Effect of a Physical Training Program on Cerebral Hemodynamics in Patients With Metabolic-associated Fatty Liver Disease

Hepatic steatosis associated with metabolic dysfunction, abbreviated as MAFLD (Metabolic Associated Fatty Liver Disease) is currently the most common liver disease and has become one of the main etiologies of chronic liver disease and hepatocellular carcinoma (HCC) in the western world, it is estimated that in a few years it will be the leading cause of liver cirrhosis1.

MAFLD encompasses a broad spectrum of liver injury of varying severity ranging from simple steatosis to steatohepatitis (NASH). In principle, simple steatosis is the accumulation of lipids in more than 5% of hepatocytes, but it can progress in approximately 10-20% of subjects with simple steatosis to steatohepatitis which is defined as steatosis accompanied by cell ballooning, lobular inflammation, and fibrosis, and if this persists without treatment, up to 10-25% may progress to liver cirrhosis and/or hepatocellular carcinoma .2,3 Epidemiology MAFLD is a common disease worldwide and presents a significant burden on health systems. It is present in 6-25% of the world population. In Latin America, the prevalence ranges between 17% to 33%, while in Mexico the prevalence is 14-30% in the general population.4 MAFLD is more common in patients in their fifth decade of life, and there are groups at higher risk, such as subjects with Type 2 Diabetes Mellitus (DM2) and patients with dyslipidemia, where the frequency of the disease increases to 62% and 83% respectively.5 Due to the increasing trend of obesity, the incidence rate of NAFLD is expected to increase significantly in the coming decades; in fact, only 3-20% of patients with this pathology are not obese, so it is concluded that obesity is the most common clinical phenotype associated with MAFLD.4,5 MAFLD-associated complications Obesity has been linked not only to simple steatosis but also to advanced disease, i.e., NASH. As a consequence, in addition to increasing all-cause mortality, obesity appears to increase liver-specific mortality in patients with MAFLD, and due to the lack of approved pharmacological interventions to counteract the disease, targeting obesity is considered an option. essential for its management.6 In NAFLD, as in other metabolic disorders, there is low-grade inflammation that is mainly determined by the expansion of visceral adipose tissue, which produces proinflammatory cytokines (IL-1B, IL-6, and TNF-a) that cause both inflammation at the hepatic level through the portal route, as well as systemic inflammation with repercussions in different organs and tissues, including muscle, heart, and brain.7 These characteristics in combination with the release of pro-coagulant, pro-oxidant, and pro-fibrogenic factors represent a risk factor for endothelial dysfunction and vascular damage. Additionally, MAFLD is associated with an increase in components of the renin-angiotensin system, particularly angiotensin II, which may contribute to vascular damage by increasing oxidative stress and accelerating atherosclerosis7,8 The mechanisms described suggest that in MAFLD, vascular alterations and endothelial dysfunction, in addition to the known manifestations of cardiovascular risk, lead to incipient alterations in cerebral blood flow (microvascular) that contribute to cognitive deterioration and increase the risk of ischemic stroke. In fact, MAFLD is associated with ischemic stroke even after adjustment for other cardiovascular risk factors such as obesity, dyslipidemia, and type 2 DM.9,10 Pharmacological and non-pharmacological treatment in MAFLD There is no registered effective pharmacological treatment for MAFLD, but guidelines support the use of vitamin E in MAFLD subjects without type 2 diabetes, or pioglitazone in NASH patients, and four drugs (obeticholic acid, elafibranor, selonsertib, and cenicriviroc) have entered phase III of development.11 Due to the above, lifestyle modification to achieve weight reduction represents the first-line treatment for MAFLD and resolves hepatic steatosis and hepatic fibrosis.12 Loss of at least 7% of body weight is associated with improvement in steatosis, but greater weight loss (>10%) is needed to reverse the histopathologic features of NASH, including fibrosis. In obese subjects a reduction of 1 kg/week is suggested and in morbid obesity 1.5-2.5 kg/week; Weight loss greater than these amounts is associated with adverse effects, such as an increased inflammatory state in these patients, and increased mobilization of intrahepatic fat, which is associated with worsening hepatic steatosis.12 Although nutritional management is the cornerstone of MAFLD treatment, physical activity should always be part of comprehensive management. Exercise intervention studies in MAFLD have shown that both aerobic and resistance exercise significantly decrease intrahepatic lipid content. In addition, there is evidence that exercise has a role in improving endothelial dysfunction in patients with MAFLD, which could have implications for the prevention of cardiovascular diseases. These benefits appear to be independent of exercise intensity and dose.13 The exact mechanisms by which exercise decreases intrahepatic lipid content are not fully understood. Some studies reported that the improvement is related to weight loss, while others reported an independent benefit.14 Several pathways could be involved in this improvement, such as decreasing insulin resistance, modifying de novo free fatty acid synthesis, and improving mitochondrial function. On the other hand, high-intensity interval training has recently been recognized as an exercise modality that demonstrated an improvement in liver stiffness (-16.8%), these benefits appear to be independent of weight loss.15 It is important to understand that the optimal exercise prescription will vary widely among patients, depending on their physical ability, personal preferences, and even their environment.

Background Arterial stiffness and endothelial dysfunction present in metabolic associated fatty liver disease (MAFLD) confer increased cardiovascular risk, which represents the leading cause of mortality in this group of patients.

Mechanisms involved in the cardiovascular complications of MAFLD have recently been found to also affect cerebral blood flow altering cerebral hemodynamics in MAFLD subjects. These alerations can be detected through transcranial Doppler, which measures the medial cerebral artery blood flow and estimates the pulsatile index (IP) and resistance (IR), markers of cerebrovascular vasoconstriction These alterations are detected by transcranial Doppler, which measures the blood flow velocity of the middle cerebral artery and estimates the pulsatility index (PI) and resistance index(RI), which are markers of cerebrovascular vasoconstriction; and the respiratory retention index (RRI), which indicates cerebrovascular autoregulation. These abnormalities are related to vascular disease in MAFLD, which plays an essential role in ischemic stroke and cognitive impairment, which explains why MAFLD patients had lower scores on cognitive function tests.

Nonetheless, there are no studies evaluating the effect of lifestyle interventions (specifically exercise) on cerebral hemodynamics in patients with MAFLD, however, it has been shown that in other pathologies that share pathophysiological similarities with NAFLD there are beneficial changes in this outcome. An example of the above is chronic heart failure and liver cirrhosis, where physical exercise attenuates the inflammatory cascade (decrease in IL6, IL8, IL12, TNFa), and decreases the activation of the renin-angiotensin system with a direct effect on endothelial function improvement. A previous report showed that a 12-week physical training program acts on this mechanism and has a beneficial effect on cerebral hemodynamics evaluated by transcranial Doppler in patients with liver cirrhosis, which leads to an improvement in neuropsychometric tests.