This study aimed at comparing, in a sample of Mexican older people, BMI distribution according to two alternative ranges as proposed by the WHO and the CDH. Results showed that having the WHO criteria a lower threshold for what is to be considered a normal weight in comparison to the CDH criteria; it classified more than half the participants as overweight, that is, the double if considering the CDH criteria. Furthermore, the CDH criteria displayed a distribution wherein half of the sample had a normal weight range. These differences in distribution patterns, in concurrence with previous research , bring into question the adequacy of the common practice of using the WHO criteria to classify normal BMI in older adults. The accuracy of BMI seems to diminish with age, as body composition changes increase adiposity and sarcopenia (i.e. decreased muscle mass) . Valid, reliable and economical assessments of BMI, with ranges adapted by age, are needed [5, 26].
Analyses of the association of BMI criteria with cognitive functioning, physical functioning and depression revealed similar though not equivalent patterns. Considering the WHO BMI criteria, overweight seems a protective factor. Results from some studies, mostly relying on the WHO BMI criteria, suggest that overweight is a protective rather than a risk factor (at least for mortality) in older people . Yet, it has been questioned whether this BMI paradox just reflects the WHO criteria’s low sensitivity for this segment of the population [25, 26]. Classifying sample by the CDH BMI criteria, adapted by age range, significant results point to the opposite direction; that is, underweight is disfavorable in older people. This concurs with studies reporting low BMI to be disfavorable in older people, and highly associated with infections, hospitalizations and predicting mortality [28, 29]. Also, a rapid and unintentional weight loss may reflect underlying illness, social deprivation, dementia or depression .
After the age of 60, average body weight and muscle mass tend to decrease. As physical activity and energy expenditure also decrease there is a tendency to fat accumulation and fat redistribution . Here, underweight older people showed a disadvantageous performance, while in other studies overweight older people showed a more favourable status . However, one must be cautious; obesity in older people is a common and serious matter of concern not to be overlooked. Not only can obesity lead to adverse health consequences and impair quality of life, but also exacerbate the age-related decline in physical function and lead to frailty, disability and autonomy limitations [29, 31,32,33]. Treatment for obesity in older persons is controversial, mainly to the misinterpretation that it may not be as harmful in older adults as it is in younger people, and the concern about the potential adverse effects of weight loss in this population [28, 31, 33,34,35]. Even small amounts of voluntary weight loss (between 5 and 10% of initial body weight) along with a healthy lifestyle may benefit older people . Weight loss in overweight/obese older people can improve risk factors, fat loss can ameliorate certain catabolic conditions of aging through impacting muscle protein synthesis and breakdown and lighter weight may also ease the mechanical burden on weak joints and muscle, thus improving mobility . Interventions aiming at voluntary weight-loss in obese older people must follow a combination of exercise and modest calorie restriction for reducing intra-abdominal fat mass while muscle mass and strength are preserved [30, 31, 33, 35]. Moreover, interventions must consider comorbidities, polypharmacy, limitation of autonomy, and social issues with a focus on the underlying medical problems, functional status and living environments .
Cognitive status, independent physical functioning, and depression are three important outcome measures in older adults that have been found related to BMI. Although some studies have found a poorer cognitive performance in the overweight and obese older people  our results concur with those finding lower BMI coinciding with a worse cognitive status . Regarding physical functioning, studies tend to support that high BMI values are associated with greater functional impediments ; yet, it has also been found that both, low and high BMI are related to a greater risk of functional impairment . The present results found poorer physical functioning in underweight older people following the CDH criteria. Depressive symptoms in older adults seem less likely to occur in overweight/obese older people , and that is the case in our study if the WHO criterion is used. If the CDH criterion is used, underweight older people seem more likely to report depressive symptoms, and that coincides with previous findings, particularly in men . Discrepancies in findings might be due to the use of diverse measures for body composition, cognition, functioning, and depressive symptoms, and the fact that these outcome measures have not been previously studied together.
Regardless of BMI criteria, the group of underweight older people had a disadvantageous outcome on all three measures in comparison to the other groups. Furthermore, results showed that considering its interaction with sex, underweight is disadvantageous for all, whereas overweight is favourable in women but disfavorable in men. These results evidence that a criterion overlooking age and sex differences in BMI may bias research findings and perhaps explain the so-called obesity paradox in older adults. More complex models including covariates that might influence outcome, such as educational level, regular cognitive stimulation, comorbidities, medication intake and mental health history should be considered to support or disclaim these results.
Besides, underweight women stand out as the more vulnerable group regarding cognitive status and independent physical functioning. Nutritional interventions must aim at helping older people to gain weight up to normal status (rather than reaching overweight) but considering a more flexible cut-off point such as suggested by the CDH. That is, a healthy BMI in older people must range between 23 and 28 in people aged 55 to 65, and between 24 and 29 in people aged 66 or older. Furthermore, priorities for intervention should be given to those at highest risk, with the primary focus on reducing the risk profile rather than weight loss per se .
It must be underscored that more weight does not equal better nutrition or good health. Given the varying contributions of bone mass, muscle mass and fluid to body weight, relying exclusively on BMI to classify individuals may result in misclassification. Anthropometric data for the potential development of reference data or standards should cover at least weight and height, plus age, sex, race, socioeconomic status, presence of disease, and smoking habits . Relying on convenience sampling limits the generalization of results as the selected group may not be comparable to others, such as older adults healthy and living independently. Moreover, when studying BMI in older adults it would be worth exploring possible differences due to receiving care from others, either at home or in care centres, and observing its evolution through time. As sex and age were recorded, their role as possible confounders was analysed; although the significance of most results was confirmed, only when adjusting by age, the association was no longer significant for depression. Further research must also consider the inclusion of other possible confounders such as disease status, smoking status, alcohol intake, physical activity, socioeconomic status and education for a better understanding of the processes regulating the associations of BMI with outcome.
Despite its limitations, this study showed that when assessing BMI in older people, a criterion adapted by age must be preferred. It seems that the WHO criteria overshadow a problem in the older population, namely that losing weight is in fact unfavorable, leading to a lower BMI. The CDH criteria are much more sensitive to that problem. Furthermore, the fact that WHO cut-off points are more restrictive may help explaining why various studies using this criterion found overweight to be favourable in older people. The use of CDH cut-off points showed that overweight is not a protective, neither a risk factor in older people, at least in relation to our 3 outcome measures. In older people, underweight is what signals a high risk of mortality, and in line, this study shows also a higher risk of cognitive and functional deterioration. Interventions for weight control in older people must monitor healthy weight gain but prevent obesity.