## 4 Discussion  
The main objective of this study was to analyse the associations between BMI and SPH, Depression Status, and Pain Degree in the Spanish population over 70 years. The initial hypotheses were that SPH, Depression, and Pain Degree would be associated with BMI in Spanish older people. Specifically, older people with Normal Weight or Overweight would have a lower risk of Negative SPH, Depression, or High Pain Degree. Indeed, the findings of this study support that the highest Negative SPH prevalence was in the Underweight groups. In contrast, Normal Weight or Overweight groups were the least likely to report Negative SPH. Previous epidemiological studies have studied the association between BMI and mortality, having found U-shaped or J-shaped relationships [35; 41; 42; 43]. In particular, in this U-shaped association, people with an Intermediate BMI tended to live more than people with a higher or lower BMI [44; 45; 46; 42]. In recent years, the evidence appears to support  
Figure 6: **(a) Odds ratio for Negative Self-Perceived Health; (b) Odds ratio for Depression; (c) Severe/Extreme Pain Degree according to the Body Mass Index in the Spanish population in the three surveys EESE2014, ENSE2017, and EESE2020. **  
that moderate obesity is associated with significantly lower all-cause mortality relative to having Normal-weight, whereas underweight people have been associated with increased all-cause mortality (strokes, heart diseases, malignant tumours, respiratory failures, others, or unknown) [37]. This phenomenon is known as the \"obesity paradox\" [42]. Although some studies have reported results that support this U-shaped relationship between BMI and mortality, it is still not well understood or controversial [41; 43; 42]. This debate about \"The Obesity Paradox\" can be due to the methodological limitations of the studies and to their heterogeneity (e.g., study population, confounding factors, control, and length of follow-up), which may contribute to a biased interpretation of this phenomenon or an incomplete understanding [34; 36]. Among the various hypotheses that emerged, a lower BMI may be associated with lower nutritional status or weight loss due to clinical or sub-clinical disease [4; 47]. In this sense, one of the most important age-related health problems is the loss of fat-free mass, i.e., sarcopenia, which causes functional impairment in older people and negative impacts on quality of life [36]. Sociodemographic and behavioural factors such as age, malnutrition, daily activities, underweight, and poor PA constitute risk factors for sarcopenia development [48]. Therefore, if underweight people also present sarcopenia, it could partially explain why a low BMI may be associated with a negative SPH found in the participants of this study. Furthermore, in this study, negative SPH risks were higher for the Underweight than for the Normal-weight participants. In this regard, authors argue that the health consequences of Underweight might be more severe in terms of mortality and quality of life, compared to Overweight [49; 50; 51]. Similarly, a study conducted on the Chinese population observed lower levels of Health-Related Quality of Life in the elderly with underweight compared to normal weight [52]. In contrast, the lowest Negative SPH prevalence was associated with people with Normal-weight and slightly increased BMI levels. In this study, obesity was associated with Negative SPH higher prevalence, again, with a higher risk in Normal-weight but less than in Underweight people. Similar results have been previously reported in Spanish and Brazilian populations, as the population with obesity or severe obesity (BMI > 35) has also shown worse quality of life and SPH compared to moderate obesity or normal weight [53; 54]. Moreover, this inverse association between BMI and self-perceived health status was more pronounced in elderly people [54].