The purpose of this study was to find potential discrepancies and correlations between vitamin D levels and different parameters in groups of children with hypertension, overweight children, overweight children with hypertension and healthy control group. We discovered statistically significant differences in vitamin D levels in all groups in relation to the control group. There were also statistically significant differences between groups of overweight children and children with hypertension, as well as between children with hypertension and overweight children with hypertension. There were no significant differences in vitamin D levels between the groups of overweight children (group C and D). Our findings suggest there is a link between obesity and arterial hypertension, as well as vitamin D, which has also been proven by other studies in this department (10, 15, 16). Considering the statistically significant differences between patients with hypertension and both groups of overweight patients (group C and D), we conclude that obesity presents a bigger negative predictive factor for vitamin D values than hypertension. The effects of both factors combined were not proven to affect vitamin D values any more than each individual factor. Significantly low levels of vitamin D in children with hypertension and normal BMI suggest vitamin D affects blood pressure independently of obesity. Similar observations have been made in other studies (10, 15, 16, 17).
We discovered statistically significant correlations between vitamin D and various parameters in our research sample of 175 children. The correlation between BMI, measured in percentiles, and vitamin D was statistically significant, which supports our previous findings when comparing individual groups of children. The lower levels of vitamin D in overweight children are probably the result of volume dilution as well as reduced sunlight exposure (18). When supplementing vitamin D in overweight children, in order to achieve appropriate vitamin D serum levels, we usually have to administer higher doses than we would when treating children who are not overweight (6, 16).
Another statistically important correlation, albeit smaller, was discovered between vitamin D and systolic blood pressure. The reason for that likely being the presence of VDR in endothelial and smooth-muscle cells of blood vessels and cardiomyocytes. Studies in recent years have provided evidence of the role vitamin D has in reducing high blood pressure. This is achieved through inhibition of the renin-angiotensin hormone system, modulation of the endothelial function and reduction of oxidative stress in blood vessels.
Together with the fact that there is a statistically significant difference between the group of normal-weight children with hypertension and the control group, we can assume that vitamin D is a non-obesity-dependent negative predictive factor of arterial hypertension and that by supplementing it, we could potentially improve blood pressure. However, it is important to keep in mind that a healthy body weight is what helps maintaining sufficient levels of vitamin D in the first place (19).
Another correlation, which proved significant, was discovered between vitamin D and homocysteine, which is again in line with findings of other studies. This is most likely the consequence of vitamin D and its regulation of cystathionine-β-synthase (CBS), an enzyme which converts homocysteine into cystathionine, which is further converted into cysteine. The net effect of this reaction is a reduction of total blood homocysteine. Because hyperhomocysteinemia can lead to endothelial disfunction, vitamin D can be seen as a potential protecting agent in preventing premature cardiovascular diseases (20, 21, 22).
We have noticed a statistically significant negative correlation between vitamin D and triglycerides (TG). Another negative correlation was found between vitamin D and LDL cholesterol and a positive one between vitamin D and HDL cholesterol, however it was statistically unsignificant. These results are in line with findings of other similar studies which proved correlation between low serum vitamin D levels and blood lipid abnormalities in children (16, 19). Statistically significant non-obesity-dependent correlation was established between vitamin D and TG, which means vitamin D is an independent negative predictive factor of hypertriglyceridemia. This is likely because vitamin D lowers fatty acid absorption in the intestine by increasing the absorption of calcium and higher calcium levels promote the hepatic conversion of cholesterol into bile. Vitamin D also reduces lipogenesis and promotes lipolysis through parathyroid hormone (PTH) inhibition, which additionally helps regulate blood lipid parameters (21, 22, 23).
There was a statistically significant positive correlation between apolipoprotein A1 (apo A) and vitamin D. Apo A is one of the main components of HDL cholesterol, its presence is therefore an indicator of cardiovascular health. In terms of the role of apo A as a marker of cardiovascular health it is important to consider its relationship with apo B because higher values of the latter indicate an increased risk for cardiovascular disease. In our study we discovered a negative correlation between vitamin D and apo B, though unsignificant. These results reflect the role of vitamin D in regulating the hepatic synthesis of apo A through its action on the VDR receptor (23, 24, 25).
The negative correlation between vitamin D and uric acid, which we found to be just slightly below the limit of statistical significance, is most likely the consequence of 1-alpha-hydroxylase inhibition which is caused by higher levels of uric acid. 1-alpha-hydroxylase is an enzyme that converts 25(OH)D into its biologically active form 1,25(OH)2D and therefore promotes its actions (28).
Despite many positive and promising findings of our study, there are still some things which could be improved. Ideally, the groups would be comprised of equal number of patients of the same sex and age. This would be very difficult to achieve in our situation, as we recruited our patients in an anterograde fashion. Another thing, which could provide us with more relevant results would be to include a larger number of participants.
We can conclude with relative certainty that the results of our study are in line with the findings of other recent studies, whether regional or international, and that they provide enough evidence to the theory that maintaining a healthy body weight and appropriate vitamin D levels is of vital importance, especially in patients subjected to cardiovascular diseases.