They've been with us as long as we've had stable agriculture. Humans seem hardwired to want to look healthy, wealthy or wise - however that "look" is conveyed in a given culture. (Remember than sun-tans, for instance, for most of human history have signaled poverty, linked with toiling in the sun. Only recently have tans been associated with leisure time away from desk work. Or Renaissance paintings and sculptures depicting full-figured women who clearly enjoyed more-than-sufficient food security as the ideal of health and beauty, before empty calories and sedentary work made a trim figure the object of desire.)
And we always want the quick fix, the easy solution, the miracle pill or one-week regimen. All the while, really, deep down, we all know they aren't healthy. And they aren't really a solution at all.
At any given time, there are hundreds (thousands?) of fad diets on the market, on bookstore shelves, in the news and our internet feeds. Each one offering quicker and more drastic results than the last. But the reality is we can't spend one month (or less!) fixing what we've spent years doing to our bodies. Real change requires consistent, long-term behavioral changes - new habits, and new ways of seeing ourselves, our lifestyles and our food.
We all know the more quickly a diet "works" the less healthy it is. But we might not know why. What many quick weight loss diets do is strategically starve the body of nutrients to induce a natural process called "ketosis", when your body burns energy stored as fat.
Ketosis, while a natural process, is the body's response to duress - a famine that the human body in engineered to survive by calling on its "rainy day" fund of energy: fat. This self-induced food shortage wreaks havoc on blood sugar levels. In the event of a genuine famine, blood sugar isn't a high priority. But spiking and plummeting blood sugar has been linked with a number of diseases like hypertension, high blood pressure, chronic fatigue syndrome and a litany of others.
Our typical diet even when eating "normally" is doing this up-and-down routine for most of our lives. Exacerbating this with faked starvation only increases the pressure on our heart, liver, spleen, other organs and muscles, and even our brain!
Ideally, these diets starve the body of only macronutrient calories. A common side-effect of quick-weight-loss diets, however, is that we inadvertently starve our body of essential micronutrients. It is very difficult to get all the vitamins, minerals and fiber we need when severely restricting our diet. Even when taking supplements our bodies don't often get the benefit of them because of what is called "bioavailability."
Bioavailability is a better indicator of what is important, because it focuses not on the mere presence of these micronutrients but on their availability to be absorbed and used by the body. The body needs certain elements in order to absorb, process or utilize other nutrients. (Calcium, for instance, can't be absorbed without vitamin D, which is why "fortifying" milk is important to make the calcium accessible.) Just swallowing a pill isn't enough to make those micronutrients "bioavailable." Vitamin supplements as part of a healthy, varied diet are in a much better position to do the body good. Fad weight loss diets generally aren't very good at providing the kind of variety and quantity of micronutrients the body needs to function well, because they are focused on restricting caloric intake.
Another complicating factor is that ketosis needs three days of energy shortage before it "kicks in." First, those three days are pretty unpleasant, as anyone on a diet will tell you. Most importantly, though, most people are able to stick to diets in three-day sprints, splurging to reward themselves or giving in under the pressure. This means that just as our body is getting to the point of starting ketosis, we short-circuit the process by giving the body the calories it craves.
Perhaps the most striking and, for me, most disturbing reality about fad diets is that most of the pounds one loses right away are from water loss. Unless you are very deliberate about consuming additional water to offset what you would normally consume in a normal diet, you won't be getting enough water. You step on the scale that first week and... miracle! You've already lost a surprising amount of weight! In reality, you are dehydrating your body. Dehydration is a dangerous and sometimes subtle state that affects cognitive function, activity level, muscle regeneration, digestion, metabolism, and organ function. Our bodies are 80% water - and if you're losing weight fast, it is likely that's where the pounds are from. That is not healthy. And it isn't real weight loss, since you haven't burned any energy stored in the fat cells. We are fooling ourselves, and playing a dangerous game at that.
The last consequence of fad diets that I'll mention here is that they are short-term. That's their selling point: just do this for so many weeks and your problem will be solved! What happens after the grueling regime has ended? Back to your normal diet and routine, which will inevitably create the same circumstances that led to the "problem" in the first place. Most diets aren't sustainable, and the benefits they offer aren't either.
No fad diet or get-thin-quick scheme is a replacement for long-term, consistent (even incremental) changes in behavior, habits and food consumption. If weight is a serious health risk for you, I would strongly recommend seeing a medical doctor, and ask for a recommendation on a number of nutritionists who might gel with your personality and goals. In weight loss, taking a longer view is a better strategy. (No matter what your weight loss goals are, keep your doctor in the loop when considering a change in eating habits or activity level!)
Weight loss should be part of an integrated plan that includes diet, activity, self-image and one's relationship with food. We eat for two reasons: to nourish our bodies and for enjoyment. There's no rule that says we can't do both at the same time. And that sounds delicious!