You might think that the obesity epidemic has fueled a horde of weight-loss diets, but really the concept of a quick, relatively painless food prescription has been around well before the uptick in adiposity became a public health focus (think the Atkins diet from back in the '70s). Of course, some weight loss diets are supported by scientific research, but then many are not. And we must admit that in no other area of public health has pure creativity played such a seminal role.
So what characterizes a fad diet? I like Wikipedia's succinct definition: "A fad diet... is a diet that makes promises of weight loss or other health advantages such as longer life without backing by solid science, and in many cases [is] characterized by highly restrictive or unusual food choices. Celebrity endorsements are frequently associated with fad diets, and the individuals who develop and promote these programs often profit handsomely."
Some of these diets seem to have a scientific basis, though they really don't. My all-time favorite in this category is the blood type diet. Based on when in human history one's blood type evolved, one should supposedly consume only the types of foods that humans consumed at that time. For example, those of us with type O blood should limit or avoid grains, beans and dairy — because this blood type evolved before humans started using agriculture and animal husbandry to provide food. Sounds pretty science-y, no? Well, sure it relies on a bit of anthropology and paleontology, but neglects to include any nutritional science. Otherwise, it can be pretty restrictive and probably requires some supplements (which the author probably sells).
Then, there's the alkaline or pH diet. This one warns that your body may be too acidic, and you need to counteract this by eliminating foods — such as meat, dairy, caffeine, alcohol — because these lead to too much acidity. So the smidgen of science here is that indeed, some such foods will make one's urine more acidic. But they won't change the acidity (or pH) of your blood — there are built-in physiological mechanisms that tightly control this, and the pH of the body as a whole.
Moving even farther away from science, we find the venerable cabbage soup diet. No one seems to know its origin — at one time it was called the "Mayo Clinic cabbage soup diet" but the MC stoutly denied having anything to do with it. This involves eating primarily cabbage soup (the recipe varies) for a week, otherwise having only a banana or skim milk on occasion. This will work to help you lose weight — unfortunately it will be mostly water. And cabbage, being what it is, will also provide plenty of gas and bloating. Not a comfortable mode of weight loss.
Another oldie is the grapefruit diet — again one that has been around for decades (probably since the 1930s). Also wrongly ascribed at one time to the Mayo Clinic (wonder why they deserve it?), it is based on the supposed presence of "fat-destroying enzymes" in grapefruit (sorry, this isn't true). This diet came in 2 versions — a 7-day and a 21 day version. With either one, the dieter consumed little besides coffee and half a grapefruit at each meal, supplemented with a little salad and lean meat. This is a very low calorie diet (VLCD), and of course will cause weight loss, but possibly also nutritional deficiencies. That we haven't heard about these is probably because no one can stick to it long enough to have such effects.
And of course there are the magic ingredient diets — including things like white bean extract, raspberry ketones, green coffee, or apple cider vinegar in certain proportions — with or without certain foods — supposedly can make weight loss a sure thing — just ask Dr. Oz.
So these are just a few of the most intriguing ideas folks have come up with to help reduce their excess padding. And they all will work — since they will all restrict calories. Unfortunately, since one goes "on" a diet, one also comes "off" it — and the weight comes back on as the usual diet is resumed. Still, it's kind of amazing to see the sheer breadth of ingenuity that people can apply to the obesity problem. Too bad they won't work!