A Critical Appraisal of the Role of Dietary Long-Chain Polyunsaturated Fatty Acids on Neural Indices of Term Infants: A Randomized, Controlled Trial

DISCUSSION

A major finding of this study was that we could not detect an effect of dietary LCPUFA on visual acuity in this group of healthy term infants who had received formula from birth. Not only were there no group effects but no LCPUFA emerged as an influencing factor in the regression model describing the relationship between diet and VEP acuity. There are 2 other full reports in the literature that show a similar lack of an effect of dietary LCPUFA in formula-fed term infants.^{6 ,7} In contrast, our previous study³and reports from other laboratories^{4 ,5} have shown in randomized clinical trials differences in visual acuity that seem to be attributable to dietary LCPUFA.

The lack of effect of dietary LCPUFA on VEP acuity in this rigorously conducted trial was surprising for a number of reasons. First, we had previously demonstrated an effect of dietary LCPUFA³ and the VEP apparatus was known to be more sensitive than the equipment used in an earlier study.¹³ Further, at the 16-week assessment, the numbers in each group allowed for sufficient power to detect changes had they been present. For these reasons, we think that it is important to reevaluate the evidence relating to dietary LCPUFA and visual function.

In the current study, we used either low EPA tuna oil or egg phospholipid containing a balance of AA and DHA but little EPA. This was different from our earlier study, in which high EPA fish oil was used.³ Other studies also have used a variety of LCPUFA sources.^4–7 One hypothesis is that in those studies in which an effect has been reported,³ the diets have generally contained high levels of total n-3 LCPUFA including EPA as well as DHA. This is not to suggest that the active ingredient in these early studies was necessarily EPA but rather that the presence of this compound may have had a sparing effect on the oxidation of DHA and thus more DHA may have been available for incorporation into neural tissues. Certainly one of the largest and most consistent effects reported in preterm infants conducted by Birch and coworkers^{14 ,15}supplemented the basic formula with nearly 1% total n-3 LCPUFA. Other studies in preterm and term infants reporting either weak effects or no effects have supplemented diets with relatively modest levels of n-3 LCPUFA (.1%-.35% DHA or total n-3 LCPUFA). However, this theory is not supported by a randomized trial of maternal DHA supplementation among breastfed infants in which increasing DHA in breast milk from ∼.2% to 1.1% total fatty acids resulted in no measurable effect on infant VEP acuity at either 12 or 16 weeks of age.¹³Alternatively, Heird et al¹⁶ proposed that visual effects of dietary supplementation with n-3 LCPUFA were seen only in studies in which the placebo diet contained <2% α-linolenic acid (ALA). However, this second hypothesis is not consistent with the current literature, because there are now 2 trials with <2% ALA in the placebo diet that demonstrate no effect of LCPUFA treatment⁷ (and the current trial) and 4 trials with >2% ALA in the placebo formula that demonstrate beneficial effects of n-3 LCPUFA supplementation on the measures of visual function.^{4 ,5 ,17 ,18} Therefore, it would seem that neither the amount of n-3 LCPUFA added or the PUFA content of the background diet (or placebo formula) is capable of explaining the disparate outcomes of LCPUFA trials conducted during infancy. We also believe that it is unlikely that the form of dietary LCPUFA (triglycerides, phospholipids) is relevant to the recipient infant provided that LCPUFA are present, because the diet-induced changes in infant LCPUFA status are similar regardless of the dietary source.

Our study was designed with sufficient power (80%) to detect a .15-.2 log unit difference in VEP acuity, allowing us to detect differences of about this magnitude between breastfed and formula-fed infants. Thus at 16 weeks, it seems unlikely that the lack of difference between the formula treatments was attributable to a type II error. However, because of the higher proportion of unsuccessful VEPs at 34 weeks, the power dropped to 72%, reducing the level of confidence in the result at this age. It is important to recognize that even in studies that report an influence of dietary LCPUFA on visual acuity, the effect is not generally large. For example, Birch et al⁵ recently reported a small effect (.1–.15 log units) of LCPUFA supplementation in VEP acuity in term infants but no effect in visual acuity measured using cards. Carlson et al⁴ reported a similarly small effect but in a measure of acuity by Teller cards. The biggest predictor of VEP acuity in our study was birth weight such that the larger the child the better the acuity score (negative association of weight with logMAR). Certainly Jorgensen and coworkers⁷have reported that birth weight was a major predictor of sweep VEP acuity. Because birth weight may be associated with greater maturity at birth, it is important that this and other potential confounding variables are adequately controlled in studies of this type. The medium- and long-term clinical significance of small differences in visual acuity during infancy remain to be established. Further, it may well be that other more sensitive and clinically relevant outcome measures may be required to establish if there is functional essentiality of dietary LCPUFA.

The secondary aim of our trial was to assess the effect of LCPUFA supplementation on Bayley's MDI and PDI at 1 and 2 years of age. The MDI and PDI scores of all randomly allocated infants were similar. This result was not surprising given the relatively small number of infants per group and the similar parental and environmental background of these groups. It is interesting to note that no study in term infants has demonstrated an effect of LCPUFA supplementation using the Bayley tests¹⁹ and only one preterm study has claimed an effect of fish oil supplementation in preterm infants.²⁰ It is, however, important to note that none of these trials have been adequately powered to measure an effect of dietary LCPUFA on the Bayley's MDI and PDI and all these studies are subject to type II error. Thus, there seems to be little evidence to prove or disprove a role for LCPUFA in global mental development as assessed by the Bayley test.

In our post hoc regression analyses including both the randomly allocated formula-fed infants and nonrandomized breastfed infants, formula feeding was negatively associated with Bayley's MDI at 2 years of age. Other studies have demonstrated that children who were breastfed as infants have better developmental scores than children who were formula-fed after adjustment for some, but not all, environmental factors that may confound the relationship between breastfeeding and developmental outcome.^21–24 In our study, birth order and gender also significantly predicted MDI at 2 years of age. We have reported similar effects of gender on Bayley's MDI scores in a separate group of infants who were all initially breastfed.²⁵

The issue that has been hotly debated is whether the differences reported between breastfed and formula-fed infants are attributable to procedural/methodological differences or factors present in breast milk that are not contained in formula. An alternate hypothesis that has been suggested is that there may be factors in formula that have a negative effect on neurodevelopment.²⁶ The complexity of interpretation of interventions of early nutrition and behavior in both animal and human studies has been addressed,²⁷ and it is clear that few firm conclusions can be made regarding the functional essentiality of any single dietary factor in the developing central nervous system.

Based solely on the results of this trial, we can find little evidence in support of an effect of dietary DHA on development of visual acuity in healthy term infants. Given that this trial had the capacity to detect differences in VEP acuity of .15 to .20 log units, it may be that any effect of dietary DHA is small and below this limit. Whether effects smaller than this can translate into developmental differences later in life or are reflected in other developmental capacities is unknown.

Our observation is consistent with a number of trials;^{6 ,7}other trials have demonstrated small positive effects of dietary DHA.^{4 ,5} The questions that surround all such results are the degree to which the reported results can be accounted for by possible bias or confounding and the degree that the reported best effect is clinically relevant in the short and long term.