Relationship Between Vitamin D Serum Levels and the Severity of Atopic Dermatitis-A Mapping Review of Evidence with Emphasis on Geography

Abstract

Atopic dermatitis (AD) is a chronic inflammatory skin disease with early-age onset. While vitamin D (VitD) has been associated with AD alleviation, geographical factors should be considered as VitD synthesis depends on sunlight exposure and dietary intake. We conducted a mapping review to identify geography-related evidence gaps in interventional and observational studies on the VitD-AD inverse association. We analyzed latitude and the Human Development Index (HDI) as background geographical factors. The review identified 38 studies (17 interventional, 21 observational), of which 26 confirmed the inverse VitD-AD association. Of all reviewed studies, 73% were from latitudes above 35° N, and 70.3% were from developed countries. The median latitude and HDI were 37.5° N and 0.915, respectively. Conversely, only 5.4% of studies were from Africa and 8.1% from Latin America. Studies that did not confirm the inverse VitD-AD association tended to be concentrated in developed countries at higher latitudes (median latitude 42.4° N, median HDI 0.937). Only 8.1% of all studies were from low-latitude developed countries, and among interventional studies this share was even lower (6.3%). In addition, 52.6% of studies lacked data on baseline VitD variability and 13.2% had no baseline VitD data at all. More thorough data reporting and additional clinical studies from countries that do not follow the high latitude/high HDI overlap pattern would facilitate future meta-analyses aimed at clarifying the role of VitD in AD treatment.

Keywords: Human Development Index; Vitamin D; atopic dermatitis; evidence gap map; latitude.

Figure 1

PRISMA flowchart of literature search. n = number of studies.

Figure 2

Distribution of reviewed studies by absolute latitude (degrees north or south of the Equator) of locations where studies were conducted, and by HDI values of countries in which studies were conducted. Green dots indicate studies that confirmed the inverse VitD-AD association, while orange dots indicate studies that did not confirm this association. (A): with all studies included, (B): with only interventional studies included, (C): with only observational studies included.

Figure 2

Distribution of reviewed studies by absolute latitude (degrees north or south of the Equator) of locations where studies were conducted, and by HDI values of countries in which studies were conducted. Green dots indicate studies that confirmed the inverse VitD-AD association, while orange dots indicate studies that did not confirm this association. (A): with all studies included, (B): with only interventional studies included, (C): with only observational studies included.

Figure 3

Comparison of latitude/HDI values between the studies that confirmed the inverse VitD-AD association (green) and the studies that did not confirm it (orange). (A): Comparison of absolute latitudes (degrees north or south of the Equator) of locations where studies were conducted with all studies included, (B): comparison of HDI values of countries in which studies were conducted with all studies included, (C): comparison of absolute latitudes of locations where studies were conducted with only interventional studies included, (D): comparison of HDI values of countries in which studies were conducted with only interventional studies included, (E): comparison of absolute latitudes of locations where studies were conducted with only observational studies included, (F): comparison of HDI values of countries in which studies were conducted with only observational studies included. Labels: x, arithmetic mean; ◦, outlier.

Figure 3

Comparison of latitude/HDI values between the studies that confirmed the inverse VitD-AD association (green) and the studies that did not confirm it (orange). (A): Comparison of absolute latitudes (degrees north or south of the Equator) of locations where studies were conducted with all studies included, (B): comparison of HDI values of countries in which studies were conducted with all studies included, (C): comparison of absolute latitudes of locations where studies were conducted with only interventional studies included, (D): comparison of HDI values of countries in which studies were conducted with only interventional studies included, (E): comparison of absolute latitudes of locations where studies were conducted with only observational studies included, (F): comparison of HDI values of countries in which studies were conducted with only observational studies included. Labels: x, arithmetic mean; ◦, outlier.

Figure 4

Geographical distribution of all reviewed studies. Green signs indicate studies that confirmed the inverse VitD-AD association, while orange signs indicate studies that did not confirm it. Numbers in violet indicate interventional studies.