How Much Cardio For Longevity
Meta-analyses of RCTsI found there meta-analyses of the effect of exercise on all-cause mortality:
|Year||# of Trials||Risk Ratio||Source|
|2016||63||0.96 (CI = 0.88 - 1.04)||Anderson|
|2011||47||0.87 (CI = 0.75 - 0.99)||Heran|
|2004||48||0.80 (CI = 0.68 - 0.93)||Exercise-based rehabilitation for patients with coronary heart disease: systematic review and meta-analysis of randomized controlled trials|
and all cardiovascular mortality:
|Year||# of Trials||Risk Ratio||Source|
|2016||63||0.74 (CI = 0.64 - 0.86)||Anderson|
|2011||47||0.74 (CI = 0.63 - 0.87)||Heran|
|2006||19||0.72 (CI = 0.55 - 0.95)||Unal|
|2004||48||0.64 (CI = 0.61 - 0.96)||Exercise-based rehabilitation for patients with coronary heart disease: systematic review and meta-analysis of randomized controlled trials|
The main downside of these meta-analyses is the same as the downside of the trials: they are overwhelming performed on older people with significant medical issues (usually cardiovascular) since they are easier to recruit to exercise for years and also yield higher death sample sizes. However, there use makes it unclear whether the conclusions generalize to the general population.
Longitudinal studies offer a way to overcome the problem of extremely skewed samples, but in looking at them we give up the ability to readily infer causality.
Inferences of causality are always troublesome in longitudinal studies, but in this case the issues are even more pronounced: as people approach death they frequently are less able to exercise, which means even very good controlling for confounders is basically guaranteed too fail.
Nevertheless, here are some brief summaries of a few:
The authors of one study Lee followed 13,485 Harvard Alumni men. After controlling for age, BMI, smoking, alcohol, and how many of their parents died before age 65, they found that exercising more than 1000 calories per week predicted 23% lower mortality relative to less than 1000 calories per week. There was essentially no gain from exercising more than that.
The authors of another study Hyde followed 16,936 Harvard Alumni men. After adjusting for age, blood-pressure status, cigarette smoking, age of parental death, and whether they had gained weight since college, they found burning 500-2000 calories per week corresponded with 1.29 years of additional life while burning 2000+ calories added an additional 0.86 years (see Table 4).
The First National Health and Nutrition Examination Survey
The First National Health and Nutrition Examination Survey followed 9,790 people Fang. After controlling for age, gender, race, income, education, diabetes history, smoking, blood pressure, cholesterol, eating disorders, caloric intake, and BMI, they found mortality decreased by between 13% and 53% between the people who described themselves with "much exercise" and "little or no exercise" (where the only other choice was "moderately active"). The difference between "much" and "moderate" exercise was between -8% and +26% (see Table 2).
The Oslo study Holme followed 14,846 men between the ages of 40 and 50 for 12 years. After adjusting for age, education, smoking, diabetes, history of heart attack, and history of stroke, they found both light-intensity exercise, hard-intensity exercise, and degree of leisure all correlated with reduced mortality. You can see Table 2 for details, but the tl;dr is that hard intensity exercise only correlated with reduced mortality for the first ~hour per week.
Since no one has studied how quantity of exercise affects mortality in RCTs, we're left groping in the dark with only longitudinal studies for company.
This chart summarizes studies Lee, Hyde, and Holme (assuming 500 calories is equivalent to an hour of hard exercise):
If you treat time spent exercising as equivalent to death, we can use the social security actuarial table Actuarial Life Table and these longitudinal studies to estimate the optimal amount of exercise per week. For a 25-year-old, the numbers come out to 0.5 hours, 3 hours, and 3 hours (for each study respectively).
However, we know that the longitudinal studies overstate the benefits from exercise because RCTs generally only find a ~13% reduction in overall mortality rather than the ~25% found in longitudinal studies. This suggests longitudinal studies overstate the effect of exercise on mortality by roughly 2x.
We can naively adjust for this by cutting the benefits suggested by longitudinal studies in half. Curiously, this doesn't actually change the optimality estimates.
Exercise, Mortality, and Age
The conclusion above suffers from two major problems. The first is rather obvious: trying to infer causation from correlation. The second is less obvious: how does exercise's effect on mortality change as we age?
The naive model is that exercise simply scales mortality. For instance, a 25-year-old has about a 0.08% chance of dying this year. 3 hours of weekly exercise might reduce that to 0.07%. We can compute the expected years of life gained by multiplying their life expectancy (54.7 years) by the change in mortality risk (0.01%) to get ~2 days. 3 hours of weekly exercise takes 10 days of conscious time per year (assuming 8 hours of sleep each night). This makes exercise look like a bad deal for the young.
Fortunately, we don't have to fully rely on the naive model since this question was briefly examined by Hyde in Table 4. They generally find that the gains in years of life decrease with age, with the 75-79 age group seeing about 5x smaller benefits than the 35-39 age group. If we assume exercise reduces mortality risk by a constant multiple, then we'd expect this number to be around ~2x. From this it appears (interpreting correlation as causation) that exercise is significantly better for older people than you'd expect even after accounting for their general increased mortality.
Still, this is all based on correlational evidence. It's possible, for instance, that older people simply die of things that make them stop exercising for years while younger people die quickly. It's also possible that exercising when you're young causes your mortality to be lower when you're significantly older.
I'd say, though, that the balance of the evidence suggests otherwise: that cardiovascular exercise probably isn't a terribly effective way for young people to live longer. It is, however, probably very effective for older people. Still, given the quality of that evidence, this conclusion is anything but certain.