Leptin is an incredibly important hormone in the body.
It is secreted by our fat cells. The bigger they get, the more leptin they secrete.
More fat = more leptin.
Leptin is supposed to send signals to the brain, informing it about how much body fat is stored.
We eat -> Body fat gets stored -> Leptin increases -> Brain "sees" the leptin and makes us stop eating.This is a basic negative feedback loop, the output (leptin) decreases the input (eating). This is how the brain is supposed to keep body fat levels within a narrow range over the long term.
The problem today is that leptin isn't working.
Obese individuals have high levels of leptin, but it isn't getting through to the brain and therefore there is no signal that the fat cells have enough energy stored and that the person needs to stop eating.
You've probably heard of insulin before.
It's the hormone secreted by the pancreas after eating a meal with carbs in it (protein affects it too, to a lesser extent).
One of the other functions of insulin is that it makes the body synthesize fats out of glucose, then tells the fat cells to pick up as much fat as they can and store it.
This is one of the reasons diabetics start gaining weight when they start shooting insulin. Insulin drives fat into storage.
If you're already overweight/obese, then you are likely to have issues with these two hormones already.
Fortunately, the solution is incredibly simple. All you have to do is eat less sugars and starches (carbs).
The failed low-fat diet may be good for lean people, but it is pretty much useless as a cure, probably because it does nothing to address the root causes of obesity, which include insulin and leptin resistance.
The largest randomized controlled trial ever conducted on diet, called the Women's Health Initiative and included 48,835 postmenopausal women, revealed some interesting results: The women on the low-fat diet were only 0.4 kg (1 lb) lighter than the women eating the standard american diet after 7.5 years (11).
This and many other studies show that the standard low-fat diet is highly ineffective.