Date of Award


Degree Type


Degree Name

Master of Science (MS)


Nutrition Science and Dietetics


Voss, Margaret

Subject Categories

Nutrition | Physiology


Objectives: The primary aim of this study was to elucidate relationships linking vitamin Dconsumption and the vestibulo-sympathetic reflex (VSR) by exploring the associations between self-reported vitamin D consumption and the cardiovascular responses to head-down flexion (HDF), a maneuver known to activate the vestibulo-sympathetic response. Background: During orthostasis, the body attempts to maintain cerebral blood flow, thereby avoiding syncope. Orthostatic intolerance, the inability to withstand postural changes, occurs when cardiac output is not maintained during orthostasis and is highlighted by symptoms including lightheadedness, impaired cognition, loss of balance, low blood pressure, and syncope. The cardiovascular and vestibular systems work in tandem to sustain cardiac output via the VSR. Due to its dependency on proper cardiovascular and vestibular function, the VSR may be vulnerable to nutrient deficiencies that impair these systems. Vitamin D, a fat-soluble vitamin, is critical to endothelial and smooth muscle function, calcium and phosphate metabolism, and vestibular otolith development and function. Though positive relationships between vitamin D serum levels and cardiovascular and vestibular health have been made, to date no data examine vitamin D status regarding the VSR. We investigated the relationship between vitamin D consumption and the VSR during HDF in healthy adults, hypothesizing that vitamin D consumption would be positively associated with the vestibulo-sympathetic reflex. Methods: Vitamin D consumption of 31 participants was measured using a food frequency questionnaire. The incidence of orthostatic intolerance within the study group was assessed using the Orthostatic Hypotension Questionnaire. Radial applanation tonometry-derived augmentation indices were used to assess arterial stiffness. Popliteal blood flow velocity and popliteal artery diameter were measured via ultrasound, and popliteal blood flow and popliteal vascular conductance were calculated to assess lower limb cardiovascular responses during HDF. A three-lead electrocardiogram was used to determine heart rate. Results: Age, sex, and change in head angle were used as covariates throughout all analyses. Mean vitamin D consumption was 3.36 ± 2.39 mcg within the study group. The mean orthostatic tolerance score was 6.18 ± 8.16. Systolic blood pressure and MAP decreased by 2.26±1.46 mmHg (P<0.05) and 1.06 ± 1.49 mmHg (P<0.05), respectively, during HDF. Heart rate decreased by 0.59 ± 0.10 bpm during HDF. Head-down flexion also decreased augmentation index by 7.6% (P < 0.05), popliteal blood velocity by 0.87 cm/s (P < 0.05), and popliteal blood flow by 10 ml/min during HDF. No other cardiovascular findings were significant. Pearson's correlations failed to detect significant relationships between vitamin D consumption and cardiovascular responses to HDF. Conclusions: This study aimed to identify novel relationships between vitamin D consumption and the vestibulo-sympathetic reflex during head-down flexion. Consistent with previous work regarding the VSR, upper and lower body responses were observed, including decreases in blood pressure and lower limb blood flow and blood flow velocity. However, the current study did not identify significant relationships between vitamin D consumption and cardiovascular responses to head-down flexion. Participants' average reported vitamin D intake was far below the National Institute of Health's daily recommended intake of 15mcg. It is thus reasonable to assume participants did not consume enough vitamin D to influence cardiovascular function. Lastly, our sample may not have achieved sufficient change in head angle to elicit the VSR. Follow-up studies should examine vitamin D status objectively, establish a minimum change in head angle to ensure excitation of the VSR, and utilize vitamin D sufficient and insufficient groups to contrast the vestibulo-sympathetic reflex during head-down flexion.


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