**1. Introduction**

The population aged 60 or over is growing at a faster rate than the total population in almost all regions worldwide. Most notably, the current challenge in demographics is the rapidly increasing population of older adults. In 2012, people aged 60 or over represented almost 11.5 per cent of our total global population of 7 billion. By 2050, the proportion is projected to nearly double to 22 per cent [1].

The number of elderly individuals in Brazil was estimated to be more than 30.2 million in 2017. Thus, they represented approximately 7 per cent of the Brazilian population of 209.3 million inhabitants

in this period. In the Northeast of Brazil, the proportion of elderly individuals increased from 5.8% in 2000 to 7.2% in 2010 [2].

A widespread health problem experienced among elderly individuals is multiple micronutrient inadequacy, including vitamin D, which may lead mainly to frailty, sarcopenia, and cognitive decline [3,4].

Vitamin D regulates aging by controlling the activity of autophagy, which slows down aging processes by removing dysfunctional mitochondria. Vitamin D also reduces mitochondrial dysfunction, oxidative stress, inflammation, calcium signaling, epigenetic changes and DNA disorders, including telomere shortening, which act upon aging processes [5].

The diagnosis of vitamin D deficiency is a controversial issue. Serum levels of 25-hydroxyvitamin D [25(OH)D] above 30 ng/mL are associated with increased efficacy of intestinal calcium absorption and stabilization of serum parathyroid hormone (PTH) values, which are suggested as being the threshold for fracture prevention [6]. The Endocrine Society suggests that the 25(OH)D concentrations that are considered deficient, insufficient, and sufficient are ≤20, ≥21 and ≤29, and ≥30 ng/mL, respectively [7]. However, the Brazilian Society of Clinical Pathology/Laboratory Medicine and the Brazilian Society of Endocrinology and Metabolism recently proposed concentrations of 25(OH)D above 20 ng/mL as being adequate for the health of the population up to 60 years old, while values between 30 and 60 ng/mL are adopted for groups at risk of vitamin D deficiency, including elderly individuals [8].

Vitamin D deficiency among elderly individuals can result in cardiovascular risk, mortality, low quality-of-life scores, decreased physical functionality, secondary hyperparathyroidism, and increased risk of fractures [3,9–11]. Age-related changes in body composition, such as decreased muscle mass and increased adipose tissue, can lead to a decrease in serum vitamin D concentrations. Furthermore, the muscular weakness presented by the elderly population can be potentiated by the deficiency of vitamin D [12]. Moreover, Pilz et al. (2012) observed a significantly increased mortality risk in the female Nursing Home (NH) residents with the lowest 25(OH)D levels [13].

Several factors are involved in changes in vitamin D status in aging, such as the seasonality, which is currently observed as being an important predictor because it has an impact on the behavior and lifestyle of individuals [6,14,15]. Even under favorable environmental conditions, vitamin D bioavailability in elderly individuals can be negatively affected by the reduced capacity of its cutaneous synthesis. Furthermore, this population can be vulnerable to vitamin D deficiency as a result of low exposure to sunlight, their particular type of skin pigmentation, adiposity, use of antiepileptic medications, reduced kidney function, and low dietary vitamin D intake [9,15–17].

In light of these considerations, hypovitaminosis D has been frequently found among institutionalized elderly individuals living in countries with low ultraviolet radiation (UV) indexes, where elderly people spend more time indoors [13,18], as well as in high-latitude regions where there is a high UV index [16,19,20].

Although several studies carried out on elderly people have revealed a poorer vitamin D status during the winter seasons because of the lower sunlight exposure [19–22], contradictory findings have been found in the literature. One study developed with elderly people from a Mediterranean area suggested significant differences in serum 25(OH)D concentrations between seasons, with the lowest concentrations occurring in the summer and the highest during the autumn, although their vitamin D intake was significantly lower in the autumn and winter [23].

Based on these findings, our study was developed to answer the question about the status of vitamin D in institutionalized elderly individuals living in a region with high levels of UV radiation, while considering various particularities of aging. For this reason, we have drawn up a theoretical model that encompasses diverse variables and contemplates these conditions.

In addition to this, institutionalization impacts the interactions of elderly individuals with the environment as well as their nutritional status, which, in turn, are influenced by their social status, psychological disorders, and general health conditions [24,25]. In this scenario, the objective of the

present study is to assess serum 25(OH)D concentrations and associated factors in institutionalized elderly individuals living in a region of northeastern Brazil with high ultraviolet radiation levels.

#### **2. Materials and Methods**
