Fatty fish and winter depression (I) – An Icelandic paradox

Lat­i­tude hypoth­e­sis needs revis­it­ing: fur­ther fac­tors – diet, genet­ics or ver­nac­u­lar cul­ture – seem to sneak into the strict lat­i­tude deter­min­ism that could be assumed in win­ter depres­sion epi­demi­ol­o­gy. Such a cor­re­la­tion of Sea­son­al Affec­tive Dis­or­der (SAD) symp­toms with prox­im­i­ty to the North Pole still holds in broad strokes: a mas­sive sys­tem­at­ic review and meta-analy­sis con­duct­ed in 2025, which aggre­gates the results of myr­i­ads of local and piece­meal stud­ies from the past, con­firms«a clear pos­i­tive asso­ci­a­tion between lat­i­tude and the preva­lence of win­ter pat­tern SAD and SSAD [sub­syn­dro­mal-SAD]». The review finds a sta­tis­ti­cal rise by 0.2 in SAD «with each 1‑degree lat­i­tude increase». (Kim, 2025) But Ice­landers seem to under­mine the hypoth­e­sis of inevitabil­i­ty.

The «Fat­ty fish and win­ter depres­sion» inves­ti­ga­tion com­pris­es two parts. This arti­cle is the first part. The sec­ond part is here: Fat­ty fish and win­ter depres­sion (II) – Inter­play of omega-3s with gene clocks.

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Not so SAD in Iceland

«Not so sad in Ice­land», head­lines the Lancet in 1994. An epi­demi­o­log­i­cal study of SAD, the only one con­duct­ed in Ice­land until 2025, shows a remark­ably low preva­lence – approx­i­mate­ly 3.8% – for a lat­i­tude of 64°N. The authors inves­ti­gate the genet­ic hypoth­e­sis, which a sub­se­quent study of pop­u­la­tions of Ice­landic descent locat­ed in two dis­tant geo­gra­phies at the same lat­i­tude with very dif­fer­ent SAD preva­lences tends rather to call into ques­tion. (Mag­nús­son, 1993; Axels­son, 2002) These stud­ies do not rule on the eti­ol­o­gy of the phe­nom­e­non: they nev­er­the­less point to the inter­play of favor­able genet­ic back­ground and envi­ron­men­tal fac­tors.

Good dietary habits? The dietary angle must be seri­ous­ly con­sid­ered, when one knows that Ice­landers are noto­ri­ous fish con­sumers: Ice­landers con­sumed 85.39kg per capi­ta of fish in 2022 ver­sus 49.66kg per capi­ta in Nor­way, 31.68kg in Fin­land, 13.05kg in Ger­many and 22.07kg in the USA. (Our World in Data, 2025)

Question of Method

A new epi­demi­ol­o­gy of Ice­landic SAD in 2025 nuances Mag­nús­son’s sur­pris­ing dis­cov­ery by report­ing a sig­nif­i­cant­ly high­er preva­lence of 17.86%. Its con­clu­sions are inter­est­ing and invite an aside: «clin­i­cal SAD [is] rather rare in Ice­land», it says, but «a sig­nif­i­cant por­tion of peo­ple [report] sea­son­al symp­toms affect­ing their well­be­ing.»

Both Mag­nús­son in 1993 and the 2025 study rely on «self-report screen­ing»: only those who report feel­ing so are con­sid­ered SAD, and objec­tiv­i­ty slips away. As the first fac­tor that may explain the com­par­a­tive­ly ele­vat­ed preva­lence, the new study states «doc­u­ment­ed ris­ing men­tal health lit­er­a­cy across West­ern coun­tries»: «pub­lic aware­ness of men­tal health issues has grown sub­stan­tial­ly in recent decades. […] increased famil­iar­i­ty with psy­cho­log­i­cal ter­mi­nol­o­gy may lead indi­vid­u­als to label sea­son­al fluc­tu­a­tions as indica­tive of a dis­or­der, even when symp­toms are sub­clin­i­cal.»

This reminds us that all epi­demi­o­log­i­cal and eti­o­log­i­cal stud­ies of SAD rely on con­cepts and diag­nos­tic meth­ods that are con­struct­ed, evolv­ing, and there­by sub­ject to cau­tion.

While it is dif­fi­cult to know defin­i­tive­ly whether Ice­landers are hap­pi­er than a lat­i­tude hypoth­e­sis would pre­dict, the link between diet and men­tal well­be­ing is amply estab­lished. (Logan, 2004; Checa-Ros, 2022) We there­fore con­tin­ue our inter­est in fish and its poten­tial for rem­e­dy­ing win­ter depres­sion.

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A disruption in circadian rhythms

As we argued ear­li­er, SAD has a strong cir­ca­di­an com­po­nent. A mis­match between cir­ca­di­an biol­o­gy and sleep/wake cycles leads to win­ter depres­sion. Typ­i­cal­ly, there is a delay in the dim light mela­tonin onset (DLMO) with respect to sleep onset. DLMO des­ig­nates the evening rise of plas­ma mela­tonin lev­els and is a key mark­er of endoge­nous cir­ca­di­an phase posi­tion. (Lewy, 2006)

Let’s briefly delve anew into the human cir­ca­di­an machin­ery. Bio­log­i­cal organ­isms pos­sess inter­nal clocks that can record the pass­ing of days or sea­sons, ensur­ing their fun­da­men­tal link to Moth­er Nature – to their ecosys­tem and the revolv­ing Earth. In any organ­isms, a num­ber of process­es and organ sys­tems are said to fol­low cir­ca­di­an rhythms.

⋙ Check the Polar Day­light Track­er by cities and lat­i­tudes.

Cir­ca­di­an rhythms exhib­it the fol­low­ing three prop­er­ties. Crit­i­cal­ly, they can be reset by exter­nal time cues or zeit­ge­bers (lit­er­al­ly, from Ger­man, time-giv­ing). The most notable zeit­ge­ber in mam­mals is the dai­ly light-dark cycle. Then, in the absence of such exter­nal time cues, for instance in dark­ness, they per­sist (or free-run) with a peri­od of approx­i­mate­ly 24 hours. Final­ly, they have an invari­ant peri­od length over a wide range of phys­i­o­log­i­cal­ly rel­e­vant tem­per­a­tures.

Circadian machinery

A tra­di­tion­al pic­ture of the human cir­ca­di­an tim­ing sys­tem may be over­ly sim­plis­tic:

• The retino­hy­po­thal­a­m­ic tract acts as an input path­way.
• It trans­duces phot­ic input to a pri­ma­ry clock or pace­mak­er, a region in the hypo­thal­a­mus called the suprachi­as­mat­ic nucle­us (SCN).
• The SCN con­veys sig­nals to down­stream effec­tor path­ways man­i­fest­ing cir­ca­di­an rhythms.

A more recent mod­el is more flu­id. Infor­ma­tion can flow in the oppo­site direc­tion, with feed­back from effec­tor to pace­mak­er to input path­way. As even iso­lat­ed indi­vid­ual cells from com­plex mul­ti­cel­lu­lar ani­mals exhib­it autonomous cir­ca­di­an oscil­la­tions, we can think of some intra­cel­lu­lar net­works as inde­pen­dent cir­ca­di­an tim­ing sys­tems. A sys­tem of «periph­er­al clocks» receives time-of-day infor­ma­tion from the pri­ma­ry pace­mak­er (SCN) to ful­fill tis­sue-spe­cif­ic reg­u­la­tion. There­by, «desyn­chro­niza­tion not only occurs between the exter­nal envi­ron­ment and the SCN rhythm gen­er­a­tor but also affects phase align­ments between the dif­fer­ent periph­er­al clocks.» (Edery, 2000)

Flavorful zeitgebers

Light ther­a­py – brief expo­sure to a high-inten­si­ty lamp in the morn­ing – or mela­tonin admin­is­tra­tion – typ­i­cal­ly in the evening – are effec­tive «stim­uli.» The lit­er­a­ture amply demon­strates their pow­er to cor­rect the cir­ca­di­an phase delays impli­cat­ed in win­ter depres­sion. (Lewy, 2006; Ter­man, 2001) What if some savory nutri­ent from Ice­landic or Nor­we­gian fish also pos­sess­es the pow­er to reset the clocks?

Fat­ty fish, notably salmon, has a very high con­tent of omega‑3 fat­ty acids – which most inter­est­ing­ly not only have proven effects on cog­ni­tion and inflam­ma­to­ry process­es but also proven inter­ac­tions with cir­ca­di­an clocks. And since we are inter­est­ed in reme­dies for win­ter depres­sion, we will focus on the mech­a­nisms of neu­ro­log­i­cal func­tion reg­u­la­tion by omega‑3 fat­ty acids via clock genes. (Tough fish has much more to offer.)

To this end, we will, in the next episode, first review how clocks work at a mol­e­c­u­lar lev­el. We will then study more close­ly those clocks at work in brain func­tion. And final­ly, we will be able to under­stand the mol­e­c­u­lar effect of omega‑3 fat­ty acids on clock genes and, ulti­mate­ly, on cog­ni­tion and mood.

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References

Kim, K., Kim, J., Jung, S., Kim, H.-W., Kim, H.-S., Son, E., Ko, D. S., Yoon, S., Kim, B. S., Kim, W. K., Lim, C., Kim, K., Lee, D.., & Kim, Y. H. (2025). Glob­al preva­lence of sea­son­al affec­tive dis­or­der by lat­i­tude: A sys­tem­at­ic review and meta-analy­sis. Jour­nal of Affec­tive Dis­or­ders, 398, 121–133. https://doi.org/10.1016/j.jad.2025.11249

Mag­nús­son, A., & Ste­fáns­son, J. G. (1993). Preva­lence of sea­son­al affec­tive dis­or­der in Ice­land. Archives of Gen­er­al Psy­chi­a­try, 50(12), 941–946. https://doi.org/10.1001/archpsyc.1993.01820240025002

Axels­son, J., Ste­fáns­son, J. G., Mag­nús­son, A., Sig­val­da­son, H., & Karls­son, M. M. (2002). Sea­son­al affec­tive dis­or­ders: Rel­e­vance of Ice­landic and Ice­landic-Cana­di­an evi­dence to eti­o­log­ic hypothe­ses. Cana­di­an Jour­nal of Psy­chi­a­try, 47(2), 153–158. https://doi.org/10.1177/070674370204700205

Our World in Data. (2025). Year­ly per capi­ta sup­ply of fish and seafood [Data set]. Food and Agri­cul­ture Orga­ni­za­tion of the Unit­ed Nations. Retrieved from https://ourworldindata.org/grapher/fish-and-seafood-consumption-per-capita

Logan, A. C. (2003). Diet and men­tal health in the Arc­tic: is diet an impor­tant risk fac­tor for men­tal health in cir­cum­po­lar peo­ples? – a review. Inter­na­tion­al Jour­nal of Cir­cum­po­lar Health, 62(3), 228–241. https://doi.org/10.3402/ijch.v62i3.17560

Lewy, A. J., Lefler, B. J., Emens, J. S., & Bauer, V. K. (2006). The cir­ca­di­an basis of win­ter depres­sion. Pro­ceed­ings of the Nation­al Acad­e­my of Sci­ences, 103(19), 7414–7419. https://doi.org/10.1073/pnas.0602425103

Edery, I. (2000). Cir­ca­di­an rhythms in a nut­shell. Phys­i­o­log­i­cal Genomics, 3(2), 59–74. https://doi.org/10.1152/physiolgenomics.2000.3.2.59

Ter­man, J. S., Ter­man, M., Lo, E. S., & Coop­er, T. B. (2001). Cir­ca­di­an time of morn­ing light admin­is­tra­tion and ther­a­peu­tic response in win­ter depres­sion. Archives of Gen­er­al Psy­chi­a­try, 58(1), 69–75. https://doi.org/10.1001/archpsyc.58.1.69