It is a good practice requirement that people who have taken part in a research project are thanked for their contribution and told about what it helped the researchers find out, where appropriate. The UK Policy Framework for Health and Social Care Research says:
Information about the findings of the research [should be] available, in a suitable format and timely manner, to those who took part in it, unless otherwise justified.
Our new guidance for researchers, chief investigators, funders, and sponsors who are responsible for sharing findings to participants will help you to do this well.
See also: