In this study, we reinforce our recent findings 
that physiological interaction among amalgamated mixtures of different FAs and/or nutrients in a diet, is critical for their impact on Aβ production/metabolism. Attributing the effects of a composite diet to a single lipid alone may not be adequate to explain most of the effects of that particular diet. Instead, it is important to investigate as well as discuss the optimal proportion of lipids/nutrients that are frequently or seldom present in a diet and might offer the most favorable biological synergy in relation to disease risk. For instance, DHA demonstrates an overall drift towards detrimental consequences - when it is administered with peptamen based liquid - in pathways important to amyloid deposition. This drift appears to be the result of a combination of several contributing factors. First, in Tg/pep+DHA mice proteolytic processing of the membrane-bound mature FL-APP was roughly augmented leading to corresponding increases in APP-CTFβ as compared to their age-matched Tg/pep and Tg/chow controls. Since the expression levels of the APP transgene and PrP promoter were unchanged, the elevated trend in FL-APP protein is most likely the result of its diminished degradation through the proteasomal pathway 
or its increased overall stability, implying that pep+DHA diet may alter Aβ generation either by altering APP trafficking to secretase-containing compartments of the membrane or secretase enzymatic activity itself. In keeping with the proposed diminishment of degradation - precedence for the impact of differential FAs on activity of the ubiquitin-proteasome pathway is seen with other proteins 
, suggesting a physiological means to regulate protein degradation where post-ER trafficking to either the Golgi or proteosomes appears the underlying basis.
Secondly, the increase inclination in BACE levels is likely due to the elevation of its FL-APP substrate - in agreement with a study demonstrating a parallel increase in BACE with levels of FL-APP 
. Alternatively, since BACE is also degraded via the ubiquitin-proteasome pathway 
– the affect of pep+DHA supplement on BACE levels may also be the result of diminished proteosomal degradation, or a combination of the two. Nevertheless, given the complexity of the in vivo
system the actual basis of this effect would be difficult if not impossible to ascertain.
Thirdly, and of particular significance, in Tg/pep+DHA mice - pep+DHA enrichment resulted in a leaning towards augmentation in Aβ levels with 57% increase in Aβ42 levels alone, more than would be expected by elevation of BACE and CTFβ alone. Consequently, the IDE levels were 54% reduced. IDE, which appears to degrade intracellular Aβ more effectively than does neprilysin in both the detergent-soluble and insoluble fractions 
Lastly, knowing that DHA and the brain are rich in oxidizable PUFA we propose that perhaps (conversely to low fat+DHA diet) some oxidizable constituents in peptamen diet might have caused lipid membrane peroxidation and/or free radical damage to the TgCRND8 mice brains 
and antagonized the beneficial effects of DHA either by increasing 4-hydroxynonenal (4-HNE), which also reports to activate BACE indirectly 
, as observed in this study or due to impaired Aβ clearance, by somehow, reducing the expression of the gene coding for IDE. Further in vitro
studies in neuronal cell lines would be required to clarify the basis of these effects.
To evaluate if DHA has protective effect when supplemented with a different composite nutritional base, we designed a diet high in DHA but low in all deleterious associations with fats, and showed that DHA indeed improve AD-type neuropathology in the brains of TgCRND8 mice. Initially, by significantly decreasing hippocampal Aβ40 and Aβ42 peptides levels and subsequently by reducing amyloid plaque burden in cortex, hippocampus as well as thalamus, endorsing our recent observation 
. Though the depletion of deleterious fat alone from the diet has shown reduced amyloidosis 
, however, DHA supplementation even enhances this protective effect further.
Peptamen is largely composed of ready to digest saturated fatty acids, which apparently perhaps the reason for ~37% increase in saturated fatty acids in Tg/pep+DHA mice. Tg/pep mice despite having lowest omega-3 (DHA) levels among all mice groups did not show as adverse plaque pathology as Tg/pep+DHA mice (with significantly higher DHA levels than Tg/pep mice). Though Tg/pep mice also showed an increase in saturated fatty acids, however, not as huge as in Tg/pep+DHA mice, perhaps supposedly due to the altered metabolism of saturated fatty acids and DHA combination or due to the toxic interaction(s) between DHA and it's metabolites (like docosanoids) and saturated fatty acids. Since the effects of such interactions are not fully understood, the possibility that they may have adverse affects on neuronal survival cannot be excluded. Linoleic acid deficiency reflects a decrease in the omega-6 fatty acids as DHA has been shown to down-regulate arachidonic acid (a member of omega-6) pathway 
. Therefore, higher levels of DHA were found to be associated with lower levels of linoleic acid in the present study. Moreover, a 2–9% increase in CNS DHA in the present study is reasonable for 4–5 months treatment, because other studies report that large (50–80%) 
alteration in brain DHA requires multiple generations of animals on DHA-depleted diets 
Our results suggest that perhaps the beneficial effects of DHA can not be seen if they are supplemented without taking the effect of other blended nutrients into consideration, especially if that nutrient affects DHA antagonistically. Our findings also explain reported controversial roles (detrimental or no effects) of diets rich in omega-3 FAs on AD pathology. For instance, high omega-3 diet was unable to improve cognitive performance in Tg mice in multiple cognitive measures 
. Further to this, the administration of DHA, EPA or placebo to a group of mild to moderate AD subjects for 6 months resulted in no difference in mini mental state examination (MMSE) or AD Assessment Scale (ADAS) scores between the groups, despite daily omega-3 doses that were many times higher than intake of omega-3 FA in fish products 
. Similarly, in non-transgenic rodents, there was no cognitive benefit to supplementation with DHA after multiple generations of omega-3 deficiency 
. Likewise, Calon et al., reported no benefit in recognition/identification memory or memory retention in either aged non-transgenic or aged Tg2576 mice supplemented with DHA following depletion 
. In the same way, MIDAS 
and ADCS 
DHA trials did not show a benefit of DHA treatment in mild to moderate AD. Similarly an increase in neuronal loss and prion formation 
in cell-culture AD models after DHA supplementation has also been reported. Ironically, none of these studies had taken into account the role and effect of other lipids or nutrients in the diet, that may had either complemented or antagonized the effects of DHA on the risk of AD.
In summary, our results from in vivo
mouse studies opened up new alleys in understanding the complex roles of PUFAs on the biochemical mechanisms leading to amyloid plaque development in patients suffering from co-morbidities. Present data do not argue the protective effects of DHA (as shown by low fat+DHA diet), rather strengthen ours 
and others 
previous findings that we may have to focus on diet mixtures rather than associating all effects to a single lipid or ingredient. We assume that cellular/molecular homeostasis mechanisms can effectively offset the effects of single lipids, rather than complex synergistic or antagonistic changes exerted by mixture of diets. Further controlled in vitro
investigations, and particularly long-term human-based retrospective studies would be required to confirm the effect of pep+DHA diet and justify dietary modifications for patients at risk of suffering from AD and HAD like co-morbidities.