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LETTER TO THE EDITOR

Omega-3 Alpha Linolenic Acid Does Not Reflect the Entire Omega-3 Fatty Acid Family

Nutrition and Food Science and the Pain Centre, McGill University, Montreal General Hospital, Montreal, QC, Canada, yoram.shir{at}muhc.mcgill.ca

In Response:

We disagree with Dr. Heller and colleagues about the study methodology employed in our analysis of the association between dietary omega-3 fatty acids and neuropathic pain (1).

1. Their claim that we can only attribute the effects of omega-3 linoleic acid and not the entire omega-3 family is correct. We did not plan to test the entire omega-3 family (as evident by not including fish oils in our protocol) nor did we state that the effects we found should be attributed to the entire family. These issues have been discussed previously (2,3).

2. We would like to remind Dr. Heller and his colleagues that arachidonic acid is not synthesized by plants because they do not have the elongase and desaturase enzyme systems needed to synthesize arachidonic acid from linoleic acid. Therefore, arachidonic acid is not found in vegetable oils. It is true that production of proinflammatory and antiinflammatory metabolites may be responsible for the effects we reported, but these metabolites are synthesized within the animal tissues from their precursors: arachidonic acid from linoleic acid of the omega 6 family and eicosapentaenoic acid from -linolenic acid of the omega 3 family. The oils we gave to the rats could not contain these anti-inflammatory metabolites. Thus, these metabolites and their possible effects on pain behavior were not reported in our article.

3. Our behavioral results were criticized on two grounds:

a. No significant difference of either intervention (i.e., the hyperalgesic versus hypoalgesic oils) in comparison with control: This is not exactly true. We are showing marginally significant differences in heat hyperalgesia between rats supplemented with water only compared with hemp oil-fed rats (P < 0.07) and corn oil-fed rats (P < 0.06).

b. The selective analgesic effect of dietary fat on thermal, but not mechanical, hyperalgesia: Several lines of clinical, anatomical, pharmacological, and molecular evidence support the notion that the physiological basis of thermal and mechanical hypersensitivity states is considerably dissociable (4). For example, the percentage of injured fibers after partial nerve injury could dictate the development of thermal versus tactile hyperalgesia (5); N-methyl-d-aspartate antagonists suppressed thermal but not mechanical hyperalgesia (6), whereas gabapentin alleviated mechanical but not cold allodynia (7) in nerve injured rats; midrange plasma levels of phytoestrogens were associated with reduced mechanical, but not thermal, pain after nerve injury in rats (8).

Therefore, our results that specific intervention is capable of suppressing thermal but not tactile hyperalgesia are well supported.

References

1. Perez J, Ware MA, Chevalier S, et al. Dietary omega-3 fatty acids may be associated with increased neuropathic pain in nerve-injured rats. Anesth Analg 2005;101:444–8.[Abstract/Free Full Text]
2. Shapiro H. Letter to the editor. Pain 2005;115:212–3.[Medline]
3. Chevalier S, Gougeon R, Pérez J, Shir Y. Letter to the editor. Pain 2005;115:213.[Medline]
4. Mogil JS, Crager SE. What should we be measuring in behavioral studies of chronic pain in animals? Pain 2005;112:12–5.
5. Obata K, Yamanaka H, Fukuoka T, et al. Contribution of injured and uninjured dorsal root ganglion neurons to pain behavior and the changes in gene expression following chronic constriction injury of the sciatic nerve in rats. Pain 2003;101:65–77.[Web of Science][Medline]
6. Tal M, Bennett GJ. Neuropathic pain sensations are differentially sensitive to dextrorphan. Neuro Report 1994;5:1438–40.[Web of Science][Medline]
7. Erichsen HK, Blackburn-Munro G. Pharmacological characterization of the spared nerve injury model of neuropathic pain. Pain 2002;98:151–61.[Web of Science][Medline]
8. Shir Y, Campbell JN, Raja SN, Seltzer Z. The correlation between dietary soy phytoestrogens and neuropathic pain behavior in rats after partial denervation. Anesth Analg 2002;94:421–6.[Abstract/Free Full Text]

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