Yet Again RCTs Fail to Confirm Promising Effects of Dietary Factors in Observational Studies

This time the dietary factors were Omega-3 fatty acids and various trace elements, and the condition of interest was cognitive decline with age.[1] The interventions were tested in a factorial RCT and the mean age of patients was 71. To be sure, cognitive function declined on average with the passing of the years, but did so equally, irrespective of whether patients had omega-3, other nutritional supplements, both, or neither. Loss to follow-up was low, the study was large (n=3,741), and follow-up averaged five years. It is possible, at least in theory, that any effect of nutrition on cognition unfolds over decades not years. It is also possible that foods rich in the various nutrients tested contain other factors that, singly or in combination, are responsible for the beneficial effects seen in observational studies. How will we ever know? Here is a thought – RCTs of nutritional factors should be thought of as testing the hypothesis that they are harmful – what some call equivalence studies. Then, so long as the nutritional factor of interest is not harmful and is good to eat, it should be included in a balanced diet.

— Richard Lilford, CLAHRC WM Director


  1. Chew EY, Clemons TE, Agrón E, et al. Effect of Omega-3 Fatty Acids, Lutein/Zeaxanthin, or Other Nutrient Supplementation on Cognitive Function. The AREDS2 Randomized Clinical Trial. JAMA. 2015; 314(8):791-801.

2 thoughts on “Yet Again RCTs Fail to Confirm Promising Effects of Dietary Factors in Observational Studies”

  1. Last night’s BBC4 repeat Horizon programme ‘ – exploring the work of scientists trying to establish how ideas come about’ brought up some interesting points that may be of relevance. Namely that: 1. Changing daily routine & 2. engagement in non brain-straining tasks e.g. sorting mixed Lego bricks into those of the same colour, resulted in greater creativity [cognitive function] with new pathways being created between the neurons.

    Another interesting point was that the left side of the brain retained pictures/words that were flashed before the left eye of participants again aiding cognitive function.

    Boredom & routine help kill the brain cells far quicker so simple stimulation would be a cheap & easy treatment as opposed to leaving the elderly just sitting in a chair all day :0)

    Hope this helps

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