ABILITY OF ENDOGENOUS FOLATE FROM SOY PROTEIN ISOLATE TO MAINTAIN PLASMA HOMOCYSTEINE AND HEPATIC DNA METHYLATION DURING METHYL-GROUP DEPLETION IN RATS

We investigated different means of achieving methyl depletion by feedi ng weanling rats modified AIN diets depleted of folate (FD), folate choline (FCD), and folate + choline + methionine (FCMD), and examined the consequent effects on folate status, homocysteine levels, and meth ylation status. Control rats were fed a 12% protein diet consisting of either casein or soy protein isolate (SPI) and containing 2 mg/kg fol ate, 0.2% choline, and 0.4% methionine. After the rats had been on the diets for 4 and 8 weeks, whole blood folate concentration was measure d and found to be significantly depleted in the folate deficient treat ments compared with controls at 4 weeks (p < 0.001), this reduction be ing significantly greater (p < 0.03) in casein-fed rats (60%) than in SPI-fed rats (32%), The omission of choline and methionine from the di et had no further influence on whole blood folate. A significant inver se correlation was observed in the casein-fed rats after 8 weeks betwe en mean plasma homocysteine concentration and decreasing methyl conten t of the diet (r(2)=0.978, p<0.002), an effect not seen in the corresp onding SPI-fed rats. Hypomethylation of hepatic DNA evidenced by a red uction in 5-methylcytosine content was present in the casein rats fed FCD and FCMD relative to control (p < 0.05). No hepatic DNA methylatio n changes were observed in the SPI-fed rats. The results obtained in t he present work demonstrate that a soy-based diet can compensate again st methyl group depletion by maintaining plasma homocysteine levels an d an adequate level of DNA methylation, a result we attribute to endog enous folate content.