Consumption of Soymilk Lowers Atherogenic Lipid Fraction in Healthy Individuals

Introduction

A high concentration of cholesterol in blood poses a risk to human health, ¹ including the risk of coronary heart disease. Soya milk is a stable emulsion of oil, protein, and water. It is a beverage made from soybean, an ancient food crop from China. It contains about 3.5% protein, 2% carbohydrate, and 0.5% ash. Several studies in both animal models and humans have shown that cholesterol levels may decrease when plant protein like soy protein is substituted for animal protein. ^{2 –4} Other reports are, however, not consistent with the latter findings. Most studies of normocholesterolemic subjects have shown little difference in effects on plasma or serum cholesterol concentrations between soy protein and control diets. ^{5 –7} Some researchers have examined soy protein, mostly as soy protein isolate, ^8,9 whereas others have studied purified isoflavone supplements. ^10,11

The mechanism by which soybean reduces cholesterol has not been ascertained, but some say it is associated with the presence of trypsin inhibitors, phytic acid, saponin, and isoflavone, all of which have been found to reduce cholesterol levels. ¹² Another attractive explanation, though, for the hypothesized hypocholesterolemic effect of soy protein involves hormone mediation. Insulin, as demostrated by Gibney, ¹³ showed increased release following consumption of a soy protein test meal. The potential effect of soy-induced increased insulin release on cholesterol metabolism is not clear, but it has been suggested that elevation of insulin may affect cholesterol synthesis or the rate of lipoprotein clearance. ¹⁴

The use of an effective non-pharmacological means to lower plasma lipid levels in humans is plausible, providing additional health benefits. This study was therefore aimed at investigating the short-term effects of standard soymilk preparation in normocholesterolemic male and female Nigerian Africans.

Subjects and Methods

The study test population, consisting of 20 male and 22 female apparently healthy volunteers between 20 and 35 years old, participated in and completed the study. Six volunteers who were initially selected withdrew from the study: five of them owing to non-compliance and the remaining one after falling sick during the study period. Forty control subjects who did not drink soymilk were selected for the study. Volunteers were randomly selected from the student population of the Department of Biomedical Sciences, College of Health Sciences, Ladoke Akintola University of Technology, Osogbo, Nigeria. Informed consent was obtained from each participant. All subjects had a body mass index of less than 25 kg/m² and were not on any kind of medication throughout the period of the study. None of the female participants was pregnant, lactating, or using oral contraceptives. Smokers, frequent alcohol consumers, and those on special diets were excluded.

Participants were also screened for proteinuria and glycosuria, and those with a history of gastrointestinal, liver, or kidney diseases were not included in the study. Five hundred milliliters of soymilk (equivalent to 50 g of soybeans) was consumed by each subject every day for a period of 3 weeks (21 days).

Venous blood samples were collected before and 21 days later following soymilk intake. Two blood samples were also collected from control subjects over the same period of time. After an overnight fast (14–16 hours), venous blood was drawn from antecubital veins with a sterile syringe and needle into an EDTA anticoagulant bottle. Plasma samples obtained by centrifugation were separated into plain bottles and stored at −20°C until time for analysis.

Plasma total cholesterol (TC) and triglyceride (TG) concentrations were determined using the modified enzymatic methods of Allain et al. ¹⁵ and Bucollo and David, ¹⁶ respectively, using chemical kits (Randox Laboratories, Crumlin, UK). High-density lipoprotein-cholesterol (HDL-C) was also determined by using the method of Allain et al. ¹⁵ after other lipid fractions were precipitated using phosphotungstate. Low-density lipoprotein-cholesterol (LDL-C) was calculated according to the formula of Friedewald et al. ¹⁷

Randox Laboratories quality control samples were used for quality assessment. The controls were included in each batch of analysis.

Results

After 21 days of soymilk consumption, the mean values of TC and LDL-C were significantly lowered (P < .001), whereas mean HDL-C was significantly increased compared to the baseline concentration (Table 1). The mean difference in TG concentration was not statistically significant. In contrast, the mean HDL-C, TC, TG, and LDL-C concentrations in control subjects who did not take the soymilk preparation were not significantly altered in comparison to corresponding baseline values (Table 2). In female test subjects, plasma TC and LDL-C levels significantly decreased (P < .001), but there were no changes in the mean HDL-C and TG concentrations. Apart from the mean concentration of plasma TC, which did not change significantly, a similar trend was observed in male soymilk drinkers. The LDL-C concentration was significantly lower (P < .05) but no changes were seen in HDL-C and TG levels compared to baseline values.

In control male and female subjects (Table 2), however, there were no significant differences in the mean plasma HDL-C, LDL-C, and TG concentrations over the 21-day period. Only the mean plasma TC level significantly decreased (P < .05) in female subjects in this non-treatment group.

Discussion

This study was aimed at ascertaining the effect of soymilk consumption on plasma lipids and lipoproteins in healthy individuals. Fewer studies ^{5 –7} have been carried out on normocholesterolemic subjects, but the results appear inconsistent. Previous reports on hyperlipidemic men, however, show more consistent lowering effect of soy protein on lipid profile. ^18,19 The result obtained from this study showed that addition of soymilk to diet significantly reduced plasma TC and LDL-C and increased mean HDL-C. The control group did not show any significant modifications of the lipid profile over the 21-day period. This corroborates the findings of Wong et al. ¹⁸ Similarly, a meta-analysis by Anderson et al. ² indicates that consumption of an average of 47 g of soy protein resulted in reductions of 9.3% in circulating TC, 12.3% in LDL-C, and 10.5% in triacylglycerol.

Dietary intervention is the best non-pharmacological method to reduce LDL-C, thereby improving cardiovascular health. The use of plant protein such as found in soymilk in order to achieve this appears desirable. Weidner et al. ¹⁹ even suggested that enriching soy drink with plant sterols could be more effective in lowering TC and LDL-C in moderately hypercholesterolemic subjects because plant sterols in addition to the hypocholesterolemic effect of soy reduce the intestinal absorption of cholesterol.

The mechanism by which soy reduces cholesterol is thought to be via a decrease in cholesterol synthesis or increased stimulation of bile acid synthesis from cholesterol. Erdman ¹² is of the view that trypsin inhibitor, which is not destroyed after heating soy, exerts a hypocholesterolemic effect by increasing the secretion of cholestokinin, which in turn stimulates bile acid and thus helps to eliminate cholesterol through the gastrointestinal tract. Cholesterol lost from the body in the form of bile shifts the liver toward providing more cholesterol for increased bile synthesis and increases low-density lipoprotein receptor levels. Other components in soy such as phytic acid, saponin, and isoflavone have also been associated with the reduction in plasma cholesterol level. Interestingly, Forsythe ²⁰ suggested that changes in plasma thyroxine may in part be responsible for the hypocholesterolemic effect of soy after observing increased thyroxine, insulin, and thyroid-stimulating hormone in subjects who took soy compared with those who took casein. Hyperthyroidism (with high plasma thyroxine) is characterized by reduced TC and LDL-C in humans.

Apart from the significantly reduced TC in female volunteers in this study, the pattern of soymilk effect on lipid profile seems to be similar in males and females. This study was well controlled. Students of the institution were preferred as volunteers because it would be easier to monitor them. Additional precautions were taken to minimize or control confounding variables. Subjects were advised not vary their normal diet; none of them smoked or took alcohol, and none of them was on medication 3 weeks before and during the duration of study. In addition, apparently healthy control subjects who were not on soy treatment were randomly selected, and their plasma lipid profiles were determined before and after the 3-week period. Moreover, young to middle-aged Nigerians with normal-range body mass index were preferred to older subjects in order to control for age and body mass index because both are known to affect circulating lipid levels.

In conclusion, the outcome of this study seems to suggest that soymilk is an important non-pharmacological cholesterol-reducing agent. A switch from animal-based products to soy-based products would therefore appear to be more appealing healthwise in the reduction of blood cholesterol levels and by extension in the management of hypercholesterolemia (reduction of the risk of cardiovascular disease). Further studies are required with respect to isolating and establishing the true bioactive component or components responsible for this effect.