In this prospective study of a large random population cohort, a higher BMI was associated with an increased risk of developing CRS during a five-year observation period. This association was present both when analysing BMI as a continuous variable (Fig. 2) and when categorising BMI according to CDC definitions and comparing normal weight (18.5 ≤ BMI < 25) with obesity (BMI ≤ 30) (OR 1.53 (1.11–2.10) (Table 2). To our knowledge, this study is the first to confirm BMI as a risk factor for the development of CRS in a prospective design. As patients with CRS have chronic disease where causal treatment is lacking and different phenotypes and endotypes exist, BMI should be taken into consideration when CRS patients are at the clinic [6].

One major strength of this study is the prospective design. By excluding all subjects reporting CRS according to the EPOS criteria, at baseline, we have only studied new-onset CRS during the five-year observation period. Our results show that obesity is a risk factor for the development of CRS, even when other known risk factors such as asthma and smoking are taken into consideration. This confirms previous cross-sectional studies indicating the existence of an association between CRS and BMI. In a large cross-sectional study from 2013, Bhattacharyya et al. found an increased prevalence of obesity in patients suffering from CRS [14]. Another cross-sectional study from 2016 found that obesity was a risk factor for CRS with nasal polyps, but its data did not support a connection between a higher BMI and CRS without nasal polyps [21]. Obesity was defined as a BMI of ≥ 25, whereas in our study we define obesity as a BMI of ≥ 30, in accordance with CDC [18]. Lee et al. found that CRS was more prevalent in individuals with the metabolic syndrome (a high triglyceride level, a reduced high-density lipoprotein level and elevated blood pressure) than in individuals without the metabolic syndrome [22]. Sidell et al. found no association between obesity and CRS in schoolchildren [23]. This study included children aged 6–17 years, whereas our study investigates adults aged 16–50 years, which could be a reason for the difference in results. Kanagalingam et al. found that obesity did not affect the severity of sinonasal disease in asthma [24]. That study used a validated questionnaire [25] to assess sinonasal disease severity, whereas in our study we use the EPOS criteria [6] to define chronic rhinosinusitis, which could explain the discrepancy in the results.

Obesity itself appears to cause a metabolic inflammatory state, defined as a low-grade, chronic inflammation orchestrated by metabolic cells and inflammatory markers in response to excess nutrients and energy [26]. Previous studies have highlighted obesity as an important risk factor for asthma, but the mechanisms behind this relationship still remain elusive [27, 28]. The connection between CRS and lower respiratory airway disease, such as asthma, is well known and was also taken into account in the analyses in this study [11,12,13]. The inflammatory response caused by obesity appears to contribute to airway inflammation, decrease in lung function and asthma exacerbation [5]. Obesity in patients with asthma also results in a higher symptom burden, as well as poorer asthma control and a higher consumption of asthma medication, which could also be a factor in CRS, but this needs to be studied in more detail [29].

The Telemark Study is a large prospective population-based cohort designed to study airway inflammatory disease and risk factors such as CRS. One of the main advantages of the population-based design is that it enables the inclusion of subjects who have CRS symptoms but who have not been in contact with health care, in contrast to other study designs based on the hospital cohorts of CRS patients. EPOS has a special symptom-based definition of CRS for the purpose of questionnaire-based epidemiological research. In the absence of a nasal inspection, it is however not possible to exclude other causes of the sinonasal symptoms or to differentiate between nasal polyps and other CRS. A strength of the study is that the questions used in the Telemark Study have been validated and frequently used before [30,31,32].

When conducting epidemiological research, it is important to consider recall bias and how it might affect the outcome. There is a risk that individuals with CRS are more prone to recall their symptoms than their counterparts who did not develop CRS; thus, resulting in a skewed distribution of the investigated symptoms or characteristics. The studied variables, such as smoking, asthma and sinonasal symptoms during the past 12 weeks, are probably less likely to be over- or underestimated and the results show that their prevalence was as expected. This may not, however, apply to BMI. Previous studies illustrate that individuals have a tendency to overestimate their height and underestimate their weight, resulting in a falsely low BMI [33]. This tendency is greater in individuals who are overweight or obese compared with individuals of normal weight. However, in this study, the application of this theory would mean that the individuals who reported being overweight or obese have a higher BMI in reality, thus resulting in a stronger correlation between CRS and obesity. Between the Nordic countries, the estimated prevalence of overweight and obesity is similar in Norway, Sweden, Denmark and Finland [34]. In the general population in Norway in 2019, the distribution of self-reported BMI was 2% underweight, 35% overweight and 16% obese. Forty-seven per cent had a normal BMI [35]. This is similar to our results of 1%, 35%, 14% and 50% (Table 1).

1,227 subjects failed to answer the questions regarding weight and height and were therefore not eligible. However, the analyses of these subjects showed that the overall five-year incidence of CRS and the distribution of smoking habits, asthma and gender were in accordance with the study population (Table 3).

Low vitamin D status has been associated with obesity [36, 37] and a recent meta-analysis by Li et al. illustrated an association between lower serum vitamin D status and CRS [38]. Vitamin D supplementation has also been shown to prevent acute respiratory infections [39]. The Telemark study did not include any data on serum vitamin D, however the role of vitamin D in regard to CRS and obesity is interesting and warrants further research.

Obesity has also been linked to sleep disturbances, in particular obstructive sleep apnea which in itself is connected with numerous chronic health conditions [40]. Prolonged sleep deficiency can lead to a chronic, systemic low-grade inflammation associated with various diseases that have an inflammatory component, such as diabetes [41]. There are data suggesting a correlation between obstructive sleep apnea and asthma [42]. Sleep disturbances and its concomitant inflammatory component may also be associated with CRS, however, this connection needs to be studied further. Finally, there is a connection between obesity and diabetes mellitus [43] and there is also data that show that subjects with diabetes mellitus and CRS who underwent functional endoscopic sinus surgery (FESS) report worse quality of life after surgery than those without diabetes mellitus [44].

As outlined by the EPOS, it is important to study different phenotypes of CRS [6]. Obesity is a sign of underlying disease and/or the interaction of genetic, environmental, social and economic factors that can facilitate the development of CRS and asthma.