a number of vegetable oils has been well established (Burr and Burr, 1930). It has also been postulated by Schantz, Elvehjem and Hart (1940) that butter fat possesses specific nutritive properties since they found greater growth of rats during the first 6 weeks following weaning when the diet consisted of a homogenized mixture of skimmed milk and butterfat than when cottonseed, corn, coconut or soybean oils were substituted for butterfat. These investigators believe that this stimulatory effect is traceable to some component of the fat itself rather than of the unsaponifiable residue. It could not be ascribed to the phospholipid (Schantz, Boutwell, Elvehjem and Hart, 1940a). But the administration of the saturated nonvolatile acids of butterfat brought about the greater growth, while the unsaturated and volatile acids were inactive (Schantz, Boutwell, Elvehjem and Hart, 1940b). Boutwell et al. (1941) found that the stimulatory factor could also be produced by hydrogenation of the unsaturated fraction in butter, but not by employing a similar procedure on the unsaturated fraction of the vegetable oils. In their most recent report, Boutwell et al. (1943) have found that the superiority of butter is only evident when the carhobydrate is lactose, while superior growth occurred with corn oil when the diet contained dextrin, sucrose, glucose or starch. Boer (1941) has also postulated that butterfat contains a specific growth-promoting property since he was able to obtain better growth in rats with butter than with olive oil diets. Gullickson, Fountaine and Fitch (1942) have reported that butterfat, tallow and lard are better tolerated by young calves and cause better growth than corn, cottonseed, soybean and coconut oils when the fats are homogenized in skim milk.

However, Euler, Euler and Saberg (1941) obtained superior growth in rats on margarine diets as compared with butter diets over a 6-week period where the average gain on margarine was 96.7 grams compared with a mean gain on the butter diet of 77 grams. Freeman and Ivy (1942) found no statistically significant differences in early growth of rats fed milk with coconut oil as compared with butter fat although the butter-fed rats were larger after 3 months. These results are diametrically opposed to those of Harris and Mosher (1940) who found more rapid growth of weanling rats on butterfat and of older rats on coconut oil. In view of these apparent contradictions the following experiments were undertaken to follow growth rate, efficiency of conversion of food calories into body tissue, the comparative composition of the body tissues of such rats, the effect of flavor on food preference and a comparison of pregnancy and lactation on such diets. The latter topics will be dealth with in later reports.

EXPERIMENTAL

Three series of tests were made on a total of 250 male and 309 female rats. In series I, the growth of 22 and 24 male rates was followed over 6 weeks on the butter and margarine diets described below; also the growth of two groups of female rats consisting of 28 and 24 in the first case and 18 and 19 in the second set was followed over a 6-week period on the butter and margarine diets respectively. In series II and III, approximately 15 rats of each sex were used with diets made up of each of the following fats over a 12-week period: a butter fat or a whole butter, corn oil, cottonseed oil, a margarine fat or a whole margarine, olive oil, peanut oil, or soybean oil. In series III whole butter and margarine were used and the diets were flavored by the addition to the fats of 4 parts per million of diacetyl which is the level preferred by the rats (Deuel and Movitt, 1944). The diets employed are listed in table 1.

The diets were prepared weekly and stored in the refrigerator to prevent rancidity. Several days' supply was placed at one time in the food cups. By the use of a special type of hopper practically no spilling of the food occurred and the food consumption could be determined with considerable accuracy. Animals were weighed weekly. Also at 3 and 6 weeks, the left tibia was X-rayed with a McFedries dental X-ray apparatus, the rats being lightly anesthetized with ether during this procedure. The length of the bones was determined by reading the X-ray photographs with a cathetometer.

The animals used were from our stock colonies. The litters were equally distributed between the various diets; when more than two in a litter developed diarrhea during the first week, the whole litter was discarded. To avoid differences in growth ascribable to environmental conditions, the animals on the various diets were so located as to prevent any segregation according to diet.

Butterfat, margarine fat 6 7, or vegetable oils supplemented as follows per 1,000 grams fat

Vitamin A: carotene 8 (40 percent of total A).

Vitamin A concentrate 9.

Vitamin D: viosterol 10

Vitamin E: a tocopherol 11

Diacetyl 12 or commercial butter flavor 13

1 Challenge brand prepared by roller process.

8 milligrams.. 133 milligrams. 801 milligrams.. ..10 milligrams. 4 milligrams..

2 Calculated to give 1.5 milligrams Fe and 0.15 milligrams each of Cu and Mn per 8 grams of food. 3 Challenge brand.

4 Fortified with 7,500 I. U. of vitamin A per pound.

5 15,000 U. S. P. XI units per gram. Calculated to furnish 28.2 units of vitamin D per 8 grams of food.

29.4

7 In series III where whole butter or whole margarine was used, these were increased so that the same amount of fat was present.

8 Fortified with 9,000 I. U. per pound.

8 S. M. A. 90 percent beta and 10 percent alpha carotene. This gives 18.8 #g. or 31.4 I. U. per 8 grams of food.

Kindly furnished by the Atlantic Coast Fisheries. Potency was 150,000 I. U. per gram or 45 I. U. per 8 grams of food.

10 Calculated to give 18.8 U. S. P. XI units per gram of food.

11 Calculated to give 23.5 ug. per 8 grams of food.

12 Obtained from Larkin & Co., Buffalo, N. Y.

13 BFA obtained from Verley Products Corp., 1621 Carroll Ave., Chicago, Ill.

The butters for the experiments in series II and III were of highest quality obtained in equal amounts from Minnesota, Wisconsin, Texas, Oregon,2 and California. The margarine, in which the fat was a hydrogenated cottonseed oil, was purchased on the open market in New York, Chicago, and Los Angeles. These samples were forwarded by express packed in solid carbon dioxide. For series II the butter and margarine were melted, the water and protein allowed to settle and the fat filtered out and mixed to give a homogenous sample. For series III, they were softened sufficiently to be mixed satisfactorily. The oils were obtained directly from the producer and shipped to us by express. They were stored in the refrigerator at approximately 38° F. in air-tight containers in the dark, which procedure kept them fresh during the course of the experiments. Inasmuch as the rat is very sensitive to even slight evidences of rancidity in fats, any specimen of fat or oil which gave positive organoleptic evidence of rancidity was not used in this investigation. The free fatty acid content, saponification number, iodine number, and thiocyanogen number of the fats were also determined. These were normal except that the free fatty acid content in the olive oil samples were especially high, being 2.20 and 4.37 percent in the samples used in series II and III, respectively, although they possessed excellent flavors.

RESULTS

The growth rate of the male and female rats receiving the butter or margarine diets (diet I) over a 6-week period is illustrated in figure 1.

The growth of the rats on the various diets in series II and series III was similar for the first 6 weeks. Figures 2 and 3 give the average gain of male and female rats respectively on the seven different fats over a 6-week period. Figure 4 gives the average total gain of the rats in series II and series III after 12 weeks on the respective diets.

The average tibia lengths of the rats in series III are given in figure 5 for 3 and 6 weeks on each diet. Similar results were obtained on series II for 6 weeks

but no measurements were made in this series at 3 weeks.

2 From farm of Oregon State College through courtesy of Dr. J. S. Butts.

3 Cotton and peanut oils from Southern Cotton Oil Co., Savannah, Ga.; corn oil from local distributor. Corn Products Refining Co.; soybean oil from Stanley Manufacturing Co., Decatur, Ill., and olive oil from Neapolitan Olive Oil Products, Los Angeles.

FIG. 1. The rate of growth of male and female rats on diets of mineralized skimmed milk powder mixed with butter fat or margarine fat. The following number of rats were used in the butter and margarine tests respectively: males 22 and 24; females in Ia, 28 and 24, and in Ib, 18 and 19.

FIG. 2.-The shaded portion represents growth of male rats for each week while the unshaded portion represents total growth from initiation of the experiment. The following number of animals were employed: butter (13), corn oil (13), cottonseed oil (14), margarine (13), olive oil (13), peanut oil (13), soybean oil (13).

FIGURE 3.-The shaded portion represents growth of female rats for each week while the unshaded portion represents total growth from initiation of experiments. The following number of animals were employed: Butter (15), corn oil (16), cottonseed oil (15), margarine (15), olive oil (13), peanut oil (15), soybean oil (15).