Fasting for stem cell rejuvenation

searched for ways to live longer and healthier lives. From the "fountain of youth" quest to novel senolytics, from alchemical recipes to modern diets, different approaches have been pursued to fulfil the human desire of prolonging life while maintaining good shape. Among all anti-aging interventions, calorie restricted diets and periods of fasting stand out as the most compelling and robust methods to prolong life and health span and to reduce the risk of diabetes, neurodegeneration, autoimmune disorders, spontaneous tumours and cardiovascular disease [1]. Furthermore, dietary interventions are also emerging as important enhancers of adult stem cell function [2]. However, little is known on how prolonged fasting alters the function and properties of adult stem cells. Since fasting outcomes are conserved across taxa [2], studying fasting in species that possess many stem cells and can cope with long periods of food deprivation can be exceedingly informative. Planarians -better known for their impressive regenerative capacitiescan be deprived of food for more than 3 months without showing an impairment in either physiology or activity levels. They handle prolonged periods of starvation or fasting by shrinking in size. Around 25% of the cells in their parenchyma are adult stem cells, which are kept in a constant ration respect their body size. Interestingly, refeeding allows fasted planarians to grow back to their original size [3]. Their stem cells do not show any signs of senescence and hence they are considered immortal. How fasting influences planarian stem cell properties is unknown. We have recently reported the effect of fasting on planarian stem cells regarding telomere length [4]. Telomeres protect chromosomes from DNA degradation and misguided repair mechanisms. Proper telomere functioning requires a minimum length that is maintain by telomerase. However, telomerase activity levels in adult tissues are not sufficient to prevent progressive telomere shortening with age [5]. Therefore, telomere length is considered a cellular marker of aging. By measuring telomere length in situ on whole planarians we found that fasted planarians present a higher percentage of stem cells with the longest telomeres, indicating that fasting rejuvenates the stem cell pool [4]. Having a population of stem cells with very long telomeres allows planarians to quickly respond to any Editorial