Hair loss, with a generalised thinning of the hair, is usually linked to emotional stress, severe illness, malnutrition, hypothyroidism, hormones, and (for a few people) extensive weight loss over a sustained period of time whatever the method of weight loss used. So, it is due to a physiological response to the actual weight loss process rather than to the Diet, and the incidence is very low. Hair does re-grow, usually thicker and glossier than before. It is the "lesser of the evils" and you will have the best of both worlds – a healthy weight and a good head of hair.
This condition should not be confused with alopecia whereby hair comes out in patches which is a medical condition needing treatment by a doctor