The Venn diagram below was generated on PubVenn, a tool for visualizing PubMed results. The size of the circle represents the number of citations to research article on the topic. As you can see, articles published on heart disease have the most number of citations compared to the other topics. The overlap represents the number of citations to the articles that are written about both these topics. For example, all the topics have overlap with one another except for Alzheimer's and obesity. The most overlap in citations is with heart disease and diabetes, which means that you can assume that many articles have been written about heart disease and diabetes with a considerable amount of citations (which is a rough proxy for research impact and influence.
The proportion of citations in PubMed by topic. Diabetes has the highest proportion of citations, followed by obesity, and finally, the combined topics of obesity AND diabetes have the lowest proportion of citations. Why proportion instead of a raw number of citations? The the astonishing increase of the biomedical literature over time makes a bare count less illustrative of changes for any given search. Proportions make it fair to compare topics to one another. For example, the topic of Alzheimer's has not been as thoroughly researched as heart disease, and has only been extensively studied in the past three decades. Therefore, Alzheimer's research has far fewer publications and thus far fewer citations than the topic of heart disease, and thus, it would be unfair to compare them by counts rather than proportions.