Medical experts have poured cold water on the suggestion that Jonah Lomu's intense boxing training contributed to his hospitalisation.

The former All Black star has been training heavily and sparring with heavyweight boxer David Tua to prepare for the Fight for Life charity boxing tournament in December.

After his admission to Auckland City Hospital's renal and transplant unit last week, a close friend reportedly said the training had taken its toll on his health.

His family are not commenting on his condition because they have a contract with a women's magazine.