Lengthy cardio training makes you fatter. This includes running. If there is no strength training included in a chronic cardio trainers' program, indeed, that person will get light, because he or she is burning off muscle and bone tissue. In 2006 a massive review of over 12,000 runners helped clarify many misconceptions about chronic aerobic exercise, especially running: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2864590/.
The summary is that as we age, we must become MORE active (and there is still a reduced yield of returned results), and that even when chronic exercisers stay very active and maintain the same weight they have a statistically significant increase in waistline. This is why people find that they keep attempting the method which “worked” when they were 20 years younger, and they just keep getting worse results.
It’s not so much that aerobic exercise is always inherently obese-making. It’s that aerobic activity is purely CATAbolic. It breaks down everything, including organs, bones, tendons, ligaments, and muscle. Thus, as we age, we are increasingly liable to lose only lean tissue when we lose weight, resulting in an ever-fatter frame, no matter how hard someone tries. This is the case even for elite ultra-endurance athletes: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3794477/. In fact, they get relatively fatter in only 5 days of an ultra-run. Most people wouldn't notice, since they begin with fairly lean frames. But endurance athletes go UP in body fat percentage the more extensively they train or compete.
How do we combat this?
A small portion of it is an inevitable function of aging. Systemic IGF and other hormones which keep us healthy and protect lean tissue WILL drop as time soldiers forward. This is an incontrovertible biological fact. So we must do everything we can to emphasize overall health and fitness. This means sleep. This means rest. This means recovery. This means heavy weights and sprints. This means protein. This means holistic sensibility.
To clarify, since your body is aching to get worse, you have to send signals which overcome that. This means intensity of exercise MUST go up, not down. And that, of course, will mean more recovery is needed and more nutrients to recover are needed.
In the 2006 survey of over 12,000 runners, we find that even the most avid runners, logging many miles per week, do not IMPROVE on average. If they’re lucky, they maintain weight, but keep losing lean tissue anyway, which ultimately makes them fatter, if only a little.
Sure. There are outliers. But the primary takeaway is this: aerobic conditioning is best utilized to improve cardiovascular health. Body composition is best changed through heavy resistance training/sprints and nutrition. If you love to run, run. If you want to get leanER, don't run.