cardiovascular fitness in women, but not in men: data from ... Health Research Institute and National Taiwan ... Nutrition Examination Survey, physical fitness.
Journal of Internal Medicine 2005; 258: 328–335
doi:10.1111/j.1365-2796.2005.01546.x
Levels of homocysteine are inversely associated with cardiovascular fitness in women, but not in men: data from the National Health and Nutrition Examination Survey 1999–2002 H.-K. KUO1,2, C.-J. YEN2 & J. F. BEAN3,4 From the 1Division of Gerontology Research, National Health Research Institutes; 2Department of Internal Medicine, National Taiwan University Hospital, Taipei, Taiwan; 3Department of Physical Medicine & Rehabilitation, Harvard Medical School; and 4Spaulding Rehabilitation Hospital, Boston, MA, USA
Abstract. Kuo H-K, Yen C-J, Bean JF (National Health Research Institute and National Taiwan University Hospital, Taipei, Taiwan; and Harvard Medical School and Spaulding Rehabilitation Hospital, Boston, MA, USA). Levels of homocysteine are inversely associated with cardiovascular fitness in women, but not in men: data from the National Health and Nutrition Examination Survey 1999–2002. J Intern Med 2005; 258: 328–335. Objectives. Cardiovascular fitness represents the ability of active skeletal muscle to utilize oxygen during aerobic exercise. Elevated homocysteine, causing tissue injury by such mechanisms as oxidative stress, endothelial damage, and protein homocysteinylation, is associated with increased risk of cardiovascular disease, dementia and osteoporotic fracture. However, the association between elevated homocysteine and cardiovascular fitness has not been reported. Design. Population-based cross-sectional study. Setting. National Health and Nutrition Examination Survey from 1999 to 2002 in the USA. Subjects. A total of 1444 noninstitutionalized adults aged 20–49 years with reliable measures of cardiovascular fitness and nonmissing values in homocysteine. Main outcome measures. Cardiovascular fitness, estimated maximal oxygen uptake or VO2max (mL kg)1 min)1), was obtained by a submaximal exercise test. Levels of homocysteine were measured by the Abbott homocysteine assay, a fully
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automated fluorescence polarization immunoassay method and were natural-log-transformed due to right skewness. Results. After adjustment for age, race and body mass index, there was a 0.70 mL kg)1 min)1 decrease (P ¼ 0.033) in the estimated VO2max for each standard deviation (SD) increase in the natural-log-transformed homocysteine level for women. Additional adjustment of hypertension, diabetes, smoking status, alcohol intake, use of lipid-lowering agents, physical activity, self-report health condition, as well as levels of folate, vitamin B12, creatinine, C-reactive protein, total cholesterol and haemoglobin seemed to influence the association. In the fully adjusted model, we observed a 1.18 mL kg)1 min)1 decrease (P ¼ 0.003) in the estimated VO2max for each SD increase in the natural-log-transformed homocysteine level in women. There was no association between cardiovascular fitness and homocysteine levels in men. Conclusion. High homocysteine levels were inversely associated with cardiovascular fitness in women, but not in men. The results suggest that homocysteine levels are important indicators of exercise tolerance amongst women and may be useful in targeting female individuals requiring endurance intervention to prevent loss of cardiovascular fitness and function. Keywords: homocysteine, National Health and Nutrition Examination Survey, physical fitness.
Ó 2005 Blackwell Publishing Ltd
ASSOCIATION BETWEEN HOMOCYSTEINE AND CARDIOVASCULAR FITNESS
Introduction Homocysteine is a sulphur-containing amino acid derived from the metabolism of methionine. Unfavourable lifestyles, such as smoking, lack of exercise, excessive alcohol intake, high coffee consumption as well as nutritional deficiencies in B vitamins are associated with elevated levels of homocysteine [1, 2]. Abundant data support the notion that elevated homocysteine is an independent risk factor for several multi-system diseases [3], including coronary heart disease [4–6], stroke [4, 7], dementia and Alzheimer’s disease [8], as well as osteoporotic fracture [9, 10]. Evidence suggests that elevated homocysteine levels may cause toxicity by such mechanisms as oxidative stress [11] and endothelial damage [11, 12]. Cardiovascular fitness represents an individual’s ability to perform aerobic exercise, that being the ability of active skeletal muscle to utilize oxygen during exercise. Cardiovascular fitness is an important correlate of several clinically important outcomes, including stroke [13], myocardial infarction [14], metabolic syndrome [15], or other cardiovascular diseases and risk factors [16–18]. Theoretically, poor cardiorespiratory capacity can be derived from pathological changes peripherally affecting the tissues and the associated vasculature or centrally perturbing the heart and coronary arteries. These pathological changes, to some extent, may be attributed to elevated levels of homocysteine and the related tissue toxicity. However, there is no existing data examining the association between cardiovascular fitness and levels of homocysteine, not to mention the fact that the association may be different for both genders. Therefore, the aim of this cross-sectional study was to examine the association between homocysteine and cardiovascular fitness in both men and women using data from the National Health and Nutrition Examination Survey (NHANES) 1999–2002.
Methods Study population The NHANES is a population-based survey designed to collect information on the health and nutrition of the US household population. The NHANES used a stratified and cluster sampling design to obtain a
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representative sample of the noninstitutionalized civilian US population. Beginning in 1999, the NHANES became a continuous, annual survey rather than the periodic survey that it had been in the past. The survey data are released every 2 years. Detailed survey operation manuals, consent documents and brochures of the NHANES 1999–2002 are available in the NHANES website [19, 20]. In NHANES 1999–2002, cardiovascular fitness test was administered to 6687 subjects aged between 12 and 49 without the following characters: (i) physical functioning limitations, such as difficulties in walking/standing, having fractures or bone/joint injuries, requiring a cane or a wheelchair, having amputations of legs or feet, blind, diabetic retinopathy, loss of balance/consciousness on a regular basis, and obese (>350 lb); (ii) cardiovascular conditions/symptoms, such as congestive heart failure, coronary heart disease, angina, stroke, myocardial infarction, pacemaker implantation, selfreported heart or stroke problems, and resting tachycardia (>100 min)1) or high blood pressure (systolic blood pressure >180 mmHg or diastolic blood pressure >100 mmHg); (iii) pulmonary conditions/symptoms, such as orthopnoea, short of breath when walking at own pace on level ground, self-reported lung or breathing problems, emphysema, frequent (>12) or severe (limit speech) attacks of wheezing during the past 12 months; and (iv) usage of certain medications including anti-dysrhythmics, beta-blocker (including eye drops), calcium channel blockers, digitalis, nitrates, central nervous system stimulants and ephedra-based weight loss medications. We excluded 3875 subjects aged 260 systolic
Ó 2005 Blackwell Publishing Ltd Journal of Internal Medicine 258: 328–335
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and/or 115 mmHg diastolic, or significant drop (>20 mmHg) in systolic blood pressure during exercise, arising during the test and resulting in safety concerns (513), not able to calculate VO2max because heart rate obtained at the end of stage 2 of treadmill test was lower than that at the end of stage 1, or the heart rate difference between stages 1 and 2 was 5 min)1 or less (15), or other reasons not specified (25). These subjects were excluded in order to have reliable data on estimated VO2max in the analysis. The subjects excluded from the analysis (1310) were more likely to have higher body mass indexes (BMIs) and more likely to be male (57.2% vs. 49.3%) and non-Hispanic black (21.5% vs. 16.4%). Amongst the 1502 subjects with reliable measures of estimated VO2max, 58 subjects were further excluded because of missing values in homocysteine examination. Therefore, the final analytic sample was confined to 1444 subjects aged 20–49 with reliable estimated VO2max and nonmissing values in the measurements of homocysteine. Measurement of cardiovascular fitness Cardiovascular fitness can be measured via an exercise tolerance test in which inspired oxygen and expired carbon dioxide are directly measured during testing. More commonly, in clinical settings including the NHANES protocol, inspired and expired gases are not measured, but cardiovascular fitness is evaluated through surrogate measures including observation of heart rate and perceived exertion. Based on age, gender, BMI, and self-reported level of physical activity, participants were assigned to one of eight treadmill test protocols. Each protocol includes a 2-min warm-up, two 3-min exercise stages, and a 2-min cool down period. The goal of each protocol is to elicit a submaximal exercise effect with heart rate that is approximately 75% of the age-predicted maximum (220 minus age) by the end of the test. By monitoring heart rate response to each of the 3-min stages, maximal oxygen uptake (VO2max) can be estimated by using measured cardiovascular responses to known levels of exercise workloads, assuming the relationship between heart rate and oxygen consumption is linear during exercise [21]. The estimated VO2max is the main outcome of cardiovascular fitness in the NHANES. The estimated VO2max was further categorized to three cardiovas-
cular fitness levels: i.e. low, moderate and high, based on gender-age specific cut-off points of estimated VO2max. The reference cut-off points used for adults aged 20–49 years are based on data from the Aerobics Center Longitudinal Study (ACLS) [16, 21]. Low level of cardiovascular fitness was defined as an estimated VO2max below the 20th percentile of the ACLS data of the same gender and age group; moderate cardiovascular fitness was defined as a value between the 20th and 59th percentile, and high cardiovascular fitness level was defined as at or above the 60th percentile. The reference ranges for VO2max are summarized in the Table 1. Measurement of homocysteine Blood specimens were collected at mobile examination centres and were frozen before analysis. Plasma homocysteine was measured by the Abbott homocysteine assay (Abbott Laboratories, Abbott Park, IL, USA), a fully automated fluorescence polarization immunoassay (FPIA) method. Plasma total homocysteine concentrations were calculated by the Abbott IMx Immunoassay Analyzer using a machine-stored calibration curve. The FPIA method, used by the NHANES as a primary tool to determine homocysteine levels, was fully equivalent to other frequently used method such as high performance liquid chromatography [22]. Table 1 Reference ranges for VO2max according to the Aerobics Center Longitudinal Study (ACLS)* Estimated ranges of VO2max (mL kg)1 min)1) Age groups
Fitness levels
Men
Women
20–29
Low Moderate High Low Moderate High Low Moderate High
44.22 42.41 39.88
36.64 34.59 32.30
30–39
40–49
*The reference cut-off points of fitness levels used for adults aged 20–49 years are based on data from the ACLS: low level of physical fitness was defined as an estimated VO2max below the 20th percentile of the ACLS data of the same gender and age group; moderate fitness as a value between the 20th and the 59th percentile; and high fitness level as at or above the 60th percentile.
Ó 2005 Blackwell Publishing Ltd Journal of Internal Medicine 258: 328–335
ASSOCIATION BETWEEN HOMOCYSTEINE AND CARDIOVASCULAR FITNESS
Covariates Age, gender, race/ethnicity and smoking status were obtained from self-report. Diabetes was defined by self-report of a doctor’s diagnosis, the presence of a plasma glucose level >200 mg dL)1 or the use of diabetic medications (including insulin injection and/or oral hypoglycaemic agents). The presence of hypertension was defined by self-report of a doctor’s diagnosis, the use of anti-hypertensive medications or averaged blood pressure >140/ 90 mmHg. The use of lipid-lowering medications was obtained by self-report. BMI was calculated as weight in kilograms divided by the square of height in metres. Self-reported general health condition was ascertained by the questionnaire ‘How would you say your health in general is?’ and was categorized to excellent, very good, good, fair, or poor. Average levels of physical activity were obtained by selfreport and categorized into the following categories: sitting during the day and not walking about very much; standing or walking about a lot during the day, but not having to carry or lift things very often; lifting light loads or having to climb stairs or hills often; and doing heavy work or carrying heavy loads. Alcohol intake was determined by the questionnaire ‘In any one year, have you had at least 12 drinks of any type of alcohol beverage?’ and was dichotomized. Nutritional markers including serum vitamin B12 and folate levels were measured by using the Bio-Rad Laboratories (Hercules, CA, USA) ‘Quantaphase II Folate/vitamin B12’ radioassay kit. Given that C-reactive protein (CRP) has been shown to be associated with cardiovascular fitness [23–25], CRP level was adjusted as a covariate and was quantified by utilizing latex-enhanced nephelometry with a Behring Nephelometer Analyzer System (Dade Behring, Deerfield, IL, USA). Total cholesterol levels, haemoglobin levels as well as serum creatinine were also obtained. Analysis The distributions of plasma levels of homocysteine in both men and women were positively skewed. Therefore, we used natural-log-transformed values, which provided the best-fitting model for analysis in which the homocysteine levels were treated as a continuous variable. For men, standard deviation (SD) scores of homocysteine were obtained from the
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formula Xi ) Xm/SD, where Xi is the natural-logtransformed homocysteine levels in the individual male subject, Xm the mean natural-log-transformed homocysteine level in the male subjects, and SD the standard deviation of the natural-log-transformed homocysteine level in the male subjects. The SD scores of homocysteine in women were obtained from the same formula. This calculation allowed us to determine the changes in the cardiovascular fitness for each increment of 1 SD in the natural-logtransformed homocysteine levels. The association between homocysteine and cardiovascular fitness was also evaluated with a quartile-based analysis. Multiple linear regression was used to examine the relationship of homocysteine levels with estimated VO2max. We used an extended-model approach for covariates adjustments: model 1 ¼ age, race, and BMI; model 2 ¼ model 1 + chronic diseases (hypertension and diabetes) + health behaviours (smoking, alcohol consumption and physical activity) + self-report health condition + use of lipidlowering agents; whilst model 3 ¼ model 2 + levels of folate, vitamin B12, creatinine, CRP, total cholesterol and haemoglobin. We also re-assessed the association between plasma homocysteine levels and odds of low cardiovascular fitness by using multiple logistic regression. Given the fact that the NHANES population weights are only applicable to analyses that use the entire population and we limited our analyses to a special subset of subjects, we did not use the NHANES 1999–2002 population weights for the purposes of this study. Data management and analysis were performed using STATA 8.0 software (STATA Corporation, College Station, TX, USA).
Results Baseline characteristics Table 2 lists the characteristics of the participants according to sex. The mean age amongst the 714 men was 33.1 years, and amongst the 730 women it was 33.5 years. Men smoked more than women and consumed more alcohol. Men had higher estimated VO2max, haemoglobin levels, serum creatinine, total cholesterol, as well as homocysteine concentrations than women. The cut-off values for homocysteine quartiles amongst men were: quartile 1 (12 alcohol drinks/year Use of lipid-lowering medications
Table 3 Regression coefficients for the association between estimated VO2max and homocysteine ba (SE)
Men Women (n ¼ 714) (n ¼ 730) P-value
33.1 26.6 15.5 7.72 459 11.2 0.11 193 0.91 44.8
(8.7) (4.6) (1.0) (2.48) (223) (6.2) (0.19) (52) (0.18) (9.0)
342 100 11 213 574
(47.9) 362 (49.6) 0.879 (14.0) 82 (11.2) 0.110 (1.5) 14 (1.9) 0.583 (29.8) 154 (21.1)
