As is further developed in Chapter 8 in relation to screening, several pieces of knowledge must be available in order to know if it is possible to prevent any chronic disorder, including diabetes. These include a knowledge of the natural history of diabetes (with a reasonably long pre-clinical phase of the natural history), an effective and simple screening test or tests for a high-risk state, and effective interventions that, if applied earlier in the pre-clinical phase, would prevent or delay the onset of the disease.
The pre-clinical stages of the development of type 2 diabetes are well known. For many years it has been shown that glucose levels become elevated prior to the development of diabetes. This stage of the natural history has been called 'chemical diabetes', IGT24,25, or most recently IFG26. However, when fasting glucose begins to rise, this may be a pathophysiologically later stage than is IGT, which usually has elevations of post-challenge glucose as the first detectable change. The use of IFG has only recently been studied, and a full understanding of its prognostic implications are not understood. Nonetheless, impairment of glucose regulation at levels which are lower than used for diagnosis increases the risk of subsequent diabetes from five- to eight-fold27. Thus, there is at least one pre-clinical stage for intervention prior to the development of diabetes.
Two major questions need to be answered: (1) Are there efficacious and effective interventions to prevent (or delay) the onset of diabetes? and (2) Does treatment at the time of early diagnosis reduce complications as much or perhaps more than waiting until the time of usual clinical intervention? This chapter will summarize the available clinical trials and observational studies relevant to the first question. The second question is dealt with in Chapter 8.
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