WEIGHT LOSS ≠ FAT LOSS, WEIGHT GAIN ≠ MUSCLE GAIN
The majority of weight loss that comes from a regular dieting might be water weight, carbohydrates depletion, muscle mass, and sometimes even bone mass. Body fat weight is decreased only to some degree, if at all. While losing body fat is beneficial - losing the same weight as a combination of fat and muscle - damages your health.
The same thing applies to muscle - if you gain weight, it might mean increased water storage and obviously body fat mass. The traditional diet's rules in this case also won't be optimal to target a growth mostly in muscle mass.
THE IMPORTANCE OF MUSCLE MASS FOR MEN AND WOMEN - NOT ONLY LOOKS RELATED BENEFITS
Preserving and increase in muscle mass size is extremely important (especially for women and elders) in terms of both looks and health. By losing muscle tissue, you reduce your metabolism - tremendously increasing chance to regain previously lost weight and an opportunity to develop a metabolic disease. Most of your internal organs are actually made of lean tissue - that is why losing muscle mass is dangerous. Besides, muscle mass is a "protective layer" of your body, not to mention the benefits of an increase in performance in everyday activities and decreased risk of getting an injury, especially with bone mass lost by certain conditions, such as osteoporosis.
Women who are afraid of getting too muscular - really shouldn't be. Their ability to build a muscle mass significant enough to look "bulky" is much lower compared to that men have (lower testosterone levels). Women who are competitive bodybuilders (which are often chosen by others as an example of possibility to get overly muscular) are taking very concrete steps to achieve that looks - often steroids on the highest professional levels.
FAT LOSS PLATEAU
A plateau is a term used to describe a state of a stalled progress (whether it is muscle gain or fat loss). In this case, I will cover fat loss stall. It usually happens a few weeks after starting a diet, with almost every nutrition regimen people use. It is determined by fat cells.
Fat cells store the fat in your body mostly around your internal organs and under your skin. Once they are full - they are signaling your body to create much more new ones. The interesting thing is that fat cells never die. Also, the body's ability to create them is unlimited - at any age.
Interestingly, while those cells are full they also send a strong signal to release the excess of body fat stores. That is why contradictory to popular belief - obese people have a HIGH metabolism and ability to lose body fat! Particularly at the beginning of dieting.
THE REASON FOR POSITIVE TESTIMONIALS AMONGST TRADITIONAL DIETS
This is exactly why you can see positive, and very often even shocking testimonials of great results with really obese people - regarding popular nutrition programs. The results they have got would be almost the same with any routine they followed - but only initially.
That is the problem - only initially and with an increased risk of regaining even more weight and developing lots of metabolic diseases.
The reason of a plateau is emptied fat cells storage. While they are empty - the body sends signals to stop using fat - both dietary and this already stored in the form of body fat. Hunger tremendously increases and your metabolism and body composition changes.
After great results for a few weeks your progress first stalls and then you regain your previous weight, but your body is in an entirely different state - you look and feel much worse.
STRESS AND CORTISOL
Dieting is usually a very psychologically taxing process - it increases stress. However, that is not the only reason why your body has changed for worse - it is the improper usage of cortisol - a "stress hormone." It is inevitably occurring hormone of your body, even without any additional stress. It naturally increases at night and during exercise (the times when it might be beneficial) and goes down some time after waking up. Its role is to break tissues of your body.
The stress, though, elevates its level and keeps it high for a long time, sometimes even for days. Not only it increases the risk of wasting muscle tissue, but also the chance to store fat and increase fat cells amount. In conclusion - popular diets regimes keep your cortisol levels elevated for a long time.
INSULIN + CORTISOL
Generally - only one of those two - insulin and cortisol are high at any given time. As previously stated - regular diet regimes may keep your cortisol elevated for longer periods. With the addition to insulin spike - it is the perfect environment for store body fat and increases the number of fat cells. Popular diets not only causes insulin spikes but also keeps its levels elevated for extended periods of time (complex carbohydrates). More on insulin later in the documentation.
To sum up. The most of popular diet regimes creates a high risk of: losing muscle, keeping getting body fat, losing energy, developing metabolic diseases (such as diabetes, thyroid dysfunction, insulin sensitivity, increased in LDL cholesterol, decrease in HDL, high blood pressure and many more - even Alzheimer's - which has much to do with metabolism). Not necessarily all of them at once, obviously - that depends on a lot of factors (i.e. genetics). You may say it is an overstatement - but it is truly not. With lots of medications hiding the real issue, people actually don't feel much change, especially when it comes to health (not so obvious symptoms, as for example becoming obese).
That is a very different matter though - treating symptoms - not the cause.