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Obesity and type 2 diabetes are so closely linked to each other, that whenever we talk about diabetes, we tend to think that obesity or overweight is the cause and an inevitable accompaniment of type 2 diabetes. While it is true that overweight and obesity are common in some forms of diabetes, e.g, in type 2 diabetes (which comprises 90 per cent of all types of diabetes) there are other forms of diabetes which are usually not associated with obesity. These include type 1 diabetes, maturity-onset diabetes of the young, fibrocalculous pancreatic diabetes and many genetic forms of diabetes. Most of these less common forms of diabetes tend to occur in normal weight or lean people.
What about type 2 diabetes itself? Does everyone who has type 2 diabetes necessarily have to be obese? While obesity and overweight are seen in some 60-70 per cent of people with type 2 diabetes, it can also occUnderur in people who are non-obese and are of ideal body weight and even in lean and frankly undernourished people.
The Body Mass Index (BMI) is usually used to quantify body weight. BMI is calculated by dividing one's body weight (in kilograms) by one's height in meters squared. To give an example, if somebody is 170 cm tall and is weighing 75 kg, the BMI will be 75/1.7 *1.7 = 25.9. A BMI between 18.5 to 22.9 is considered an ideal body weight. BMI between 23.0-24.9 is considered overweight, and a BMI of 25 and above is considered obese in India. Conversely, a BMI of less than 18.5 is considered lean or undernourished or underweight.
Body mass index (BMI) classification:
As mentioned above, usually type 2 diabetes occurs in those who are overweight and obese i.e. those with BMI>230 or 250. However, it can occur in those of normal body weight with a BMI between 18.5-22.9.
Occasionally, type 2 diabetes can also occur in those who are lean or underweight, i.e., in undernourished people with BMI<18.5 also. Earlier diabetes in underweight people was considered to be a separate form of diabetes called 'Malnutrition Related Diabetes Mellitus (MRDM)', which was also called 'Protein Deficient Diabetes Mellitus or PDDM'. After years of research, this entity was dropped from all the classifications as it does not have a definite marker to define it. Thus, people who are diagnosed to have 'MRDM' may actually be having type 1 diabetes or lean type 2 diabetes.
In a recent major research development, we classified type 2 diabetes into four subtypes.
The first entity known as Severe Insulin Deficient Diabetes (SIDD) usually occurs in young and thin people and many of them can be frankly underweight. These people will have more severe insulin deficiency and insulin resistance may not be marked. On the other hand, the IROD variety occurs in overweight and obese people who mainly have insulin resistance and the CIRDD also occurs in overweight people, who have both insulin deficiency and insulin resistance. The MARD variety occurs in those who have late-onset diabetes i.e. diabetes, usually sets in after 50 or 60 years of age.
It has been shown that those with lean type 2 diabetes (SIDD) are more prone to develop certain complications of diabetes like diabetic eye disease (retinopathy) and diabetic nerve disease (neuropathy). Those with the SIDD variety of diabetes also usually require insulin earlier than obese people with diabetes. They also respond better to medicines like DPP4 inhibitors and sulfonylureas, ie., insulin secretagogues which help improve their insulin secretion. In contrast, in the IROD variety weight reduction through diet, exercise and insulin sensitizers like metformin or SGLT2 drugs are the preferred line of treatment.
The bottom line is that when we see a lean patient, we need not rush to the conclusion that they are having type 1 diabetes. We should remember that type 2 diabetes can also occur in lean people and that it is one of the peculiar features of diabetes in Indians.
(The article is contributed by V. MOHAN, M.D., PhD, D.Sc, Chairman & Chief Diabetologist, Dr Mohan's Diabetes Specialities Centre)
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