Pr. Daniel TOMÉ
François MARIOTTI

 While being a traditional food in Asia, soybean became in the West an industrial raw material in the processing of food ingredients. It essentially contains lipids, proteins and minerals. Some of its components, such as isoflavonoids, could have beneficial effects on health. New interests in these compounds should not overcast the importance of a diversified and balanced diet.

Soy is a legume traditionally consumed in Asia and recently introduced in western countries. The development of soy by-products adapted to western taste has been favoured by the good nutritional reputation of the bean. Soy is used mainly in the form of ingredients, derived from the bean, in industrial food processing. Following the technological and economical development of non-meat food products, soybean extracts now occupy an important place on the market of vegetable protein sources. These ingredients are used for their functional organoleptic properties (water retention, binding, emulsion, texture, etc.) or their nutritional properties (low in fat, high in proteins). The unique characteristics of soy raise growing interest for this food both as an important source of nutrients in a balanced diet, and also for the role it may play as part of a "preventive" diet.

Soy germ has a relatively low nutrient density. However, the bean and its by-products (see Table 1) represent a substantial source of proteins, lipids and minerals (see Table 2) in a diversified and balanced diet. Soybean is a rich protein source: it contains about 40% protein. Undesirable component of the bean, such as trypsin inhibitors and haemagglutinin, are destroyed by traditional or industrial cooking methods. Recent data indicates that after an adequate technological treatment, soy proteins have a satisfying biologic value for man, yet lower than milk proteins. As for essential amino acids, soy has a higher lysine content than many other plant sources while being limited in sulphur amino acids (methionine, cysteine). Cereals and legumes, in particular soybeans, have been traditionally associated to one another: soy complements cereals that are lysine deficient which, in turn, provide a high source of sulphur amino acids. Soy-based infant formulas supplemented in methionine are used in cases of lactose intolerance and allergies to milk proteins. However, the use of these formulas is limited by the existence of a cross-allergy to soy proteins, that might occur in 20 to 30% of children allergic to milk proteins. Soybean is characterised by a high lipid content: it contains about 20% fat. Soybean oil, which is little consumed in western countries, is a source of essential polyunsaturated linoleic and alpha - linolenic fatty acids. It also contains lecithins that represent a source of choline and that are extensively used as emulsifying agents. The main minerals in soybean are calcium, zinc and iron. However, bioavailability of calcium is reduced by the presence of phytic acid and oxalic acid. Bioavailabitily of zinc is reduced by the presence of phytic acid. Iron, although present in important amounts, shows little bioavailability. Hence, phytates are susceptible to reduce mineral bioavailability in soy-based infant formulas.

The industrial use of soy led to research to identify the "protective" properties associated with certain fractions of soybean.

Compounds of interest:
Soybean compounds that have been investigated are found in its protein and lipid fractions, in fibre and especially among the specific molecules produced from intermediary metabolism of plants. In spite of the abundant literature on soy, many questions still remain unanswered regarding its properties.
Soy is a source of soluble and insoluble fibre with well-known effects on the digestive transit, the onset of colon cancer, blood sugar levels and the elimination of cholesterol. Fermentable ploysaccharides induce flatulence but could beneficially modify the intestinal flora.
Soy contains isoflavones (genisteine, aidzeine, and glyceteine) known as phyto-œstrogens and which have lately been attracting growing attention. These isoflavones show anti-oxidant properties and possess œstrogenic or anti - oestrogenic activities. They would take part in various biologic "protective" actions attributed to soy extracts.

NUTRITIVE VALUE OF SOYBEAN AND SOY BY-PRODUCTS
Commercialised Products (100 g) :
	Soy Germ (raw)	Seed (raw)	Soybean oil	Soy Milk
Energy (Kcal)	122	416	884	33
Proteins (g)	13,1	36,5	-	2,75
Lipids (g)	6,7	19,9	100	1,9
Polyunsaturated fatty acids (g)	3,8	11,3	37,6	0,8
Carbohydrates (g)	9,6	30,2	-	1,8
Fibre (g)	-	9,3	-	1,3
Calcium (mg)	67	277	-	4
Iron (mg)	2,1	15,7	-	0,6

	Miso	Natto	Soy sauce (tamarin)	Tempeh	Tofu (raw)
Energy (Kcal)	206	212	60	199	76
Proteins (g)	11,8	17,7	10,5	18,9	8,1
Lipids (g)	6,1	11	0,1	7,7	4,8
Polyunsaturated fatty acids (g)	3,4	6,2	0,04	4,3	2,7
Carbohydrates (g)	28	14,4	5,6	17	1,9
Fibre (g)	2,5	5,4	0	-	1,2
Calcium (mg)	66	217	20	93	105
Iron (mg)	2,7	8,6	2,4	2,3	5,4

Ingredients for the agri-food industry (100 g) :
	Soy concentrate	Full-fat flour (baked)	Defatted flour	Soy isolate	Soy lecithin
Energy (Kcal)	332	441	329	338	763
Proteins (g)	58,1	34,8	47	80,7	-
Lipids (g)	0,5	21,9	1,2	3,4	100
Polyunsaturated fatty acids (g)	0,2	12,4	0,5	1,6	45,3
Carbohydrates(g)	31,2	33,7	38,4	7,4	-
Fibre (g)	3,8	-	17,5	5,6	-
Calcium (mg)	363	188	241	178	-
Iron (mg)	10,8	5,8	9,2	14,5	-

Investigated properties:
Hypocholesterolemic effects of various plant foods (particularly soy) have been studied for numerous years. It has been shown that soy reduces circulating cholesterol levels by decreasing LDLs (atherogenic fraction) and the LDL/HDL ratio (atherogenic fraction / anti - atherogenic fraction). This reducing effect is increasingly important, as the initial cholesterol level is high. This property is probably largely due to isoflavonoids through their œstrogenic or anti-oxidant action. It may also be due to the protein fraction of the bean. Even if no mechanism has been clearly explained yet, in October 1999 the United States' Food and Drug Administration allowed the health claim that soy-containing products reduce the risk of cardiovascular diseases.
The effect of soy in the prevention of cancer is still unclear. It would mainly be associated with isoflavones. Bowman-Birk trypsin inhibitor and phytic acid (through anti-oxidant properties) also seem to inhibit the onset of cancer, particularly colon cancer.
Oestrogenic properties of soy isoflavones are studied in pre-menopausal women, especially for their effect on hot flushes and bone loss.
In children however, presence of isoflavones in soy-based infant formulas raise the issue of the consequences of their potential endocrine effect.
Protective properties of soy still have to be shown in physiological settings, and as part of a diversified diet.

MAIN SOY BY-PRODUCTS AND THEIR USE

Soy and soybean by-products are studied for their nutritional quality but also for the possible "preventive" properties of some of their specific components, particularly proteins and isoflavones. With our diet becoming more diversified, soy consumption may increase in the years to come. Food fads must however be avoided. The recent interest in the role of vitamins and mineral and in some protective "micro - nutrients" such as isoflavones is due to the fact that the importance of fruits and vegetables in a diversified diet has long been neglected. We should not repeat this error now by minimising the nutritional importance of animal products in a diversified and balanced diet.

Anderson J.W. Johnstone B.M.
and Cook-Newell M.E.
A meta-analysis of effects of soy protein intake on
serum lipids in human.
N. Engl. J. Med. 1995; 333 : 276-282.

Cassidy A., Griffin B.
Phyto-oestrogens: a potential role in the prevention of CHD?
Proc. Nutr. Soc. 1999 ; 58 : 193-199.

Lucas E.W. and Riaz M.N.
Soy protein products : processing and use.
J. Nutr. 1995 ; 125 : 573S-580S.

Mariotti F., Mahé S., Benamouzig R., Luengo C., Daré S., Gaudichon C., and Tomé D.
Nutritional value of [15N]- soy protein isolate assessed from ileal digestibility and postprandial protein utilization in humans.
J. Nutr. 1999 ; 129 : 1992-1997.

Setchell K.D.R. and Cassidy A.
Dietary isoflavones : Biological effects and relevance to human health.
J. Nutr. 1999 ; 129 : 758S-767S.