A fad diet is any diet plan that is promoted by publicity and word-of-mouth rather than scientific study. Most such diets focus around weight loss, but many claim to cure diseases such as autism, attention-deficit hyperactivity disorder, and even cancer.
Such diets are often created by a single person, sometimes a medical practitioner of some sort, but often a lay person with no particular credentials. Some are often propagated as anonymous faxlore or email lore in much the same manner as many urban legends, but others are distributed and heavily promoted by major media corporations.
The diets' effectiveness is generally anecdotal and the claimed mechanisms a mix of plausible could-be science and fairly blatant magical pink unicorns. Actual validity (or lack thereof) is often established or disproven long after the diet has become popular due to the lack of peer review in the popular press, especially since many fad diet promoters have thrown in their lot with alternative medicine.
 Where fad diets come from
Fad diets are generated from a confluence of sincerity and stupidity:
- Person does thing A.
- Shortly after, person experiences thing B.
- Person concludes A caused B.
- Person writes book generalising this as the solution for everyone else in the whole world.
This process produces diets suffering certain fairly obvious and important epistemic and scientific deficiencies. People who think correlation equals causation tend not to be very good at understanding medical or scientific literature in a useful manner. Compare broscience.
Generally, when you remove the Snake Oil from any "fad diet" and read the fine print, you'll find that most fads suggest that you drink at least 8 glasses of water, take a vitamin, and "get plenty of exercise and sleep." These habits, in and of themselves, are often enough to lose up to 10 lbs (and often quite quickly) due to the way the body handles water. This "water weight" gives people the sensation that the diet is better than anything they've ever tried, and they rush off to Facebook to share the good news. Of course, two weeks later, the weight is back on, but that's "surely not the diet's fault... I just started to eat real food again."
Some fad diets emerge out of real medical needs. The gluten-free diet is the only current method of dealing with coeliac disease, but some woo promoters have "concluded" that if gluten is bad for some people, it must be bad for everyone. This is especially popular amongst Autism denialists.
 Weight loss
Civilisation has more or less solved the food problem (barring occasional distribution problems), but our genes don't know this, so we pack on the fat in anticipation of lean times that never come. To lose weight in a world of abundant food, we need to behave unnaturally. And different people's metabolisms seem to require different unnatural courses of action.
Fad diets for weight loss tend to involve cutting out a swathe of the local typical diet in its entirety, usually with associated calorie restriction or exercise.
The most high-profile fad weight loss diets tend to be associated with a specific personality or catchy name. Currently popular ones include low-carb diets like the Atkins diet (based on drastically reducing carbohydrate intake), the South Beach Diet by Dr. Arthur Agatston, and the Zone Diet. The Jesus Diet, using ingredients and foods mentioned in the Bible, exists in several different versions. The Lemonade Diet has gone through on-and-off periods of popularity since the 1970s. The A.T.W. Simeons diet, which relies on a highly restrictive 500 calorie/day diet plus daily injections of the HCG hormone, has recently come back into popularity thanks to promotion by Kevin Trudeau. Past examples include the Hay diet (separating meat and vegetables to avoid supposedly-unfavorable acid imbalances), the Beverly Hills diet (eating nothing but fruit for 10 days), and Fletcherism (chewing your food for way too long).
Some of these diets do result in weight loss if followed, usually just because of reduced calorie intake. It's always easiest to follow a diet at the beginning, when your motivation is high and your glycogen stores are still sizeable. In fact, weight loss during the first week or two of any restricted-calorie diet is almost always going to be precipitous, since the weight you lose in that early phase is water weight rather than fat.
Some of the diets are completely worthless even for weight loss. Even those which are effective often do not provide a healthy intake of necessary nutrients, or potentially unhealthy levels of some substances (such as saturated fats in the case of some low-carb diets).
Most fad diets worked for their author, per the four-step process above. If your metabolism just happens to match theirs, great! If not, then it must be your fault for being noncompliant.
Bottom line: Long-term weight loss comes about by permanently reducing calorie intake and increasing physical activity. Be wary of anyone promising fast weight loss from any other method.
These are similarly lacking in scientific substance, but also claim to build muscle, usually through lots of protein. Most are made of broscience.
 Health risks
The health risks associated with fad diets overseen by unqualified 'dieticians' were highlighted by a recent BBC News article which concludes with a quote from kidney specialist, Professor Graham MacGregor, saying that:
“”In normal circumstances, then people should drink when their body tells them to — when they get thirsty. Anything else is completely unnecessary, and will just leave you standing in the queue for the toilet. Detox diets are a complete con in that respect.
 Examples of Fad Diets
- Raw foodism - The belief that cooking or any kind of processing "kills" the nutrients in food.
- Macrobiotics - Expanding the concept of yin and yang to food and is all about "balance".
- Alkaline diet - Foods affect the blood pH level, really, really, they do.
- Fasting - Starving the body, especially if combined with cleanses is a wonderful way to not only lose weight but stay healthy!
- Jesus Diet - What Would Jesus Eat!
- Lemonade diet - Drinking lemon juice, maple syrup, cayenne pepper, and water will cure all your ills.
- Low-carb diet - Carbs are more evil than Satan. Low-carbing ranges from a more healthy approach like South Beach, to a truly heart attack generating approach like Atkins.
- Paleo diet - Obviously, modernity sucks. We should live like our caveman ancestors and we will all lose weight.
- Weigh down diet - Pray yourself thin.
- The Virgin Diet - You secretly have food sensitivity which is giving you all sorts of inflammation, which makes you fat. The problem is 7 foods which you need to avoid, the identities of which you'll just have to buy the book to find out about. Named after the author, JJ Virgin, it presumably can be followed even by non-virgins, and (we hope) the name doesn't refer to the type of food to be consumed.
 See also
- A humorous website on fad diets run by ShopInPrivate.com
- One blogger's listing of 25 of the most ridiculous diets ever
- "A Steady Diet of Detox": Forbes.com article (via MSN Health & Fitness) about extreme detoxifying diets.