Role of Dietary Soy Protein in Obesity

 Abstract: Soy protein is an important component of soybeans and provides an abundant source of dietary protein. Among the dietary proteins, soy protein is considered a complete protein in that it contains ample amounts of all the essential amino acids plus several other macronutrients with a nutritional value roughly equivalent to that of animal protein of high biological value. Soy protein is unique among the plant-based proteins because it is associated with isoflavones, a group of compounds with a variety of biological properties that may potentially benefit human health. An increasing body of literature suggests that soy protein and its isoflavones may have a beneficial role in obesity. Several nutritional intervention studies in animals and humans indicate that consumption of soy protein reduces body weight and fat mass in addition to lowering plasma cholesterol and triglycerides. In animal models of obesity, soy protein ingestion limits or reduces body fat accumulation and improves insulin resistance, the hallmark of human obesity. In obese humans, dietary soy protein also reduces body weight and body fat mass in addition to reducing plasma lipids. Several potential mechanisms whereby soy protein may improve insulin resistance and lower body fat and blood lipids are discussed and include a wide spectrum of biochemical and molecular activities that favorably affect fatty acid metabolism and cholesterol homeostasis. The biologic actions of certain constituents of soy protein, particularly conglycinin, soyasaponins, phospholipids, and isoflavones, that relate to obesity are also discussed. In addition, the potential of soy protein in causing food allergy in humans is briefly discussed.

Keywords: soy protein, obesity, human studies, animal studies, mechanisms, soy protein allergy

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1. Introduction

Obesity has become a worldwide epidemic and its prevalence continues to increase at a rapid rate in various populations and across all age groups. Obesity poses a major public health challenge since it is a well recognized independent predictor of premature mortality. Moreover, it often coexists with other cardiovascular risk factors, namely, diabetes, dyslipidemia, and hypertension, which further add to the burden of cardiovascular disease. The dramatic increase in the occurrence of overweight and obesity over the past several decades is attributed in part to changes in dietary and lifestyle habits, such as rapidly changing diets, increased availability of high-energy foods, and reduced physical activity of peoples in both developed and developing countries. 

Obesity is a complex metabolic disorder that is thought to result from an imbalance of energy intake and energy expenditure leading to the excess accumulation of fat in various adipose tissues and organs. The development of obesity is associated with hyperinsulinemia, insulin resistance, and abnormalities in lipid metabolism. Insulin resistance is considered the most common underlying abnormality in human obesity and is influenced by genetic and environmental factors, and in particular, changes in diet and physical activity. Lipid abnormalities associated with obesity include increased overall production of lipids with elevated concentrations of fatty acids, triacylglycerols, and low-density lipoproteins (LDL), as well as very-low density lipoproteins (VLDL). Excess sugar intake especially in the form of high sugar containing and high fructose corn syrup containing colas leads to the formation and deposition of lipids in various fatty tissues. Elevated plasma concentrations of free fatty acids (FFA) have been shown to play a key role in contributing to the development of insulin resistance in obesity and in type 2 diabetes mellitus. In addition, there is evidence that suggests that accumulation of excess fat and FFAs in non-adipose tissues, such as the liver, heart, skeletal muscle, kidneys, and blood vessels may impair their functions, and contribute to cell dysfunction or cell death, a phenomenon known as lipotoxicity. Preventive or therapeutic strategies to control obesity should target these abnormalities. Various dietary modifications designed to control excess body weight and dyslipidemia have focused on the manipulation of the amount and nature dietary energy and fat intakes. In recent years, increased attention has shifted toward the role of dietary protein intake in the management of obesity. 

2. Dietary protein and effects on food intake and body weight

Ingestion of foods with high protein content is well known to suppress appetite and food intake in humans. Among the three macronutrients (carbohydrate, fat, and protein), protein has the most suppressing effect on food intake. In addition, dietary protein has been shown to induce higher satiating and thermogenic effects and greater weight loss than carbohydrates. In a randomized trial in overweight and obese subjects, consumption of high protein (25% of total energy) in ad libitum fat-reduced diets for 6 months produced greater weight loss and body fat loss, compared to consumption of high carbohydrate (12% of total energy). These effects were not related to changes in fat intake since the amount of dietary fat (30% of total energy) was maintained constant during the intervention. Similarly, in a 4-week randomized dietary intervention trial of male obese hyperinsulinemic subjects, a high protein hypoenergetic diet (45% protein, 25% carbohydrates, and 30% fat) also induced greater weight loss and resting energy expenditure, compared to a high carbohydrate hypoenergetic diet (12% protein, 25% carbohydrates, and 30% fat). In a recent 12-week trial conducted in healthy adult subjects, increasing the amount of dietary protein content from 15% to 30% of total energy while maintaining the carbohydrate content (50%of total daily caloric intake) in the diet resulted in sustained losses in weight and body fat. The favorable effects on body composition in this study appear to be due to sustained decrease in appetite and ad libitum caloric intake induced by the high-protein intake. More recently, Batterham et al examined the effects of dietary protein on satiety and the responses of gut hormones, particularly the gut hormone peptide YY (PYY), a known inhibitor of food intake in humans and rodents. These investigators showed that high-protein intake induced an increase in plasma PYY levels and marked satiety in normal-weight and obese human subjects. Furthermore, in studies of obese Pyy null mice, which were selectively resistant to the satiating and weight-reducing effects of protein, exogenous administration of PYY in these animals reversed their obesity. These findings suggest that modulating the release of endogenous satiety factors, such as PYY treatment, plays an important role in mediating the satiating effects of dietary protein. 

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