Because running can reduce the muscles of the whole body, especially for people who often run long distances.

Running for a long time often consumes a lot of protein in the body, and the most important substance for synthesizing muscle fiber is protein. When the intake of protein can't meet the needs, it will consume muscle tissue and make people thin.

Suggestion:

If you don't want to lose weight, you can usually run for 30 minutes. If your body fat content is not very high, you don't need to run every day, you can run every other day or two.