What if the difference is because of pre-existing health state and conditions of those people i.e. those who's health is poor enough they are likely to die soon, just don't have energy to walk a lot?
Meaning, this may be a classic case of correlation which is not a causation.
I would speculate that in particular case of the U.S., there is also a social factor in play that is, people with different health outcomes and longevity are also people from different social classes who may or may not have time and space/environment appropriate for these activities?
This is addressed in the full text, which you can find on a certain hub.
>The inverse association persisted in the subgroups defined by age, geographical location, follow-up duration and devices used for counting steps, as well as after adjustments were made for important confounders including body mass index, alcohol drinking, smoking status, step intensity, and pre-existing comorbidities
>These findings were supported by the findings of intervention studies wherein increasing steps per day resulted in significant improvement in cardiometabolic risk factors [41, 42].
Less obvious but still interesting: At which point does diminishing returns set in? At some point does one extra hour of exercise yield less than one hour of increased average lifespan, and so not worth the additional effort, for someone that doesn't find exercise to be intrinsically rewarding?
Not at all. The only question is whether number of steps is a valid measure of how much exercise you're getting. Higher step count means people are walking more which means more general activity which is good. However, there are other relevant factors to consider such as walking speed, cardiac frequency, the intensity of the exercise. There are also other ways to exercise which don't involve steps at all. Weight lifting for example.
In other words, people who can’t walk or can barely walk are likely not that healthy - is probably a more intuitive theory than walking causes better health.
> a more intuitive theory than walking causes better health.
Why? Genuine question. In todays world, people drive themselves in cars and then park themselves in chair for hours. Then, on other end there are people having physical work that is literally body destroying - or people who literally go too much all in sports.
Why would it be unintuitive that normal walking around, mild exercise related to easy day to day activity would be better for health?
Obviously that will be the case to some extent, but it doesn't necessarily explain the whole effect, given this part:
> Dose-response meta-analysis indicated a strong inverse association, wherein the risk decreased linearly from 2700 to 17,000 steps per day.
I expect (especially in the US) that many or even most people who could walk several thousand steps a day, don't. They talk about the 70+ age group separately, but most of it is talking about the general population and says the relationship holds.
Of course, this meshes with what we already know about the clear links to exercise causing better health - not just things like balance, less risk of injury, bone density etc. in older people but for anybody prevention of or improvements to things like non-alcoholic fatty liver disease (combined with diet), improvements for those with metabolic syndrome, etc. which all contribute to a lot of early mortality.
> It’s entirely possible that people who are healthier simply tend to walk more than people who are less healthy, and daily step counts are therefore merely a proxy for general health, and don’t have an inverse causal relationship with all-cause mortality. However, I don’t think that’s the case – at least not entirely. For example, a 2015 meta-analysis found that group-based walking interventions, all lasting one year or less, led to significant decreases in systolic blood pressure, diastolic blood pressure, resting heart rate, body fat percentage, body mass index, total cholesterol, and depression scores, while increasing VO2max, 6-minute walk distance, and score on the SF-36 physical functioning inventory. Most walking intervention studies don’t use particularly strenuous walking programs either – generally 20-30 minutes of walking per day, which works out to ~2,400-3,600 steps for most people. So, if a bit of walking can beneficially modify ten different risk factors for all-cause mortality in less than a year, I think we can make a pretty strong case that the inverse relationship between step counts and all-cause mortality is more than mere association.
> those who's health is poor enough they are likely to die soon, just don't have energy to walk a lot?
True, and there's a feedback loop in there as well. Being inactive leads to significant losses in muscle mass which leads to even more inactivity which compounds the problem. Sarcopenia is a serious problem especially in the elderly.
>The HR was 0.87 (95% CI 0.78–0.97, I 2 = 59%, n = 3) in adults older than 70 years, and 0.88 (95% CI 0.82–0.95; I2 = 86%, n = 4) in the general population of adults (p subgroup difference = 0.11).
Meaning, this may be a classic case of correlation which is not a causation.
I would speculate that in particular case of the U.S., there is also a social factor in play that is, people with different health outcomes and longevity are also people from different social classes who may or may not have time and space/environment appropriate for these activities?