The major finding of this study is the clear association between obesity and diabetes in a large, representative sample of the US population. In this cross-sectional survey of adults with diabetes, the lowest prevalence for diabetes was found in individuals with normal weight (BMI <25.0). The prevalence of diabetes increased throughout the range of obesity classes. Nearly a quarter of adults with diabetes have poor glycemic control and nearly half of the individuals with diabetes are considered obese.
The prevalence of diabetes from our study is similar to the findings from Cowie and colleagues who reported the crude prevalence of total diabetes in adults age = 20 years was 9.6% with another 3.5% of adults at high risk for diabetes with hemoglobin A1C between 6.0% and 6.5% [1]. The prevalence of obesity among diabetic in our study is much higher than the reported prevalence of obesity in the general US adult population. In our study, the prevalence of obesity among adults with diabetes was 49.1% while Flegal and colleagues reported the prevalence of obesity was 32.2% among US adult men and 35.5% among adult women [8]. The glycemic control for individuals with diabetes is improving during the past decade. Saaddine et al. reported that the hemoglobin A1C level between 6% and 8% increased from 34.2% in 1998–1994 to 47.0% in 1999–2002 [9]. In our study, the proportion of individuals with hemoglobin A1C level between 6% and 8% was 77%. Similarly, the mean hemoglobin A1C level among individuals with diabetes was reported by Hoerger and colleagues to decrease from 7.82% in 1999–2000 to 7.18% in 2003–2004. The mean hemoglobin A1C level in our study was 7.2% [10].
In this study, we also found the mean fasting glucose and HbA1c levels were highest for diabetics with BMI <25.0, suggesting a state of higher severity of disease. Mean insulin and c-peptide levels were highest for diabetics with BMI equal to 35.0, suggesting a state of insulin resistance. These results may suggest that more diabetics within normal weight range have type 1 classification and diabetes in the obese are mostly type 2 with associated insulin resistance. Type 2 diabetes account for 90–95% of those with diabetes and encompasses individuals who have insulin resistance and usually have relative insulin deficiency.
The finding of a high prevalence of obesity in adults with diabetes from this study suggests that more effort should be taken to combat obesity since obesity is a modifiable risk factor for the development of diabetes. These efforts should include prevention of obesity in combination with medical and surgical treatments of obesity. The look AHEAD trial is one of the largest trials of intentional weight loss in diabetics. This is a multicentered, randomized, controlled trial of 5,145 individuals with type 2 diabetes with BMI >25 who were randomized to undergo intensive lifestyle intervention compared with diabetes support and education (control group). Participants randomized to lifestyle intervention lost an average of 8.6% of their initial weight compared to 0.7% in the control group [11]. A greater proportion of the individuals in the lifestyle intervention group had reduction in diabetes, hypertension, and lipid-lowering medications. The mean hemoglobin A1C level improved significantly more in the lifestyle intervention than control participants [11]. Bariatric surgery has also been shown to improve and even induce remission of diabetes in the morbidly obese. In a large study on 240 patients with diabetes or have impaired fasting glucose levels, Schauer et al. [12] reported that resolution of diabetes was achieved in 83% of patients and patients with a short duration of the disease (<5 years), mildest form of diabetes, and the greatest weight loss were more likely to achieve complete remission of diabetes. In a meta-analysis of 19 studies and 11,175 patients on the effect of bariatric surgery on diabetes, Buchwald et al. reported that 78.1% of diabetic patients had complete resolution and diabetes was improved or resolved in 86.6% of patients; diabetes resolution was greatest for patients undergoing biliopancreatic diversion/duodenal switch, followed by gastric bypass, and least for banding procedures [13]. In addition to the findings of improvement of diabetes with bariatric surgery, several recent studies have shown improved survival rates for morbidly obese individuals who underwent bariatric surgery compared to the control individuals without surgical intervention [14, 15]. Adams and colleagues reported a retrospective cohort study comparing 7,925 morbidly obese patients who underwent bariatric surgery to 7,925 severely obese control subjects and found that the cause-specific mortality related to diabetes decreased by 88% in the surgical group during a mean follow-up of 7.1 years [14]. In a landmark paper published in 2004 comparing 2,010 severely obese subjects who underwent bariatric surgery compared to 2,037 control subjects, Sjostrom and colleagues reported that the incidence of diabetes was significantly lower within the surgery group compares to control group at 2 and 10 years follow-up [16]. Of the 1,056 patients with a 10-year follow-up, the incidence of diabetes in the control group was 24% while the incidence of diabetes in the surgical group was 7% [16]. Lastly, prevention of obesity is a key public health initiative in an attempt to reduce the incidence of obesity and diabetes risk. Currently, Head Start is the largest federally funded early childhood obesity education program in the United States. In a survey of 1,583 Head Start programs, Whitaker and colleagues reported that most Head Start programs are doing more to support healthy eating and gross motor activity than required by federal requirement standards [17].
Our study shows a clear association between obesity and diabetes, using a large nationwide database. However, there are notable limitations in the use and interpretation of this observational data. The NHANES survey is a series of cross-sectional analyses of the US population. Therefore, longitudinal data across study years is not provided, and no temporal analyses can be conducted. As with all survey data, there are inherent limitations in the collection methods, which can lead to sampling error, measurement error, and reporting bias.