For today’s edition of Dear Mark, I’m answering two questions. First up, what happens when a brisk walk isn’t enough to attain the optimal fat-burning heart rate zone? It’s a good problem to have—better fitness—but it still needs a response. What activities can a person do to slightly increase the intensity without going over the target heart rate? And second, are fermented foods a potential cause of depression? If they have any effect on serotonin, could this cause problems rather than improvements?