15 May 1998

The Editor,

The Lancet,

42 Bedford Square

London WC1B 3SL

Dear Sir,

Joseph R Hibbeln (18 April, p 1213) points out an interesting inverse correlation between fish consumption and major depression and suggests some excellent rationale. The possibility of confounding factors is also mentioned. Electromagnetic radiation[1]^{, [2]} and amount of daylight exposure[3] may have some relation to prevalence of depression. A simple measure of these factors is the geographic latitude.

We compared, for the same countries, the annual rates of depression with the latitude of the country and found that they are directly related to each other. The prevalence of depression rises as one gets farther away from the equator (R=0.65, p=0.055). If we exclude Puerto Rico, the least Westernised of these countries, from the calculations, the correlation crosses the conventional level of statistical significance (R=0.72, p=0.040). Perhaps mood is simply dependent on daylight exposure acting through various mechanisms such as changing gene expression[4] or vitamin D levels[5].

See Graph

Yours sincerely,

Jayant S Vaidya Michael Douek

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[1]. George MS, Wassermann EM, Kimbrell TA, et al. Mood improvement following daily left prefrontal repetitive transcranial magnetic stimulation in patients with depression: a placebo-controlled crossover trial. Am J Psychiatry 1997 Dec;154(12):1752-1756

[2]. Verkasalo PK, Kaprio J, Varjonen J, Romanov K, Heikkila K, Koskenvuo M. Magnetic fields of transmission lines and depression. Am J Epidemiol 1997 Dec 15;146(12):1037-1045

[3]. Molin J, Mellerup E, Bolwig T, Scheike T, Dam H J. The influence of climate on development of winter depression. Affect Disord 1996 Apr 12;37(2-3):151-155

[4]. Partonen T The shortening of the photoperiod may alter gene expression in winter depression. Med Hypotheses 1994 Jan;42(1):13-14

[5]. Michelson D, Stratakis C, Hill L, et al. Bone mineral density in women with depression. N Engl J Med 1996 Oct 17;335(16):1176-1181