Soyabean (Glycine max)

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Latin Name Glycine max, G. soja, G. hispida
Sanskrit Name Soyabean
English Name Soybean, Soya
Common Name Soyabean, Raam Kurthi, Bhat
 

 

Phytochemistry:

Soybean is rich in protein, oil and minerals, but low in carbohydrates. It also contains water-and fat-soluble vitamins. The major portion of soy protein is composed of glycinin and betaconglycinin. Soy saponins are divided into three groups according to their respective type of aglycon, soyasapogenol A, B and E.

The main constituents are a natural mixture of phosphatides, mainly phosphatidylcholine (2031.6%), phosphatidylethanolamine and phosphatidylinositol, in combination with fatty acids, carbohydrates, and other substances. Soybean lecithin contains 11.7% palmitic, 4% stearic, 8.6% palmitoleic, 9.8% oleic, 55.0% linoleic, 4.0% linolenic, and 5.5% C20 to C22 acids.
It also contains Protein Carbohydrate, Fibre, Ca, P, Fe, Na, K, Vitamin A and Vitamin C.

Pharmacological Actions:

It is antioxidation, antifatigue, antiobesity and hypoglycaemic.

Medicinal Use:

Soy lecithin (phospholipids extracted from the seeds of G. max)—used for moderate disturbances of fat metabolism, especially hypercholesterolaemic (if dietary measures are not sufficient). (German Commission E.)
Published studies suggest that phospholipids may be useful in the treatment of menopause and post-menopausal conditions, cancer, hypertension, aging, and benign prostatic hyperplasia.

Clinical / experimental study:

Saponin A and AB group fraction protects the liver against antioxidation and improved lipid metabolism in the injured liver.

Administration of a small peptide derived from soybean showed antifatigue, antiobesity and hypoglycaemic activity in mice.

Feeding soy protein to hamsters, consistently, resulted in significantly reduced incidence of gallstones.

In studies of experimental carcinogenesis in animals, soybean isoflavones exhibited protective effect in 65% animals.

It has antioxidant property. Tyrosine autophosphorylation effect of small peptides derived from soybean on fatigue, obesity and glycemia in mice investigated. It contains saponins which is inhibited the lipid peroxidation.

Soy may also lower blood lipids. Soy is recommended for hypercholesteremia in patients whose cholesterol levels do not respond to exercise or weight loss regimens. A recent meta-analysis of 38 studies noted that when dietary meat protein was supplanted with vegetable protein, risks for coronary artery disease were reduced.

The soy-based diets reduced serum levels of total cholesterol, LDL cholesterol, and triglyceride, without affecting HDL cholesterol.

Dietary supplementation with soy phospholipids may help patients with liver disease, alcoholism, or chronic parenteral nutrition reduces their risk of LA deficiency. Soy phospholipid 7379% (3-sn-phosphatydyl) choline products, in addition, are reported to reduce symptoms of liver disease, chronic hepatitis, or liver dysfunction due to malnutrition, such as loss of appetite and abdominal pain.

Soy lecithin is appears to act by improving the metabolism of cholesterol in the digestive system.

Cardiovascular Disease There are many potential mechanisms by which soy may improve cardiovascular outcomes, including reduction in total cholesterol, LDL, HDL, triglycerides, lipoprotein a, blood pressure, C-reactive protein, homocysteine, endothelial function, systemic artery compliance, and oxidised LDL.

A review by the North American Menopause Society suggests that the most convincing health effects of soy can be attributed to the actions of isoflavones on lipids, with studies finding statistically significant reductions in LDL and triglycerides, as well as increases in HDL.

Consumption of soy protein may produce cardiovascular benefits through multiple mechanisms, including the low methionine content reducing serum homocysteine concentration, reduction of the insulin/glucagon ratio, downregulation of the hepatic transcription factor sterol regulatory element binding protein-1, which in turn reduces lipotoxicity in the liver, regulation of hepatic lipid metabolism through upregulation of LDL receptors and increase in bile acid secretion, reducing hepatic fatty acid and triglyceride biosynthesis & increasing fatty acid oxidation, preventing the transfer of fatty acids to extra adipose tissues by increasing the adipocyte hormone adiponectin, and increasing bile acid secretion and bacterial conversion of cholesterol to the non-absorbable coprostanol.

Soy protein peptides may also act to decrease intestinal cholesterol absorption and bile acid uptake, reduce aortic accumulation of cholesterol esters and suppress food intake and gastric emptying by increasing cholecystokinin, and inhibiting angiotensin-converting enzyme. Soy protein is also reported to have beneficial effects on renal function, with suggestions that the isoflavones genistein and daidzein reduce glomerular damage by protecting LDL from oxidation and the high arginine content acts as a precursor for NO thus improving renal flow.

Soy isoflavones have, however, been found to improve systemic arterial compliance in perimenopausal and menopausal women. Soy protein, regardless of isoflavone content, modulates serum lipid ratios in a direction beneficial for cardiovascular disease risk in healthy young men.

Cardiovascular Disease
Substantial data from epidemiological surveys and nutritional interventions in humans and animals indicate that soy protein reduces serum total and LDL cholesterol and triglycerides, as well as hepatic cholesterol and triglycerides18. Based on this data the US FDA has approved a food label health claim stating that a diet with a daily intake of 25 g of soy protein, and low in saturated fat and cholesterol may reduce the risk of heart disease.

At least six systematic reviews have assessed the effects of soy isoflavones on lipid levels, and suggest that a diet supplemented with soy protein isolate containing isoflavones reduces LDL-cholesterol by approximately 0.15 mmol/L, but without clear effects on triglycerides or HDL-cholesterol26. In one of the most recent systematic reviews that included a total of 68 RCTs, it is suggested that soy has small to moderate effects on lipids with significant reductions in total cholesterol (by 2.5%), LDL (3%), triglycerides (6%) and no significant change on HDL, and that the effect is independent of isoflavone content. This review further suggests that higher doses of soy protein are associated with greater LDL reduction in those with elevated baseline LDL levels.

In a meta-analysis of 23 RCTs published from 1995 to 2002, soy protein with isoflavones was associated with significant decreases in total cholesterol (by 3.8%), LDL (by 5.25%), and triacylglycerols (by 7.27%) and significant increases in serum HDL-cholesterol (by 3.03%), with the observed changes being greater for those having higher baseline cholesterol levels or taking more than >80 mg/day isoflavone and the greatest lowering effects on total cholesterol and LDL-cholesterol occurred within short time frames, whereas improvements in HDL-cholesterol were only observed in studies of longer than 12 weeks duration.

Since these reviews a number of studies have confirmed the effects of soy on blood lipids, yet results continue to be mixed. In a double-blind RCT of 117 patients with hypercholesterolaemia 15 or 25 g/day soy protein was found to significantly reduce LDL, total serum cholesterol and apolipoprotein B levels without affecting HDL, triglycerides, homocysteine, folic acid, or vitamin B12, with 25 g/day of soy protein being twice as effective as 15 g/day28. Similar results were obtained in a similar study, with a preparation combining isolated soy protein with soy fibres and phospholipids showing twice the lipid-lowering effect of a preparation containing isolated soy protein alone. A soy protein substitution was also reported to have lipid-lowering effects in hyperlipidaemic but not normolipidaemic haemodialysis patients.

Blood Pressure
In addition to beneficial effects on lipids, epidemiological data suggest that soy may affect blood pressure. In an observational study of 45,694 participants of the Shanghai Women’s Health Study, aged 40–70 years with no history of hypertension, diabetes or cardiovascular disease, the intake of soy foods over 2–3 years was inversely associated with both SBP and DBP, particularly among elderly women. Results of this study found that compared to women consuming less than 2.5 g/day of soy, consumption of more than 25 g/day was associated with a significant reduction in SBP of 1.9 mmHg and a significant reduction in DBP of 0.9 mmHg and that the inverse association between soy consumption and blood pressure became stronger with increasing age, with significant reductions of –4.9 mmHg for SBP and –2.2 mmHg for DBP in women aged over 60 years.

Cardiovascular disease/lipid alterations
Soy isoflavones have strong biological properties in animals33, causing arterial vasodilation, lowering serum cholesterol, and inhibiting atherosclerosis in postmenopausal monkeys.

Soy protein has gained considerable attention for its potential role in improving risk factors for cardiovascular disease.

In October 1999, the US Food and Drug Administration approved labeling for foods containing soy as protective against coronary heart disease. Soybean phytosterols lowered total and LDL cholesterol in a randomized, controlled trial of men with elevated LDL cholesterol36. Increased consumption of soy in Asian populations is associated with decreased rates of cardiovascular disease37. A vegetarian diet consisting of soy-based products was given to 32ߙcoronary heart disease patients who discontinued their conventional hyperlipidemic medications. The diet resulted in normalization of serum lipids, with the best results associated with the group who maintained this diet for the longest period of time.

In 1995, a meta-analysis of 38 controlled studies39 concluded that substituting soy protein for animal protein lowered total cholesterol, LDL cholesterol, and triglycerides without affecting high-density lipoprotein (HDL) cholesterol. These effects were greater in subjects with higher baseline cholesterol levels. Daily soy protein consumption resulted in a 9.3% decrease in total serum cholesterol, a 12.9% decrease in LDL cholesterol, and a 10.5% decrease in triglycerides. This cholesterol lowering effect was additive to the effect seen with a low-fat/low-cholesterol diet. The average amount of soy protein consumed to achieve these results was 47 g/day.

Overall dietary replacement of animal protein with soy protein may have a favorable, yet variable, effect on serum lipid values in men and women. The addition of soy protein to the diet may be useful for patients requiring only modest reductions in cholesterol. The precise mechanism by which soy improves the blood lipid profile is unknown. One possible mechanism is altered hepatic metabolism with enhanced removal of LDL and very low density lipoprotein cholesterol by hepatocytes.

The biological properties of soy isoflavones in animals led to the assumption that only soy products high in isoflavones produced favorable results. Consequently, in a review by the American Heart Association (AHA) in 2006, studies were grouped according to whether isoflavones were present or not. In 22 randomized trials, isolated soy protein with isoflavones was compared with casein or milk protein, wheat protein, or mixed animal proteins. The range of soy protein was 23 to 135 g/day; the range for isoflavones was 40 to 318 mg. LDL or non-HDL cholesterol concentrations decreased in most studies and were statistically significant in 8 trials, with an overall effect of approximately 0.3% (weighted average).

A meta-analysis41 that included 10 studies published from 1995 through 2002 found a similar percentage reduction (4%) in LDL cholesterol with no dose effect. Studies with 50 g of soy protein showed a drop in LDL cholesterol concentrations similar to studies using a smaller amount of soy35. No effects were evident for HDL cholesterol or triglycerides in most of the studies; the weighted average effects were very small: 1.5% for HDL cholesterol and -5% for triglycerides. In 7 trials, soy protein washed with alcohol to remove isoflavones was compared with casein, milk protein, or various animal proteins. Two of the studies showed small decreases in LDL cholesterol; however, other well-controlled studies did not find effects of soy protein on LDL cholesterol. Changes in HDL cholesterol and triglycerides occurred in 1 out of 7 trials. No dose effect was evident. A meta-analysis concluded that isoflavones do not affect blood lipid concentration.

The effect of soy protein or isoflavones on LDL cholesterol does not appear to be modulated by the saturated fat and cholesterol content of the diet.

Influence of initial blood LDL cholesterol
In a meta-analysis39, the degree of LDL cholesterol reduction was related to initial cholesterol levels: subjects with severe hypercholesterolemia more than 335 mg/dL (8.66 mmol/L) had reductions of 19.6%; 259 to 333 mg/dL (6.7 to 8.61 mmol/L) had reductions of 7.4%; 200 to 255 mg/dL (5.2 to 6.6 mmol/L) had reductions of 4.4%; and less than 200 mg/dL had reductions of 3.3%. However, in a 2006 review35, a larger percentage reduction in LDL cholesterol in hypercholesterolemia was not evident in 22 trials. In studies of isoflavones, no relation was evident between initial cholesterol and cholesterol reduction. The AHA now states that the direct cardiovascular health benefit of soy protein or isoflavone supplements is minimal. A very large amount of soy protein, more than half the daily protein intake, may lower LDL cholesterol by a few percentage points when it replaces dairy protein or a mixture of animal proteins. No benefit is evident on HDL cholesterol, triglycerides, lipoprotein (a) or blood pressure.

Effects on blood pressure
Several studies have tested the effect of soy protein with isoflavones on blood pressure. Blood pressure was reduced in only 1 of 6 studies. The weighted average change in systolic blood pressure was −1 mm Hg. Soy isoflavones showed no effect on blood pressure

 

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