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Background: Growing evidence suggests that antibiotic-induced dysbiosis of gut microbiota potentially contributes to colorectal cancer development and oncological outcomes. However, the role of antibiotics in colorectal cancer incidence, survival and postoperative outcomes at a population level remains incompletely understood.

Aims: The overall aim of the thesis is to investigate prescription antibiotics use in relation to colorectal cancer risk, survival and postoperative complications, particularly surgical site infections including anastomotic leakage.

Methods: The thesis work includes matched case-control and cohort studies, leveraging complete population-based data from Swedish national registers. Paper I is a matched case-control study that consists of 40 545 colorectal cancer cases and 202 720 matched controls, aiming to investigate antibiotics use and risk of incident colorectal cancer. Multivariable conditional logistic regression was used. Paper II is a cohort study, including 47 303 colorectal cancer cases, investigating antibiotics use in relation to cancer-specific survival. Stratified Cox proportional-hazards regression was used. Paper III includes 38 839 colorectal cancer cases who had undergone abdominal tumour-resection surgery and assesses antibiotics use in relation to surgical site infections, including anastomotic leakage, within 30 days after surgery. Logistic regression with multi-level mixed-effects models was used.

Results: In paper I, a dose-response association between antibiotics use and a higher risk of proximal colon cancer was found, whereas a slight inverse association with rectal cancer was observed, mainly in women. A null association was found between methenamine hippurate, assessed as a negative control due to no known effect on gut microbiome, and the risk of colorectal cancer. In paper II, the findings did not support any substantial negative effect of antibiotics on cancer-specific survival, except for very high cumulative exposure (>180 days) in stage I-III diseases. In stage IV colorectal cancer, modest inverse relationships between antibiotics use and survival were noted. In paper III, prescription antibiotics use up to 4.5 years before surgery was associated with a higher risk of surgical site infections, including anastomotic leakage, after colon cancer surgery but not rectal cancer surgery. A null association was observed between methanamine hippurate and the risk of surgical site infections. For cardiovascular and/or neurological complications, also considered as a negative control due to expected negligible or null effects of gut microbiome on these outcomes after surgery, associations were null in both colon and rectal cancer.

Conclusion: These studies provided further support for antibiotics use as a modifiable risk factor for proximal colon cancer and identified antibiotics taken long before surgery as a novel risk factor for surgical site infections, including anastomotic leakage, after colon cancer surgery. In contrast, we did not find any substantial negative impact of antibiotics on cancer-specific survival. Taken together, the findings described in this thesis provide etiological insights and may contribute to strategies to prevent colon cancer and improve postoperative outcomes through prudent use of antibiotics, thereby aiding in the reduction of colorectal cancer incidence and mortality.

Resistin is a protein involved in inflammation and angiogenesis processes and may play a role in the progression of colorectal cancer (CRC). However, it remains unclear whether resistin is associated with increased mortality after CRC diagnosis. We examined pre-diagnostic serum resistin concentrations in relation to CRC-specific and all-cause mortality among 1343 incident CRC cases from the European Prospective Investigation into Cancer and Nutrition cohort. For CRC-specific mortality as the primary outcome, hazard ratios (HRs) and 95% confidence intervals (95% CI) were estimated from competing risk analyses based on cause-specific Cox proportional hazards models and further in sensitivity analyses using Fine–Gray proportional subdistribution hazards models. For all-cause mortality as the secondary outcome, Cox proportional hazards models were used. Subgroup analyses were performed by sex, tumor subsite, tumor stage, body mass index and time to CRC diagnosis. Resistin was measured on a median of 4.8 years before CRC diagnosis. During a median follow-up of 8.2 years, 474 deaths from CRC and 147 deaths from other causes were observed. Resistin concentrations were not associated with CRC-specific mortality (HRQ4vsQ1 = 0.95, 95% CI: 0.73–1.23; Ptrend =.97; and HRper doubling of resistin concentration = 1.00; 95% CI: 0.84–1.19; P =.98) or all-cause mortality. Results from competing risk (sensitivity) analysis were similar. No associations were found in any subgroup analyses. These findings suggest no association between pre-diagnostic circulating resistin concentrations and CRC-specific or all-cause mortality among persons with CRC, and the potential insignificance of resistin in CRC progression.

Gut microbiota composition has been implicated in surgical site complications after colorectal cancer surgery. Antibiotics affect gut microbiota, but evidence for a role in surgical site complications is inconclusive. We aimed to investigate use of prescription antibiotics during the years before surgery in relation to the risk of surgical site infections, including anastomotic leakage, within 30 days after surgery. Cardiovascular/neurological complications and the urinary antiseptic methenamine hippurate, for which there is no clear link with the microbiota, were used as negative controls. We conducted a patient cohort study using complete population data from Swedish national registers between 2005 and 2020. The final study population comprised 26,527 colon cancer and 12,312 rectal cancer cases with a 4.5 year exposure window. In colon cancer patients, antibiotics use was associated with a higher risk of surgical site infections (adjusted odds ratio (aOR) for any versus no use = 1.20, 95% confidence interval (CI) 1.10–1.33) and anastomotic leakage in particular (aOR =1.19, 95% CI 1.03–1.36), both with dose–response relationships for increasing cumulative antibiotics use (Ptrend = <0.001 and Ptrend = 0.047, respectively). Conversely, associations in rectal cancer patients, as well as for the negative controls cardiovascular/neurological complications and methenamine hippurate, were null. In conclusion, prescription antibiotics use up to 4.5 years before colorectal cancer surgery is associated with a higher risk of surgical site infections, including anastomotic leakage, after colon cancer but not rectal cancer surgery. These findings support a role for antibiotics-induced intestinal dysbiosis in surgical site infections.

BACKGROUND: Antibiotics use is associated with higher colorectal cancer risk, but little is known regarding any potential effects on survival.

METHODS: We conducted a nationwide cohort study, using complete-population data from Swedish national registers between 2005 and 2020, to investigate prediagnostic prescription antibiotics use in relation to survival in colorectal cancer patients.

RESULTS: We identified 36,061 stage I-III and 11,242 stage IV colorectal cancer cases diagnosed between 2010 and 2019. For stage I-III, any antibiotics use (binary yes/no variable) was not associated with overall or cancer-specific survival. Compared with no use, moderate antibiotics use (total 11-60 days) was associated with slightly better cancer-specific survival [adjusted HR (aHR) = 0.93; 95% confidence interval (CI), 0.86-0.99)], whereas very high use (>180 days) was associated with worse survival [overall survival (OS) aHR = 1.42; 95% CI, 1.26-1.60, cancer-specific survival aHR = 1.31; 95% CI, 1.10-1.55]. In analyses by different antibiotic types, although not statistically significant, worse survival outcomes were generally observed across several antibiotics, particularly macrolides and/or lincosamides. In stage IV colorectal cancer, inverse relationships between antibiotics use and survival were noted.

CONCLUSIONS: Overall, our findings do not support any substantial detrimental effects of prediagnostic prescription antibiotics use on cancer-specific survival after colorectal cancer diagnosis, with the possible exception of very high use in stage I-III colorectal cancer. Further investigation is warranted to confirm and understand these results.

IMPACT: Although the study findings require confirmation, physicians probably do not need to factor in prediagnostic prescription antibiotics use in prognosticating patients with colorectal cancer.

BACKGROUND: Antibiotics use may increase colorectal cancer (CRC) risk by altering the gut microbiota, with suggestive evidence reported. Our study aims to investigate antibiotics use in relation to subsequent CRC risk.

METHODS: This is a nationwide, population-based study with a matched case-control design (first primary CRC cases and 5 matched, cancer-free controls). Complete-population data, extracted from Swedish national registers for the period 2005-2016, were used to calculate odds ratios and 95% confidence intervals.

RESULTS: We included 40 545 CRC cases and 202 720 controls. Using the full dataset, we found a positive association between more frequent antibiotics use and CRC, excluding antibiotics prescribed within 2 years of diagnosis attenuated results toward the null. In site-specific analyses, excluding the 2-year washout, the positive association was confined to the proximal colon (adjusted odds ratio for very high use vs no use = 1.17, 95% confidence interval = 1.05 to 1.31). For rectal cancer, an inverse association, which appears to be driven by women, was observed. Quinolones and sulfonamides and/or trimethoprims were positively associated with proximal colon cancer, whereas a more general inverse association, across antibiotics classes, was observed for rectal cancer. We found no association between methenamine hippurate, a urinary tract antiseptic not affecting the gut microbiota, and CRC risk.

CONCLUSIONS: This register-based study covering the entire population of Sweden found a robust association between antibiotics use and higher risk of proximal colon cancer and an inverse association with rectal cancer in women. This study strengthens the evidence from previous investigations and adds important insight into site-specific colorectal carcinogenesis.

Background: Body mass index (BMI) and cardiometabolic comorbidities such as cardiovascular disease and type 2 diabetes have been studied as negative prognostic factors in cancer survival, but possible dependencies in the mechanisms underlying these associations remain largely unexplored. We analysed these associations in colorectal and breast cancer patients.

Methods: Based on repeated BMI assessments of cancer-free participants from four European countries in the European Prospective Investigation into Cancer and nutrition (EPIC) study, individual BMI-trajectories reflecting predicted mean BMI between ages 20 to 50 years were estimated using a growth curve model. Participants with incident colorectal or breast cancer after the age of 50 years were included in the survival analysis to study the prognostic effect of mean BMI and cardiometabolic diseases (CMD) prior to cancer. CMD were defined as one or more chronic conditions among stroke, myocardial infarction, and type 2 diabetes. Hazard ratios (HRs) and confidence intervals (CIs) of mean BMI and CMD were derived using multivariable-adjusted Cox proportional hazard regression for mean BMI and CMD separately and both exposures combined, in subgroups of localised and advanced disease.

Results: In the total cohort of 159,045 participants, there were 1,045 and 1,620 eligible patients of colorectal and breast cancer. In colorectal cancer patients, a higher BMI (by 1 kg/m2) was associated with a 6% increase in risk of death (95% CI of HR: 1.02–1.10). The HR for CMD was 1.25 (95% CI: 0.97–1.61). The associations for both exposures were stronger in patients with localised colorectal cancer. In breast cancer patients, a higher BMI was associated with a 4% increase in risk of death (95% CI: 1.00–1.08). CMDs were associated with a 46% increase in risk of death (95% CI: 1.01–2.09). The estimates and CIs for BMI remained similar after adjustment for CMD and vice versa.

Conclusions: Our results suggest that cumulative exposure to higher BMI during early to mid-adulthood was associated with poorer survival in patients with breast and colorectal cancer, independent of CMD prior to cancer diagnosis. The association between a CMD diagnosis prior to cancer and survival in patients with breast and colorectal cancer was independent of BMI.

Background: Despite global achievements in reducing early childhood mortality, disparities remain. There have been empirical studies of inequalities conducted in low- and middle-income countries. However, there have been no epidemiological studies on socioeconomic inequalities and early childhood survival in Myanmar.

Objective: To estimate associations between two measures of parental socioeconomic status - household wealth and education - and age-specific early childhood mortality in Myanmar.

Methods: Using cross-sectional data obtained from the Myanmar Demographic Health Survey (2015-2016), univariate and multiple logistic regressions were performed to investigate associations between household wealth and highest attained parental education, and under-5, neonatal, post-neonatal and child mortality. Data for 10,081 children born to 5,932 married women (aged 15-49 years) 10 years prior to the survey, were analysed.

Results: Mortality during the first five years was associated with household wealth. In multiple logistic models, wealth was protective for post-neonatal mortality. After adjusting for individual proximate determinants, the odds of post-neonatal mortality in the richest households were 85% lower (95% CI: 50-96%) than in the poorest households. However, significant association was not found between wealth and neonatal mortality. Parental education was important for early childhood mortality; the highest benefit from parental education was for child mortality in the one- to five-year age bracket. After adjusting for proximate determinants, children with a higher educated parent had 95% (95% CI 77-99%) lower odds of death in this age group compared with children whose parents' highest educational attainment was at primary level. The association between parental education and neonatal mortality was not significant.

Conclusions: In Myanmar, household wealth and parental education are important for childhood survival before five years of age. This study identified nuanced age-related differences in associations. Health policy must take socioeconomic determinants into account in order to address unfair inequalities in early childhood mortality.