But losing spare weight is usually a good thing, so why the warning?
Well crash dieting usually involves severe calorie restriction, with diets for women of usually 1300 calories or less. Whilst you might accept some tiredness, light headedness, lack of concentration and headaches as a temporary side effect of following such a diet most people probably aren't aware of the long term impact, particularly on the thyroid gland.
You see when you severely cut your calories your body thinks you're being starved and so it slows down your metabolism to help sustain you on a low calorie diet. The way it does this is by suppressing thyroid function reducing your production of thyroid hormones.
When you go back to a normal calorie intake your thyroid hormone production goes back up, but repeated dieting can overtime lead to a general suppression of the thyroid gland. This then causes weight gain, which becomes increasingly difficult to reverse, leading to people resorting to even stricter diets. This is why people often find diets that used to work for them are less effective when they try them again at a later date.
Under functioning thyroids are increasingly common with a lot of hypothyroidism undiagnosed and large numbers of individuals with borderline hypothyroidism which won't be treated with thyroxine.
Whilst it's possible to restore thyroid function with proper nutrition and lifestyle changes, it can be a fairly slow recovery - much better to skip the crash diet and not get to that point in the first place.
But I'm a realist and I know that whatever health warning I give some NITC dieters will still resort to a crash diet. In my opinion a low-carb diet is actually a much better idea than a calorie restrictive diet. True, the weight lost will be mostly water which come back on when you eat carbs again, but you don't have to restrict calorie intake and the body only detects food scarcity in the first few days whilst you move into ketosis, plus you won't be hungry. Alternatively a detox diet can be a healthy way to lose weight more rapidly whilst actually having some positive effects on health.