slope of the line at about 14 weeks of age for rats 

 fed the rapid-growth-producing Anderson-Smith 

 diet, and suggested that this deflection may be 

 related to the beginning of a normal adult period. 

 Copping, Crowe, and Pond (4-4) observed a deflec- 

 tion at 11 weeks on two of the eight diets that 

 they investigated, and suggested that this deflec- 

 tion may be due to the rapid early growth of rats 

 on these two diets that contained more than ade- 

 quate amounts of protein. 



When Zucker's formula was applied to the 

 data here reported for rats on SP 8 HVO and SPE 

 diets, a straight-line relationship was found to 

 hold reasonably well until the rats reached 15 

 weeks of age. During this period no deflection 

 was apparent that would indicate any nutritional 

 deficiencies. The K values of 3.8 for SP 8 HVO 

 and of 4.0 for SPE diet were similar to the 3.8 

 value reported by Zucker for male rats. The 

 change in the slope of the line after 15 weeks 

 provided further evidence that the age range over 

 which this formula applies may be limited when 

 very rapid growth occurs in the young rat. 



Everitt (58) and Berg and Harmison (20), 

 reporting data relating body weight to age 

 throughout life, observed a rapid period of growth 

 followed by a plateau similar to the results re- 

 ported here (fig. 3) for rats fed SP 8 HVO and 

 SPE diets. The subsequent decline in body 

 weight which these authors observed was also 

 noted for the majority of rats fed SP 8 HVO and 

 SPE diets, as seen in figure 2 in the examples for 

 individual rats. Although there is general agree- 

 ment that such weight loss frequently occurs 

 before death, there still seems to be some question 

 as to whether or not weight loss is a necessary 

 accompaniment of the aging processes. Everitt 

 (58) reported an average decrease in weight from 

 381 to 249 grams during the last 200 days of life 

 of 68 male rats. Eighty percent of the animals 

 had lung abscesses, but their loss in weight was 

 similar to that in a comparable number of rats 

 with healthy lungs. No data were reported on 

 the incidence of other lesions except for three 

 tumors. According to Everitt and Webb (59), 

 this weight loss may be due to disease or to 

 senescence. 



Results obtained with BHE rats in this labora- 

 tory, however, as well as those reported by Berg 

 and Harmison (20), indicate that weight loss in 

 older animals generally reflects some pathological 

 condition. These authors separated the results 

 for rats with no lesions from those with lesions. 

 The average age of the group without lesions was 

 681 days. The body weight of these rats increased 

 with age up to 522 days, with no marked difference 

 thereafter. A progressive decrease in weight with 

 age was observed in the older rats with lesions. 



Growth and food intake of young rats (ail diets) 



A pattern of weekly weight gain and food intake 

 similar to that discussed for rats fed SP 8 HVO 

 and SPE diets was observed for rats on the other 



dietary regimens under investigation. Data for a 

 minimum of 10 rats were generally obtained with 

 each of the experimental diets. Although there 

 was considerable variation in the response of 

 individual rats to any one diet, the data reported 

 for SP 8 HVO and SPE diets summarized in 

 appendix tables 79 and 80 indicate that average 

 values for groups of approximately 10 rats 

 generally agree well with those obtained for the 

 larger number of animals. 



In table 8 are summarized data on the weight 

 gains during the first 12 weeks on each of the 

 experimental diets, and except where indicated, 

 the data reported have been confined to groups of 

 littermates. Included also are data obtained on 

 food intake in grams and in calories and on the 

 efficiency with which these diets were used as 

 measured by the calories-per-gram gain. Calories 

 are reported as gross calories throughout this 

 publication. To determine available calories from 

 these diets, data are needed on the digestibility 

 by the rat of the various diets under investigation, 

 on the energy from protein that is stored in the 

 body of the growing animal, and on the loss of 

 energy in the urine either as protein or as incom- 

 pletely oxidized products from protein. The 

 factors 4, 9, 4, frequently used for calculating 

 available energy from dietary protein, fat, and 

 carbohydrate, were developed for use with man 

 from the extensive investio'ations of Atwater and 

 Bryant (14). Metta and "Mitchell (131) showed 

 that these factors are not applicable to the rat. 



Modifications of semipurified diet — with 

 selected foods. — Young rats grew well on all 

 of the experimental diets. The most rapid growth 

 was observed with animals fed the diets in which 

 egg, milk, beef, or peanut butter replaced 20 to 

 25 percent of the semipurified diet. Weight gains 

 were similar for all of the diets containing these 

 foods and were consistently greater than were 

 those observed on the semipurified diet. The 

 difference between 16.8 Calories-per-gram gain on 

 SP 8 HVO diet and 15.3 on SPE diet was highly 

 significant (P <^0.01). Differences in digestibility 

 of these two diets do not explain the differences 

 observed in food utilization. Marshall and Hilde- 

 brand (125) recently reported that BHE rats 

 were able to digest 'SP 8 HVO and SPE diets 

 equally well. No significant differences were 

 noted in the utilization of the diets containing 

 egg, milk, beef, or peanut butter. 



SPE DIET WITH PURIFIED NUTRIENTS. No 



change in rate of growth or in calories-per-gram 

 gain was observed as the result of adding the 

 various nutrient supplements to SPE diet. 



Kind and level of fat. — Data for the group of 

 diets containing different kinds and levels of fat 

 were limited, and the calorie values of the diets 

 were less accurately established than those for 

 the semipurified diet or for the SP diets containing 

 egg, milk, beef, or peanut butter. There was a 

 consistent trend for rats fed diets containing 8 

 percent fat to grow more slowly than those fed the 



14 



