The incidence of non-insulin-dependent diabetes mellitus (NIDDM) and its risk factors were analysed from data of a follow-up study conducted on Japanese-Americans living in Hawaii and Los Angeles areas known to have a high prevalence of NIDDM. There were 1144 Japanese-Americans aged 40 years or more (mean (+/- SD) age: 60.7 +/- 10.1) available for follow-up, and who were non-diabetic based on glucose tolerance test (GTT) results conducted at the time of the initial examination. During the study period (6.3 +/- 3.2 yr), 124 cases of NIDDM occurred with an incidence rate as high as 10.8% or 17.2 persons per 1000 person-years. The incidence of NIDDM was 1.25 times higher in males than in females, and the rate increased with age. The incidence in obese people (body mass index: BMI > or = 25) was approximately twice that in the non-obese, the rate increasing with the level of obesity. Using Cox's proportional hazards model, the risk factors for NIDDM were analysed after adjusting for sex and age. Significant risk factors were the serum glucose (SG) level (fasting, 1-h, 2-h), the serum immuno-reactive insulin (IRI) level (1-h, 2-h) during GTT, BMI, serum triglycerides, high density lipoprotein (HDL)-cholesterol, serum uric acid, diastolic blood pressure, systolic blood pressure, serum total protein, and delta IRI/delta SG (0-30 min) level. In particular the hyperinsulinaemia was a significant risk factor even after adjusting for sex, age, and obesity. The incidence of NIDDM in the sub-group whose delta IRI/delta SG (0-30 min) level was under 0.3 was higher than that of the over 0.8 sub-group by a factor of approximately 12. Likewise, after dividing the subjects into five sub-groups according to the fasting or 2 h IRI level, the incidence of NIDDM was higher in the highest quintile than in the lowest quintile sub-group by a factor of approximately 3 and 7, respectively. The results of the GTT of the subjects who developed NIDDM during the follow-up period were analysed longitudinally and compared with the results obtained 8 years before occurrence of NIDDM. An increase in the 2-h serum IRI was observed initially, followed by an increase in the 2-h serum glucose level, preceding the appearance of NIDDM.
In conclusion: 1. A high incidence of NIDDM is clearly observed in the population of Japanese-Americans, who are genetically indistinguishable from native Japanese; causes of this increase, from the survey results, are suspected to include westernization of lifestyle, particularly the reduction in the level of physical activity, conversion to a diet containing markedly more animal fat, simple carbohydrates, and less complex carbohydrates. 2. In the Japanese-Americans, diminished early insulin release to an oral glucose challenge and increased insulin resistance characterized by hyperinsulinaemia are suspected to be some of the important risk factors for NIDDM. Consequently, this study strongly suggested the possibility that the development of NIDDM in Japanese persons may be influenced by environmental factors.