There are still two very distinct groups of dieters in the world. The low carb camp who limit carbohydrates and typically eat protein and fat indiscriminately. The low fat group limit fats of all kinds and typically eat carbohydrates indiscriminately. Most of the time these two ends of the dietary spectrum are diametrically opposed. And I'm going to step out on a limb and say both of these approaches used exclusively and long term are wrong for most people.
While both of these shotgun approaches won't necessarily hurt you in the short run, they also don't optimize your nutrition. And they may be hampering your metabolism and your fat loss and / or muscle building progress.
While most people can lose weight with either dietary approach they overlook the importance of tailoring their individual daily diet to their daily activity level and nutritional needs. At any given time your body's nutritional needs depend on diverse factors including activity level (how much activity you engage in), but other factors including what types of activities you're engaging in (aerobic, anaerobic, sedentary,) whether you're healing from illness or injury, stress levels, hormone levels, and even environmental factors like weather.
Here's a few generalized examples of why certain dietary approaches can be wrong in certain situations:
Scenario One, The Low Fatter (Pun Intended)
A typical yo yo dieter who has struggled with weight for years decides to go on a low fat diet because everybody knows fat is bad for you and cutting fat grams at 9 calories each (versus 4 calories per gram of protein and carbohydrate) gets you more calorie cutting for the buck. (Myth number one: fat is bad. If you truly believe this "fat is bad" nonsense read this. Myth number two: cutting fat calories is the best approach. If you believe this "cut the dietary fat" nonsense read this).
Mrs. Low Fat suffers through hunger and denial, overlooks her thinning hair and dry skin, and bad things she cannot see like unbalanced hormones and vitamin deficiencies due to a lack of essential dietary fats. She manages to lose some weight. But she hits a wall, quits losing, and perhaps binges followed by a self conflagration over her perceived failure and probable weight gain. A lack of calories and unbalanced nutrition has lead to a compromised metabolism (already lowered by less body weight and loss of muscle mass).
Desperate, she moves on to a crash diet of less than 1200 calories a day and less than 30 grams of fat a day. Unless her willpower is irrational and her determination insane she will succumb to her body's protestations through manifesting hunger and lack of energy. (Unless this dieter has developed a form of anorexia) a starvation or semi-starvation diet never works. Both are unsustainable and go against nature.
(Note: low fat and low calorie almost always go hand in hand. Both, when followed too long, are bad dieting tactics. While low carbohydrate dieters often cut calories also, increased dietary fat and protein appear to have a positive effect on appetite - and improved 24-hour blood glucose profiles, insulin sensitivity - in the short term).
Scenario Two, The Low Carber
An Atkins style low carber who never limits fat nor counts calories loses weight easily for several months. Within twenty pounds of goal weight, the dieter's weight loss stalls. They hit a plateau, maybe even gain weight, and decide to cut even more carbs. They may even return to an induction phase of low carb. But this time induction only nudges a small loss or no loss at all.
One obvious but usually overlooked clue as to why their weight won't budge is calories. The other is a reduced metabolic rate due to their previous weight loss. The less you weigh the less calories you need. Even if you eat zero carbohydrates, if you eat more calories than you burn you won't lose weight. Fat and protein calories are not magical. Eaten in excess they can and will be stored as body fat.
Both the above dieters are making crucial mistakes. And there are solutions to these mistakes that are relatively simple and easy to incorporate.
I'll tell you what they are in part two. (Cue the Hitchcockian suspense music).
