Assessing the impacts of intermittent fasting on arterial stiffness and SARS-CoV-2 infection in mice

Abstract

Intermittent fasting (IF), regulated periods of caloric abstinence followed by ad libitum energy intake, has emerged as one of the most popular dietary strategies globally. Although there are many factors facilitating IF's growing popularity, a primary reason could be its purported health benefits. Notably, an extensive amount of research has shown that IF reduces cardiovascular disease (CVD), the leading cause of death worldwide, by reducing prominent CVD risk factors including hypertension, hypercholesterolemia, and hypertriglyceridemia. Obesity-related arterial stiffness is another independent cardiovascular disease risk factor, yet whether IF impedes arterial stiffening in obesity is poorly understood. Our lab has shown that the gut microbiota is a key determinant of obesity-related arterial stiffness. Mechanistically, obesity broadly perturbs gut microbial composition and function, which can lead to systemic inflammation. This overactive inflammatory response provokes vascular injury and arterial stiffness. Interestingly, by impeding aberrant gut-immune signaling, IF has been shown to reduce blood pressure and improve endothelial function in overweight and obese individuals. Therefore, IF may be leveraged to impede obesity-related arterial stiffness through similar mechanisms, yet this has not been explored. Moreover, there are other pertinent questions on IF and arterial stiffness that have yet to be answered; including 1) how many bouts of IF are needed to reduce arterial stiffness, and 2) once arterial de-stiffening occurs, how long after the fasting period is it maintained? While IF may have benefits on the cardiovascular system, it might also have some drawbacks. One potential concern is IF's immunosuppressive effect, which may be beneficial for long-term vascular health, but might increase the risk of acute infections. SARS-CoV-2 (SC2), the culprit of the COVID-19 pandemic, is still a prominent virus that can lead to mild-to-severe health complications. Furthermore, data suggest that a robust immune response and changes to the gut microbiota at the onset of SC2 infection predict infection severity and long-term sequelae. Thus, understanding whether IF-induced modifications to the gut microbiota and immune system lead to worse SC2 infection is essential, but has not yet been studied. Chapter 3 of this dissertation investigated differences in arterial de-stiffening between a single fasting-refeeding cycle and a multi-week, once-weekly, 24-hour IF protocol. Surprisingly, we found that a single 24-hour fast acutely increased arterial stiffness in lean, but not obese mice. However, this was not due to acute changes within the gut microbiota or immune system and may be the result of other acute factors (i.e., blood pressure, nitric oxide signaling, vascular smooth muscle cell function). Furthermore, the elevated arterial stiffness was diminished after refeeding, suggesting no long-term vascular effects. In contrast, a prolonged period of once-weekly, 24-hour IF only impacted (reduced) arterial stiffness in obese mice. These vascular improvements occurred without changes in body weight or food intake, and we also show that the arterial de-stiffening effects of multi-week IF lasted for at least 5 days after fasting. Mechanistically, reductions in obesity-related arterial stiffness were associated with compositional and functional changes to the gut microbiota, reduced circulating chemokines, and reduced perivascular adipose tissue lymphocyte accumulation. In the second study (Chapter 4), we investigated whether there is a discordant impact of multi-week, once-weekly, 24-hour IF on arterial stiffness and SC2 infection in mice. After 10 weeks of IF, we once again observed arterial de-stiffening in obese mice. After 12 weeks of IF, we infected mice with a mouse-adapted SC2 strain. We found that IF before SC2 infection did not substantially impact SC2 burden in the lungs, circulating SC2-specific antibodies, gut microbial ecology, or lung and spleen immune cell populations in either lean or obese mice. Overall, these findings reveal that once-weekly 24-hour fasting could be a safe and feasible dietary strategy that improves a putative determinant of cardiovascular health with minimal impacts on acute SC2 infection severity. Future studies should uncover how the "gut-immune axis" modulates obesity-related arterial stiffness, as well as translate the arterial de-stiffening effects of IF to human vascular health.