A somewhat systematic review of previous systematic reviews and meta-analyses on nutrition and Alzheimer’s disease

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A team from the International Education College of Zhejiang University of Chinese Medicine, Hangzhou, China, conducted a review of systematic reviews and meta-analysis studies around the topic of nutrition in disease.

In the article “Effect of nutrition in Alzheimer’s disease: a systematic review”, published in Frontiers in Neurosciencethe researchers compiled previous systematic reviews and meta-analyses of randomized clinical trials investigating the association between nutritional interventions and Alzheimer’s disease published between 2018 and 2022.

Excluding non-human research and studies that included other forms of dementia or pharmacological interventions, the current meta-study selected 38 studies, of which 17 were randomized clinical trials and 21 were systematic reviews or meta-analyses.

The compiled review of previous findings showed that specific nutritional interventions were observed to correlate with a slowing of the rate of progression of Alzheimer’s disease, improving cognitive function.

While a typical meta-analysis will compile many individual studies into one for analysis, there is a risk that a systematic review of multiple metadata studies may artificially weight specific study results if they are included multiple times in the various meta-analyses.

For example, a 2012 Australian study, “Adherence to a Mediterranean diet and risk of Alzheimer’s disease in an Australian population”, appears in several meta-studies analyzed by the current study. The 149 patients with Alzheimer’s disease in this study could have an amplified signal in the global assessment if it was not taken into account.

Additionally, the article highlights two meta-studies that found no association between omega-3 fatty acids and reduced symptoms of Alzheimer’s disease. The current article states that “meta-analyses performed by Zhu et al. and Araya-Quintanilla et al. found no decreased risk of dementia or improved cognitive function with supplementation of these fatty acids. ”

What is perhaps important to know is that these two meta-analyses included the same 2015 study, titled “Intakes of fish and polyunsaturates and risks of mild to severe cognitive impairment: a dose- response from 21 cohort studies”. studies”, which is itself a meta-analysis that could contain studies included in other meta-analyses covered by the current study.

Interestingly, the authors state that “…the objective of this systematic review was to identify and map updates over the past 5 years regarding the nutritional status and nutritional interventions associated with patients with AD.” Although the meta-analysis studies and systematic reviews they selected may have been published within the last five years, they are retrospective studies. The research contained in these articles is often more than ten years old.

The authors point to studies that show vitamin D supplementation improves cognitive function, while other studies find no link. The article cites meta-studies that show omega-3 fatty acids do not improve cognitive and functional decline, and references another meta-analysis that found supplementation with these nutrients may improve decline. cognitive, functional connectivity and brain atrophy.

Often a systematic review asks a specific question and provides an analysis of the selected combined works. In the current study, the analysis aspect is unclear, although it shows that more primary research is needed, if only to prevent new meta-analyses of past meta-analyses.

More information:
Inmaculada Xu Lou et al, Effect of nutrition in Alzheimer’s disease: a systematic review, Frontiers in Neuroscience (2023). DOI: 10.3389/fnins.2023.1147177

Journal information:
Frontiers in Neuroscience

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