Describing the landscape of nutrition- and diet-related randomized controlled trials: meta-research study of protocols published between 2012 and 2022.

Background: Publishing protocols promotes transparency and reproducibility. The scope and methods of protocols for nutrition- and diet-related randomized controlled trials (RCTs) have not been investigated yet.

Objective: Map the landscape of nutrition- and diet-related interventions research.

Methods: We conducted a meta-research of nutrition-and diet-related RCT protocols published between January 2012 and March 2022, in any language, targeting human participants, evaluating nutrition interventions isolated or combined. A systematic search of the literature was conducted in six online databases. Bibliometric information, study characteristics, and research transparency practices data were collected from the included publications. The instructions for authors of journals with publications in our sample were checked for endorsement of reporting guidelines. Mentions to reporting guidelines in the included protocols were also checked.

Results: Among the 62,319 records retrieved, 1,068 were eligible. The number of published protocols increased annually, with a mean of 103 (range: 32-163) publications/year. Protocols were published in 148 journals, 50 of them (33.8%) endorsed SPIRIT, 111 (75.3%) CONSORT, and 4 (2.7%) TIDieR, while 343 (32.1%) protocols mentioned SPIRIT, 297 (27.8%) CONSORT, and 20 (1.9%) TIDieR. Most protocols reported the RCT registration number (n=1,006; 94.2%) and included statements about conflicts of interest (n=952; 89.1%) and funding (n=994; 93.2%). About one third of protocols focused on adults or elderly participants (n=677; 63.4%). Most protocols described one isolated nutrition- or diet-related intervention (n=724; 67.8%), which were most frequently "supplementation, supplements or fortification" (n=405; 37.9%) or "nutrition education, counselling or coordination of care" (n=354; 33.1%). The most frequent primary outcomes reported were related to clinical status (n=308; 28.8%).

Conclusions: The number of protocols for nutrition- or diet-related RCTs published is increasing, supporting the raising awareness and the importance of promoting these publications. The support and mention of relevant reporting guidelines by journals and researchers, respectively, remain far from ideal.