## 4 Discussion
Although obesity is a well-recognised risk factor for impaired health-related quality of life, few studies have investigated the underlying mechanisms of this relationship [55]. A possible contributing factor is that in patients with obesity, there is also sarcopenia, so-called sarcopenia obesity [6]. In these cases, obesity might exacerbate sarcopenia, leading to increased disability, accelerated functional decline, frailty, morbidity, and mortality [6; 36]. In summary, in this study, concerning SPH and age, an inverted J or U-shaped relationship is observed, with low and high BMI levels associated with a worse SPH, and a negative SPH with a higher risk in Underweight people. A second objective was to test the associations between BMI, Depression Status, and Pain Degree in older Spanish adults. Similar results to those found between BMI and SPH were observed. The highest prevalence and risk of suffering from Depression and Severe/Extreme Pain occurred in Underweight people. Likewise, the lowest Depression and Severe Pain prevalence corresponded to the Normal-weight and Overweight groups. Moreover, Obesity presence again slightly increased the Depression or Severe/Extreme Pain risks compared to Normal-weight. In a previous study, a sample from 18 to 90 years old (N = 44,374 individuals) explored the association between BMI and Depression, showing results in the same line. Concretely, they showed a U-shaped association in which Obesity and Underweight were associated with higher Depression levels [56]. However, a previous longitudinal study observed that the risk of moving from a normal BMI to an underweight state increased by 62% among participants with depressive symptoms, so unhealthy weight loss could be a side-effect of depressive symptoms [57]. Concerning BMI and Pain Degree associations, another investigation [58] studied the chronic pain prevalence in older people aged 65+ with similar outcomes. The greatest chronic pain estimation was observed in Underweight individuals (24.6%), followed by Obese people (20.2%) and Overweight older adults (14.2%). Furthermore, several studies support that obesity is a risk factor for chronic pain. However, the current findings along with those reported in this study also support that Underweight people may exhibit the same risks as people with Obesity. In this context, some authors [59; 60] suggest that pain associated with Underweight people tends to be generalised throughout the body and may also be linked with osteoporosis or other diseases. Based on our results and other authors' statements, it seems that the relationship between BMI, Depression Status, and Pain Degree also presents a U-shaped relationship. Thus, health and educational professionals should consider these issues, so both people with Obesity and Underweight would be at risk of Depression and Pain [56; 61]. Additionally, physical training professionals must include resistance training or multicomponent exercise interventions to optimise functional outcomes in older adults and adults with frailty [62; 63; 64]. Finally, physical inactivity, also a risk factor for sarcopenia, is, in general, more connected with Underweight and people with Obesity, which would contribute to this negative SPH, Depression Status, and Pain Degree [36; 65]. In this respect, a growing body of work indicates that optimal PAL is significantly associated with a lower incidence of depressive and anxiety symptoms, reduced stress, and improved mood, which may translate into good SPH [66]; thus, PA or exercise may also be a therapeutic tool of choice to achieve optimal BMI in older people [65]. A tool to achieve an optimal BMI in older people is PA or exercise. Some authors suggest that in the elderly population, BMI recommendations should be higher (those categorised as Normal-weight and Overweight) [43]. Current evidence for the PA and exercise benefits in the prevention and treatment of many chronic diseases such as sarcopenia and Depression, among many others, is strong [67].