The consumption of
soy protein was shown to reduce blood
lipids in humans and other animal species. Furthermore, it was shown that the ingestion of
soy protein maintains normal insulinemia. Thus, the purpose of the present study was to determine whether
soy protein affects the synthesis of
lipids in the liver through
sterol-regulatory
element binding protein-1 (SREBP-1) due to modulation of
insulin levels. We first conducted a short-term study in which rats were fed a diet containing 18 g/100 g
soy protein or
casein for 10 d. Rats fed
soy protein had significantly lower serum
insulin concentrations than rats fed
casein, and this response was accompanied by an elevation in hepatic SREBP-1
mRNA that was 53% lower than that in rats fed
casein at d 10. The increase in SREBP-1
mRNA occurred 30 min after consumption of the
casein mean, and increased steadily for the next 2 h. We then conducted a second study to assess the long-term effect of
soy protein consumption for 150 d on hepatic SREBP-1 expression. Long-term consumption of
soy protein maintained normal
insulin concentrations compared with rats fed
casein, which were hyperinsulinemic. Thus, rats fed the
soy protein diet had significantly lower expression of SREBP-1
mRNA than rats fed the
casein diet.
Soy protein intake also reduced the expression of
fatty acid synthase (FAS) and malic
enzyme, leading to low hepatic
lipid depots of
triglycerides and
cholesterol, whereas rats fed the
casein diet developed
fatty liver. These data suggest that
soy protein regulates SREBP-1 expression by modulating serum
insulin concentration, thus preventing the development of
fatty liver.