Harare - Zimbabwe President Robert Mugabe's secret to a long life is simple: no cigarettes or alcohol and lots of exercise, his sister said in an interview published on Sunday. Mugabe turns 91 next month and is one of Africa's longest-serving leaders."He has always known that things like alcohol and smoking are debilitating and degrade the body," Regina Gata, Mugabe's only surviving sister said in an interview with the Sunday Mail."Even up to now, he exercises a lot," Gata told the state-controlled publication in an interview from her home in Mabelreign, a modest suburb in the capital Harare."I always hear from Amai [Mother] Grace Mugabe that he exercises a lot everyday and he remains disciplined in this respect," Gata, 68, said.Gata is Mugabe's half-sister from his father's second wife.Mugabe's asceticism is well-known. He has previously revealed that he takes a long walk every morning at 5 am, eats very little meat and lots of vegetables. His wife takes pride in growing her own for the family.He regularly travels to Singapore for what the authorities insist are "eye check-ups", and, despite a very occasional stumble, appears to be in remarkably good health.Gata said Mugabe had used his savings to pay for her nursing course in the UK in the 1970s, when he was still in prison.