Great research, unexpected conclusion – Why fish is so good for you (?)

7 May 2013 by Danny Haelewaters, posted in Marine biology

A few weeks ago I encountered two articles about unsaturated omega-3 fatty acids and their mechanism. Great research, but the tagline that was coupled with these studies - Why fish is so good for you - came rather unexpected. What follows are some commentary thoughts that have been crossing my mind since then.

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In fatty fish species, as are anchovy, herring, mackerel, and salmon, unsaturated omega-3 fatty acids like docosahexaenoic acid (DHA) and eicosapentaenoic (EPA) are abundantly found. Their health-promoting effects are widely recognized, although until recently the molecular effectors of omega-3 fatty acids and the mechanisms of the interactions remained unclear.

In a current issue of PNAS, Hoshi et al. (1,2) show that the hypotensive effect of DHA lies in the rapidly and reversibly activation of Ca2+-dependent K+ (Slo1+ß1, Slo1+ß4) channels. The large effect of DHA in vascular Slo1+ß1 and neuronal Slo1+ß4 channels is due to the presence of two residues (Arg11, Cys18) in both ß1 and ß4 subunits (2). Moreover, the efficiency of natural-derived DHA and EPA versus their ethyl ester derivatives found in dietary supplements is emphasized (1). So far for the physiological (molecular/cellular) effects and mechanisms.

While I do not want to deny the importance of the above conclusions, I regret that the authors did not include other available natural sources for unsaturated omega-3 fatty acids, in addition to (fatty) fish, and this for the reasons as outlined below.

In both human consumption and aquaculture industry fish oil is a key source of omega-3 fatty acids. It is extracted from the world’s oceans, yielding around one million ton of fish oil annually (3). Although the scientific community recognizes these specialized fisheries as sustainable (pers. comm. Giovanni Turchini), the problem is that they cannot expand to meet the currently ever-growing demand for fish oil.

This demand for fish oil as well as for fish and invertebrates for consumption is supplied by marine capture fisheries and aquaculture industry, which heavily relies on input from marine catches. Fisheries globally are facing crisis (4); as large predator species are depleted, fisheries need to turn to increasingly smaller fish species, and will eventually be left with previously dismissed small fish and invertebrates – a concept referred to as fishing down the food web (5). Currently, more than 70 to 75% of all commercial fishing grounds are severely overfished. It is estimated that 0.97 to 2.7 trillion wild fish are caught globally each year (6), adding up to 77.4 million tonnes of fish species (7).

Given the rapid worldwide drop in fish stocks since the 1950s (4) and the 133 confirmed (local, regional, and global) extinction events of marine populations, most of which due to (over-)exploitation (8), it has become irresponsible to drive people in First World countries to consume fish for their intake of long-chain omega-3 fatty acids.

A last but not insignificant point is that the vast majority of DHA and EPA found in fish is actually not produced by them; these fats originate from and make up half the weight of certain species of phytoplankton and from then on become increasingly concentrated in marine organisms higher on the food chain. Phytoplankton, single-cell photosynthesizing algae like Crypthecodinium cohnii, are easy to farm and therefore may play and important role in future, more sustainable directions for natural-derived omega-3.

 

References

(1) Hoshi T, Wissuwa B, Tian Y, Tajima N, Xu R, Bauer M, Heinemann SH, Hou S (2013) Omega-3 fatty acids lower blood pressure by directly activating large-conductance Ca2+-dependent K+ channels. Proc Natl Acad Sci USA.

(2) Hoshi T, Tian Y, Xu R, Heinemann SH, Hou S (2013) Mechanism of the modulation of BK potassium channel complexes with different auxiliary subunit compositions by the omega-3 fatty acid DHA. Proc Natl Acad Sci USA.

(3) De Silva S, Francis DS, Tacon AGJ (2011) Fish oil in aquaculture: in retrospect. In: Turchini GM, Ng WK, Tocher DR (eds.) Fish oil replacement and alternative lipid sources in aquaculture feeds, CRC Press, Boca Raton, Florida: 1-20.

(4) Jenkins DJA, Sievenpiper JL, Pauly D, Sumaila UR, Kendall CWC (2009) Are dietary recommendations for the use of fish oils sustainable? Canadian Medical Association Journal 180 (6): 633-637.

(5) Pauly D, Christensen V, Dalsgaard J, Froese R, Torres Jr F (1998) Fishing down marine food webs. Science 279 (5352): 860-863.

(6) Mood A 2010. Worse things happen at sea: the welfare of wild-caught fish. Fishcount.org.uk, 2010: 1-139.

(7) FAO 2009. Capture Production 1950-2007 (Release Date: February 2009). Food and Agriculture Organization of the United Nations, Rome (Italy).

(8) Dulvy N, Sadovy Y, Reynolds JD (2003) Extinction vulnerability in marine populations. Fish Fishery 4: 25-64.


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