One of the primary rules of the body is “Use it or lose it”. In other words, the body will typically stop devoting resources to structures and functions that you aren’t using. There are several examples of how this works.
Probably the most familiar example is our muscles. If we don’t exercise and don’t use our muscles, they gradually get weaker and weaker over time. I see this quite often in patients who are particularly sedentary. Some will come in concerned about weakness they’ve recently noticed in their legs and are worried that they are developing some dreaded neurodegenerative disorder like MS or Parkinson’s. While these conditions do of course need to be ruled out, most of these patients don’t have anything abnormal going on at all. Declining strength is perfectly normal when you don’t move around very much.
As with muscles, bones require activity to stay strong and healthy. Much attention has been placed on calcium intake as the means to prevent osteoporosis, but calcium supplementation without also doing weight bearing exercise is minimally effective at best. Bones are in a constant state of turnover, with a balance between bone formation and bone resorption under normal conditions. Without exercise to stimulate bone formation though, bone resorption will overcome bone formation, and the bones become thinner and thinner.
Another common scenario is in people who use antibiotics for every little sniffle. Over time, their immune systems get weaker and weaker. Meanwhile, the “bugs” become more and more resistant to the antibiotics, so over time such people tend to just stay sick most of the time. Getting sick is a part of life, and if you want a healthy immune system, you have to let it do some work once in a while. Antibiotics may be neccessary to help overcome the rare severe infection, but routine use of antibiotics is extremely counterproductive in the long run.
One last example (although there are many others) is sexual health. Lack of regular sexual activity in males is associated with prostate enlargement and erectile dysfunction. In women, it is associated with poor lubrication and associated pain with sexual activity.
What it comes down to is if you want to live a healthy life, you have to be active.