I'll try to keep this one short.
Detox and cleansing diets are a scam.
Too bad that's not enough info to keep people from trying these silly diets. Not that going into detail why they are a scam or don't work is going to make a dent in the popularity of detox diets, but I'll give it a shot.
There are toxins in your body that only special detox/ cleans diets can get rid of.
These toxins make you tired, give you "brain fog", cause arthritis, and whatever long list of medical maladies the purveyor of the special diet makes for the day.
Is it true?
Can't be. Could it be that a constantly lousy diet devoid of all the essential vitamins and minerals, as well as too many calories, poor fitness, and poor sleep cause people to feel tired and worn out?
Adress these variables (you know, eat right and exercise, get a good nights sleep), and.. don't be shocked here.. a person will instantly start feeling better. No whacked out short term diet needed.
If a person was having an actual medical crisis where toxic levels of metabolic waste products were accumulating in the body it would mean a trip to the emergency, not going on a detox diet.
Frequently these diets also claim that the body will be too acidic and will need to be brought back into a less acidic state through eliminating fruits, nuts, grains, proteins and whatever else the scam artists arbitrarily decides to add to the list.. although they will try to make "scientific" rationalizations for these recommendations.
Trouble is there is no research that shows these diets have any detoxification affect, or any positive health affect of any kind.
Some of these diet take another approach and don't recommend a severe restriction of any normally healthy foods such as fruits and grains, but rather suggest you cut coffee, excessive alcohol, refined foods, and decrease portion sizes, and eat more veggies, nuts, grains. This "special" diet (which is really a simple healthy diet) will "detoxify" your body and make you feel good.
No, a healthy diet will make you feel good because you are shifting from not meeting basic dietary needs to meeting dietary needs, not because of a "detox" effect.
Detox is taken care of largely by the kidneys, liver, and lungs. If these organs were not working properly you would need a doctor, not diet, although of course anyone with an unhealthy diet will benefit from switching to a healthy diet.
These diets are short term and extremely restrictive. They usually attract those who believe that since nothing they have tried has worked for them yet, maybe this will help. It's the promise of hope; the hope to feel good, the hope to rid the body of these claimed toxins, the hope of something better in return for a short term effort.
The reality is most who feel they "have tried everything", have either only tried other fad diets that can't work anyway, or have not stuck to healthy habits long term.
Wikipedia has some good notes on these diets and what acidosis really is..