I personally have found that I do feel "run down" which turned out to be lower blood pressure. I attributed this to low sodium intake. So, I drink two very low sugar (6g) recovery drinks for my first caloric intake around noon.
One thing that wasn't covered, and likely isn't in many of these weight loss recaps, is not just caloric deficit; but also food content. Sugar's (and carbs) relationship to fat and our bodies cannot be understated. There's plenty of reading on how high sugar diets are detrimental. Here's an interesting listen: https://www.foundmyfitness.com/episodes/refined-sugar
This image alone scares the snot out of me: http://reachingutopia.com/wp-content/uploads/2013/06/sugar-i...
I think most people are aware of this in principle, but don't often associate normal foods as being high in sugar. Orange juice is probably the worst offender. All the sugar, none of the fiber.