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I think the reason people fail in their diets is because they crash diet.  The trick is to stay above your Basal Metabolic Rate.  Some people go well below that and are starving, essentially.  This in turn sends their brain messages to binge and then they “fail” in their diet (really it’s not sustainable so they didn’t fail).  
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I also don’t think it’s helpful to assume people are overweight because they just eat junk food all the time.  Many people have hunger signals and cues that are completely out of wack from crash dieting and overeating, not to mention genetics.    

You don't crash diet until you're already overweight in the first place. And genetics just plain and simply cannot make you obese barring extremely rare disorders.
I'm not saying a few extra pounds at 40 means you're eating like ****. But severely overweight at 20 absolutely does.

Another factor in all of this is messed up hormones, specifically the thyroid, which regulates metabolism and energy.  Things which destroy the thyroid (and which are very difficult to avoid):
1.  Fluoride in water
2.  the flour used in most breads is also a thyroid inhibiter.
3.  Various other chemicals like MSG, which affects your adrenal glands, and indirectly hurts your thyroid because of the adrenal-thyroid connection.
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Bottomline:  When people have a weak thyroid, then their adrenals compensate to try to attain balance.  When people's adrenals get overworked, they feel tired, so they resort to caffeine, carbs or both for energy.  This whole unbalanced process leads to poor sleep, loss of nutrients and lots of stress.  
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Stress leads to further cravings for Vitamin C (most people choose sweets) and Omega 3 oils (most people choose bad, fatty foods).  This leads to weight gain and further stress on the thyroid/adrenals.  The cycle is hard to break.  But nutrition, avoiding thyroid inhibitors, and exercise are the key.
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Thyroid nutrients - Get IODINE through Kelp vitamins, or iodide supplements, detoxified iodine liquid drops (look up Edgar Cayce), or the supplement "Atomidine".
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Adrenal nutrients - Vitamin C.  Good, healthy salts ("real salt" company).  Good, omega3 oils.

The only diet that works:

Very funny.  Of course, there is a lot of wisdom in not going too extreme with calorie restriction.  People get impatient, so they cut down to starvation levels.  But the best way to lose weight is in fact to go no more than like 500 calories below your total energy expenditure.  Then it can become a life habit.  When people go into starvation mode, they inevitably crash and burn.  It also slows the metabolism, and you start losing muscle mass, so when the weight comes back, your body composition ends up being more fat than before.

Yes, counting calories is tough and can be inexact.  But there are some apps out there that can help you keep track.

Also you can buy a very good reverse osmosis water purifier on Amazon now for about $140.  And then try to get natural/organic stuff if you can.