In response to the Heart Scan Blog post, Fish oil makes you happy: Psychological distress and omega-3 index, Dr. David Grimes offered the following argument.
Dr. Grimes is a physician in northwest England at the Blackburn Royal Infirmary, Lancashire. He is author of the wonderfully cheeky 2006 Lancet editorial, Are statins analogues of vitamin D?, questioning whether the benefits of statin drugs simply work by way of increased vitamin D blood levels.
There is a fashionable interest in Omega-3 fatty acids, and these become equated with fish oil.
But fish oil is much more. Plankton synthesise the related squalene (shark oil) which, in turn, is converted into 7-dehydrocholesterol (7-DHC). The sun now comes into play and it converts 7-DHC into vitamin D (a physico-chemical process).
Small fish eat plankton, large fish eat small fish, and we eat large fish. So vitamin D passes through the food chain.
This has been a vital source of vitamin D for the the Inuits and also for the Scots and other dwellers of northwest Europe. (Edinburgh is on the same latitude as Hudson Bay and Alaska, further north than anywhere in China). In these locations there is not adequate sunlight energy to guarantee synthesis of adequate amounts of vitamin D, again by the action of sunlight on 7-DHC in the skin.
When the Scots moved from coastal fishing villages to industrial cities such as Glasgow, they became seriously deficient in vitamin D, and so the emergence of rickets. This was followed by a variety of other diseases resulting from vitamin D deficiency: tuberculosis, dental decay, coronary heart disease, and even multiple sclerosis and depression (the Glasgow syndrome).
And so it was with the Inuits. When their diet changed from fish for breakfast, fish for lunch, fish for dinner, they became deficient of vitamin D and they developed diseases characteristic of industrial cities, where there is indoor work for long hours, indoor activities, and atmospheric pollution.
It is the vitamin D component of fish and fish oils that is important.
I recently saw an elderly lady from Bangladesh living in northwest England. I would have expected her to have a very low blood level of vitamin D, as her exposure to the sun was minimal. However the blood level was 47ng/ml, not 4 as expected. She eats oily fish from Bangladesh every day, showing its value as a source of vitamin D with subsequent good health. I expect her blood levels of omega-3 fatty acids would also be high.
But it is unfashionable vitamin D that is important, not fashionable omega-3.
Excellent point. The health effects of omega-3 and vitamin D are intimately intertwined when examining populations that consume fish.
In this study of Inuits, it is indeed impossible to dissect out how much psychological distress was due to reduced vitamin D, how much due to reduced omega-3s. My bet is that it's both. Thankfully, we also have data examining the use of pure omega-3 fatty acids in capsule (not intact fish) form, including studies like GISSI Prevenzione.
Nonetheless, Dr. Grimes reminds us that both vitamin D and omega-3 fatty acids from fish oil play crucial roles in mental health and other aspects of health, and that it's the combination that may account for the extravagant health effects previously ascribed only to omega-3s.