The main results of the present study showed that vitamin D insufficiency and, mainly, the presence of co-occurring depressive symptoms increased the incidence of functional disability and reduced performance in both ADLs and IADLs. The incidence of functional disability increased with age. However, for the combination of vitamin D status and the presence of depressive symptoms, this increase did not occur to the same extent in ADLs and IADLs.
A study by Koning et al.29A study of older Dutch adults found that 17.0% of participants had vitamin D deficiency. At the start of the study, 15.1% of the sample had depressive symptoms, which increased to 19.8% after 6 years. Cross-sectional analyses did not find a significant association between vitamin D and depressive symptoms. However, longitudinal analyses found that older women with vitamin D concentrations below 75 nmol/L had 17.0% to 24.0% more depressive symptoms than women with vitamin D concentrations above 75 nmol/L. The Brazilian older adults in this study showed higher levels of vitamin D insufficiency and higher depressive symptoms. However, the association with functional decline was similar.
Rafiq et al.42 Participants with lower vitamin D levels were older, had poorer physical performance, and showed more depressive symptoms. Chronic diseases, depressive symptoms, and physical performance were found to be mediators explaining the relationship between vitamin D and quality of life. The characterization and classification values of the study sample were also different from our study, with <25 nmol/L considered deficient, 25-50 nmol/L insufficient, and >50 nmol/L sufficient, which prevented a proper comparison. However, similar to our study, older people with vitamin D insufficiency and depressive symptoms showed worse functional outcomes.
Low vitamin D levels have been reported26 The cause of the decline in function due to muscle weakness, osteomalacia and the appearance of hyperparathyroidism, which in turn leads to increased bone loss and the development of osteoporosis and osteopenia. These factors lead to reduced mobility and a higher incidence of falls and fractures. On the other hand, functional impairment prevents vitamin D from actively and effectively performing its functions.43In the present study, subjects were active and functional and therefore did not show impairment in ADLs, but in the case of depression, the association between vitamin D deficiency and depressive symptoms further increased functional impairment.
Depressive symptoms may increase the likelihood of ADL and IADL impairment44Symptoms of depression can lead to a lack of an active lifestyle.45,46 The damage caused by vitamin D deficiency is further exacerbated by reduced exposure to sunlight, reduced nutritional intake, and slower nutritional metabolism as we age.47.
The authors also note an inverse relationship in which vitamin D deficiency may contribute to depressive symptoms.26,27The presence of vitamin D receptors in the brain where serotonin is produced explains the development of cognitive impairment, depression, and anxiety following vitamin D deficiency.27,48.
More than half of the participants in our study did not report worsening depressive symptoms. A possible reason for this low percentage is that our sample population was younger and therefore less prone to show factors influencing the development of depressive symptoms. Our sample was also characterized by being active and functional, not showing the functional impairments seen in the oldest old. Physical activity is a protective factor for depression, and similarly, the higher the physical activity level and the more functional you are, the lower the risk of depressive symptoms.49.
Studies of elderly people in Brazil and abroad often point to vitamin D deficiency. Even in the sunniest regions, serum levels of this vitamin are often low, as sun exposure alone is not enough. In addition, the skin of elderly people becomes less effective, further reducing its ability to synthesize vitamin D upon sun exposure. Another factor that reduces serum levels of vitamin D in old age is overweight. Increased body fat reduces the bioavailability and makes it more difficult to distribute vitamin D.47.
We investigated the extent of the increase in disability across all age groups and how it relates to depressive symptoms and vitamin D insufficiency. The observed increase in disability may be explained by the fact that the presence of depressive symptoms often increases with age.50Physiological changes affect physical function, impairing basic skills such as sitting, walking and getting up, resulting in a reduced quality of life due to lowered self-esteem and increased depression.51Regarding vitamin D, age-related decline in renal function may result in decreased renal production of 1,25(OH)2D.47In addition, the concentration of vitamin D receptors in the intestine decreases with age, which may be one of the causes of resistance to 1,25(OH)2D, resulting in52.
Previous studies carried out in different Brazilian states found similar characteristics in the profile of older adults as in our study:53,54have a partner53,55low level of education56Data from the Brazilian Institute of Geography and Statistics (IBGE) also confirms that the majority of older people are women and white.57The justification for this finding is the longevity characteristic of the aging process, where women outnumber men. In this regard, the United Nations (UN) estimates that by 2040, there will be 6.2 million more women than men.58.
In our study, we targeted the age group 50 to 59 years old to track the onset of aging. However, some studies54,55,56the participants included were over 60 years old, with the majority in the age group 60 to 69 years. Although this study shows a very important point, its limitations should also be highlighted. First, in this type of study, it is not possible to establish temporal relationships. Second, there is a non-participant bias that causes sample loss in the study, since the weakest people tend not to participate. Third, although the instruments used have been widely used in other studies and allow comparison with world-level studies, they are self-reported measures and may be influenced by social, environmental and psychological factors, in addition to being affected by memory and information biases. Although vitamin D insufficiency alone rarely causes functional impairment, it is necessary to maintain sufficient serum vitamin D levels, because the functional loss is even greater if the elderly person has functional impairment, whether the initial factor is symptoms of depression or other physical conditions resulting from the aging process. Therefore, strategies to keep this population active and maintain sufficient vitamin D levels are important. Vitamin D supplementation should be considered, including for those at risk of vitamin D deficiency and for those for whom sun exposure is contraindicated.
Daily activities such as walking to the market and keeping the home clean and tidy are crucial for maintaining the independence of older adults. However, access to physical activity is essential for a quality aging process. In tandem with maintaining vitamin D serum levels, physical activity performed outdoors in groups promotes social interaction. Adding integrative, multitasking, playful and recreational practices can help prevent/recover from depression as well as maintain cognitive function.
Maintenance and restoration of functional capacity is the result of the interaction of multiple factors, including physical and mental health, the ability to independently perform common daily tasks, community participation, economic independence, the availability of family support, and access to health professionals. Similarly, symptoms of depression are not simply a physiological response to the aging process but are a multifactorial outcome that must be monitored closely.
We encourage new studies to investigate the longitudinal relationship between depressive symptoms and vitamin D functionality, taking into account the region of residence and the season of data collection. Consider analyses taking into account categories such as vitamin D deficiency, insufficiency, and sufficiency. It is noteworthy that Brazil is a country with a large area, climate, and livelihoods. In this sense, food availability itself varies from region to region and is directly related to vitamin D. In the present study, we did not include data on nutrition, but it strengthens our suggestions for future research.
Final thoughts
The coexistence of vitamin D deficiency and depressive symptoms increases the likelihood of functional impairment in adults. Healthcare professionals can prioritize early detection and treatment of vitamin D deficiency and depressive symptoms in adult patients to reduce the risk of functional impairment.
