Crash diets are usually deficient in some nutrients

This may be true. It is difficult to obtain all the essential nutrients from food alone if we consume less than 4200 kilojoules (1000 kilocalories) per day, and certainly if we have less than 3400 kilojoules (800 kilocalories). Of course, we can often get away with this for a few days, but repeated and persistent practices of this kind are nutritionally risky.

The nutrients which may be lacking in a crash diet are protein, the vitamins folacin, thiamin (vitamin B-1), riboflavin (vitamin B-2) and pyridoxine (vitamin B-6), the minerals potassium, magnesium and zinc, and dietary fibre.

It is possible to increase the nutritional quality of low energy diets by including nutrient-rich foods like liver, wheat germ and yeast.

Energy intakes lower than 3400 kilojoules (800 kilocalories) per day must be accompanied by nutrient supplements or be provided by formula feeds, under medical supervision.