Also, I worry about the health-related question part - aggregating data sets of medical data is a serious business; it's not just a case of pooling it all toget

Also, I worry about the health-related question part - aggregating data sets of medical data is a serious business; it's not just a case of pooling it all together. If there is really a specific question of interest, either gather data from a valid meta-analysis (like a Cochrane review) or stick to a single sufficiently-large dataset so that you can understand the biases in the data collection.