We examined the effect of altering the linoleic acid (LA, 18:2n-6) to ␣ -linolenic acid (ALA, 18:3n-3) ratio in the dietary fats of 3 day old piglets fed formula for 3 weeks. The LA-ALA ratios of the experimental formulas were 0.5:1, 1:1, 2:1, 4:1, and 10:1. The level of LA was held constant at 13% of total fats while the level of ALA varied from 1.3% (10:1 group) to 26.8% (0.5:1 group). Incorporation of the n-3 long chain PUFA EPA and 22:5n-3 into erythrocytes, plasma, liver, and brain tissues was linearly related to dietary ALA. Conversely, incorporation of DHA into all tissues was related to dietary ALA in a curvilinear manner, with the maximum incorporation of DHA appearing to be between the LA-ALA ratios of 4:1 and 2:1. DHA is found in high concentrations in neural tissue including brain cortex and the retina. The proportion of DHA in brain cortex is partly dependent on diet, because cortical DHA is higher in breastfed infants when compared with those fed standard formula (1, 2). A number of clinical trials have demonstrated that preterm and term infants fed breast milk or formula supplemented with DHA perform better at a range of neurological and visual tests (3, 4) than their counterparts fed unsupplemented formulas, although some trials have demonstrated no effect of supplementation (5-8). It has been hypothesized that this may be explained by the fact that placebo groups had inadequate precursor ␣ -linolenic acid (ALA, 18:3n-3), and hence, insufficient endogenous synthesis of DHA occurred (9). This line of thought is based on the premise that increased ALA will result in improvements in DHA synthesis and hence tissue DHA. Infant studies allow a limited range of ALA levels to be tested as regulatory committees limit the linoleic acid (LA)-ALA ratio to between 5:1 and 10:1 (10). Thus, we planned a study in formula fed piglets, as they are accepted models of human metabolism.As linoleic acid (LA, 18:2n-6) must be added to all formulas to provide essential n-6 fatty acids, testing the effect of increasing the dietary supply of ALA also results in changing the ratio of LA to ALA in the diet. Several different means of altering the LA-ALA ratio have been studied in human and animal models. Most studies have altered both the LA and ALA levels in order to attain a range of LA-ALA ratios (11-15), and there has been little consistency in the percentage of total PUFA of the diet in the various studies. Some investigators have kept the LA constant and varied the level of ALA (16-18), others have kept the ALA constant while varying the LA level (19). Thus, there was a need for a systematic dose-response study over a broad range of ALA intakes.We chose to examine the effect of altering the LA-ALA ratio in the diet of newborn piglets, as the timing of the brain growth spurt is similar to that reported for humans. In this experiment we tested LA-ALA ratios ranging from 10:1 (found in many infant formulas) through to 0.5:1 by keeping the LA level constant and varying the ALA concentration as a first step in elu...