While the total number of diabetics in the country, which is known as prevalence, is estimated to be 35-40 million, the number of people who become diabetic every year was not known. Not anymore. The “first incidence study on diabetes and pre-diabetes in India” has been published in the March 2008 online edition of the Journal of Association of Physicians of India (JAPI).
Unlike the prevalence study undertaken by many researchers, studying the incidence — the rate at which the disease is occurring in a population within a certain period — is a challenging task. For one, it has to be done over a long period of time, usually many years, and on the same set of people.
While all the individuals are healthy at the beginning of the study, some become afflicted with the disease during the years of follow-up. Incidence study “reflects the rate at which healthy people in the population acquire the disease.”
The Chennai Urban Population Study (CUPS) undertaken by Dr. V. Mohan and others from the Dr. V. Mohan’s Diabetes Specialities Centre, Chennai, has focused on two residential colonies representing the middle and lower income groups. The study was started in 1996.
Though 1,061 individuals were studied in 1996, the follow-up study, undertaken after eight years, was done on 513 individuals who were still living in the same area.
The team found that the overall incidence of diabetes has been 20.2 per 1,000 person years of follow-up; impaired glucose tolerance (IGT), a stage prior to becoming diabetic, has been 13.1 per 1,000 person years of follow-up.
Unlike prevalence, which is measured in terms of percentage, the incidence rate is calculated based on the number of people followed up for a certain period of time. More the number of people studied for a longer duration, more the incidence rate will be and vice versa.
Number of converters
Further scrutiny of the results has thrown up some interesting trends.
The incidence rate of diabetes is arrived at by taking into consideration the healthy individuals and those already with IGT in 1996. The IGT incidence rate is arrived at by looking at the number of healthy people (in 1996) who have now developed glucose intolerance.
Of the 513 who were followed up, 476 were healthy in 1996 and 37 already had IGT. Further break-up of numbers shows that 64 of the 476 who were healthy in 1996 have now become diabetic; 48 have developed glucose intolerance. Similarly 15 of the 37 (who already had IGT in 1996) have now become diabetic.
Commenting on the research, the Editorial published in the same issue notes: “The publication of data such as this augurs well for epidemiology in India. While excellent prevalence studies have been published from India, the time has come when India also publishes world class incidence data.”
Though the first of its kind, it should be noted that the number of subjects studied is not big and there has been only 52 per cent follow-up of the subjects. “…This [52 per cent] still is good follow-up rate,” the Editorial has commented.
“The study is not about knowing the incidence rate alone,” said Dr. V. Mohan, Chairman of Dr. V. Mohan’s Diabetes Specialities Centre, Chennai. “It has shown that the Indian Diabetes Risk Score (IDRS) developed by us is a good predictor of the disease.”
The data collected at the start of the study shows that those who are younger, with low BMI and smaller waist circumference have a fair chance of remaining healthy. While it is the reverse in those where these parameters are not in their favour.
“The proportion of subjects with IDRS greater than or equal to 60 was higher among converters [those who became diabetic] compared with non-converters,” the paper notes. Even in the case of those who have developed glucose intolerance, the IDRS has been slightly on the higher side.
“The rule of thirds, as we call it, says that one-third of people with IGT will develop diabetes, one-third will remain as IGT and one-third will revert to normal state,” Dr. Mohan said. “Lifestyle changes will help people not to progress to a diabetic state. It is a golden window period to prevent diabetes if people are detected early at the IGT stage.”
What causes a reversion to a normal state is not known. IGT could have probably been a temporary phenomenon.
According to him, the number of people who have developed diabetes has been smaller in one of the colonies where active intervention resulted in lifestyle changes.
The biggest advantage by doing an incidence study is to know what exactly causes a disease. Though prevalence studies have shown that people with diabetes tend to be obese, have a sedentary lifestyle, higher BMI, bigger waist circumference and family history, there has been no way of confirming that these are indeed the parameters that cause the disease.
An incidence study brings out clear picture of the risk factors.
Unlike a prevalence study which points out to “…associations between these different variables [higher waist circumference, age, sedentary lifestyle], in an incidence study … we can move beyond association to causation,” the Editorial points out.
Dr. Mohan’s incidence study has indeed shown that age, BMI and waist circumference are clear risk indicators. But family history did not appear to be a risk factor.
“It [family history] did not reach statistical significance. It may probably be because the number of people studied has been small,” explained Dr. Mohan. A bigger study is warranted as family history is believed to be a big risk factor.