The weight of the evidence to date suggests that diabetes mellitus (DM) does increase the risk of pneumonia-related hospitalization.^1-3

A large population-based study involving over 30,000 patients found an adjusted relative risk (RR) of hospitalization with pneumonia of 1.26 (95% C.I 1.2-1.3) among patients with DM compared to non-diabetics.  Of note, the risk of pneumonia-related hospitalization was significantly higher in type 1 as well as type 2 DM and among patients whose A1C level was ≥9.¹  Another population-based study found a high prevalence of DM (25.6%) in patients hospitalized with CAP, more than double that in the population studied.²  A 2016 meta-analysis of observational studies also found increased incidence of respiratory tract infections among patients with diabetes (OR 1.35, 95% C.I. 1.3-1.4).

Not only does DM increase the risk of pneumonia-related hospitalization, but it also appears to adversely affect its outcome with increased in-hospital mortality.² Among patients with type 2 DM,  excess mortality has also been reported at 30 days, 90 days and 1 year following hospitalization for pneumonia. ^4,5 More specifically, compared to controls with CAP, 1 year mortality of patients with DM was 30% (vs 17%) in 1 study. ⁴

Potential reasons for the higher incidence of pneumonia among patients with DM include increased risk of aspiration (eg, in the setting of gastroparesis, decreased cough reflex), impaired immunity (eg, chemotaxis, intracellular killing), pulmonary microangiopathy and coexisting morbidity. ^1,3,5,6

Bonus Pearl: Did you know that worldwide DM has reached epidemic levels, such that if DM were a nation, it would surpass the U.S. as the 3rd most populous country! ⁷

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References

1. Kornum JB, Thomsen RW, RUS A, et al. Diabetes, glycemic control, and risk of hospitalization with pneumonia. A population-based case-control study. Diabetes Care 2008;31:1541-45. https://www.ncbi.nlm.nih.gov/pubmed/17595354
2. Martins M, Boavida JM, Raposo JF, et al. Diabetes hinders community-acquired pneumonia outcomes in hospitalized patients. BMJ Open Diabetes Research and Care 2016;4:e000181.doi:10.1136/bmjdrc-2015000181. https://drc.bmj.com/content/4/1/e000181
3. Abu-Ahour W, Twells L, Valcour J, et al. The association between diabetes mellitus and incident infections: a systematic review and meta-analysis of observational studies. BMJ Open Diabetes Research and Care 2017;5:e000336. https://drc.bmj.com/content/5/1/e000336. 
4. Falcone M, Tiseo G, Russo A, et al. Hospitalization for pneumonia is associated with decreased 1-year survival in patients with type 2 diabetes. Results from a prospective cohort study. Medicine 2016;95:e2531. https://www.ncbi.nlm.nih.gov/pubmed/26844461
5. Kornum JB, Thomsen RW, Rus A, et al. Type 2 diabetes and pneumonia outcomes. A population-based cohort study. Diabetes Care 2007;30:2251-57. https://www.ncbi.nlm.nih.gov/pubmed/17595354
6. Koziel H, Koziel MJ. Pulmonary complications of diabetes mellitus. Pneumonia. Infect Dis Clin North Am 1995;9:65-96. https://www.ncbi.nlm.nih.gov/pubmed/7769221
7. Zimmet PZ. Diabetes and its drivers: the largest epidemic in human history? Clinical Diabetes and Endocrinology 2017;3:1 https://clindiabetesendo.biomedcentral.com/articles/10.1186/s40842-016-0039-3