Whole body synthesis rates of docosahexaenoic acid (DHA) from α to linolenic acid are greater than brain DHA accretion and uptake rates in adult rats.

Docosahexaenoic acid (DHA) is important for brain function, however, the exact amount needed for the brain is not agreed upon. While it is believed that the synthesis rate of DHA from α  linolenic acid (ALA) is low, how this synthesis rate compares with the amount of DHA required to maintain brain DHA levels is unknown. The objective of this work was to assess whether DHA synthesis from ALA is sufficient for the brain. To test this, rats consumed a diet low in  n3 PUFA, or a diet containing ALA or DHA for 15 weeks. Over the 15 weeks, whole body and brain DHA accretion were measured, while at the end of the study, whole body DHA synthesis rates, brain gene expression and DHA uptake rates were measured. Despite large differences in body DHA accretion there was no difference in brain DHA accretion between rats fed ALA and DHA. In rats fed ALA, DHA synthesis and accretion was 100 fold higher than brain DHA accretion of rats fed DHA. Also, ALA fed rats synthesized approximately 3 fold more DHA than the DHA uptake rate into the brain. This work indicates that DHA synthesis from ALA may be sufficient to supply the brain. (Authors abstract)

α linolenic acid (ALA) is the most accessible and sustainable source of  n3 polyunsaturated fatty acids ( n3 PUFA) in the global diet.  ALA is also a precursor to docosahexaenoic acid (DHA), an  n3 PUFA that is particularly enriched within the brain. While it is generally accepted that DHA is important for normal brain function, the amount of DHA required by the brain is not agreed upon.  n3 PUFA cannot be synthesized by mammals de novo, therefore, DHA must be consumed from dietary sources or be synthesized from shorter chain  n3 PUFA (i.e ALA). To date, reports suggest that the synthesis rate of DHA from ALA is low and perhaps even below detection. However, plasma concentrations of DHA in vegans are only 0 to 40 percent lower than fish eaters despite having no dietary DHA. Furthermore, vegan and vegetarian populations do not have an increased risk of neurological disorders. The lack of concordance between the low DHA synthesis rates and the relatively normal plasma DHA concentrations in vegans may be due to the methods used to measure DHA synthesis.

Since the human adipose half to life can be longer than one year, it is possible that the tracer is unavailable for DHA synthesis during the study period.  Rapoport and colleagues developed a new method in rats to estimate the DHA synthesis rate from ALA. This method requires a steady to state infusion of labeled ALA and uses non to linear regression to determine the DHA synthesis rate. Importantly, this method estimates the DHA synthesis rate in rats to be 9.8 microMole/day matching the 11 microMole/day synthesis and accretion rate of longer chain  n3PUFA, which we estimated from a published balance study performed in growing rats. The steady to state infusion method may have advantages because: 1) infusing a tracer to achieve steady to state in the plasma eliminates issues around adipose tissue storage of the tracer and 2) it allows for a quantitative determination of the DHA synthesis rate.

The goal of this study was to determine if DHA synthesis from ALA can maintain brain DHA in rats. We addressed this objective by measuring (i) brain and whole body DHA accretion, (ii) DHA synthesis rates from ALA, and (iii) brain DHA uptake rates in rats fed 3 different diets, a control diet (low  n 3PUFA), an ALA diet (2 percent ALA), or a DHA diet (2 percent DHA). It was found that rats fed ALA and DHA accreted similar amounts of brain DHA, which together with kinetic findings suggest DHA synthesis from ALA is likely sufficient to maintain brain DHA levels. In rats, the methods used in humans to determine DHA synthesis from ALA were used by subjecting rats to a gavage with labelled ALA. The rates from this experiment, in rats, were comparable to the results of previously published human studies. Collectively, these results indicate that the rat is an appropriate model for measuring brain DHA synthesis and that brain DHA can be supplied from dietary ALA.

This was supported by the finding that dietary ALA and DHA resulted in the same level and accretion of brain DHA after 15 weeks. A comparison of these daily brain accretion rates to the whole body DHA synthesis rate in the ALA fed rats in the balance study, showed that DHA synthesis rates exceed brain uptake rates by 100 fold.
 
Despite sizeable differences in  n3 PUFA concentrations and accretions in the bodies, the study was unable to detect differences in brain DHA concentrations and accretions between rats consuming diets with DHA or ALA as the only  n3 PUFA source. The finding that brain DHA concentrations are not different between rats fed the ALA and DHA diet is in contrast to previously published work. But the results of kinetic studies support the idea that rats consuming the ALA diet were able to synthesize enough DHA to supply the brain.  Despite the similar brain DHA concentrations in rats fed the ALA and DHA diet, rats consuming DHA had an almost 2 to fold greater uptake rate of DHA into the brain. This means that the brains of rats fed the DHA diet took up and metabolically consumed more DHA. Despite increased metabolism of DHA in the brain, there were no differences in gene expression between rats on either diet. It is conceivable that exposing these rats to a stressor (such as brain trauma, neuroinflammation, etc.) would result in differential gene expression in the brains of these rats. Therefore, future experiments should measure the effect of diet on brain gene expression in rats that have been exposed to stress.

When ALA is consumed orally, the majority will be oxidized or stored in the adipose. This study as well as other balance studies found that the majority of ALA intake is metabolically consumed and not accumulated in the tissues. Approximately 90 percent of dietary ALA was metabolically consumed. It is generally accepted that the rat can synthesize more DHA than the human, however, the methods used to measure DHA synthesis in the human have not been validated in the rat. In humans, DHA synthesis is measured by administering an oral bolus of labeled ALA and measuring the appearance of labeled DHA in plasma. These calculations can be applied to the data from the gavage study to mimic the ―human‖ method for determining DHA synthesis. The results from the oral gavage study are inconsistent with the belief that the rat is more efficient at synthesizing DHA than the human. Also, the fact that applying different kinetic calculations to the same data gave markedly different results for DHA synthesis indicates that the calculations used to measure DHA synthesis are inconsistent and should be used largely to compare relative DHA synthesis between experimental groups within a study. While pilot data indicated that labeled DHA peaked within 6 hours of an oral gavage in a rat, if this study was extended beyond 6 hours the DHA synthesis rates measured could be higher.

This study showed that despite large differences in fatty acid accumulation in the body, rats fed a diet containing DHA or ALA making up 2 percent of the fatty acids did not have differences in brain DHA accumulation. As the uptake rate of DHA into the brain has been shown to match rates of brain DHA metabolism, it is likely that decreased brain DHA metabolism, in combination with an increased rate of DHA synthesis from ALA is the reason that brain DHA accretion in rats fed the ALA diet did not differ from the rats fed the DHA diet. The overall results from this study indicate that DHA synthesis from ALA in the rat may be sufficient to maintain brain DHA concentrations in the absence of dietary DHA consumption. Importantly, the steady to state infusion method can be used in humans to calculate an actual DHA synthesis rate that can be compared to brain DHA uptake rates measured in humans with positron emission tomography scanning. (Editors comments)