The results of the present research which was conducted on non-menopausal overweight and obese female adults, showed a reduction in waist circumference following four weeks consumption of soy milk. We found that soy milk replacement in the diet had no significant effect on weight, blood pressure, liver enzymes, glycemic control and serum fibrinogen level among overweight or obese female adults. Previous studies regarding the effects of soy had focused on premenopausal and post menopausal women or patients. However, few studies are available on the female adults or overweight or obese subjects.
Soy milk replacement did not enhance weight loss but provided waist circumference reduction. Previous researches showed no significant changes on weight with different soy products in Iran.[24
] Therefore, the beneficial components in soy products could be effective on cardiometabolic and cardiorenal abnormalities independent of weight change.[24
] A six weeks trial by soy drinks showed beneficial effects on weight and waist circumference reduction in 90 overweight and obese subjects.[28
] The results of this short time trial was same as some study which had not shown beneficial effects of soy product on weight. Most researches which shown positive effects in this regard worked on animals or larger number of subjects in longer time trials.[28
] In this study, waist circumference reduction may be related to soy milk phytoesterogen content and soy milk protein which may play an important role on reducing fat accumulation.[33
] This might be related to a soy protein like Beta conglycinin.[34
] The results of this study is similar to Sites study. In both two researches waist circumference reduced without any changes in weight.[17
Some soy contents like polyphenols have beneficial influences on controlling blood pressure.[19
] Following soy product consumption, serum nitric oxide level increases and blood pressure reduces.[35
] Some studies showed no relation between soy product consumption and blood pressure.[15
] Soy isoflavones had beneficial effects on blood pressure in hypertensive subjects but no significant result in normotensive subjects.[36
] Soy isoflavones might act like estrogens, especially in postmenopausal women who have low endogenous estrogen levels.[37
] In this study, soy milk replacement did not affect on blood pressure. This result may be related to conducting research on normotensive and non-menopausal subjects.
We were unable to find significant differences between diet with soy milk and diet with cow's milk with regard to glucose and insulin metabolism in our study. Similar to our findings, Villa and colleagues found no difference in glycemic control indices between genistein and placebo[38
] In contrast, Jayagopal and colleagues reported that in postmenopausal type 2 diabetics, soy supplementation resulted in significant decreases in insulin.[39
] It is possible that the effects of soy isoflavones on glycemic control indices vary depending on the glycemic status of the individual.[40
Fibrinogen is a glycoprotein, the plasma component of which is synthesized in the liver. Fibrinogen is changed by thrombin to output fibrin monomers that are the primary constituent of the fibrin clot.[41
] Increased plasma fibrinogen levels have been clearly connected with an increase in risk of cardiovascular disease, including ischemic heart disease, stroke, and other thromboembolic events, because increased fibrinogen levels enhance thrombus forming by altering the kinetics of the coagulation cascade, resulting in increased fibrin forming, increased platelet aggregation, and increased plasma stickiness.[42
] There is no evidence for the effects of soy milk on Fibrinogen in previous studies. We did not find any effect of soy milk on fibrinogen levels in this study.
Comparing the two diets with soy milk and cow's milk showed that serum ALT and AST had no significant reduction during the diet with soy milk. Previous studies have shown that higher liver enzyme levels are related to higher risk of cardiovascular diseases.[9
] Furthermore, serum liver enzyme levels are considered as new cardiovascular risk factors and they are associated with glycemic control abnormalities.
Most studies have focus on only some components of soy and there are few researches on all parts of the soy and whole soy. Recent researches indicate higher positive effects from complete forms such as soybean[45
] and soy milk.[18
] It sound that combination of soy protein, fatty acids, and phytoestrogens together are more effective than the isolated soy, purified phytoestrogens and protein alone.[46
A positive point of this research was cross-over design for conducting this trial. Our trial should have good external validity, since this study was conducted on a sample of non-menopausal overweight or obese female adults with no special disorder.
Limitations of the study
We did not prepare food for each patient except soy milk and cow's milk. This limitation should be considered in explaining the results. What the female adults ate was controlled by analyzing the dietary intake of patients’ food records. The results indicated that soy milk could have beneficial effects on waist circumference, despite the diet in our study may not be followed as carefully as the trials which prepared food was available.