The definition of what constitutes a disability is elaborately explored in the Technical Assistance Guidelines on Disability: "People who have a long-term or recurring physical or mental impairment which substantially limits their prospects of entry into, or advancement in, employment."
For a specific case to meet the definition of a disability (for EEA1 purposes, reasonable accommodation measures, incapacity proceedings) it must satisfy the elements of the definition above. Clearly some cases of diabetes may constitute a disability (temporary or permanent) while others not. The USA Equal Employment Opportunity Commission (EEOC) has addressed this question in some detail at http://www.eeoc.gov/facts/diabetes.html
They may have a slightly different take on what constitues a disability and counterbalance reasonable accommodation measures with the principle of "undue hardship" as opposed to our wider term "unjustifiable hardship", but the underlying discussion is certainly relevant. And so, as with so many legal matters, "each matter will be decided on a case by case basis".
"The respondent is guilty of assigning characteristics which are generalised assumptions about groups of people to each individual who is a member of that group, irrespective of whether that particular individual displays the characteristics in question. It is treating all insulin dependent diabetics the same and imposing a blanket ban on the employment of that group as fire-fighters, irrespective of whether the particular individual - such as Murdoch, who is physically fit and in optimal control of his diabetes - displays any susceptibility to uncontrolled hypoglycaemic episodes." (JS178/09)  ZALCJHB 7; 2011 (2) SA 638
Of course, in South Africa, many employees do not have the resources and finances available to regulate their insulin levels and offer suffer debilitating illnesses and ailments associated with disabetes. In this regard see the article below.
- Six challenges that impede entrepreneurs with disabilities in South Africa
- Disabled people in Africa get a raw deal. What’s been done to fix this
- South Africa's new higher education disability policy is important, but flawed
- Why employing persons with disabilities is good for business
- ILO GBDN: Bringing business together to promote disability in the workplace