Low hemoglobin A(1c) in nondiabetic adults: an elevated risk state?

OBJECTIVE: To identify predictors of low hemoglobin A(1c) (HbA(1c)) (<5.0%) and to investigate the association of low HbA(1c) with cause-specific mortality and risk of liver disease hospitalization. RESEARCH DESIGN AND METHODS: Prospective cohort study of 13,288 participants in the Atherosclerosis Risk in Communities Study. Logistic regression was used to identify cross-sectional correlates of low HbA(1c), and Cox proportional hazards models were used to estimate the association of low HbA(1c) with cause-specific mortality.
RESULTS: Compared with participants with HbA(1c) in the normal range (5.0 to <5.7%), participants with low HbA(1c) were younger, less likely to smoke, had lower BMI, lower white cell count and fibrinogen levels, and lower prevalence of hypercholesterolemia and history of coronary heart disease. However, this group was more likely to have anemia and had a higher mean corpuscular volume. In adjusted Cox models with HbA(1c) of 5.0 to <5.7% as the reference group, HbA(1c) <5.0% was associated with a significantly increased risk of all-cause mortality (hazard ratio [HR]: 1.32, 95% CI: 1.13-1.55) and of cancer death (1.47, 95% CI: 1.16-1.84). We also noted nonsignificant trends toward increased risk of death from cardiovascular causes (1.27, 95% CI: 0.93-1.75) and respiratory causes (1.42, 95% CI: 0.78-2.56). There was a J-shaped association between HbA(1c) and risk of liver disease hospitalization.
CONCLUSIONS: No single cause of death appeared to drive the association between low HbA(1c) and total mortality. These results add to evidence that low HbA(1c) values may be a generalized marker of mortality risk in the general population.

Hemoglobin A1c (HbA1c) is the standard measure of glucose control in persons with diagnosed diabetes mellitus and is now recommended for use as a diagnostic test for diabetes (1,2). The 2010 American Diabetes Association recommendations for use of HbA1c as a diagnostic test will likely increase its use in persons without a prior diagnosis of diabetes. A number of studies have demonstrated that HbA1c values, even below the diagnostic threshold of 6.5%, are associated with clinical outcomes including cardiovascular events (3–5), kidney disease (6), and total mortality (3,7–9). Previous studies in nondiabetic populations have also reported a J-shaped association of HbA1c with all-cause mortality (3,7,10,11). The objectives of this study were to examine predictors of low HbA1c (i.e., <5.0%) and investigate the association of low HbA1c with all-cause and cause-specific mortality in a community-based population. Because recent studies have shown a high prevalence of liver disease among persons with low HbA1c (10,12), we also examined the association between low HbA1c and risk of liver disease hospitalization in this cohort.

RESEARCH DESIGN AND METHODS

Study population

The Atherosclerosis Risk in Communities (ARIC) Study is an ongoing community-based prospective cohort study of 15,792 middle-aged adults from four U.S. communities: Washington County, MD; suburban Minneapolis, MN; Jackson, MS; and Forsyth County, NC. The first study visit occurred between 1987 and 1989 with three follow-up visits that occurred approximately every 3 years (13,14). Visit 2 (1990–1992) was attended by 14,348 participants and is the baseline for the present analysis. Participants were included in our analyses irrespective of the previous occurrence of nonfatal events. We excluded participants who self-identified as other than white or black race (n = 48) or who were missing data on HbA1c or other covariates of interest (n = 970), leaving a final sample size of 13,288 participants in this analysis. Institutional review boards at each clinical site approved the study protocol, and written informed consent was obtained from all participants.

Measurement of HbA1c

Frozen whole-blood samples collected at ARIC visit 2 were thawed and assayed for the measurement of HbA1c using high-performance liquid chromatography (Tosoh A1c 2.2 Plus Glycohemoglobin Analyzer method in 2003 to 2004 and the Tosoh G7 method in 2007 to 2008; Tosoh Corporation) (15). Both instruments were standardized to the Diabetes Control and Complications Trial assay (16).

Outcomes

ARIC Study investigators conduct continuous surveillance for all hospitalizations and deaths among participants via annual phone calls to participants or proxies, and detailed information on deaths is obtained from family members, coroner reports, or health department death certificates. Methods for the ascertainment of death and its causes in ARIC have been published previously (13). We classified deaths according to underlying cause, on the basis of coding from the ICD-9 and -10. We divided causes of death into the following major diagnosis categories defined by the ICD codes: 1) cancers (ICD-10 codes of C00-D48, ICD-9 codes of 140–239); 2) cardiovascular system (ICD-10 codes of I00–I99, ICD-9 codes of 390–459); 3) respiratory system (ICD-10 codes of J00–J99, ICD-9 codes of 460–519); 4) digestive system and liver (ICD-10 codes of K00–K93, ICD-9 codes of 520–579); and 5) genitourinary system and kidney (ICD-10 codes of N00–N99, ICD-9 codes of 580–629). We also identified incident liver disease hospitalizations from hospital discharge records with an ICD-9 code for liver disease using the following ICD-9 codes (listed anywhere in the hospital discharge record): 570.0–573.9.

Covariates

All covariates were assessed during visit 2 (1990–1992), except education and physical activity, which were assessed during visit 1 (1987–1989). Covariates evaluated as potential confounders included: age (continuous), race/field center (Washington County, MD whites; Minneapolis, MN whites; Forsyth County, NC whites, Forsyth County, NC blacks; and Jackson, MS blacks), sex (male/female), education (less than high school, high school or equivalent, or more than high school), BMI (continuous), waist-to-hip ratio (continuous), cigarette smoking (current, former, or never), alcohol intake (current, former, or never), physical activity [assessed with the use of Baecke index (17)], family history of diabetes, presence of hypertension (defined as systolic blood pressure ≥140 mmHg, diastolic blood pressure ≥90 mmHg, or use of antihypertensive medication during the previous 2 weeks), total cholesterol (continuous), HDL cholesterol (continuous), prevalent coronary heart disease, prevalent stroke, fasting glucose (continuous), hemoglobin concentration (continuous), red-cell mean corpuscular volume (MCV; continuous), total leukocyte count (continuous), and plasma fibrinogen levels (continuous). Further details about data collection methods in ARIC are described elsewhere (13,18).

Statistical analysis

We divided the baseline study population into groups according to clinical categories of glycated hemoglobin (<5.0, 5.0 to <5.7, 5.7 to <6.5, and ≥6.5% or diagnosed diabetes) (2). With the HbA1c category of 5.0 to <5.7% as reference, we used logistic regression models to identify predictors of low HbA1c (<5.0%) at baseline among persons with HbA1c <5.7% (n = 9,254). To characterize the prospective association of low HbA1c with all-cause and cause-specific mortality and risk of liver disease hospitalization, we used Cox proportional hazards models to estimate the hazard ratios (HRs) and their corresponding 95% CIs across baseline categories of HbA1c. We verified that the proportional hazards assumption was met (19). Two models were used: model 1 was adjusted for age, sex, and race/field center, and model 2 was adjusted for the variables in model 1 plus total and HDL cholesterol, BMI, waist-to-hip ratio, hypertension, family history of diabetes, education level, alcohol use, physical activity, smoking status, hemoglobin, MCV, fibrinogen, and leukocyte count. We performed sensitivity analyses with additional adjustment for serum glucose, pulmonary function tests, white-cell differential, platelet count, and mean corpuscular hemoglobin. We formally tested for interaction by hemoglobin level. We also looked separately at the association among persons with and without anemia (defined as hemoglobin <13.5 g/dL for males and <12.0 g/dL for females) and after restricting the population to persons with ≥3 years of follow-up to address the potential for reverse causality. In all analyses, participants were censored if the participant was lost to follow-up, died of other causes, or reached the end of the follow-up period (31 December 2008). We used a restricted cubic spline model to investigate the continuous association between HbA1c and risk of liver disease hospitalization. All reported P values are two-sided, and P < 0.05 was considered statistically significant. All analyses were performed using Stata Statistical software version 11 (StataCorp, College Station, TX).

RESULTS

Of the 13,288 participants included in the study, 3,078 (23.2%) died by the end of follow-up period (maximum follow-up was 18 years). The demographic, clinical, and lifestyle characteristics of the study population are shown stratified by HbA1c categories in Table 1. Overall, the mean age was 57 years, 55% were women, and 24% were black. Persons with higher HbA1c were more likely to be older and current or former smokers.

Table 1

Baseline characteristics of the study population by HbA1c category at baseline (ARIC visit 2)

The odds ratios and 95% CIs for predictors of low HbA1c are shown in Table 2. Compared with those with HbA1c values 5.0 to <5.7%, participants with HbA1c values <5.0% were younger, had a lower BMI, were less likely to smoke, had lower total cholesterol levels, and were much less likely to have hypertension, a family history of diabetes, or a history of cardiovascular disease. Participants with HbA1c <5.0% were more likely to have either low (<12.9 g/dL) or high hemoglobin (>14.7 g/dL).

Table 2

Adjusted* associations of low HbA1c (<5.0% vs. 5.0 to <5.7%) among persons without diabetes (N = 9,254)

Of the total 13,288 participants included in this study, 1,113 (8.4%) participants died of cancer, 1,085 (8.2%) died of cardiovascular disease, 235 (1.8%) died of respiratory system disease, 82 (0.6%) died of digestive system or liver disease, and 61 (0.5%) died of genitourinary system or kidney disease.

The HRs for all-cause and cause-specific mortality by baseline HbA1c category are shown in Table 3. Comparing models 1 and 2, we observed that multiple adjustments strengthen the association of low HbA1c with cause-specific and all-cause mortality, which is consistent with negative confounding. Using the group with HbA1c of 5.0 to <5.7% as the reference group, HbA1c values <5.0% were associated with a significantly increased risk of all-cause mortality (HR: 1.32, 95% CI: 1.13–1.55) and of cancer death (1.47, 95% CI: 1.16–1.84) in model 2. Low HbA1c was also associated with an increased risk of cardiovascular system deaths (1.27, 95% CI: 0.93–1.75) and respiratory system deaths (1.42, 95% CI: 0.78–2.56), but these associations were not statistically significant, possibly owing to the smaller number of deaths due to these causes. Low HbA1c was not significantly associated with risk of deaths related to the digestive system or liver disease (0.96, 95% CI: 0.34–2.71) or deaths due to conditions of the genitourinary system and kidney disease deaths (1.01, 95% CI: 0.23–4.42). Similar results were observed after adjustment for fasting serum glucose, pulmonary function tests, white-cell differential, platelet count, and mean corpuscular hemoglobin and when the analysis was restricted to persons with ≥3 years of follow-up (data not shown). Tests for interaction by hemoglobin level were not significant (all P values for interaction >0.1). However, in sensitivity analyses among persons with anemia (10.2% of the study population), we observed stronger associations of low HbA1c with all-cause mortality (1.60, 95% CI: 1.07–2.39), cancer mortality (1.94, 95% CI: 1.03–3.65), and cardiovascular death (1.51, 95% CI: 0.74–3.09). Among participants without anemia, the results were somewhat attenuated but remained significant for all-cause mortality (1.22, 95% CI: 1.03–1.46) and cancer death (1.35, 95% CI: 1.05–1.74).

Table 3

Adjusted HRs (95% CI) for cause-specific mortality by HbA1c category

Compared with persons with HbA1c 5.0 to <5.7%, those persons with high HbA1c (≥6.5%) also had an increased risk of all-cause mortality and deaths from cancer, cardiovascular disease, diseases of the respiratory system, digestive system and liver disease, diseases of the genitourinary system, and kidney disease (Table 3). Similar results were observed among persons with and without anemia, after adjustment for fasting serum glucose, pulmonary function tests, white-cell differential, platelet count, mean corpuscular hemoglobin, and when the analysis was restricted to persons with ≥3 years of follow-up (data not shown).

There were 353 hospitalizations for liver disease during follow-up. Fig. 1 presents the adjusted HRs and 95% CIs from the restricted cubic spline model for the association of baseline HbA1c with risk of hospitalization for liver disease. We observed a pronounced J-shaped association; both very low and high HbA1c values were associated with an increased risk of liver hospitalization in this population.

Figure 1

Adjusted HRs (95% CI) for liver disease hospitalization by baseline HbA1c value. The HR is expressed per absolute increase in one percentage point in the HbA1c value at baseline. The shaded area is the 95% CI from the restricted cubic spline model. The model is centered at the median (5.5%) with knots at the 20th, 40th, 60th, and 80th percentiles. The HRs were adjusted for age, sex, race, field center, total and HDL cholesterol, BMI, waist-to-hip ratio, hypertension, family history of diabetes, education level, alcohol use, physical activity, smoking status, hemoglobin, MCV, fibrinogen, and leukocyte count. The HRs are shown on a natural log scale.

CONCLUSIONS

Compared with persons with HbA1c in the normal range, we found that low HbA1c values (<5.0%) were associated with an increased risk of all-cause mortality and death from various causes, including cancer. This finding is consistent with previous studies that have documented a J- or U-shaped association between HbA1c and all-cause mortality (3,7,8,10,11). Our results are contrary to a recent analysis of the European Prospective Investigation of Cancer-Norfolk Study that reported no increase in risk of mortality at low normal HbA1c values in a relatively homogenous population of Caucasians (20). The debate regarding the HbA1c–mortality association also extends to the mortality curves for fasting and 2-h glucose, which are reported to be J-shaped in some studies (21–24). No single cause of death appeared to drive the association between low HbA1c and risk of death in our study. Prior studies have suggested that liver disease may be an important contributor to low HbA1c values (10,12). Consistent with this hypothesis, we observed that low HbA1c values were associated with an increased risk of hospitalization due to liver disease in the ARIC Study population.

The exact mechanisms underlying the association of low HbA1c values with increased risk of death are not known. We found that low HbA1c values were significantly associated with higher mean red-cell volume and low hemoglobin. Other studies have demonstrated associations of red-cell indices with long-term mortality in the general population (25,26). The etiology of these relationships is largely unclear but may reflect that some disease processes affect red-cell turnover and thus HbA1c. Whereas high HbA1c values are almost entirely determined by circulating glucose levels (27), it is probable that nonglycemic factors such as red-cell turnover may be of disproportionate importance in the low range of HbA1c. Our observations suggest low HbA1c values may be a general marker of ill health, analogous to the well-documented U-shaped association between cholesterol and mortality (28–30). There is strong evidence that some disease processes reduce circulation of cholesterol-bearing lipoproteins (28). Low HbA1c values may characterize a mix of healthy people and a group in which low HbA1c is a marker or signal of underlying health issues, such as liver disease or early stages of cancer (3,10). This hypothesis is consistent with the negative confounding observed in our study; once traditional risk factors for death were added to our models, the association of low HbA1c with all-cause mortality and cause-specific mortality became stronger.

Certain limitations should be considered in interpreting the results of this study. We were limited to information available from the death certificate for the classification of the underlying cause of death, which may have resulted in misclassification, particularly for the noncancer and noncardiovascular outcomes (31–33). We also did not have data available on iron indices, reticulocyte count, or red-cell distribution width. Nonetheless, the ARIC population is a large, diverse cohort with virtually complete follow-up for vital status and comprehensive information on important confounders and major risk factors for mortality. To our knowledge, this study is one of the first comprehensive examinations of the association of low HbA1c with cause-specific mortality in a general population.

In conclusion, our results add to the evidence that low HbA1c values may be a marker of elevated mortality risk in the general population. With new recommendations for the use of HbA1c for diagnosis of diabetes, HbA1c testing will undoubtedly increase. To the extent that HbA1c is adopted for diabetes screening, clinicians will frequently observe HbA1c values in the normal range, including low normal values. Additional work should focus on understanding the nonglycemic determinants of HbA1c and the mechanisms that explain why low HbA1c values in some individuals may reflect an elevated risk state.