My hypothesis is that, people who've been cutting back on an essential nutrient for a while do better if they get more of it.
Thus, if you've been trying the low-fat diet for a while, then your body will appreciate the change when you finally start giving it enough fat.
If you've been on a low-protein diet (yes, it existed for a while), then your body will appreciate the change when you finally start giving it enough protein.
If you've been on a low-carb diet for a while, then your body will appreciate the change when you finally start giving it enough carbohydrates.
I read some scientists a few years back who suggested instead of looking at whether we should cut back on particular nutrients, we should instead look at the quality of the nutrients we are getting. Because all three nutrients are essential.