Effects of dietary fats on learning, pain threshold, thermoregulation and motor activity in rats: interaction with the length of feeding period.

The effects of both a semisynthetic diet containing 20% fat from various sources (soybean oil, sunflower oil and lard) and a control diet on learning capacity, motor activity, pain threshold and thermoregulation were studied in rats which were fed on these diets for various lengths of feeding periods (1, 2, 3 and 4 weeks). Two weeks feeding period of soybean oil source induced an improvement in learning capacity, which was further enhanced by increasing the length of the feeding period. A 3-week feeding period was required to obtain an increase in the pain threshold, by which time the rats were also protected from d-amphetamine induced hypothermia. The analgesia induced by the diet is naloxone-dependent. None of the other diets, including the sunflower oil diet, which is richer in polyunsaturated fatty acids, differed from control diet. While the mode of action of this diet is still unknown, the effects of the soybean oil source diet cannot be attributed to nutritional factors such as changes in energy consumption or body weight.