According to the American Heart Association (AHA), obesity is directly associated with an increased incidence of cardiovascular risk factors.¹ However, the impact of obesity on health is not uniform, and the localization of excess body fat is more crucial than the overall body weight.² In fact, intra-abdominal (visceral) adipose tissue has been identified as the most strongly fat depot linked to metabolic complications, forming a key correlation of various atherogenic, prothrombotic, and inflammatory abnormalities collectively referred to as the metabolic syndrome.^3,4

Vitamin D, also known as cholecalciferol, is a steroid hormone primarily responsible for regulating calcium homeostasis and skeletal health. Prolonged deficiency of vitamin D leads to conditions like rickets and an increased risk of fractures associated with osteoporosis.⁵ On the other hand, 25-hydroxyvitamin D (25(OH)D) concentrations between 20 ng/ml and 50 ng/ml (50–125 nmol/l) have been considered safe and sufficient for skeletal health in the general population.⁶

In recent years, emerging evidence has revealed additional functional roles of vitamin D, linking its deficiency (25(OH)D < 20 ng/dl) and suboptimal status (25(OH)D ≥ 20 and < 30 ng/dl) to various diseases and unfavorable metabolic factors such as Insulin Resistance (IR), Type 2 Diabetes Mellitus (DM2), and Cardiovascular Disease (CVD). Epidemiological studies have shown that vitamin D deficiency is associated with an increased risk of future cardiovascular events and is correlated with the presence of endothelial dysfunction and arterial stiffness, particularly in hypertensive individuals.^7–9

Obesity and vitamin D deficiency are commonly associated with health hazards. Several potential explanations account for this relationship. One possibility is that the liposoluble nature of vitamin D leads to volumetric dilution in obese individuals.¹⁰ Additionally, the larger amount of adipose tissue in obese individuals can act as a reservoir, sequestering vitamin D and contributing to its reduced availability.^11,12 Reduced sun exposure in obese populations further limits dermal synthesis of prohormones, resulting in lower serum vitamin D levels.¹³ Furthermore, the response to vitamin D supplementation is less pronounced in obese subjects, suggesting the need for dose adjustments based on body size.¹⁴ The impaired response to vitamin D supplementation in obesity may be attributed to various factors. For instance, patients with Nonalcoholic Fatty Liver Disease (NAFLD) may experience lower hepatic 25-hydroxylation of vitamin D.¹⁵ Moreover, obese individuals may exhibit reduced gene expression of specific enzymes in the cytochrome P450 system responsible for the conversion of vitamin D into its bioactive form in subcutaneous adipose tissue.^16,17 While adverse dietary habits are common in obesity, they are considered a less likely contributor to low serum vitamin D levels.¹⁸ Overall, the relationship between obesity and vitamin D deficiency involves complex biochemical mechanisms that warrant further investigation for a comprehensive understanding of their health implications.

This study explores the potential interplay between obesity, vitamin D deficiency, and their collective influence on inflammation and vascular dysfunction. Through the modulation of vasoconstriction factors and vasodilator substances, such as nitric oxide, these conditions are believed to contribute significantly to vascular inflammation and endothelial activation.^19,20 We aim to investigate the association between body composition, altered biochemical parameters, and vascular function, assessing indices that could be linked to impaired endothelial function, arterial stiffness, and autonomic imbalance in overweight and obese individuals with inadequate vitamin D status.

A total of 90 individuals were selected after having been screened by BMI and Vitamin D levels. The mean age of the study population was 52 years, and the mean BMI was 31 kg/m². The median 25(OH)D was 22.6 [interquartile range: 17.8–26.5] ng/ml. Clinical and anthropometric characteristics of the participants sorted by median serum 25(OH)D levels are shown in Table 1. Although BMI was similar between the groups (SUB-D 30.8 ± 4.0 vs. LOW-D 31.6 ± 3.6, p = 0.858), there were significant gender differences in %BF, which was found to be significantly higher in males than in females in the LOW-D group. On the other hand, females showed higher values of VAI and LAP indices in the same LOW-D group.

Biochemical variables are expressed in Table 2. As expected, the mean Vitamin D levels were significantly lower in the LOW-D group compared to those in the SUB-D group. Regarding the glycidic profile, the LOW-D group presented higher parameter levels, including insulin resistance HOMA-IR > 2.71 (p = 0.049), although glycated hemoglobin did not reach statistical significance (p = 0.058).

In relation to endothelial function, impaired microvascular reactivity was shown by a smaller % AUC increase in the LOW-D group (Table 3). Concerning the central hemodynamic parameters and HRV, no significant differences were found between the groups, although the LOW-D group patients presented sympathetic hyperactivity with higher LF/HF and SD2/SD1 ratio (Tables 3 and 4).

Concentrations of 25(OH)D showed weak inverse correlation to TG (r = -0.231 p = 0.032), PTH (r = -0.259 p = 0.015), and to WHtR, and moderate correlation with Female's VAI and LAP. On the other hand, the increase in area under the curve in post-occlusion reactive hyperemia (AUC-PORH increase, %) was weakly positively correlated to 25(OH)D levels (Figure 1). Considering 25(OH)D concentrations as a dependent variable, those significant parameters were assessed using multiple linear regression and remained statistically significant after adjusting for age and sex (Table 5).

Gender differences were considered in correlations among 25(OH)D concentrations and all continuous variables. In male subjects, some anthropometric indices, such as WC, WHR, WHtR, and CI, were weakly, and the %BF were moderately inversely correlated to 25(OH)D levels. On the other hand, vitamin D in females presented moderated inverse correlations with VAI, LAP, and TG, and weak AIP and TG/HDL. Females also presented a positive weak correlation with the increase in AUC-PORH (Table 6).

In this cross-sectional study, the lowest 25(OH)D concentrations were associated with anthropometric, metabolic, and vascular changes in middle-aged and obese adults with suboptimal vitamin D levels.

The connection between reduced vitamin D concentrations and obesity is classic and well-documented in observational studies.³⁰ These sample characteristics, with low vitamin D levels and high BMI, are consistent with previous studies in other populations that have found an inverse relationship between 25(OH) D levels and BMI.³¹

Our results showed significant gender differences in body adiposity content in the LOW-D group. Men with the lower vitamin D serum levels presented a significantly greater content of fat mass than women, as shown by higher adiposity (%BF) in BIA analysis. These findings agree with previously published reports,^32–35 but do not with an Italian report of 500 subjects, in which females presented higher body adiposity.³⁶ Our females’ main characteristics were higher visceral adiposity and the accumulation of lipid products. Visceral depots represent around only 6% of total body fat in women compared to 20% in men.³⁷

Obesity's related cardiovascular risks are more closely related to the location of the excess body fat rather than to an elevated body weight per se.^24,38,39 VAI and LAP are seen as better indicators of dysfunctional adipose tissue than BMI for recognizing cardiovascular risk in some illness conditions and in the general population.^25,40,41 Our women's sample does not follow Vague's definitions of the most common phenotype as gynoid obesity.³ So, it seems that our women are at increased health risks as their VAI and LAP indices were significantly higher in the LOW-D group. These findings support that modern dietary habits may overcome phenotypic traits.

The LOW-D group presented biochemical data of insulin resistance and slightly higher HbA1C levels, albeit still within the normal range. Defects of vitamin D and insulin resistance have a superimposable epidemiological distribution that can be considered the direct or indirect consequence of vitamin D deficiency.⁴² Also, this finding aligns with the Saudi study that investigated diabetic and non-diabetic adults, which reported decreased vitamin D levels associated with increased HbA1C in non-diabetic individuals.⁴³ Additionally, participants in the LOW-D group exhibited notably lower levels of vitamin D, as anticipated, and higher levels of PTH, although not significant. These results are likely attributable to secondary reflex hyperparathyroidism.

The LOW-D group exhibited impaired microvascular reactivity in vascular function, as evidenced by a smaller increase in AUC from baseline to the PORH period. Previous studies have demonstrated patterns of endothelial dysfunction in various populations with vitamin D deficiency, including those with ischemic heart disease, as well as in healthy subjects from diverse ethnic backgrounds. In other studies, these patterns are predominantly characterized by lower increments in flow-mediated dilation of the brachial artery^8,44,45 and, in some cases, by evaluating the fingertip reactive hyperemia index using the Endo-PAT 2000, as described elsewhere.^46,47Clique ou toque aqui para inserir o texto. A few publications have evaluated reactive hyperemia, including one of the most recent ones that evaluated overweight post-menopausal women and could only associate lower PORH with age, not with vitamin D levels, although this one has shown a weak inverse correlation with one oxidative stress blood marker (3-nitrotyrosine).⁴⁸ Therefore, to the best of our knowledge, no studies have yet utilized this reactive hyperemia using our LSCI system methodology in populations with our specific characteristics.

Concerning HRV, the LOW-D group patients presented sympathetic hyperactivity with higher LF/HF and SD2/SD1 ratio, although no significant differences were found between the groups. These findings align with a randomized double blind clinical trial where daily vitamin D supplementation resulted in an improvement in autonomic imbalance.⁴⁹

The correlation tests by gender showed an association of lower vitamin D levels with worse anthropometric parameters in males. Our findings align with some results from a large cross-sectional study where the conicity index was negatively associated with vitamin D levels in men. However, there were conflicting findings, as % BF and WHtR were negatively associated with vitamin D levels in both sexes.⁵⁰ In another large cross-sectional Chinese study, mean values of WC and WHtR tended to increase with 25(OH)D insufficiency in both sexes.⁵¹ In contrast, our correlation tests indicate that these parameters were inversely associated with vitamin D levels only in men. It is important to note that WC and WHtR are traditional indicators of abdominal obesity but do not directly reflect metabolic dysfunction. The conicity index, on the other hand, is more closely related to body fat and fat mass,⁵² and WHR has a better correlation with subcutaneous adipose tissue.⁵³ These findings diverge from the current literature, where not only the amount of fat mass is increased in males, but visceral adiposity as well.^32–35

Interestingly, in our female sample, indicators of dysfunctional visceral adiposity were inversely associated with vitamin D levels. To the best of our knowledge, we could not find published data where visceral adiposity was higher in women. Furthermore, in the correlation analysis by sex, we observed a negative association between serum vitamin D levels and TG in women. However, no significant correlation was found between vitamin D status and other lipid markers, as previously reported in non-obese populations.⁵⁴ This inverse association seems to be related only to increased caloric intake in obese individuals.

Lower levels of vitamin D were associated with a smaller percentage increase after post-occlusion reperfusion area, suggesting compromised endothelial function, and we found a positive correlation between vitamin D levels and endothelial dysfunction. Vitamin D plays a crucial role in regulating the synthesis of Nitric Oxide, a potent endothelium-dependent vasodilator, thereby conferring vaso-protective effects. Conversely, vitamin D deficiency represents a risk factor for endothelial dysfunction, which reinforces the significance of our findings.⁵⁵

This study needs to be analyzed for its limitations and strengths. The sample size could be a limitation, but it was sufficient to achieve significant results using different methods of structural and functional assessment. Concerning physical activity, all the participants were considered inactive. Diet and sun exposure were not collected as variables, as in other vitaminD studies. The oscillometric method used to assess PWV is not considered the gold standard, although it has already been validated and used in clinical practice. Assessment of the autonomic nervous system may be influenced by several factors related to the patient's resting environment. However, the HRV measurements provide several parameters related to sympathetic and parasympathetic activities. This cross-sectional design, which impairs causal inference, was elaborated for a specific population of obese individuals and cannot be extrapolated to non-obese individuals or those with sufficient vitamin D levels. Although these findings are crucial to better understand some vascular dysfunctions in this very prevalent condition within the general population, mostly related to sex differences.

In this sample of overweight and obese individuals, endothelial function was impaired in those with the lowest vitamin D levels. Additionally, noteworthy gender differences emerged, with males showing an excess of fat mass irrespective of body distribution. However, the most intriguing findings were related to the female subjects, who presented a metabolically unfavorable condition characterized by dysfunctional visceral adipose tissue, adverse biochemical markers, and impaired endothelial function.