How obesity or weight loss can affect singing?

According to the director Franco Zeffirelli, it seems that Maria Callas used amphetamines to “find the strength for both singing and diet, while, to sleep and counteract the effect of amphetamines, was forced to take increasing doses of barbiturates”.

But even when following a balanced diet we must remember that the variations in fat and muscle mass caused by rapid weight loss, as well as rapid weight gain, have an impact on at least four anatomic-functional factors related to the quality of the voice: on the vocal tract morphology, on breathing (even without respiratory problems), on the balance of sex hormones (which are steroidal hormones or lipid-based hormones) and on the appearance or aggravation of gastro-oesophageal reflux.

Regarding the vocal tract morphology in obese subjects, there is a tendency towards vocal strain, induced by an excess of fat deposits along the vocal tract causing difficult breathing.