Effects of Vitamin D Supplementation on a Diet-induced Obesity Model: Cognitive Behavior

ABSTRACT

Obesity is a disease that affects 1/3 of the US population that could lead to several comorbidities. Obesity induces cognitive and behavioral impairment. Low plasma vitamin D levels are associated with increased body mass index (BMI). Therefore, vitamin D supplementation has shown benefits in BMI, glucose homeostasis and cognitive/behavioral function;however, contradictory data exists in animal models and remains poorly studied. Obesity in males increases 5.7-fold the risk for severe complications and death due to SARSCoV-2. Brain RAS activity, especially activation of the Angiotensin II receptor (AT1R), has been implicated in cognitive impairment and may participate in persistent neurological symptoms seen in 1 in 3 severe COVID-19 cases, i.e., disorientation, inattention and ataxia. Supplementation with Vitamin D (VitD), known to ameliorate respiratory infections and thromboembolism, may protect against COVID symptoms by increasing ACE2 and Masr expression. To generate a diet-induced obesity (DIO) model, C57Bl6/J male mice (7 wks of age) were randomly assigned to 3 groups (n=10/group) and were fed ad libitum with a) chow diet (CD), b) lard-based high-fat diet (HFD, 60% fat;D12492;Research diet, New Brunswick, NJ), or c) HFD supplemented with vitamin D (60% fat plus 15,000 IU/kg diet, HVitD). Behavioral and metabolic endpoints were measured starting at 7 wks on diet. The HFD diet was effective in generating DIO vs CD despite equivalent food intake consumption on a kcal-basis. Body composition analysis (EchoMRI) showed increased fat mass in HFD and HVitD when compared to CD. Contrary to our hypothesis, ip glucose (0.25 mg/kg) caused greater glucose intolerance in HVitD relative to HFD at 15 (P<0.05) and 30 min (P<0.01) post injection. HFD and HVitD groups showed insulin resistance relative to CD while having an equal glucose clearance post insulin bolus (3.75 U/kg;ip). Cognitive ability was assessed using novel object recognition (NORT), passive avoidance (PAT) and transfer latency (TL) on the elevated plus maze (EPM) tests. There was no effect of DIO on NORT. Surprisingly, in the PAT, VitD supplementation reduced conditioned anxiety as seen by a decreased step-through latency to aversive chamber (3 d retention), suggesting poorer fear-based learning relative to HFD (P<0.01). No effect was seen on TL. Increased anxiety was observed on EPM in HFD (P<0.04), as indicated by decreased time in the open arm relative to that in CD. Reduced locomotor activity was observed on the open field test (OF) in HFD and HVitD groups in the last 4 min of a 10 min test duration. Molecular analysis of RAS genes in the brain using RNAscope is currently underway. In conclusion, Vitamin D supplementation (15,000 IU/kg diet) was not effective in protecting against DIO and glucose intolerance. Importantly, VitD also reduced fear-based learning.