I do love a good diet bun-fight thread (make my bun a paleo, coconut, grain free one! 
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MBJ How is eating 1800 calories a day 'depriving' yourself?
It 'feels' that way, because I'm always hungry. It's bringing me down
Well, easy to see why you feel deprived and hungry on the diet you described - very processed, high carb and too little fat and protein. Honestly I dieted that way, "the proper way" ie low fat, low protien, high carb, high grain, for 20 years are got fatter and fatter and sicker and sicker (which is what is happening to all western populations!). Now I'm Paleo (7 years, started as low carb, developed into Paleo) and 6 stones 
lighter, and a whole lot healthier.
Eating the way we do now as a society, chronically screws up our bodies insulin response, and as a knock on effect, leads to a lot of metabolic disorders and autoimmune disease, including, but not limited to -obesity, diabetes (type 1 and type 2), gallstones, gout, non-alcoholic fatty liver disease, heart disease, basically all the "diseases eof civilisation", which were very rare or non-existent in pre-agricultural humans. Western eating habits are not natural or healthy.
WRT Grains - these are a very recent addition to the human diet, before agriculture, probably small amounts were gathered by people, but only after cooking was developed - raw grains are indigestible. Also the grain we mostly eat now (semi-dwarf wheat) is hideously unnatural and was developed only a few decades ago. It it a hexaploid variety (6 sets of genes), and contains a lot of novel compounds which seem to make it much more inflammatory than ancient grain species (which are not very good for us either!)
So why is 5:2 better than a "traditional" low fat diet? Probably because it improves you insulin response because on 2 days a week, if you are sticking to around 500 calories, you will be pumping out a lot less insulin and this is a good thing for your whole biochemical system. On 5 days a week you will be eating a decent amount of fat (much better for your insulin response than carbs).
A 7/7 low fat, low protein, high carb diet EVEN WITH EXACTLY the same amount of calories as the 5/2 diet (over the course of the week), will spike your insulin far more, leading to constant hunger (but more importantly it's potentially damaging your health).
Considering doing 5:2?
IMO, 5:2 in conjunction with paleo/primal eating sounds like a very natural way to eat, but I don't think I'd recommend anyone to do 5:2 with a "normal" junk food - you are not going to get the best results to your health, or your weight.
At the very least, 5:2 should be combined with eating as much fresh and cooked from scratch food as possible, even if you don't want to cut out carbs from grains and do the whole paleo thing (yet! 
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