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Low Hemoglobin A1c and Risk of All-Cause Mortality Among US Adults Without Diabetes

Originally publishedhttps://doi.org/10.1161/CIRCOUTCOMES.110.957936Circulation: Cardiovascular Quality and Outcomes. 2010;3:661–667

Abstract

Background—

Among individuals without diabetes, elevated hemoglobin A1c (HbA1c) has been associated with increased morbidity and mortality, but the literature is sparse regarding the prognostic importance of low HbA1c.

Methods and Results—

National Health and Nutrition Examination Survey III (NHANES III) participants, 20 years and older, were followed up to 12 years (median follow-up, 8.8 years) for all-cause mortality. Cox proportional hazards regression was used to calculate hazard ratios (HR) and 95% confidence intervals (CI) for the association between HbA1c levels and all-cause mortality for 14 099 participants without diabetes. There were 1825 deaths during the follow-up period. Participants with a low HbA1c (<4.0%) had the highest levels of mean red blood cell volume, ferritin, and liver enzymes and the lowest levels of mean total cholesterol and diastolic blood pressure compared with their counterparts with HbA1c levels between 4.0% and 6.4%. An HbA1c <4.0% versus 5.0% to 5.4% was associated with an increased risk of all-cause mortality (HR, 3.73; 95% CI, 1.45 to 9.63) after adjustment for age, race-ethnicity, and sex. This association was attenuated but remained statistically significant after further multivariable adjustment for lifestyle, cardiovascular factors, metabolic factors, red blood cell indices, iron storage indices, and liver function indices (HR, 2.90; 95% CI, 1.25 to 6.76).

Conclusions—

In this nationally representative cohort, low HbA1c was associated with increased all-cause mortality among US adults without diabetes. Additional research is needed to confirm these results and identify potential mechanisms that may be underlying this association.

Introduction

Elevated hemoglobin A1c (HbA1c) is associated with an increased risk of cardiovascular events among individuals without diabetes.1,2 Additionally, a recent study identified HbA1c as a better predictor of cardiovascular events compared with fasting plasma glucose.3 Recently, the American Diabetes Association updated its clinical practice recommendations to include HbA1c as a diagnostic test for diabetes,4 which may lead to its widespread use. Along with the broad use of this assay, low HbA1c values, including very low values <4.0%, may be frequently detected. HbA1c ≥6.5% has been shown to be reasonable for diagnosing diabetes,5 but it is currently uncertain how to interpret low HbA1c values.

Very low HbA1c values among persons without diabetes may reflect underlying biological processes. Certain health conditions that decrease erythrocyte life span (eg, iron-deficiency anemia) are known to alter HbA1c values and make them unreliable.6 However, little is known about other biological factors that result in low HbA1c values among individuals without diabetes. Low HbA1c may not reflect metabolic control among individuals without diabetes but may be reflecting other biological factors, such as red blood cell markers, inflammation, or decreased liver function. The purpose of the present study is to describe the health characteristics of individuals with low HbA1c levels, evaluate the association between low HbA1c and all-cause mortality, and assess the underlying biological processes that explain the association in a nationally, representative sample of US adults without diabetes.

Methods

Study Population

The National Health and Nutrition Examination Survey (NHANES III) is a stratified, multistage probability survey designed to select a representative sample of the civilian noninstitutionalized US population. Overall, 18 825 adults 20 years of age and older completed the NHANES III interview and examination between 1988 and 1994. Through linkage with the National Death Index, participants were followed for mortality through December 31, 2000. We excluded 3022 participants without plasma glucose or HbA1c measurements and 19 participants who did not have follow-up information. We also excluded 1685 participants with diabetes, defined as a fasting plasma glucose ≥126 mg/dL, a nonfasting plasma glucose ≥200 mg/dL, an HbA1c ≥6.5%, and/or a self-reported history of diabetes with concurrent use of antidiabetes medication. After these exclusions, 14 099 NHANES III participants without diabetes were included in the current analyses. The protocol for NHANES III was approved by the National Center for Health Statistics of the Centers for Disease Control and Prevention Institutional Review Board. All participants gave informed consent.

  • WHAT IS KNOWN

  • The widespread use of hemoglobin A1c as a diagnostic test for diabetes may lead to the frequent detection of low hemoglobin A1c values.

  • The prognostic importance of low hemoglobin A1c values is unknown, but the natural assumption of clinicians may be that low HbA1c is beneficial.

  • WHAT THE STUDY ADDS

  • Hemoglobin values as low as 2.8% were detected in this nationally representative study.

  • Low hemoglobin A1c (<4.0%) was associated with an increased risk of all-cause mortality.

  • Other biological processes, such as inflammation and liver function, may underlie the association between low hemoglobin A1c (<4.0%) and all-cause mortality.

Baseline Data Collection

NHANES III baseline data were collected during an in-home interview and a subsequent visit to a mobile examination center when a physician conducted a medical evaluation. During the in-home interview, researchers collected sociodemographic, behavioral, and health-related information including prior diagnoses and current medications using a standardized questionnaire. The medical evaluation included a physical examination, anthropometric and blood pressure measurements, and blood and spot-urine collection.

HbA1c was analyzed at the University of Missouri at Columbia using standardized laboratory protocols and quality control procedures. HbA1c was measured using a Bio-Rad Diamat ion exchange high-performance liquid chromatography system. If hemoglobin variants (hemoglobin C, D, F, and S) were present, an alternative standardized affinity chromatographic method was used. HbA1c results <4% or >11% were repeated for verification. Two quality control approaches were used for HbA1c: (1) batch quality control specimens placed in each run, and (2) 5% sample specimens randomly selected from each batch which were reanalyzed in another batch. The coefficients of variation ranged from 2.0% to 3.1% for low HbA1c quality control pools and 1.1% to 2.3% for high HbA1c quality control pools.7

Mortality Follow-Up

The current analyses use participant follow-up for mortality through December 31, 2000. The method of probabilistic matching was used to link NHANES III participants with the National Death Index to ascertain vital status. Identical matching methodology applied to the NHANES I Epidemiological Follow-up Study for validation purposes found that 96.1% of deceased participants and 99.4% of living participants were correctly classified.8

Statistical Methods

Characteristics of participants were calculated by a priori–defined groups of HbA1c levels (<4.0%, 4.0% to 4.4%, 4.5% to 4.9%, 5.0% to 5.4%, 5.5% to 5.9%, and 6.0% to 6.4%) as means for continuous variables and proportions for categorical variables. All-cause mortality rates, standardized to the age distribution of US adults without diabetes, were calculated by HbA1c levels. Follow-up for each study participant was calculated as the time between their NHANES III examination and the date of death or December 31, 2000, for those still alive at the end of follow-up.

Cox proportional hazards regression was used to obtain the hazard ratios (HR) and 95% confidence intervals (CI) for all-cause mortality associated with HbA1c levels. The variables for the models were selected and grouped into 7 categories, based on possible associations with HbA1c and all-cause mortality: (1) demographic factors including age, race-ethnicity, and sex; (2) lifestyle factors including education, income, current smoking, alcohol consumption, physical inactivity, body mass index, and aspirin use; (3) cardiovascular factors including systolic blood pressure, antihypertensive medication use, total and HDL cholesterol, log-transformed triglycerides, C-reactive protein, and history of cardiovascular disease (CVD); (4) metabolic factors including prior diagnosis of thyroid disease, thyroid-stimulating hormone, estimated glomerular filtration rate, and albuminuria; (5) red blood cell indices including hemoglobin, red blood cell distribution width, mean cell volume, and serum folate; (6) iron storage indices including serum albumin, ferritin, and transferrin saturation; and (7) liver function indices including hepatitis C seropositivity, aspartate aminotransferase (AST), and alanine aminotransferase (ALT). The initial model adjusted for demographic factors. The next set of models adjusted for demographic factors plus each of the other variable groups listed above, separately. The final model adjusted for demographic factors plus all of the other variable groups, simultaneously. To further explore the HR for all-cause mortality across the full range of HbA1c levels, we used restricted quadratic splines with knots at the 2.5, 10, 50, 90, and 97.5 percentiles (4.3%, 4.7%, 5.3%, 5.9%, and 6.2%, respectively) in a full multivariable adjusted Cox regression model.

Sensitivity analyses were conducted after excluding participants with cancer, CVD, anemia, and hepatitis C in the full multivariable adjusted model. The proportional hazards assumption was assessed using Schoenfeld residuals, and no violations were noted. Data were analyzed using SUDAAN (version 9.0; Research Triangle Institute, Research Triangle Park, NC) and R (version 2.6.1; R Foundation for Statistical Computing, Vienna, Austria) to account for the complex NHANES sampling design, including unequal probabilities of selection, oversampling, and nonresponse.

Results

Characteristics of Participants Without Diabetes by HbA1c Level

The range of HbA1c values was 2.8% to 6.4%, and 97 participants had an HbA1c <4.0%. Participants with an HbA1c between 4.0% and 4.4% were the youngest; those with an HbA1c of 6.0 to 6.4% were the oldest (Table 1). A greater proportion of participants with HbA1c <5.0% were women. J-shaped or U-shaped associations were present for age, non-Hispanic black race-ethnicity, less than a high school education, and annual household income <$20 000. Mean cell volume, ferritin, transferrin saturation, and liver function measures were elevated in the HbA1c <4.0% group, whereas mean systolic blood pressure, diastolic blood pressure, total cholesterol, and median triglyceride levels were lowest among those with an HbA1c <4.0%. Also, the proportion of participants with a history of CVD disease was lowest among those with an HbA1c <4.0%, whereas the proportion of participants with hepatitis C was the highest in this group.

Table 1. Characteristics of NHANES III Participants Without Diabetes (n=14 099) by HbA1c Levels, 1988 to 1994

HbA1c%
<4.0 (n=97)4.0–4.4 (n=469)4.5–4.9 (n=2675)5.0–5.4 (n=5691)5.5–5.9 (n=4035)6.0–6.4 (n=1132)
Demographics
    Age, y40.1 (2.3)34.6 (0.7)36.5 (0.4)42.9 (0.5)51.6 (0.7)57.3 (0.9)
    Women, %57.160.259.850.446.150.4
    Race-ethnicity
    Non-Hispanic white, %64.381.483.578.471.658.9
    Non-Hispanic black, %27.010.25.98.314.525.2
    Mexican-American, %2.83.04.45.35.64.7
    Other, %5.95.46.28.08.311.3
Lifestyle factors
    Less than high school education, %18.113.517.521.632.239.7
    Income <$20 000, %42.331.626.830.437.744.8
    Current smoker, %20.519.525.329.932.436.6
    Consumes alcohol, %52.765.062.156.847.838.0
    Body mass index, kg/m225.4 (0.9)24.7 (0.2)25.1 (0.1)26.1 (0.1)27.5 (0.1)28.7 (0.4)
    Physically inactive, %17.218.117.319.427.130.3
    Aspirin use, %21.536.836.237.939.439.5
Cardiovascular factors
    Systolic blood pressure, mm Hg115.0 (2.6)115.0 (0.6)115.8 (0.4)120.8 (0.5)127.0 (0.5)132.3 (0.9)
    Diastolic blood pressure, mm Hg67.2 (1.6)72.2 (0.5)72.2 (0.3)74.3 (0.2)75.6 (0.2)76.5 (0.5)
    Antihypertensive medication use, %3.35.36.68.914.628.0
    Total cholesterol, mmol/L4.42 (0.13)4.81 (0.08)4.98 (0.03)5.25 (0.03)5.54 (0.03)5.73 (0.05)
    HDL cholesterol, mmol/L1.39 (0.05)1.42 (0.03)1.36 (0.01)1.33 (0.01)1.27 (0.01)1.22 (0.02)
    Triglycerides, mmol/L, median (interquartile range)1.00 (0.73–1.26)1.03 (0.75–1.54)1.12 (0.79–1.68)1.20 (0.86–1.80)1.42 (0.98–2.08)1.55 (1.07–2.37)
    Elevated CRP, %22.418.622.026.535.849.4
    History of cardiovascular disease, %0.01.01.63.26.89.5
Metabolic factors
    Prior diagnosis of thyroid disease, %1.94.32.74.15.54.9
    Thyroid-stimulating hormone, mU/L1.9 (0.1)1.7 (0.1)1.8 (0.04)2.0 (0.04)2.4 (0.2)2.6 (0.3)
    Estimated GFR, mL/min/1.73 m2104.6 (4.6)100.6 (1.9)98.6 (0.6)93.5 (0.6)87.8 (0.6)83.1 (1.1)
    Albuminuria, %13.99.35.37.710.717.5
    Fasting plasma glucose, mmol/L*4.93 (0.09)4.98 (0.05)5.10 (0.02)5.23 (0.02)5.49 (0.02)5.74 (0.04)
Red blood cell indices
    Hemoglobin, g/L132 (2)138 (1)140 (1)142 (1)142 (1)141 (1)
    Red blood cell distribution width, %13.54 (0.26)12.65 (0.06)12.71 (0.02)12.88 (0.02)13.22 (0.03)13.52 (0.04)
    Mean cell volume, fL93.3 (1.0)91.7 (0.4)90.6 (0.1)90.1 (0.2)89.3 (0.2)88.4 (0.3)
    Serum folate, nmol/L18.4 (2.7)15.2 (0.9)15.2 (0.5)15.2 (0.5)15.6 (0.7)17.4 (0.9)
Iron storage indices
    Serum albumin, g/L39.7 (1.4)42.7 (0.5)42.4 (0.2)42.1 (0.2)41.3 (0.2)40.5 (0.2)
    Ferritin, μg/L192.4 (38.7)129.7 (11.1)108.9 (3.3)119.8 (2.5)134.5 (3.4)150.7 (4.5)
    Transferrin saturation, %36.3 (3.4)28.9 (0.8)27.8 (0.5)26.4 (0.3)25.0 (0.3)24.1 (0.6)
Liver function indices
    Hepatitis C seropositivity, %11.12.31.42.42.33.1
    AST, U/L33.9 (5.9)20.8 (0.8)20.7 (0.3)21.3 (0.3)21.6 (0.2)22.2 (0.6)
    ALT, U/L20.1 (3.3)16.6 (1.0)16.9 (0.4)17.7 (0.5)17.8 (0.5)17.8 (0.8)

Values are means (SE) unless otherwise noted.

GFR indicates glomerular filtration rate; CRP, C-reactive protein; albuminuria, urinary albumin-to-creatinine ratio ≥30 mg/g; and elevated CRP, CRP ≥3 mg/L.

To convert SI units to conventional units, divide by the following conversion factors: cholesterol (0.02586); triglycerides (0.01129); fasting plasma glucose (0.05551); serum folate (2.266).

*Limited to the fasting subsample of NHANES III participants. The sample size from lowest to highest HbA1c levels was n=45, 196, 1213, 2582, 1819, and 487.

HbA1c and All-Cause Mortality Among Participants Without Diabetes

Over a median follow-up of 8.8 years, 1825 deaths occurred among participants without diabetes. The age-standardized mortality rates by HbA1c level for participants without diabetes and the HRs (95% CIs) for all-cause mortality associated with HbA1c levels are presented in Table 2. Using the group with HbA1c of 5.0% to 5.4% as the reference, HbA1c levels <4.0% were associated with an increased risk of all-cause mortality after adjustment for age, race-ethnicity, and sex (HR, 3.73; 95% CI, 1.45 to 9.63). The HRs increased after separate adjustment for lifestyle factors, cardiovascular factors, or iron storage. In contrast, the HRs were attenuated after separate adjustment for metabolic factors (HR, 3.47; 95% CI, 1.36 to 8.82), red blood cell indices (HR, 2.68; 95% CI, 1.08 to 6.66), or liver function measures (HR, 3.09; 95% CI, 1.20 to 7.92). In the multivariable adjusted model including all variable categories, the HR for all-cause mortality among those with an HbA1c <4.0% versus 5.0% to 5.4% remained almost 3 times as high and statistically significant (HR, 2.90; 95% CI, 1.25 to 6.76).

Table 2. Age-Standardized Mortality Rates and HRs (95% CI) for the Association Between HbA1c and All-Cause Mortality Among NHANES III Participants Without Diabetes

HbA1c, %
<4.0 (n=97)4.0–4.4 (n=469)4.5–4.9 (n=2675)5.0–5.4 (n=5691)5.5–5.9 (n=4035)6.0–6.4 (n=1132)
Mortality rate*39.313.59.98.811.312.5
    Model 13.73 (1.45–9.63)1.31 (0.68–2.53)1.08 (0.80–1.46)1 (ref)1.23 (1.06–1.42)1.28 (1.06–1.54)
    Model 2a3.97 (1.54–10.2)1.37 (0.74–2.54)1.13 (0.84–1.52)1 (ref)1.19 (1.02–1.38)1.25 (1.04–1.51)
    Model 2b3.97 (1.54–10.2)1.27 (0.66–2.44)1.11 (0.81–1.50)1 (ref)1.22 (1.05–1.42)1.25 (1.03–1.51)
    Model 2c3.47 (1.36–8.82)1.36 (0.71–2.60)1.16 (0.87–1.55)1 (ref)1.21 (1.04–1.42)1.23 (1.02–1.47)
    Model 2d2.68 (1.08–6.66)1.28 (0.69–2.35)1.01 (0.76–1.34)1 (ref)1.27 (1.09–1.47)1.41 (1.16–1.70)
    Model 2e3.87 (1.53–9.80)1.30 (0.67–2.50)1.12 (0.83–1.52)1 (ref)1.18 (1.00–1.38)1.26 (1.05–1.51)
    Model 2f3.09 (1.20–7.92)1.29 (0.67–2.48)1.11 (0.82–1.50)1 (ref)1.22 (1.06–1.42)1.31 (1.10–1.58)
    Model 32.90 (1.25–6.76)1.26 (0.69–2.28)1.16 (0.86–1.55)1 (ref)1.26 (1.05–1.50)1.30 (1.06–1.60)

*Mortality rate standardized to age distribution of participants without diabetes, per 1000 person-years.

Model 1: Adjusted for age, race-ethnicity, and sex.

Model 2a: Adjusted for Model 1 variables plus education, income, current smoking, alcohol consumption, physical activity, body mass index, and aspirin use.

Model 2b: Adjusted for Model 1 variables plus cardiovascular factors (systolic blood pressure, antihypertensive medication use, total cholesterol, HDL cholesterol, log triglycerides, C-reactive protein, and history of CVD).

Model 2c: Adjusted for Model 1 variables plus metabolic factors (prior diagnosis of thyroid disease, thyroid stimulating hormone, estimated glomerular filtration rate, and albuminuria).

Model 2d: Adjusted for Model 1 variables plus red blood cell indices (hemoglobin, red blood cell distribution width, mean cell volume, and serum folate).

Model 2e: Adjusted for Model 1 variables plus iron storage indices (serum albumin, ferritin, and transferrin saturation).

Model 2f: Adjusted for Model 1 variables plus liver function indices (hepatitis C seropositivity, AST, and ALT).

Model 3: Adjusted for age, race-ethnicity, sex, education, income, current smoking, alcohol consumption, physical activity, body mass index, aspirin use, cardiovascular factors (systolic blood pressure, antihypertensive medication use, total cholesterol, HDL cholesterol, log triglycerides, C-reactive protein, and history of CVD), metabolic factors (prior diagnosis of thyroid disease, thyroid-stimulating hormone, estimated glomerular filtration rate, and albuminuria), red blood cell indices (hemoglobin, red blood cell distribution width, mean cell volume, and serum folate), iron storage indices (serum albumin, ferritin, and transferrin saturation), and liver function indices (hepatitis C seropositivity, AST, and ALT).

Elevated HbA1c among participants without diabetes was also associated with an increased risk of all-cause mortality. An HbA1c of 6.0% to 6.4%, compared with an HbA1c of 5.0% to 5.4%, was associated with an increased risk of all-cause mortality after adjustment for age, race-ethnicity, and sex (HR, 1.28; 95% CI, 1.06 to 1.54). This association persisted after separate adjustment for the different variable groups and in the full multivariable adjusted model (HR, 1.30; 95% CI, 1.06 to 1.60).

Using a multivariable adjusted quadratic spline with HbA1c=5.2% as the cut-point, low and high HbA1c values were associated with an increased risk of all-cause mortality among individuals without diabetes (Figure).

Figure.

Figure. Adjusted hazard ratios for the association between HbA1c and all-cause mortality among participants without diabetes using a quadratic spline with knots at the 2.5, 10, 50, 90, and 97.5 percentiles. Adjusted for age, race-ethnicity, sex, lifestyle factors (education, income, current smoking, alcohol consumption, physical activity, body mass index, and aspirin use), cardiovascular factors (systolic blood pressure, antihypertensive medication use, total cholesterol, HDL cholesterol, log triglycerides, elevated C-reactive protein, and history of CVD), metabolic factors (prior diagnosis of thyroid disease, thyroid-stimulating hormone, estimated glomerular filtration rate, and albuminuria), red blood cell indices (hemoglobin, red blood cell distribution width, mean cell volume, and serum folate), iron storage indices (serum albumin, ferritin, and transferrin saturation), and liver function indices (hepatitis C seropositivity, AST, and ALT). Knots were placed at 4.3%, 4.7%, 5.3%, 5.9%, and 6.2%, representing HbA1c levels at the 2.5, 10, 50, 90, and 97.5 percentiles; shaded area represents 95% CI.

Sensitivity Analyses

Sensitivity analyses excluding participants with cancer, CVD, anemia, or hepatitis C are presented in Table 3. Among participants with an HbA1c <4.0%, excluding those with anemia increased the effect estimate (HR, 3.56; 95% CI, 1.38 to 9.17), whereas excluding those with hepatitis C attenuated the association (HR, 2.26; 95% CI, 0.96 to 5.42). After excluding participants with cancer, CVD, anemia, or hepatitis C, the association between low HbA1c and all-cause mortality persisted (HR, 2.81; 95% CI, 1.03 to 7.67). For participants with an HbA1c of 6.0% to 6.4%, compared with participants with an HbA1c of 5.0% to 5.4%, the association with all-cause mortality also persisted (HR, 1.41; 95% CI, 1.05 to 1.89) after excluding participants with cancer, CVD, anemia, or hepatitis C.

Table 3. Sensitivity Analyses for the Association Between HbA1c and All-Cause Mortality Among NHANES III Participants Without Diabetes

HbA1c, %
<4.0 (n=97)4.0–4.4 (n=469)4.5–4.9 (n=2675)5.0–5.4 (n=5691)5.5–5.9 (n=4035)6.0–6.4 (n=1132)

Model*HR (95% CI)
Model 12.88 (1.17–7.09)1.27 (0.70–2.33)1.21 (0.90–1.64)1 (ref)1.33 (1.11–1.59)1.40 (1.13–1.74)
Model 22.87 (1.25–6.62)1.08 (0.56–2.05)1.17 (0.85–1.60)1 (ref)1.31 (1.08–1.60)1.34 (1.04–1.73)
Model 33.56 (1.38–9.17)1.15 (0.64–2.05)1.24 (0.88–1.75)1 (ref)1.34 (1.09–1.64)1.29 (1.03–1.63)
Model 42.26 (0.96–5.42)0.92 (0.50–1.62)1.06 (0.78–1.42)1 (ref)1.23 (1.04–1.44)1.27 (1.04–1.54)
Model 52.81 (1.03–7.67)0.95 (0.54–1.94)1.16 (0.79–1.87)1 (ref)1.40 (1.10–1.76)1.41 (1.05–1.89)

Model 1 excludes n=530 with cancer.

Model 2 excludes n=814 with CVD.

Model 3 excludes n=1741 with anemia.

Model 4 excludes n=349 with hepatitis C.

Model 5 excludes n=3086 with cancer, CVD, anemia, or hepatitis C.

*Adjusted for age, race-ethnicity, sex, education, income, current smoking, alcohol consumption, physical activity, body mass index, aspirin use, cardiovascular factors (systolic blood pressure, antihypertensive medication use, total cholesterol, HDL cholesterol, log triglycerides, C-reactive protein, and history of CVD [except models 2 and 5]), metabolic abnormalities (estimated glomerular filtration rate, albuminuria, prior diagnosis of thyroid disease, and thyroid-stimulating hormone), red blood cell indices (hemoglobin, red blood cell distribution width, mean cell volume, and serum folate), iron storage indices (serum albumin, ferritin, and transferrin saturation), and liver function indices (hepatitis C seropositivity [except Models 4 and 5], AST, and ALT).

Discussion

In the present study of US adults, very low HbA1c was associated with an increased risk of all-cause mortality among persons without diabetes. Specifically, an HbA1c <4.0% was associated with a large, statistically significant association with all-cause mortality that persisted after multivariable adjustment for demographic, lifestyle, cardiovascular factors, metabolic factors, red blood cell indices, iron storage, and liver function indices. Although participants with HbA1c <4.0% had a generally favorable cardiovascular profile at their baseline NHANES III examination, red blood cell indices, iron storage, and liver function measures were generally adverse compared with participants with higher HbA1c levels. However, participants with an HbA1c <4.0% still had an increased risk of all-cause mortality after adjusting for these differences.

Low HbA1c values are likely to be detected among persons without diabetes as the use of HbA1c as a diagnostic test for diabetes becomes part of routine clinical practice, as recently recommended by the American Diabetes Association.4 However, the clinical relevance of low HbA1C values remains unclear, with little known about the biological underpinning of such low values or the diagnostic and prognostic utility of such observations. Although several studies have investigated the association of HbA1c levels with morbidity and mortality among persons without diabetes,914 they primarily have focused on the association between higher levels of HbA1c and health outcomes. Elevated HbA1c has been associated with an increased risk of cardiovascular morbidity1,10,12,15 and mortality9,14,1618 among persons without diabetes, although some of these associations were attenuated after adjustment for cardiovascular factors. In contrast, a couple of studies have also reported that elevated HbA1c is not associated with cardiovascular morbidity13 and mortality11 among persons without diabetes. In the present study, participants with an HbA1c of 5.5% to 5.9% or 6.0% to 6.4% had an increased risk of all-cause mortality that remained after adjusting for various biological factors.

Few of the studies investigating outcomes by HbA1c level for individuals without diabetes have reported the morbidity and mortality risk associated with low levels. The lowest HbA1c quintile evaluated in the Women's Health Study included values between 2.27% and 4.79%, with a median HbA1c of 4.7%.18 The association between HbA1c and mortality, modeled using a cubic spline, indicated an increased risk for mortality at higher levels of HbA1c, but no increased risk was observed at lower HbA1c levels. In contrast, in the Atherosclerosis Risk in Communities Study, low HbA1c levels (<5.0%) were not associated with coronary heart disease, ischemic stroke, or diabetes but were associated with an increased risk of all-cause mortality after adjusting for cardiovascular risk factors and baseline fasting glucose.3

It is not clear what biological processes may be underlying the association between low HbA1c and all-cause mortality. In the present study, participants with low HbA1c values had unfavorable profiles of red blood cell related factors, iron storage, and liver function. Red blood cell distribution width has been associated with an increased risk of morbidity and mortality1922 and with inflammation.21 Younger red blood cells are typically larger and have more size variability compared with older red blood cells,20 which could affect red blood cell distribution width and HbA1c values.6,23 Also, iron stores were elevated among those with low HbA1c in the present study. Iron-deficiency anemia has been associated with increased HbA1c values among individuals without diabetes; however, patients with iron-deficiency anemia who were treated with iron therapy had decreases in HbA1c values.24,25 Also, ferritin was elevated among those with low HbA1c in the present study. Elevated ferritin has been associated with an increased risk of myocardial infarction26 and atherosclerosis,27 but it is also an acute-phase reactant and could be reflecting an inflammatory response. Finally, liver function enzymes were elevated and hepatitis C was more prevalent among those with an HbA1c <4.0% in the present study. Elevated ALT is frequently associated with fatty liver and an adverse cardiometabolic risk factor profile. ALT was positively associated with HbA1c levels among participants with and without diabetes in the British Women's Heart and Health Study, with a stronger association noted among women without diabetes.28 Certain combination therapies for hepatitis C have been reported to temporarily lower HbA1c levels29; however, the low HbA1c-mortality association observed in the present study was still evident after excluding participants with hepatitis C. The overall biomarker profile of the participants without diabetes and an HbA1c <4.0% is suggestive of red blood cell markers, inflammation, and liver function as part of the biological underpinning for the association noted in this study. However, the low HbA1c and all-cause mortality association observed in this study persisted after adjustment for lifestyle, cardiovascular, metabolic, red blood cell, iron storage, and liver function indices. The association also persisted after excluding participants with cancer, CVD, anemia, or hepatitis C in sensitivity analyses. Additional research is needed to explore the potential health effects of aberrations in red blood cell markers, inflammation, and liver function indices and associations with all-cause mortality among individuals with a low HbA1c.

The present study should be interpreted in the context of possible limitations. HbA1c results may be affected by hemoglobin variants and conditions that affect erythrocyte life span (eg, hemolytic anemia).6,30 The study protocol included an alternative testing method for hemoglobin variants, but data were not available to assess decreased red blood cell survival. When participants with anemia and other conditions were excluded from the analysis, low HbA1c remained associated with all-cause mortality. Despite these limitations, this study has multiple strengths including the large, ethnically diverse, and representative sample of US adults without diabetes. Additionally, NHANES III included a broad collection of data using a standardized protocol and core laboratory measurement of HbA1c with quality control procedures that allowed for the assessment of extensive participant characteristics by HbA1c level.

The present findings may have important clinical implications as the use of HbA1c as a diagnostic test for diabetes becomes more common. Low HbA1c levels will be identified in clinical practice and the natural assumption may be that low HbA1c is beneficial. However, the findings from the present study suggest that there may be a threshold below which HbA1c is associated with an increased all-cause mortality risk among persons without diabetes. Additional research is needed to confirm these results and identify the putative mechanisms involved in this association.

Sources of Funding

Dr Laclaustra is supported by CP08/00112 “Miguel Servet” Grant (Instituto de Salud Carlos III, Spain).

Disclosures

Dr McGuire received consultancy fees <$10 000 from Tethys Bioscience, Biosite, Inc, F. Hoffmann La Roche, and Daiichi Sankyo.

Footnotes

Correspondence to April P. Carson, PhD,
Department of Epidemiology, University of Alabama at Birmingham, 1530 3rd Ave South, RPHB 230N, Birmingham, AL 35294-0022
. E-mail

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