The basic principle is that instead of going to the trouble to invent a clever con, you invent a really stupid one, send out to a a large number of people and you will find the the occasional person dumb, curious or greedy enough to fall for it. If only one in hundred thousand people are stupid enough to fall for something you will still get ten responses to an e-mailing of one million.
The specific con varies - most commonly, people are asked for bank information so that money can be transferred in, the account is emptied instead. But once somebody is foolish enough to reply - any of a number of cons can be tried. The key point of it is to obtain not just suckers - but super-suckers.
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Ok - now re food questions. What changes constantly are the fads - ultra high carbohydrate, ultra-low-fat, ultra-low protein. The mainstream recommendation has remained, lots of veggies, a few fruits, whole grain carbs in reaonable quatities, veggie and fish oils in reasonable quanites, and protein either from veggie, animal or fishe source in reasonable amounts. Animal fats, sugars and quick absorbing carbos in very low qunaities.
As is pointed out if you eat more carbs than you can burn it gets turned into fat. If you eat more fat than you need it stored. If you eat very little fat, your makes fat - and of a worse type than if you ate fat. Fish oils and certain veggie oils (olive or safflower oild), and milk fat are good for you in moderate quanties. Protein is good for you, but you need 75 grams of protein at most. Unused protein is excreted. If you eat a lot of protein over what you need, you put a strain on your kidneys.
For diabetics: while it is true that carbs are what your body has trouble processing, it is also true that a diet extremely low in carbs actually makes it harder for your body to deal with carbohyrates and will make your diabetes worse. There is a good reason why the American Diabetic Association recommends a moderate carbohydrate diet rather than a low carb diet. (A high carb diet is of course deadly to a diabetic.) And yes carbs from green leafy vegatables have some of the lowest glycemeic affects, But certain whole grains really aren't that bad.
And whatever the balance between carbs, fat and protein - calories still count. If you are not active, and eat a 3,500 calorie a day diet you are going to be fat. (Unless you eat most of those calories in the form of fat free protein in which case you will end up with kidney damnage.)
And given that most coverage of diet - even in so-called respectable media - focuses on fad diets, you cannot blame people for being mis-informed. After all, the promotion of ignorance is what are media does best. Remember those studies that people who watched a lot CNN (and used it as their primary information source) knew less about politics than those who had no regular source of political information?