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.
In a study of older Dutch adults by Koning et al., 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 showed 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 showed that participants with lower vitamin D levels were older, had poorer physical function, and had more depressive symptoms. Chronic diseases, depressive symptoms, and physical function were found to be mediators explaining the relationship between vitamin D and quality of life. The characterization of the study sample and categorization values were also different from our study, with <25 nmol/L as deficient, 25–50 nmol/L as insufficient, and >50 nmol/L as sufficient, which prevented appropriate comparisons. However, similar to our study, worse functional outcomes were observed in older individuals with vitamin D insufficiency and depressive symptoms.
Low vitamin D levels have been reported as a cause of functional decline due to muscle weakness, osteomalacia, and the appearance of hyperparathyroidism26, which in turn leads to increased bone loss and contributes to 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 impairments prevent vitamin D from actively and effectively performing its functions43. In the present study, the sample was active and functional, so no impairment in ADL was observed, but in the presence of depression, functional impairments were further increased due to the association between vitamin D deficiency and depressive symptoms.
Depressive symptoms may lead to a lack of an active lifestyle45,46 and, combined with reduced sun exposure, reduced nutrient intake, and reduced nutritional metabolism that generally declines with age, may potentiate the damage caused by vitamin D deficiency, thus increasing the likelihood of ADL and IADL impairment47.
The authors also discussed an inverse relationship, where vitamin D deficiency may be responsible for depressive symptoms.26,27 The presence of vitamin D receptors in the brain where serotonin is produced explains the cognitive impairment, depressive symptoms, and anxiety that occur after 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 onset of depressive symptoms. Our sample did not present the functional impairments seen in the oldest old, and therefore was also characterized as active and functional. Physical activity is a protective factor for depression, and similarly, the higher the level of physical activity and the better the functioning, the lower the risk of depressive symptoms49.
Vitamin D deficiency is common in studies of elderly people in Brazil and abroad. Even in the sunniest areas, serum levels of this vitamin are often low, as sun exposure alone is not enough. In addition, the skin of elderly people loses its efficacy, 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, as increased body fat reduces the bioavailability and distribution of vitamin D47.
We investigated the extent of the increase in disability across all age groups and to what extent it is associated with depressive symptoms and vitamin D insufficiency. The observed high degree of disability can be explained because the presence of depressive symptoms often increases with age50, while physiological changes affect physical function, impairing basic skills such as sitting, walking and getting up, resulting in a decreased quality of life due to lower self-esteem and increased depressive symptoms51. With regard to vitamin D, ageing may decrease renal function, which may result in a decrease in renal production of 1,25(OH)2D47. It has also been suggested that the concentration of intestinal vitamin D receptors decreases with age, which may be one of the causes of resistance to 1,25(OH)2D, resulting in a decrease52.
Previous studies carried out in different Brazilian states found similar characteristics in the profile of older adults as in our study: female gender53,54, having a partner53,55 and low educational level56. Data from the Brazilian Institute of Geography and Statistics (IBGE) also confirm that the majority of older adults are female and white57. This finding is supported by the characteristic that the number of women living longer than men during the ageing process. In this regard, the United Nations (UN) estimates that by 2040, there will be 6.2 million more women than men58.
In our study, we included the age group 50 to 59 years to track the onset of aging. However, in some studies,54,55,56 the participants were over 60 years old, with the 60 to 69 years age group being the mainstream. Although this study shows a very important point, its limitations should also be highlighted. First, this type of study does not allow 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 comparisons with studies at a global level, they are self-reported measures and, in addition to being affected by memory and information biases, can be affected by social, environmental, mental and emotional factors. Although vitamin D deficiency alone rarely causes functional impairment, it is necessary to maintain sufficient vitamin D concentrations in serum, because the functional decline is even greater when older people become disabled, regardless of the initial factors, such as symptoms of depression and other physical disabilities resulting from the aging process. Therefore, strategies to keep this population active and maintain good vitamin D concentrations are important. Vitamin D supplementation should be considered for individuals at risk for vitamin D deficiency, including those with contraindications to sun exposure.
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 serum levels of Vitamin D, 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 ability to count on 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.