Soy - Its role in the prevention and treatment of the diseases
Bibliographic Revision in charged to: Ma. José Borreguero
Doctor in Chemical Science

From a nutritional point of view, the soy and its by-products, were identified basically for their protein contents, but during last years, there has been a growing interest between scientists to know the causes and the potential of the soy in the treatment and prevention of a lot of diseases.
Recently, the scientific studies have been focused on the presence of isoflavones. In differents works it has been proved that the isoflavones inhibit the growth of the carcinogenic cells, low the cholesterol level and reduce the osteoporosis.

Proteins

The concentration of proteins in the soy is the highest between the legumes. But the quality is more important than quantity.
The protein content in the vegetables has a low content in sulphurated amino acids (metionine and cystein), except in the soy, where the level of these amino acids is enough to give the quantities required for the human body (Young 1991).
The soy protein receives a PDCAAS (method to evaluate the quality of the protein) between 0.95 and 1.0, the highest possible value.

Fats

An approximate 53% of the fat content of the total of the fat acids belongs to linoleic acid. However, the most important fact is that approximately between the 7% and 8% belongs to the essential fat acid, alfa-linolenic. There are relatively a few vegetable sources, so the consumption of soy milk is a convenient way of covering the daily requests.
The ratio between fat acids n-6 and polinsaturated fat acids n-3, suggested for the OMS is between 4:1 and 10:1 (HOW/FAO 1994). The ratio between the linolenic and the alfalinolenic acids in the soy is between 7-8:1

Carbohydrate

Among them we need to emphasize the oligosacarids. The soy is the legume that contains the highest concentration of oligosacarids.
Last years it has been proved the benefits of consuming oligosacarids due to they promote the growth of the bifidobacterium, related to the reduction of the colon cancer (Tomomatsu 1994) (Burkitt and Trowell 1975).
Respect to the soluble carbohydates, the rafinose and the estaquinose are the most important, principally because their presence is related to the flatulence and abdominal strain related to human consumption of soy milk.

The human being doesn´t have the enzyme alfa-galactosidose necessary to hydrolize the alfa-galactosidic chemical bond in these oligosacarids. For this reason they can not hidrolyze in the duodenum and they go intact to the large intestine where they are metabolized for microorganisms that have this enzyme. The result is a production of gas like carbon dioxide, hydrogen, nitrogen, methane, etc., depending on the diet and the microflora typical for each individual (Cristofaro et al. 1974, Liener 1994).

In spite of the presence of oligosacarids in the by-products of the soy is considered generally undesirable, recent studies have proved beneficial effects in the human diet (Masai et. al. 1987, Takasoye et al. 1991, Tomomatsu 1994). These are:

1.- Increase of the population of bifidobacterium in the colon, which have an antagonistic effect, suppress the effect of the bacterium with its putrefied activity.

2.- Reduce the toxic metabolit and damaging enzymes.

3.- Prevent the patogenic and autogen diarrhoea thanks to the same mechanisms described in the reduction of of the damaging bacterium.

4.- Prevent the constipation due to the production of the high levels of short chain fat acids.

5.- Have a function of hepatic protection since they reduce the toxic metabolits.

6.- Reduce the blood pressure.

7.- Have anticarcinogenic effects.

8.- Produce nutrients like the vitamins since they increase the activity of the bifidobacterium.

Soy isoflavones

During last years the scientific community has begun to recognize the importance of the micronutrients present in the plants, usually called fitochemicals. In the case of the soy, we need to talk about the presence of isoflavones.

The isoflavones have a very limited presence in the nature and in practice we can consider that the only natural source of these components is the soy (Steinmetz and Potter, 1991). For this reason we are not surprised for the fact that the isoflavon levels found in blood and the urine in the Asian people (Adlercreutz at al. 1993) and in vegetarian western people (Adlercreutz at 1995) is between 10 and 100 times higher than the levels in individuals with a traditional western diet.
A glass of soy milk contains approximately 30mg of isoflavones, which is considered the necessary daily quantity (Hertog et al. 1995).

One of the most important properties of the isoflavones is their estrogenic activity. Recent publications have proved the estrogenics and antiestrogénic effects of the isoflavones of the soy (Makela et al. 1995).

First, they can act as antiestrogens when the endogens levels of estrogens are relatively high, like in premenopausic women (Lonkovaara et al. 1995, Cassidy et al. 1994, 1995, Fujimoto et al. 1995). Secondly, they can act astrogen agonists, when the levels of estrogens are low, in the same way as we find postmenopausic women (Biggers and Curnow 1954, Bickoff 1962, Martin et al. 1978, Tang and Adams 1980, Mayr et al. 1992, Markiewicz et al. 1993).

Soy in the prevention and treatment of cancer

Some studies prove that the consumption of soy is related to an effective protection against some types of cancer (lung, colon, rectal, breast, stomach and prostate) (Akiyama and Ogawara 1991, Kiguchi et al. 1990, Kondo et al. 1991, Okura et al 1988, Schweigerer et al. 1992, Watanabe et al. 1989, 1991).

It is believed that the isoflavones prevent the cancer, for their antioxidant properties (Wei et al. 1993) and for the inhibition of some enzymes that regulate the cellular growth (Ogawara et al. 1986, Akiyama et al. 1987, Thorburn and Thurburn 1994, Linassier et al. 1990, Markowits et al. 1989, Constantinou et al. 1990).

Breast cancer

Breast cancer is the most common cancer among women and the second cause of death (American Cancer Society 1994). the risk of breast cancer among women is between 1 and 9.

We have found a relation between the soy consumption and the decrease in the prevalence of this type of cancer. So in Japan, where people consume regularly soy food, the prevalence of breast is four times less than the one in the USA (Yuan et al. 1995, Hirose et al. 1995). Besides, it has been proved that the antiestrogenic action of the isoflavones of the soy (Peterson and Barnes 1991).

Prostate cancer

Among men the most common cancer is the prostate one (American Cancer Society 1994). The incidence in the USA is approximately 1 in 11.

In this type of cancer we have observed as well that the incidence is between 10 and 15 times higher in the USA than in Japan (Yatani et al. 1989). Some scientific researches among which we need to mention the Makela one (1995) in which it was proved that a marked decrease of the incidence of the prostate cancer in animals feeded with with a diet that contains soy in comparison with the control group. These discoveries make us suppose that the soy consumption is the element which contributes to the low incidence of the prostate cancer in the Japan men.

Although the specific function of the estrogens in the prostate cancer is not well defined. The potential estrogenics effects of the isoflavones let us expect a protection effect, specially after having used successfully the hormonal therapy of the prostate cancers with metastasis (Pienta and Esper 1993).

The scientific researches of Peterson and Barnes 1993 and Evans et al. 1995, prove that the isoflavones of the soy can inhibit the growth of the prostate cancer thorugh the mechanisms hormonal and no hormonal (inhibiting the action of some enzymes).

Treatment of the cancer

Some data demonstrate that the soy can be used in the treatment of carcinogenic tumours together with the conventional chemotherapeutics agents, increasing their action. So, the the researches of Takeda et al. 1994 and Clark et al. 1989, show that in vitro the isoflavones of the soy potentiate the effectiviness of the chemotherapeutics and of the other anticarcinogenic drugs.

Soy and Osteoporosis

The osteoporosis is a big world problem. The osteoporosis is particularly problematic in developed countries. In the USA, for example, between 15 and 20 milion people suffer from osteoporosis (Osteoporosis Consensus Panel 1884).

Animal protein, soy protein and osseous health

It´s believed that one of the factors that is adverse for the osseous health is the daily consumption of protein, so Wachman and Bernstein (1968) were the first in proposing that a higher protein consumption can provoque osteoporosis for an increase of the urine excreción of Calcium. In spite of the high content of soy protein, it seems that the substitution of the animal protein for the soy protein is advantageous.

A very important point to fight against the osteoporosis is the reduction of the excrecion of calcium, optimizing in this way the retention of calcium in bones.

Although other factors can be important as well, it seems that the hipercalciuric effect of the protein is due to the metabolism of the sulfoaminoacids, metionine and cystein. So the low content of sulfoaminoacids of the soy protein can help to reduce the excrecion of calcium in comparison with the animal protein (Remer and Manz 1994, Green and Kleeman 1991), since the soy protein contains minor quantities of sulfoaminoacids than the milk or animal protein (Pennington 1994).

But, apart from the protein, other compounds in the soy that help to a good state of the osseum structure exist. Some experiments suggest that the isoflavones of the soy inhibit the resorcion of the bone and stimulate the formation.

An interesting observation is the similarity in chemical structure, between the isoflavones of the soy and ipriflavone (medicine used to inhibit the resorcion of the bone, through an estrogenic mechanism) (Tsutsumi et al.1994).

In different scientific researches done in animals an increase of the osseum mass with the consumption of soy has been observed (Arjmandi et al.1995, Anderson et al. 1987 and Blair et al. 1996). Fanti et al. 1996 suggests that the “isoflavonas” of the soy increase the osseum density, through the stimulation of the formation of the bone more than inhibiting the resorcion.

We need to remark that the soy is rich in calcium (15 mg/100 g) easily absorbible for the organism (Heatney et al. 1991, Weaver and Plawecki 1994).

Soy and kidney diseases

In kidney diseases, a restriction in the protein contribution is recommended, since it has been proved that this provoques a worsening of the renal function (Brenner et al. 1982, Klahr 1990). However, not all the proteins affect in the same way the renal functions and replacing the animal proteins for proteins of soy benefits the nephrotic patients and helps to reduce the risk of developing kidney diseases to the high risk individuals.

Some scientific researches have proved that a vegetarian feeding in which the proteins of soy are a very important part of the protein contribution, some parameters of the renal function improve respect to the these individuals that consume a diet where the proteins derive from animal sources (Kontessis et al. 1995).

It seems that the effect of the soy on the renal function is due to the protein and to the hipocolesteronemic effect of the soy. This fact has become very important recently since it has been recognized that the pathology of the arteriosclerosis and of the renal disease is similar, consequently controlling the lipid levels in the renal disease is extremely important (Guijarro and Keane 1994, Wheeler et al. 1994).

Soy and heart diseases

It´s known that the approximately a 20% of the population has high levels of cholesterol, fact that provoques a high risk of suffering a heart disease (National Cholesterol Education Program 1993).
Recently some publications (Carroll 1991) have proved the property of reducing the levels the cholesterol in blood of the protein of soy. Yet since the decade of the 70s, the Government Department Responsible for Public Health in Italy supplied protein of soy for the treatment of the high cholesterol levels in blood (Sirtori et al. 1993).

The first test in humans proving the hipocolesterolemic effects of the soy was published in 1967 (Hodges et al. 1967), since then approximately 40 clinical studies have been published studying this relation.

Another point to take in account is the relation between LDL and HDL Cholesterol. The diets aimed to reduce the cholesterol levels generally reduce the levels of the LDL and HDL Cholesterol. In the case of the protein of soy, however, only the value of LDL is reduced, in this way we get a relation LDL:HDL favourable (Nilausen and Meinertz 1996, Potter et al. 1996, Kurowska et al. 1996).

It seems that the effect of the soy on the heart diseases works for two ways:

1.- The hipocolesterolemic effect of the “isoflavonas” (Kanazawa etal. 1995)
2.- The decrease of the risk of heart disease independent of the reduction of the cholesterol levels (Murphy et al. 1993, Hollenberg 1994)

Soy and menopause

We know that the incidence of the menopausal symptoms, like night sweats and hot flushes, in Japanese women is a third of the reported ones between United States women (Lock 1994). The consumption of soy has been proposed as an explanation for this low incidence (Adlercreutz et al. 1992). The biological base would be the “estrogenico” effects of the isoflavones of the soy.
The role of the soy in the reduction of the own symptoms of the menopause is important, due to the millions of women that are arriving at the menopause in the coming years and only a few of them choose the therapy of the hormonal replacement.

In 1990, Wilcox et al. (1990) indicated the estrogenic effects observed for the consumption of soy in postmenopausic women fixed by the rate of the vaginal maduration.

The hormonal treatment apart from eliminating the menopausal symptoms (Woods et al. 1996, Dalais et al. 1996, Burke 1996) reduces the “cardiovascular” risk and the osteoporosis and protects at the same time from the colon cancer (Davidson 1995). The effect of the consumption of soy in the elimination of the “menopausico” symptoms has been proved but from one clinical point of view it hasn’t been proved that the consumption of soy has the same effects as “sustitutivo” hormonal treatment.