, this was a short study with respect to bone,
and these findings should be confirmed by more ex-
tensive studies (e.g. 2-3 years).
Arjmandi (1998) reported improvement in femoral
bone density in rats that were fed soy protein isolate
for 35 days compared to rats fed a casein-based diet.
However, he recommends additional long-term stud-
ies to determine the effects of soy beans on maintain-
ing bone health. Adlercreuz and Mazur (1997) sug-
gest that isoflavonoids may to some degree inhibit os-
teoporosis but may be insufficient for complete pro-
tection as single prevention strategy.
EFFECTS OF SOY ON THE MANAGEMENT OF DIA-
BETES MELLITUS
Both Jenkins et al (1984) and Anderson et al (1984)
did extensive research on the role of dietary fibre in
the management of diabetes. Recent studies suggest
that blood glucose may be influenced by various die-
tary fibres, although usually the most effective fibre
sources for control of diabetes are soluble fibres such
as guar gum and pectin (Slavin, 1991). However, they
are neither highly palatable nor acceptable for long-
term therapy of diabetes. It is considerably easier to
incorporate soy fibre in a meal without greatly affecting
the texture and palatability of the meal.
Tsai et al (1987) studied seven obese patients with
noninsulin-dependent diabetes mellitus (NIDDM). The
subjects were given a standard meal with and without
10 g of soy fibre. The soy fibre supplement signifi-
cantly enhanced the return of serum glucose levels to
fasting levels during the latter half of the test meal.
Soy fibre had no effect on plasma insulin. Verster
(1993) studied the long-term effect of either an energy-
restricted high-carbohydrate, high-fibre, low-fat
(HCHFLF) diet with a daily addition of 150 g (cooked
weight) of dry beans compared to the influence of the
same diet with the addition of 50 g (raw weight) of soy
protein isolate on the metabolic control of sixteen
NIDDM patients for a twelve-week period. Both diets
improved glycaemic control as indicated by decreased
glycated haemoglobin (HbA1) concentrations. Lo et al
(1986) conducted glucose tolerance tests on patients
with hyperlipidemia. Adding 25 g of soy fibre in a
cookie to the diets of subjects significantly reduced
their fasting glucose levels by 8,5%. Thus, although
some studies found a positive effect on control of dia-
betes, more research is needed in this area.
RECOMMENTATIONS FOR OPTIMUM DIETARY IN-
TAKE OF SOY PRODUCTS
Oriental populations consume 20 mg to 80 mg of the
isoflavone genistein per day, almost entirely derived
from soy, whereas the dietary intake of genistein in the
United States is only 1 mg to 3 mg per day (Barnes et
al, 1996). Soy beans contain 100 mg to 300 mg of the
isoflavones genistein and daidzein (Herman et al,
1995). According to Craig (1997), it is possible to ob-
tain substantial levels of dietary isoflavones through
the daily consumption of 30 g to 60 g of soy protein.
One half-cup of soy beans, one cup of soy beverage
or 120 g of tofu provide 30 mg to 40 mg of gen-
istein. Soy bean concentrates prepared by alcohol ex-
traction display no oestrogenic activity due to genistein
extraction in aqueous ethanol, and soy protein isolates
contain only small amounts of isoflavones (Nash et al,
1967).
CONCLUSIONS
Researchers are just beginning to understand the
physiological and biochemical effects of soy consump-
Health benefits of soy beans and soy products: a review
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31
tion. Considerable progress has been made since the
First International Symposium on the Role of Soy in
Preventing Chronic Disease was held in 1995
(Messina et al, 1998). The potentially beneficial ef-
fects of soy consumption clearly indicate both the
need and the justification for more clinical and experi-
mental studies. Further studies are required to exam-
ine the effects of soy beans and soy bean products on
cardiovascular risk factors, cancer, osteoporosis and
the relief of menopausal symptoms. Although multiple
factors are driving research on soy, the single most
important factor is arguably that soy beans are a con-
centrated source of isoflavones. Whereas relatively
little research on soy bean isoflavones had been con-
ducted before 1993, well over 1 000 studies dealing
with isoflavones were published between 1994 and
1996 (Messina et al, 1998). Definite data about the
relationship between soyfoods and isoflavones and
the risk of chronic disease may be many years away.
However, the foundation has now been laid for re-
search to determine not only the effects of soy and
isoflavones on serum lipids, but also on the incidence
of heart disease; not only on bone mineral density, but
also on fracture risk, and not only on biomarkers of
cancer risk, but also on cancer rates.
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