Humans don't have a way to comprehend good health except through its experience.
Currently, digital products such as fitness trackers only provide us with a symbol of health - such a number, a score, or a badge. We experience a symbol of health more like a reward, than like as an immersive experience - just as the experience of a photo is not the same as the experience of the location in the photo. Our cognitive system simply doesn't work that way.
So a symbol of health is a sign of progress or lack thereof, not progress itself. It is a landmark of health, not the landscape of health itself. All that a symbol that comes from doing a 'healthy behaviour', such as walking the treadmill, provides is - a picture of health on a scale. A non-professional human being approximates the reading on this scale in experiential terms quickly in terms of 'lousy', 'ok', 'good', 'great, etc. .
So the user reads the symbol ('number of calories burned' for example) that she sees on her device or app as an emotional 'ah, thats good!' or 'oh crap! that's not so good'. She then instinctively seeks the real evidence of the gains or loss indicated by the symbol in her life experience that follows'. If she feels good, then she attributes it to her treadmill walking, and compares the feeling to the score she got.
This repetitive cycling through : behaviour performed > symbol obtained > life experienced - is the core behavioural pattern that drives the adoption and entrenchment of health products.
Users who experience plateauing or decline in the value gained from symbol / score driven-products at their core, such as the Nike Fuel Band or Fjuul app are basically unable to anymore find consistency between the score and the lived experience of health.
There are fundamentally only three core health experiences: the experience of deterioration, the experience of progress, and the experience of being in the moment ('state of flow'). Of these the first two are reflective states, and only the third is really perceived as an 'experience' by the human.
Gamification-driven health offerings need to be designed carefully keeping this behavioural model in mind, for they embed a social reward of acceptance or rejection into the symbol. The product needs to evolve over time to support the actual behaviour and goals of the user - is it he social reward or the tangible health experience, or a combination?
Users would form such cohorts over time. Many would drop off entirely, and others would learn to find a replacement reward and start to perform a real world or digital behaviour that would function as a proxy for the behaviour originally developed on the product. Others would sink into apathy from lack of progress and social reward. Especially those that opted in just for the social benefits - from peer pressure in an office setting is a common example - will be the most affected in a non-optimal way in the long run, or worse, may even start to openly dislike / become the product's critics.
Some users will simply start to take for granted the abstract symbol as evidence of good health. Just seeing the number or score would start make them feel better. Drivers for such associations could be the brand, peer sentiment and reinforcement, etc. This is a case where the user changes her perception to match reality, enabling herself to 'feel' better when a goal is reached. She'd learn to infuse that feeling of wellness into her life context immediately following the reward, and over time learn to enjoy it, then start to seek it out. These behaviours start to become the foundation of habits, lifestyle changes and ultimately such products start to be monetised over time.