The Science of Sungrazers, Sunskirters, and Other Near-Sun Comets

This review addresses our current understanding of comets that venture close to the Sun, and are hence exposed to much more extreme conditions than comets that are typically studied from Earth. The extreme solar heating and plasma environments that these objects encounter change many aspects of their behaviour, thus yielding valuable information on both the comets themselves that complements other data we have on primitive solar system bodies, as well as on the near-solar environment which they traverse. We propose clear definitions for these comets: We use the term near-Sun comets to encompass all objects that pass sunward of the perihelion distance of planet Mercury (0.307 AU). Sunskirters are defined as objects that pass within 33 solar radii of the Sun’s centre, equal to half of Mercury’s perihelion distance, and the commonly-used phrase sungrazers to be objects that reach perihelion within 3.45 solar radii, i.e. the fluid Roche limit. Finally, comets with orbits that intersect the solar photosphere are termed sundivers. We summarize past studies of these objects, as well as the instruments and facilities used to study them, including space-based platforms that have led to a recent revolution in the quantity and quality of relevant observations. Relevant comet populations are described, including the Kreutz, Marsden, Kracht, and Meyer groups, near-Sun asteroids, and a brief discussion of their origins. The importance of light curves and the clues they provide on cometary composition are emphasized, together with what information has been gleaned about nucleus parameters, including the sizes and masses of objects and their families, and their tensile strengths. The physical processes occurring at these objects are considered in some detail, including the disruption of nuclei, sublimation, and ionisation, and we consider the mass, momentum, and energy loss of comets in the corona and those that venture to lower altitudes. The different components of comae and tails are described, including dust, neutral and ionised gases, their chemical reactions, and their contributions to the near-Sun environment. Comet-solar wind interactions are discussed, including the use of comets as probes of solar wind and coronal conditions in their vicinities. We address the relevance of work on comets near the Sun to similar objects orbiting other stars, and conclude with a discussion of future directions for the field and the planned ground- and space-based facilities that will allow us to address those science topics

Insulin resistance in nonalcoholic fatty liver disease

Abstract: Nonalcoholic fatty liver disease (NAFLD) refers to a spectrum of liver damage ranging from simple steatosis to nonalcoholic steatohepatitis (NASH), advanced fibrosis and cirrhosis. NAFLD is considered the hepatic component of the metabolic syndrome and insulin resistance represents its pathophysiological hallmark. Insulin resistance in NAFLD is characterized by reduced whole-body, hepatic, and adipose tissue insulin sensitivity. The mechanism(s) underlying the accumulation of fat in the liver may include excess dietary fat, increased delivery of free fatty acids to the liver, inadequate fatty acid oxidation, and increased de novo lipogenesis. Liver fat is highly correlated with all the components of the metabolic syndrome, independent of obesity, and NAFLD may increase the risk of type 2 diabetes and atherosclerosis. Overproduction of glucose, very low-density lipoproteins, C-reactive protein and coagulation factors by the fatty liver could contribute to the excess risk of cardiovascular disease. The reason(s) why some patients will develop NASH are poorly understood. Circulating free fatty acids may be cytotoxic by inducing lipid peroxidation and hepatocyte apoptosis. Insulin resistance is often associated with chronic low-grade inflammation, and numerous mediators released from immune cells and adipocytes may contribute liver damage and liver disease progression. Understanding the molecular mediators of liver injury would promote the development of mechanism-based therapeutic interventions. This article briefly summarizes the recent advances in our understanding of the relationship between NAFLD/NASH, insulin resistance and the metabolic syndrome

The effect of lifestyle changes in non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) is a clinical/biochemical condition associated with the metabolic syndrome. As the disease stems from excess calorie intake and lack of physical activity, the correction of unhealthy lifestyles is the background of any prevention and treatment strategy; drugs should remain a second-line treatment. Several studies have shown that weight loss and physical activity, the cornerstones of a healthy lifestyle, have a specific therapeutic role in NAFLD, preventing disease progression and reducing the burden of disease. Prescriptive diets have a limited longterm efficacy; after a short period, most patients resume their old habits and weight regain is the rule. Physical activity, usually in combination with diet, but also independent of weight loss, improves liver enzymes and reduces liver fat, with uncertain results on hepatic necroinflammation; however, making patients increase their physical activity is very difficult. Only a behavioral approach may give patients the practical instruments to achieve their eating and exercise goals, incorporate them into lifestyle, and maintain the results for a long period, thereby possibly guaranteeing longterm durability of change. Cognitive-behavior treatment should be provided to patients at risk of advanced liver disease, and this action should be coupled with prevention strategies at the population level. Only a synergistic approach and a global societal response might be effective in reducing the burden of advanced liver disease and premature death due to NAFLD/NASH (non-alcoholic steatohepatitis)