There are two major types of diabetes, Type 1 ("Juvenile diabetes" or "insulin-dependent diabetes mellitus") and Type 2 ("Adult onset" or "non-insulin dependent" diabetes mellitus (NIDDM). This article discusses a newer classification, Type 1.5.
Type 1.5 is one of several names now applied to those who are diagnosed with diabetes as adults, but who do not immediately require insulin for treatment, are often not overweight, and have little or no resistance to insulin. When special lab tests are done, they are found to have antibodies, especially GAD65 antibodies, that attack their beta cells. This sort of diabetes is sometimes called Slow Onset Type 1 or Latent Autoimmune Diabetes in Adults (LADA).
One study performed in Bruneck, Italy [Diabetes, October 1998] found that 84% of the people diagnosed as Type 2 had insulin resistance, but the other 16% did not, suggesting these individuals had Type 1.5 diabetes. Several other studies have shown similar results as well as the presence of antibodies, especially those against glutamic acid decarboxylase (GAD), characteristic of Type 1 in this group of people diagnosed with Type 2.
Type 1.5 diabetes has virtually the same underlying cause as type 1. The difference is that type 1.5 happens in people older than 25, whereas type 1 happens in childhood, the teen years and young adulthood. People as old as 80 have been diagnosed with type 1.5.
A misdiagnosis is easy to make when the patient is older and responds well at first to treatment with oral medications. If someone does not clearly fit the model for Type 1 diabetes, they may be mistakenly placed on oral agents even though limited capacity for insulin production remains. The immune system's slower and more selective attack on the beta cells allows these cells to function to a high degree for a few years. On average, insulin is required in half of those with Type 1.5 diabetes within four years of diagnosis, compared to over ten years in those with true Type 2. [Endocrine Practice, 7(5), Sept/Oct 2001, pp.339-45]
Since insulin resistance is minimal or non-existent, medications designed to reduce insulin resistance such as Avandia and Actos are not effective. Other medications that stimulate the pancreas to produce insulin, slow digestion of carbohydrates, or reduce excess glucose production by the liver are often effective in controlling the blood sugar for a few years.
Knowing your diabetes type can give you a better understanding of the changes that may occur as you age and the disease progresses. For example, if you have had insulin-resistant diabetes for several years that has become harder to control on a sulfonylurea medication and your C-peptide level (a lab test that measures insulin production) is now low, the addition of insulin will be needed. However, if your control of the diabetes is poor and your C-peptide is normal, adding another oral agent and paying closer attention to your food and exercise choices may be all that is required.
As insulin production falls, insulin becomes necessary to maintain control of the disease. One indication that people have Type 1.5 rather than Type 2 is their appearance, which is more likely though not always slender and physically fit. They often do not have other signs of Type 2 diabetes, such as the Syndrome X cluster of high TGs, low HDL or high blood pressure. Fortunately, in these early stages, diabetes treatment is not significantly different for slow-onset Type 1.5 patients than for truly insulin-resistant Type 2 patients. The only exception is that drugs designed to increase insulin sensitivity like the glitazones do not work because insulin sensitivity is normal.
One major benefit for patients with Type 1.5 diabetes is that when their blood sugars are controlled they usually do not have the high risk for heart problems more often found with the high cholesterol and blood pressure seen in true Type 2 diabetes.
It is not clear yet if people with type 1.5 have the same high risk for cardiovascular problems as individuals with type 2.
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