Considerable interest has been evidenced in the 

 plant sterols, such as are present in peanut butter, 

 and their role in lipid metabolism, because of 

 their possible value in reducing blood cholesterol 

 levels. Although there is evidence that plant 

 sterols do result in reduced blood cholesterol under 

 many conditions, the findings on this subject have 

 not been entirely consistent. Here again, the 

 presence or absence of dietary cholesterol seems to 

 be a factor in determining the response to these 

 sterols. 



Several recent reports (76, 99, 178) indicate 

 that plant sterols are absorbed by the rat. Swell, 

 Boiter, Field, and Treadwell (178) investigated 

 some of the factors influencing the absorption of 

 these sterols and have suggested that they are 

 absorbed through the same mechanism as cho- 

 lesterol. A maximum absorption of 22.9 percent 

 was observed when soybean sterols were fed with 

 25 percent oleic acid and 1 percent sodium 

 taurocholate. Ivy, Lin, and Karvinen (99) 

 reported a comparable value for the absorption 

 of soybean sterols and a decrease in absorption of 

 cholesterol when a mixture of the two sterols was 

 fed. 



There appears to be little evidence for a decrease 

 in serum cholesterol values when plant sterols 

 are fed to rats on cholesterol-free diets. Swell, 

 Boiter, Field, and Treadwell (178) obtained an 

 appreciable elevation in the serum level as the 

 result of including 2 percent soybean sterol in a 

 diet containing oleic acid and bile salts. Liver 

 sterols tended to be lowered when plant sterols 

 were included in the diet. Chromatographic 

 analysis provided no evidence of appreciable 

 amounts of plant sterols in either blood or liver, 

 and the rise in the sterol concentration in blood 

 appeared to be due to cholesterol or to a sterol 

 with the same R £ as cholesterol. 



When soybean sterols were added to a diet 

 containing cholesterol, Swell, Boiter, Field, and 

 Treadwell (177) obtained a reduction in blood 

 cholesterol values in comparison with those 

 observed in the absence of the plant sterol. Serum 

 cholesterol levels were found to decrease with 

 increasing concentrations of the plant sterol. 

 Alfin-Slater, Wells, Aftergood, and others (7) and 

 Ivy, Lin, and Karvinen (99), however, found no 

 appreciable change as the result of adding soybean 

 sterols to a cholesterol-containing diet. The basic 

 diet used by Swell was one that resulted in a 

 marked hypercholesteremia in the absence of 

 plant sterols, whereas the diets used by Alfin- 

 Slater, Wells, Aftergood, and others (7), and Ivy, 

 Lin, and Karvinen (99) produced blood cholesterol 

 levels only slightly higher than normal. 



In view of the results reported for plant sterols 

 fed in the absence of cholesterol, it is possible that 

 phytosterol as well as the unsaturated fat present 

 in peanut butter may be a factor in the elevated 

 serum cholesterol levels reported in this publica- 

 tion for BHE rats fed SPPB diet. 



The kind and level of dietary protein (61, 134, 



135, 142, 144, 147) and the type of dietary carbohy- 

 drate (3, 82, 103, 152, 153) have been implicated as 

 factors of importance in determining serum choles- 

 terol levels in the rat. The investigations reported 

 in this publication were not planned to determine 

 the role of either of these dietary components. 



The level of protein was relatively constant in 

 the diets of BHE rats except for the diets consist- 

 ing of 100 percent whole egg or egg yolk. The data 

 available were insufficient to determine whether 

 the high level of protein in Y100 diet was a factor 

 in lowering the serum cholesterol of these rats. 

 The possible effect of specific proteins in combina- 

 tion with other dietary ingredients as factors in- 

 fluencing serum cholesterol levels has not been 

 excluded. 



Sucrose was the dietary carbohydrate for most 

 of the diets fed to BHE rats. The only diets with- 

 out high levels of sucrose were the stock diet, 

 E100, and Y100 diets. Differences in the response 

 of rats to diets containing 25 percent whole egg 

 (SPE) and those containing 100 percent egg (El 00) 

 or 100 percent egg yolk (YlOG) may be related to 

 the lack of sucrose in these last two diets. 



The thyroid gland has long been recognized as 

 exerting appreciable influence on lipid metabolism 

 and is the most important of the endocrine glands 

 as regards the control of cholesterol metabolism. 

 Handler (84) reported a marked increase in the 

 cholesterol concentration of the liver and serum of 

 the rat in the hypothyroid state. Thyroid feeding 

 resulted in a decrease in the cholesterol concentra- 

 tion of the liver and effected a relatively small 

 decrease in serum levels. Although cholesterol 

 synthesis and absorption have been found to in- 

 crease in the hyperthyroid rat, it appears that the 

 excretory or destructive processes concerned with 

 cholesterol dominate in the hyperthyroid state 

 (36). 



However, as the result of injecting anti-rat kid- 

 ney serum (AKS) into rats previously fed with 

 thyroid to produce hyperthyroidism, Rosenman 

 and Smith (165) reported a marked increase in 

 plasma cholesterol. Under these conditions the 

 hypothyroid rat showed a lowering of the plasma 

 cholesterol when compared with the control ani- 

 mal. It appears that this effect on the hyperthy- 

 roid rat was due to a metabolic block to lipid 

 egress resulting from the anti-rat kidney serum 

 which permitted the accumulation of cholesterol 

 in the blood and not to any change in the hyper- 

 thyroid state. 



The enlarged thyroids here reported for BHE 

 rats fed SPE and SPPB diets were generally ac- 

 companied by elevated serum cholesterol levels. 

 There was no evidence microscopically of any 

 abnormality in the thyroids of the rats that were 

 maintaining weight, and no data were available to 

 determine the possible influence of these diets on 

 the excretion of this sterol. 



There has been considerable evidence associating 

 hypercholesterolemia with nephrosis in man and 

 in animals. Hyperlipemia has also been observed 



84 



