While vitamin D deficiency is considered an important public health problem, few studies on the subject have been conducted in sub-tropical countries.
At the same time, predictors of vitamin D deficiency in those with healthy kidney function have not been well researched.
D-ficient degree holders
Based on this, researchers at Taiwan's Chang Gung Memorial Hospital and Chang Gung University conducted a cross-sectional study between August 2013 and 2017 to evaluate the prevalence and factors linked to vitamin D deficiency in northern Taiwan.
They recruited 3,954 participants without chronic kidney disease, aged 30 and above, in northern Taiwan. They then recorded the participants' serum 25-hydroxyvitamin D 25(OH)D levels, biochemistry, sociodemographic variables (such as age, sex, education level and occupation) and lifestyle habits (tea and coffee consumption, and physical activities).
Those who had been drinking tea and / or coffee regularly for over five years were classified as tea and / or coffee drinkers, while their physical activity level was determined using reported hours per day of activities such as walking, dancing, gardening, hiking and swimming.
Subsequently, the researchers reported a "significantly higher vitamin D deficiency ratio in the women compared to the men (22.9% versus 9.9%)".
In addition, vitamin D deficiency was most prevalent (38.4%) among the participants aged 30 to 39 years, with those holding a graduate degree having the highest rate of vitamin D deficiency (31.5%).
According to the findings, the main predictors of vitamin D deficiency were female sex, young age, a high education level, residing in an urban area, and physical inactivity.
However, the researchers also found that tea consumption was negatively correlated with vitamin D deficiency, a relationship that few prior studies had assessed.
While there was no significant difference in the incidence of vitamin D deficiency between those who consumed tea regularly and those who did not, the researchers stated that “tea consumption appeared to be a protective factor against vitamin D deficiency after multiple regression analysis adjusting for confounding variables such as age, education and residential districts".
They added that these factors had likely confounded the relationship between vitamin D status and tea consumption: among the study population, those who drank tea regularly also tended to be younger (between 53 and 65 years old) than those who did not (between 59.7 and 72.2 years old), but vitamin D deficiency was more common among the younger participants.
This, the researchers said, may have concealed the benefit of tea consumption for the participants' 25(OH)D levels.
On the other hand, coffee consumption was positively correlated with a higher prevalence of vitamin D deficiency, compared to no coffee consumption at all.
However, following multiple logistic regression analysis, the researchers determined that coffee-drinking was not an independent risk factor for vitamin D deficiency.
Still, despite the positive association between tea consumption and vitamin D status, the underlying mechanism remained unclear to the researchers, who stated that more studies were warranted to clarify this relationship.
Determining the data
They added that the large study population and the absence of chronic kidney disease among them lent strong support to its findings, but acknowledged that they had not obtained information about the participants' dietary intake of vitamin D.
They had also not taken into account the study subjects' amount of sun exposure or the factors that may have influenced it, including clothing, length of time spent outdoors, sunscreen use, and skin colour — all of which could also have affected their 25(OH)D levels.
At the same time, data on exercise and tea and coffee consumption were obtained through questionnaires, which could have resulted in reporting or recall bias. The researchers also did not estimate the impact of the current season or month when collecting blood samples from the participants.
Furthermore, they reported that the method they had used to measure the participants' 25(OH)D values (radioimmunoassay) could have led to lower values than the gold standard method (liquid chromatography tandem mass spectrometry), thus resulting in an overestimation of the prevalence of vitamin D deficiency among the study population.
Finally, the cross-sectional data made it impossible for them to analyse longitudinal changes in the participants' vitamin D status.
In conclusion, they wrote: "Our data demonstrated that vitamin D deficiency is prevalent in subtropical areas such as northern Taiwan, even in healthy individuals without chronic kidney disease.
"The prevalence was particularly high in women, those who were younger, better educated, and who lived in an urban area. Vitamin D supplements are thus an important issue in this group of people.
"Furthermore, we also found that tea consumption had a protective effect on vitamin D deficiency. Further studies are needed to confirm our findings."
Source: BMC Public Health
"Vitamin D deficiency in northern Taiwan: a community-based cohort study"
Authors: Ming-Jse Lee, et al