Soy Protein for the Metabolic Syndrome
3. Effect of Soy Protein in Weight Loss and Adiposity Reduction
Animal models have demonstrated that soy protein has a positive effect on weight and fat loss that was especially prominent when comparing casein - based diets to soy - based diets (Table 6.2 ). Some of these studies suggested that β - conglycinin fraction is responsible for the effect of soy protein in weight and fat loss (Moriyama et al. 2004 ; Koba et al. 2007 ; Martinez - Villaluenga et al. 2008 ).
A recent epidemiological study examined the rela- tion between soy consumption and body mass index (BMI) among 1,418 women in Hawaii (Maskarinec et al. 2008 ). Women consuming more soy during adulthood had a lower BMI. Studies investigating weight loss in overweight and obese humans suggest
Table 6.2. Effect of soy protein intake on body weight and adiposity reduction and metabolic parameters in recent animal studies. Model Diet Duration (Weeks) Body Weight Metabolic Parameters References Obese (KK yellow) mice (n = 10) Casein vs. soy protein isolate (20% protein) 2 Signifi cant lower body weight and less adipose tissue (mesenteric, epididymal, and brown fat) in soy protein – fed group.
Plasma cholesterol, TG, FFA, and glucose levels were decreased more effectively by soy diet.
Nagasawa 2002 Obese (KK yellow) mice (n = 10) Energy - restricted diets (20% protein, 30% fat) Casein vs. β - conglycinin vs. glycinin
2 Mice fed β - conglycinin diet had lower body weights and lower liver weights than corresponding groups fed casein diet.
Soy β - conglycinin – fed animals showed lower serum TG, insulin, and glucose levels.
Moriyama 2004 Rats with metabolic syndrome (n = 10) Casein vs. soy protein (20%) 36 Soy protein – diet group (with either low or high isofl avone content) had signifi cantly lower body and liver weight.
Low soy protein diet exhibited better glycemic control. High isofl avone soy protein – fed rats had higher glucose and TG levels and lower plasma insulin.
Davis 2005 Male Sprague Dawley rats (n = 10) Casein vs. soy protein (20% protein) 2 Epididymal adipose tissue was lower in soy protein – fed rats. Not determined Morifuji 2006 Rats (n = 10) Casein vs. soy protein in high - fat diet (% protein not specifi ed)
25 Reduction in epididymal adipocyte number and area and lower serum leptin and FFA concentration in rats fed soy protein.
Soy protein – fed animals showed lower serum FFA levels. Torre - V et Obese male ZDF rats (n = 10) Casein vs. low or high isofl avone soy protein (23%)
11 Body weight and liver adiposity was only signifi cantly reduced in high isofl avone soy protein – fed group.
Soy protein – fed animals showed lower fasting glucose, improved plasma insulin level, and improved dyslipidemia.
Davis 2007 Male rats (n = 10) Soy protein isolate (SPI) β - conglycinin (CON) (20%) vs. casein (CAS)
4 Higher reduction in epididymal and perineal adipose tissues in SPI and CON - fed groups compared with CAS - fed group.
Soy protein – fed animals showed lower serum TG and higher serum adiponectin levels.
Koba 2007 Diet - induced obese rats (n = 8) Low - fat diets (5% fat) and high - fat diets (25% fat) containing casein or soy protein (30%)
12 Rats fed soy protein diet showed smaller adipocyte area, which was associated with a lower body fat content, signifi cantly lower lipid droplet area in adipose tissue, and lower weight gain.
Soy protein – fed animals showed higher glucagon, lower serum leptin, cholesterol, and TG.
Torre - V et
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may reduce hepatic lipotoxicity reducing adipocyte hypertrophy, increasing perilipin expression, and preventing the transfer of FFA to extra adipose tissues (Torre - Villalvazo et al. 2006, 2008 ).
Moreover, soy protein feeding in rats decreased hepatic triacylglycerol levels and epididymal adipose tissue weight mediated by an increased activity and mRNA levels of hepatic and skeletal muscle enzymes involved in fatty acid oxidation, including carnitine palmitoyltransferase (CPT1), beta - hydroxyacyl - CoA dehydrogenase (HAD), acyl - CoA oxidase, and medium - chain acyl - CoA dehydrogenase (Morifuji et al. 2006 ; Takahashi and Ide 2008 ). In addition, Histological analysis of epididymal adipose tissue
from rats fed soy protein revealed that there were more adipocytes per area but they were smaller in size than those fed casein (Torre - Villalvazo et al.
2006, 2008 ). This indicates that soy protein prevents hypertrophy and stimulates adipocyte hyperplasia associated with high peroxisome proliferator acti- vated receptor gamma (PPAR γ) gene expression, preventing the release of excessive amounts of FFA to the circulation (Torres and Tovar 2007 ; Takahashi and Ide 2008 ). PPAR γ ) is responsible for adipocyte differentiation, fatty acid storage, and sensitizing the body to insulin (Torres and Tovar 2007 ). Soy protein
Figure 6.3. Proposed molecular mechanism of soy protein action on adiposity and caloric intake reduction (adapted from Torres and Tovar 2007 ). CCK = Cholecystokinin; FFA = Free fatty acids; GLP - 1 = Glucagon - like peptide 1; PPAR = Peroxisome proliferator activated receptor; SREBP = Sterol regulatory element binding protein; TG = Triglycerides; UCPs = Uncoupling proteins; VLDL = Very low density lipoproteins.
glucose metabolism; this relationship was also reviewed by Cope et al. (2008) . A summary from these two sources shows that there is evidence for positive effects of soy protein on glucoregulatory function in in vitro and animal models. It is not clear whether soy protein is more benefi cial than other protein sources for glucoregulatory function during weight loss, nor is there enough evidence to show that soy directly affects glucose metabolism inde- pendent of weight and fat loss in animal models.
However, epidemiologic data in postmenopausal women indicate that soy protein consumption ( > 13 g/d) decreases the risk of glycosuria. Further- more, clinical evidence demonstrated that a soy - based meal replacement protocol can be used for weight reduction in diabetic patients, causing a posi- tive effect on insulin secretion and glycemic control.
Wagner et al. (2008) treated male monkeys for 8 weeks using diets containing casein/lactalbumin, soy protein with a high isofl avone concentration, or soy protein that was alcohol - washed to deplete iso- fl avones. It was observed in this study that con- sumption of isofl avone - containing soy proteins increased insulin responses to the glucose challenge and decreased plasma adiponectin. On the other hand, isofl avone - depleted soy protein decreased body weight.
An epidemiological study conducted in a population - based prospective cohort of middle - aged Chinese women found an association between soymilk consumption and lower risk of type 2 dia- betes mellitus (Villegas et al. 2008 ). A longitudinal randomized clinical trial was conducted among 41 type 2 diabetes patients ( ≥ 126 mg/dl) with nephropa- thy. The soy protein group consumed a diet contain- ing 70% animal proteins and 30% vegetable proteins for 4 years. Soy protein consumption signifi cantly reduced fasting plasma glucose ( − 18 vs. 11 mg/dl for control group) and signifi cantly improved levels of serum total cholesterol, low - density lipoprotein (LDL) cholesterol and triglycerides, and proteinuria and urinary creatinine (Azadbakht et al. 2008 ).
Although the exact mechanism behind the benefi - cial effect of soy proteins on glycemic control and IR is not well known, several of the proposed mechanisms of action are presented in Figure 6.4 . PPAR γ and PPAR α mRNA levels were also found
to be elevated, suggesting that soy protein intake stimulates hepatic and skeletal muscle fatty acid oxi- dation by activating PPAR pathways leading to reduced accumulation of body fat in rats (Takahashi and Ide 2008 ; Torre - Villalvazo et al. 2008 ) and in male monkeys (Wagner et al. 2008 ). Moreover, brown adipose tissue was lower in rats fed a soy and high - fat diet probably due to higher mRNA levels of uncoupling proteins (UCP 1, 2, and 3). Therefore, it is possible that an increase in energy expenditure through the increased mRNA levels of PPAR γ and subsequent upregulation of UCPs is involved in the antiobesity effect of soy protein (Takahashi and Ide 2008 ; Torre - Villalvazo et al. 2008 ).
4. Effect of Soy Protein in Reduction of