Metabolic disorders, including obesity and high blood glucose and serum lipid levels, can significantly increase the risk of diabetes. A cohort study involving 51405 Korean men5 showed that individuals who developed T2DM were likely to have a higher BMI, FBG and serum lipid levels, which is consistent with our findings. Similar conclusions were drawn in the meta-analysis by Lotta et al,6 who showed that compared with healthy individuals, metabolically unhealthy individuals had a higher risk of T2DM, irrespective of BMI category.
While, in the univariate regression analysis, individuals with the habit of smoking and drinking more frequently had a lower diabetes risk. Maybe it was because individuals without the habits of smoking and drinking gave up smoking and drinking after they were combined with multiple chronic diseases and they were also more susceptible to be suffered from diabetes in the future.
Our study showed that older adults with higher BMI at baseline had a higher risk of developing diabetes. Overweight at baseline was a strong predictor of diabetes risk, independent of weight gain. The relative risk (RR) of diabetes for people with BMI 30.0-34.9 kg/m2 or ≥ 35 kg/m2 was up to 20.1- or 38.8-times higher than that among those with BMI < 23 kg/m2.7 In a retrospective cohort study of 1257 parous women,8 initial BMI and BMI after 28–48 years of follow-up were found to be strongly associated with diabetes risk, which increased with weight gain.
Among overweight or obese older adults, the risk of diabetes was higher in abdominally obese individuals than in those with normal WC, and obese older adults with abdominal obesity had the highest risk. The WC mainly reflects visceral fat content. The increase in body fat percentage is associated with a higher risk of diabetes, even in those with normal weight or underweight.9 National Diabetes and Metabolic Disease Survey10 showed that compared with people with normal weight and WC, people with obese, obesity and combination of the two had 1.88-, 1.12-, and 2.19-times higher odds of diabetes, respectively. This could be because an increase in WC results in the release of more free fatty acids from adipose tissue, causing lipid toxicity in beta cells, which in turn leads to a further decline in islet cell secretion.
The increase in WC (> 3 cm or > 5%) was also associated with an increased risk of diabetes, so increase in WC may be a predictor of T2DM. Moreover, the American Health Professionals Study11showed that 20% of the risk of diabetes could be attributed to an increase of > 2.5 cm in WC. The diabetes risk in people with a WC increase of 14.6 cm or more increased by 70% during a 4-year follow-up period.
The risk of new-onset diabetes in obese older adults with abdominal obesity was higher in women than in men. The proportion of older adults with central obesity is higher among women, and that the risk of diabetes is higher among adults with central obesity than among those with low BMI and WC10. While women generally have a higher percentage of fat, including total fat mass, subcutaneous thigh fat, and subcutaneous abdominal fat, they are more sensitive to the negative effects of excess fat accumulation. Increasing levels of visceral fat were associated with an approximately 3-fold increase in diabetes risk in women, while the risk in men increased by a modest 20%12. The central distribution of adipose tissue had a greater influence on the incidence of non-insulin-dependent diabetes in women than in men and may contribute to an increased risk of diabetes13. Together, these findings suggest that abdominal obesity may be a stronger risk factor for diabetes among women.
In our age-dependent subgroup analysis, the RR of diabetes in obese or abdominally obese adults aged 75 years and older was lower than that in those aged less than 75 years. According to a cross-sectional survey of Tianjin residents in 2017, the incidence of diabetes is the highest in people aged 75 years (0.75%) and then rapidly declines with age (0.4% in people aged 85 years). However, in our study, this trend only appeared in obese or abdominally obese older adults. The effect of overweight and obesity on the incidence of diabetes gradually weakens with increasing age. Further, older adults may not be as easily affected by a slight increase in risk owing to weight gain as younger people. Considering the close connection between obesity and several life-threatening conditions such as cardiovascular disease and stroke14, older obese people may be more likely to have a combination of these diseases and thus experience early death. The survivors may not be as susceptible to such disorders, including diabetes, which could explain the low incidence of diabetes in adults aged over 75 years.
Weight gain and large weight fluctuations were independent risk factors of diabetes, regardless of baseline BMI levels15, and this risk was positively correlated with the extent of weight gain16,17.The diabetes risk was higher among those who became obese, remained obese, or achieved a normal BMI than among those who remained non-obese. 9.1% of diabetes cases could be avoided if obese individuals became non-obese18 Some studies on weight loss through lifestyle interventions, such as the Da Qing study from China and the Diabetes Prevention Program, a 43% and 34% decrease in diabetes risk, respectively, was observed in individuals with prediabetes after long-term follow-up19,20. But when we analysed how the variation in weight and BMI and a high increase or decrease in weight or BMI affects the risk of new-onset diabetes, a U-shaped curve was found. Older adults who became obese or overweight had the highest risk of diabetes, followed by those who remained overweight or obese and those who achieved normal weight or became underweight, the corresponding risk of diabetes also increased in older adults with high weight loss (> 6 kg during follow-up). The possible reason is that the individuals were older adults in our study. During weight loss, older adults may lose proportionately more muscle mass than younger individuals. This loss of skeletal muscle and the increased proportion of fat may contribute to increased insulin resistance, which may weaken the benefit derived from the weight loss. And weight change across adulthood increased all cause and cause specific mortality.21
Physical activity is important for diabetes prevention. Physical activity can effectively reduce the risk of diabetes22–25and an appropriate increase in energy consumption can effectively regulate postprandial insulin secretion and improve the glucose metabolism status.26 This effect can persist for up to 10 years. However, the target population that can gain the most benefits from regular exercise is yet to be identified. The effect of moderate- or high-intensity exercise on diabetes prevention is more obvious in obese people.27,28 People with normal weight can benefit more from exercise or gain the same benefits as obese people, and prediabetes.29 In our study, daily exercise can reduce the risk of diabetes by 19% for overweight older adults, but the risk of diabetes did not show a significant decline for people with normal weight who exercised daily. This may be because people with normal body weight already had a low risk of diabetes. Moreover, the body fat content in people with normal weight was lower, and hence a further significant decline in body weight and fat would be challenging.
Our study has some strengths. It was the first large-scale prospective cohort study on the effect of obesity indicators and their changes on the incidence of diabetes in older adults from urban areas of northern China. Our study included over 60000 individuals, and the data were maintained in an updated database, which ensured the accuracy and integrity of this large amount of data. Moreover, subgroup analyses were performed according to sex, age, and baseline BMI levels, and confounders were adjusted for to minimise their influence. Furthermore, the participants enrolled in our study were community residents, and not inpatients, and our cohort was thus well-representative of the general older adult population.
Nevertheless, our study also had several limitations. First, the follow-up was only 4 years, and the long-time effect of changes in weight, BMI and WC did not appear, and next we will prolong the follow-up time. Second, our participants were from a coastal city in northern China; hence, our data may not be generalisable to individuals of other ethnicities and those living in other areas.