Weight Loss Myth #1: Fad diets = permanent weight loss.

Myth: Fad diets work for permanent weight loss.
Fact: Fad diets are not the best way to lose weight and keep it off.
Fad diets often promise quick weight loss or tell you to cut certain
foods out of your diet. You may lose weight at first on one of these
diets. But diets that strictly limit calories or food choices are hard
to follow. Most people quickly get tired of them and regain any lost
weight.
Fad diets may be unhealthy because they may not provide all of the
nutrients your body needs. Also, losing weight at a very rapid rate
(more than 3 pounds a week after the first couple of weeks) may
increase your risk for developing gallstones (clusters of solid material
in the gallbladder that can be painful). Diets that provide less than 800 calories per day also could result in heart rhythm abnormalities, which
can be fatal.
Tip: Research suggests that losing 1/2 to 2 pounds a week by making
healthy food choices, eating moderate portions, and building physical
activity into your daily life is the best way to lose weight and keep it
off. By adopting healthy eating and physical activity habits, you may
also lower your risk for developing type 2 diabetes, heart disease, and
high blood pressure.

Ref: National Institute of Diabetes and Digestive and Kidney Diseases