I'm a 74 yr old cyclist. Over the past few years, I have been noticing a gradual decline in my ability to go longer distances at a reasonable pace. Of course, a certain amount of decline is 'expected' due to age. Then I tried CircO2, and began to notice a gradual improvement. Now my ability is once again more dependent on my training instead of my advancing age. At this age, I get a constant bombardment of products pitched; you would think that given enough money, I should be able to live forever in perfect health. It is just nice to find at least one product that seems to do what it says it will.