## 3 Results and Discussion
### General
#### 3.1.3 Is It Possible to Establish a BMI Range for COPD Patients?  
**Recommendation 3**.: _GARIN suggests maintaining a body mass index (BMI) above 21 kg/m\({}^{2}\) and below 30 kg/m\({}^{2}\)._  
Grade of recommendation B, (90%) Consensus.  
Comment:  
In many studies, BMI correlates positively with lung function [8; 13; 14; 37; 38] and negatively with exacerbations [38]. Patients with a BMI above 22 kg/m\({}^{2}\) have been found to have a better prognosis and fewer exacerbations. Possible reasons include better nutrient intake, better nutritional and muscle status, and less inflammation. In some studies, overweight or obese patients even had better lung function, fewer exacerbations, and less inflammation than those with a BMI of less than 21 kg/m\({}^{2}\)[8; 38; 39].  
Low BMI has been associated with an increased risk of hospital admissions and long-term hospital stays [7] and is considered an independent risk factor for mortality in patients with COPD [7; 8; 30; 34; 35]. In the study by WU [38], overweight and obesity were associated with lower mortality rates compared with normal weight among smokers with COPD, but this association was not present among non-smokers with COPD. This could be explained by physiological differences between smoking-associated COPD and COPD in non-smokers [38]. Smoking is a risk factor for COPD and is associated with weight loss and other pathologies that increase the risk of mortality. It is a known central anoretic agent and also conditions the appearance of other cofactors that may influence weight loss [40]. Early implementation of smoking cessation measures can help slow the progression of the disease and reduce the risk of cardiovascular disease and mortality [41; 42].  
BMI is one of the determining factors of bone mineral density, and the effect of body weight seems to be influenced by both fat mass and lean mass. In addition to a low BMI, patients with COPD often present sarcopenia or low FFMI, and this association is mediated not only by mechanisms such as decreased skeletal load but also by hormonal and other specific factors such as systemic inflammation, vitamin D deficiency, and the use of oral or inhaled corticosteroids [43; 44]. A relationship has been found between decreased bone mineral density and low BMI, with significantly high values as BMI and FFMI increase [45].  
Despite this, the relationship between obesity and COPD is unclear, and it is difficult to establish whether obesity actually has a detrimental impact on COPD patients. The prevalence of obesity in COPD patients is estimated to range from 10% to 50% [46; 47]. Some studies have reported worse respiratory symptoms, increased severe exacerbations, increased comorbidities, worse prognosis, greater restriction of daily activities, worse health-related quality of life, and more use of medical care in obese COPD patients [48; 49].  
It is clear that the exclusive assessment of BMI does not provide sufficient data to clarify the association of BMI with COPD. Assessing changes in body composition and muscle quality is mandatory. It is also essential for future research to explore the relevance of different phenotypes [15; 50; 51], taking into account different types of obesity, different symptoms (emphysematous, chronic bronchitis, or both), groups of smokers and non-smokers, and other relevant factors [51].