The practice of heterochronic parabiosis, linking the circulatory systems of old and young mice and observing the results, produces measurable beneficial changes in the tissues of the old mice. Researchers have identified a few proteins such as GDF-11 where changes in the circulating levels occur with age, and artificially resetting these levels - such as via parabiosis - can alter the behavior of stem cells to make them more active in tissue maintenance. A range of other experiments are currently ongoing to try to better understand and catalog these results first identified via parabiosis studies. Some of these experiments are going to produce null or ambigious results, as is the case here, because nothing in biology is simple or straightforward. You'll probably want to skip ahead to the discussion section at the end of this open access paper, as that is where the interesting information can be found:
Aging is for now an irreversible process that affects multiple organs and is the leading cause of age-associated mortality and morbidity. The search for an efficient way to counter age-related changes in an organism is a task of high importance. Recently, scientific evidence of a rejuvenating effect of young blood on different tissue and organ functions was published. Among these studies, heterochronic parabiosis was particularly interesting: the model demonstrates the possibility of constant exchanges of cellular and humoral factors through the blood between animals of different ages.
Still, after such mostly positive reports, the question of the overall beneficial effect becomes extremely intriguing: Instead of looking at a specific parameter or a short period of time, is maintaining a young milieu globally beneficial over time? This can be asked through a simple measure: Does it increase lifespan? In previous experiments, we looked at the survival of mice following temporary isochronic and heterochronic parabiosis (unpublished data). It was found that aged mice tended to live longer after a period of heterochronic parabiosis than isochronic parabiosis, suggesting a globally beneficial effect of the young milieu. However, the difference was not statistically significant, and lifespans were not in the range of those of untreated animals, possibly due to the traumatic condition of parabiosis. Therefore, general conclusions were very uncertain regarding "anti-aging" effects, and another model for long-term effects was sought, namely, plasma injections.
To assess the anti-aging effect of young blood we tested the influence of repeated injections of plasma from young mice on the lifespan of aged mice. One group of 36 CBA/Ca female mice aged 10-12 months was treated by repeated injections of plasma from 2- to 4-month-old females. Their lifespan was compared to a control group that received saline injections. The median lifespan of mice from the control group was 27 months versus 26.4 months in plasma-treated group; the repeated injections of young plasma did not significantly impact either median or maximal lifespan.