Serum lactate dehydrogenase (LDH) is known pathologic marker for a diversity of diseases, including myocardial ischemia. Strenuous and enduring physical activity can transiently induce a greater total LDH level, still within its normal range. To date, however, it has not been determined whether normal-range LDH might be inversely associated with coronary artery disease (CAD) in the low-cardiovascular-risk, physically active, adult population. We conducted a retrospective cohort analysis. A total of 5,519 healthy adults aged 34 to 86 years were followed up for a mean period of 4.2 years. The cohort incidence of CAD was 6.1% (338 cases) from 2001 to 2009. In the present cohort, greater mean LDH levels were significantly associated with a greater number of years, days/week, and minutes/week of leisure time activity (p = 0.02, p = 0.04, and p = 0.01, respectively). These associations were externally validated successfully by analysis of all 5,064 healthy participants aged ≥40 years with normal-range LDH from the 2007 to 2010 National Health and Nutrition Examination Surveys combined. For instance, the mean LDH level was significantly greater in those engaged in 6 to 7 versus 1 to 5 days/wk of vigorous-intensity work activity (138.0 ± 20.7 IU/L vs 133.3 ± 21.7 IU/L, respectively, p = 0.007). In our cohort, the hazard ratio for CAD according to the normal total serum LDH tertiles, adjusted for multiple risk and protective CAD factors in a Cox proportional hazards model, was 0.70 (95% confidence interval 0.54 to 0.92) in the greater versus lower tertile (p for trend = 0.01). In conclusion, we suggest that increased normal-range total serum LDH is associated with reduced short-term risk of CAD outcome in this low-risk, physically active population.
Lactate dehydrogenase (LDH) is a cytoplasmic enzyme found in all body cells that catalyzes the reversible conversion of pyruvate, the primary end-product of glycolysis, to lactate as a part of the lactic acid (Cori) cycle. This metabolic pathway is accelerated under anaerobic physiologic conditions such as during increased power output of skeletal muscle activity. Using the cell-to-cell lactate shuttle and their tissue-specific LDH isoenzyme, both skeletal and cardiac muscles avidly oxidize lactate as a fuel, accounting for ≤60% of the energy supply in the myocardium. Physical activity can immediately and transiently induce between minimal, nonsignificant, and ≤25% elevation of total serum LDH after the most strenuous and enduring types of exercise, but still within the normal laboratory values. Abnormal serum LDH levels, greater than the upper laboratory limit, is a nonspecific, clinical, pathologic marker of many medical conditions, including myocardial ischemia, muscle injury, hemolysis, and malignant melanoma. We wondered whether, because it is related to strenuous physical activity and reversible ischemic conditions, all within the normal body physiology, normal-range LDH could be inversely associated with coronary artery disease (CAD) in the low-risk, physically active, ambulatory adult population. To test this a priori hypothesis, we conducted a retrospective cohort analysis of a highly selected, healthy, ambulatory population.
Methods
The Executive Screening Survey performs annual screening examinations of apparently healthy adult men and women of the general ambulatory population at the Chaim Sheba Medical Center (Tel Hashomer, Israel). The full methods used by this routine program have been previously published. The computerized database, established in 2000, was the data source for the present study.
Included in the present retrospective analysis of this convenience sample were all men and women, aged >34 years, who met all the following criteria at baseline: (1) no history of type 1 or 2 diabetes mellitus; (2) no history of heart, cardiovascular, cerebrovascular, or vascular illness, in particular, CAD, congestive heart failure, arrhythmia, valvular heart disease, ischemic cerebral stroke, peripheral vascular disease, or hypertension; (3) no history of hyperlipidemia; (4) not taking permanent medications; (5) measured serum LDH value at baseline within the normal range of the medical center’s central laboratory (100 to 260 IU/L during the entire study period); (6) a record of ≥1 follow-up visit that was ≥1 year apart from the baseline visit; and (7) complete demographic data (age and gender). The final retrospective cohort included 5,519 apparently healthy ambulatory persons with a relatively low risk of cardiovascular disease, who were followed up from May 2001 to December 2009.
We defined a composite study end point of the time-to-event of any of the following events: (1) newly diagnosed CAD (the main morbidity outcome measure, as detailed in the following section); (2) new cerebrovascular accident or transient ischemic attack, as determined by a documented neurologic deficit; and (3) new-onset type 1 or 2 diabetes mellitus diagnosed by 2 consecutive fasting glucose tests >125 mg/dl performed at the examination center or diagnosed in the community and reported by the subject at any annual follow-up visit.
The main outcome measure (dependent variable) was the diagnosis of new-onset CAD. The outcome was concluded if the subject presented, at any annual follow-up visit, with documented new CAD. This was determined if the subject had presented with angiography proven stenosis >50% in ≥1 coronary artery, positive perfusion thallium-201 scintigraphy findings, or positive ergometry test using the Bruce protocol. Examinees with pathologic ergometry or thallium-201 scintigraphy results were usually referred for coronary angiography.
The 2 other major disease outcomes, cerebrovascular disease and diabetes mellitus, also prevalent in the adult population, were used for censoring, if either appeared before the development of CAD, to avoid their potential effect on a future co-morbid CAD diagnosis. This could occur with the greater frequency of medical follow-up visits for already morbid subjects (surveillance bias) or because both the LDH level and the CAD outcome might be affected in various ways by these systemic co-morbidities (confounding bias).
All the independent variables, including those determined from the blood samples, were measured or determined at baseline. Blood samples were obtained in the morning after a 12-hour nocturnal fast and before the exercise test. The age-predicted maximal heart rate-percentage (APMHR%) was calculated as the percentage of the age-predicted maximal heart rate (defined by the formula [220 − age]) achieved during ergometry test. The ergometry fitness level was categorized according to standard gender-adjusted categories of the routinely calculated maximal oxygen uptake, using the Bruce protocol. Regular leisure time physical activity as a dichotomous variable was defined as reporting regular engagement in ≥1 specific leisure time activity, such as walking or jogging. The baseline body mass index was calculated using the formula, weight in kilograms divided by the height in square meters. A family history of cardiovascular disease was defined as reporting CAD in a first-degree relative.
The United States National Health and Nutrition Examination Surveys (NHANES) of 2005 to 2006, 2007 to 2008, and 2009 to 2010 were combined for the purpose of external cross-sectional validation and examination of possible associations between normal LDH levels and physical activity or alcohol consumption characteristics in a large representative sample of the general ambulatory population. The NHANES is a routine, nationwide program of studies that includes personal interviews, physical examinations, and various laboratory tests.
We included in the analysis all NHANES 2005 to 2010 participants aged ≥40 years (a blood sample for LDH was only drawn from those aged ≥40 years), who reported no history of coronary heart disease, heart attack, diabetes or borderline/prediabetes, or stroke, and had normal LDH levels (range 93 to 198 IU/L, inclusive in all 3 NHANES surveys) at the survey. For the analysis of various physical activity characteristics, we included the participants of the 2007 to 2010 surveys only, because the different set of physical activity questions used in the NHANES 2005 to 2006 survey.
The frequencies and percentages were calculated for the categorical variables, and the mean ± SD for continuous variables. For the triglycerides levels, the median and interquartile range were also calculated. In the univariate analysis of baseline characteristics, the chi-square test, with continuity correction for 2 × 2 tables in the case of dichotomous variables, was used to measure the significance of categorical variable associations with the main dichotomous outcome measure (CAD). The Student t test was used to measure the significance of the mean differences of the continuous variables with the CAD outcome and to measure the differences in mean continuous LDH levels across the physical activity characteristics, dichotomized. One-way analysis of variance was used to test the differences in the mean continuous LDH levels across physical activity or alcohol consumption characteristics of >2 groups. The baseline categorical independent variables and the incidence of CAD were analyzed for the significance of trends across the normal-range LDH tertiles using the chi-square linear-by-linear association test or the nonparametric Kruskal-Wallis 1-way analysis of variance test for baseline continuous variables.
We used multivariate Cox proportional hazards analysis to estimate the hazard ratio (HR) and 95% confidence interval for having the CAD outcome according to the tertiles of normal-range LDH. The proportional hazards assumptions were evaluated using log-minus-log plots. The models were adjusted for variables that had a significant association with CAD outcome on univariate analysis or if they were a previously known risk, or protective, factor for CAD. The significance of trend between HRs of LDH tertiles was measured as the overall given p value of the LDH tertiles’ variable within each model. All p value calculations were 2-tailed and were considered statistically significant if their value was p <0.05. The statistical analyses were performed with IBM SPSS, version 19.0 (IBM, SPSS, Chicago, Illinois). The institutional review board approved the present study.
Results
The baseline characteristics of 5,519 adult men and women (mean age 47.0 years, range 34 to 86), who were followed up for a mean of 4.2 years (23,415 person-years), are presented according to CAD outcome ( Supplementary Table 1 ) and total serum LDH tertiles ( Table 1 ).
| Characteristic | Tertile | p Value for Trend | ||
|---|---|---|---|---|
| 1 (n = 1,860) | 2 (n = 1,888) | 3 (n = 1,771) | ||
| Serum lactate dehydrogenase (IU/L) | — | |||
| Mean | 135.4 ± 10.7 | 159.6 ± 6.0 | 190.4 ± 16.9 | |
| Median | 137 | 159 | 186 | |
| Range | 100–149 | 150–170 | 171–260 | |
| Age (years) | 46.2 ± 7.3 | 47.0 ± 7.3 | 47.7 ± 7.7 | <0.001 |
| Men | 70.8% | 75.5% | 78.7% | <0.001 |
| Blood pressure (mm Hg) | ||||
| Systolic | 118.0 ± 14.0 | 121.0 ± 14.9 | 123.6 ± 15.6 | <0.001 |
| Diastolic | 75.5 ± 9.1 | 77.5 ± 9.6 | 79.1 ± 9.9 | <0.001 |
| Low-density lipoprotein cholesterol (mg/dl) | 120.3 ± 26.5 | 124.8 ± 26.7 | 127.2 ± 28.0 | <0.001 |
| High-density lipoprotein cholesterol (mg/dl) | 46.8 ± 11.6 | 47.3 ± 11.5 | 47.8 ± 12.2 | 0.04 |
| Triglycerides (mg/dl) | ||||
| Median | 104 | 108 | 111 | <0.001 |
| Interquartile range | 75–147 | 79–151 | 82–158 | |
| Body mass index (kg/m 2 ) | 25.1 ± 3.5 | 25.8 ± 3.4 | 26.7 ± 3.9 | <0.001 |
| Percentage of age-predicted maximal heart rate | 0.06 | |||
| <85% | 3.0% | 2.3% | 2.8% | |
| 85%–100% | 72.0% | 69.8% | 69.5% | |
| ≥100% | 21.4% | 24.2% | 25.3% | |
| Leisure time activity (min/wk) | 159.6 ± 104.7 | 162.2 ± 113.2 | 175.4 ± 116.7 | 0.02 |
| Cardiovascular family history ⁎ | 22.2% | 21.9% | 22.8% | 0.64 |
| Current smoker | 21.8% | 17.7% | 16.3% | <0.001 |
⁎ Defined as reporting coronary artery disease in first-degree relative.
The overall incidence of CAD outcome in the cohort was 6.1% (338 cases) during the follow-up period. In addition, 208 incident cases of diabetes mellitus (3.8%) and 14 incident cases (0.3%) of cerebrovascular disease were recorded. At baseline, those who later developed CAD were significantly older than those who did not have this outcome, were more often men, had greater systolic and diastolic blood pressures, greater low-density lipoprotein cholesterol and triglyceride levels, lower high-density lipoprotein cholesterol levels, poorer ergometry fitness levels, lower APMHR%, and also were less engaged in regular physical activity by 18.8%. The baseline mean LDH level was significantly lower in those who developed CAD during the follow-up period ( Supplementary Table 1 ).
Those in the greater LDH tertile also had significantly greater rates of these risk factors for CAD, apart from having a significantly lower rate of smokers, greater high-density lipoprotein cholesterol, greater rate of APMHR% ≥100%, and greater mean weekly minutes of leisure time physical activity ( Table 1 ).
All variables of leisure time physical activity collected in our cohort population are listed in Table 2 . Regular engagement in running, followed by bicycling and walking, was significantly associated with greater mean total serum LDH levels. Greater mean LDH levels were also significantly associated with more days of the week practicing any regular leisure time activity, a greater number of total weekly minutes of activity, and practicing ≥5 years of regular leisure time activity.
| Characteristic | % | LDH (IU/L) | p Value |
|---|---|---|---|
| Regular leisure time activity | 0.001 | ||
| Running | 6.5% | 165.9 ± 26.3 | |
| Bicycling | 3.1% | 162.6 ± 28.2 | |
| Walking | 30.2% | 162.2 ± 25.3 | |
| Gym workout | 8.4% | 161.1 ± 26.7 | |
| Swimming | 3.6% | 157.6 ± 25.4 | |
| Any other leisure time activity | 8.2% | 159.2 ± 23.3 | |
| No regular leisure time activity | 40.0% | 160.6 ± 24.9 | |
| Weekly leisure time activity (days/wk) | 0.04 | ||
| 1 | 5.0% | 159.7 ± 24.6 | |
| 2 | 16.2% | 160.4 ± 25.9 | |
| 3 | 13.6% | 161.2 ± 24.8 | |
| 4 | 5.2% | 163.3 ± 25.3 | |
| 5 | 2.9% | 164.9 ± 27.8 | |
| 6 | 1.5% | 163.8 ± 28.4 | |
| 7 | 1.3% | 168.2 ± 27.9 | |
| Intervals of leisure time activity (min/wk) | 0.01 | ||
| ≤180 | 30.4% | 160.6 ± 25.2 | |
| 181–240 | 4.2% | 163.5 ± 25.4 | |
| 241–360 | 4.8% | 165.0 ± 28.3 | |
| >360 | 1.9% | 166.1 ± 28.7 | |
| Leisure-time activity duration (years) | 0.02 | ||
| <5 | 15.3% | 159.9 ± 25.4 | |
| ≥5 | 18.3% | 162.7 ± 26.0 |
Adjusted for age and gender, the HRs for CAD demonstrated a statistically significant descending HR gradient across the increased total serum LDH levels by tertiles compared to the reference lower tertile ( Table 3 ). Additional adjustments for APMHR% and minutes per week of leisure time physical activity, and the fully adjusted multivariate model did not substantially change the HRs ( Table 3 and Figure 1 ). The HR for continuous LDH (per 10-IU/L increment) in the same fully adjusted multivariate model presented in Table 3 was 0.94 (95% confidence interval 0.90 to 0.99, p = 0.01). No significant effect modification in regard to CAD morbidity outcome was found for the interaction between LDH and gender within the same fully adjusted multivariate model presented in Table 3 (data not shown).
