Mediocristan or Extremistan?

I woke up to a Gary Taubes' Twitter text with a link to A New Yorker article "Is Fat Killing You, Or Is Sugar?" by Jerome Groopman. It ended with a typical refrain of 'everything in moderation'. Granted it was very good at pointing out the complexity of diet and health but the refrain triggered my struggles with understanding Jensen's inequality as it applies to mediocristan and extremistan. These places are creations by Nassim Taleb author of The Black Swan and Antifragile. Ok, you are in mediocristan. You have to cross a river. So Jensen implies don't cross a river if on average it is 4 feet deep. I get it. Now that body that just avoided that river and did not die because it didn't cross is in extremistan when it comes to health and sickness. Applying the average/moderation may bring undesirable results because of the complex environment of extremistan. The application of varying lets say sleep, calories, fat, sugar etc. on a body can bring benefits or harm depending on your fragility to what you are varying. Each body is unique. So the typical refrain of moderation avoids the application of Jensen's inequality in extremistan.