Interesting question, Payam. I suspect that your "face fat" is not fat, but subcutaneous fluid, for 2 reasons:

1) You said that it goes away in a few days. The speed of resolution suggests fluid, not fat.

2) Since "spot reduction" is difficult, I suggest that simultaneous spot reduction and "spot gain" of fat is well-nigh impossible.

My theory (this should not be construed as medical opinion or advice) is that carbs are acting as a diuretic for you (Payam, singular "you"). Bingeing on carbs may be dehydrating in some cases. The solution is to add back some relatively benign carbs to your diet, such as rice or sweet potato, or to find another diuretic. Reputed natural diuretics that you can try include dandelion, ginger, juniper, celery, onion, eggplant, asparagus, watermelon, hawthorne, corn silk and parsley. Also, you can try lowering your sodium consumption.

I eat alot of root vegetables, potatoes included, which can be pretty starchy. The paleo diet eliminates alot of unnecessary carbs like bread, sugar, some starches, pasta...stuff that didn't exist 100,000 years ago. It's semi-low carb by the natural process, but through veggies and fruit, you should be getting carbs, just don't over-do it. It's important to have carbs in our diet daily, not only for energy, but for hormone maintenance, particularly testosterone. So don't try to think of it in terms of hi or low carb, but be sensible about what you're eating from the paleo perspective.

I think Ed has some great points in support of the possibility of simple fluid gain rather than fat gain. Due to the speed at which it appears and resolves, fat does seem unlikely.

Reducing added sodium may be a very good idea. In addition, and of equal importance is ingesting sufficient potassium (K) in ratio to kcal intake and Na intake. It's very rare to find someone getting sufficient K intake without a specific effort to do so.

Setting aside the subq fluid possibility for a moment and considering fat gain, the first thing that comes to mind is cortisol. Some individuals, while in ketosis or readapting to ketosis will experience cortisol elevation for a time as the body tries to access fat for fuel. If for some reason, yours is chronically elevated, it would be possible (in theory) to see an increase in fat distribution in the same places one sees fat accumulation with oral steroid supplementation--ie the 'moon' face and fat storage (eventually) in the abdomen and upper body. It does seem to be a very unlikely possibility though due to (again) the speed at which it both occurs and resolves.

Perhaps there are some vitamins or minerals in the the carby things that you are lacking when you cut those carbs out. My advice would be to type in yoru food intake into fitday.com and see if you are low on any nutrients while eating lower carb. Another thing I would try is eating only one carb source, like say potatoes, and see if that fixes it. If it fixes it, then you know it is either the nutrients in potatoes or carbs themselves, but not sugars. YOu need to narrow down what exactly functions to fix the problem. This will give you a clue as to what is causing the problem.

Wow, very interesting question!

Recently I thought about it by myself. I wonder if it's possible to change one's face structure on a diet. I'm not just talking about losing face fat, but highlighting the overall bone structure (high cheekbones).

Maybe your body accumulates more fluid when carbs are too low, and therefore body temperature decreases. It's self-protection from the cold.

Look at Johnny Depp: http://www.thefablife.com/2010-05-14/sad-news-america-johnny-depp-is-now-old-and-ugly/ . His face changed a lot! I wonder how his diet changed.

here's my theory; you are gaining some facial fat during you're off times and when you lose it, the skin is left w/o the fullness and has no place to go but downward. maybe it's water maybe it's fat but whatever it is, it is filling you out during those off paleo times.