METABOLIC SYNDROME: AN OVERVIEW
This column (Part I) will provide an overview of metabolic syndrome (MetS), the defining criteria for MetS and discuss the benefits resistance training (RT) may have on each component of MetS. The one on one column (Part II) will provide evidence-based RT program guidelines for managing or preventing MetS and discuss RT modifications / safety precautions for the individual components of MetS.
Metabolic syndrome is a name for a group of cardiovascular disease risk factors that occur together. Having MetS increases the risk for a number of health consequences including coronary artery disease, heart attack and stroke (2). MetS is identified in a person by the presence of 3 of 5 criteria including an elevated waist circumference, blood pressure, blood glucose, or triglycerides and low high-density lipoproteins (HDL). At this time, the criteria set by the American Heart Association and National Heart, Lung, and Blood Institute (AHA/NHLBI) are currently recommended in the United States (13). This definition represents an updated version of the National Cholesterol Education Program Adult Treatment Program III criteria (Table 1). An individual must have at least 3 of the defining criteria to be diagnosed by a physician with MetS.
It is interesting to note that at the present time, there are no universal criteria for diagnosing MetS (7). Various governing bodies have slightly different diagnostic criteria. Recently, a harmonized definition or unifying criteria for MetS has been proposed (1).
Although genetics play a role in the development of MetS (e.g., family history of type 2 diabetes [T2D] and obesity) (22), lifestyle or environmental issues such as low physical activity levels, poor diet, and progressive weight gain contribute significantly to the risk of developing MetS. Sufficient levels of physical activity/exercise, including RT can play a major role in the prevention and management of MetS (2,9,24).
RESISTANCE TRAINING EFFECTS ON METABOLIC SYNDROME
In recent years, there has been increased attention to the effects RT has on preventing and managing metabolic diseases. Although aerobic training is not discussed in detail in this column, it is important to note that a combination of aerobic and resistance exercise may be superior compared with either exercise modality alone, for improving components of MetS, such as glycemic control (6). Although aerobic exercise will always be an integral part of the exercise prescription for individuals with cardiovascular disease risk factors, it is important to understand the effects and benefits of RT on the various components of MetS. Recent evidence shows that RT performed at least 2 days/wk can reduce the risk and prevalence of MetS and its individual components in U.S. adults (8, 9). Resistance training has a clinically significant effect on MetS risk factors and should be part of the lifestyle management of MetS (24).
IMPAIRED FASTING GLUCOSE/TYPE 2 DIABETES
There is evidence that RT can have a positive impact in improving glycemic control (4,5,20). A recent study showed that 10 weeks of resistance exercise was associated with significantly better glycemic control in adults with T2D compared with a treadmill exercise group (4). Additional studies have shown RT to be effective in improving glycemic control in older adults (5) and women (20). Muscle contraction-mediated glucose uptake and both increased intramuscular glucose transport (GLUT4) protein content and increased lean body mass have all been identified as mechanisms for enhanced glycemic control (11,14,20). Resistance exercise has been shown to improve ability to perform activities of daily living (18), and this is important to help increase daily physical activity, thus helping prevent or manage impaired fasting glucose, T2D, and MetS.
Improvements in total cholesterol, low-density lipoproteins (LDL), and triglycerides have been observed from RT (16,20). The results of studies examining the effects of RT on various blood lipids (e.g., increases in HDL) have reported inconsistent findings, which may be partly because of varying RT programs used in the studies (e.g., different volumes of RT) (12,25). A reduction in abdominal adiposity is a proposed mechanism for these improvements observed in both T2D and dyslipidemia patients after RT (5,11,15,16,20,23). Healthy weight loss and weight maintenance is generally emphasized for persons with dyslipidemia. Although RT does contribute to overall caloric expenditure, it is generally recommended as an adjunct to aerobic or endurance training in persons with dyslipidemia (2).
RT has been shown to reduce both resting and ambulatory blood pressure in persons with hypertension (19,21). The effects of RT on resting blood pressure are generally modest (3). A meta-analysis by Kelley and Kelley (17) indicates an approximate 2% reduction in resting systolic blood pressure and an approximate 4% reduction in resting diastolic blood pressure. This analysis looked at RT programs (circuit-training and a more conventional format [multiset, longer rest periods]) lasting at least 4 weeks. However, the majority of outcomes in this meta-analysis and a second one (10) were in subjects with resting blood pressures of <140/90 mm Hg. More research is needed regarding the effects of RT on blood pressure in persons with hypertension. Recent data indicate that adults performing resistance exercise at least twice weekly may reduce the risk of developing prehypertension (9).
Regular RT may have other benefits for persons with prehypertension/hypertension. Activities requiring physical exertion (e.g., physical activities involving lifting or carrying) may be safer to perform, because hemodynamic responses (e.g., increases in blood pressure and heart rate) may be less as a result of increased muscular strength and endurance from regular RT. Resistance exercise has been shown to decrease the blood pressure response to maximal exercise and improve heart rate and blood pressure recovery following cardiorespiratory exercise (26).
Although aerobic activities should be emphasized for weight loss and abdominal fat loss (2), RT has a demonstrated role with this component of MetS. Resistance training, performed twice weekly, in the absence of a weight loss diet, has been shown to improve insulin sensitivity and fasting glycemia and decrease central adiposity in older men with T2D (15). In addition, a recent meta-analysis showed that RT could have a significant impact on reducing abdominal and total adiposity in individuals with abnormal glucose metabolism (24). Although RT should not be the primary mode of exercise for weight loss and total and abdominal fat loss, RT does expend calories and can result in increases in lean body mass, which can increase resting metabolic rate. In addition, increased muscular strength and endurance will likely enable overweight and obese individuals to be more physically active on a regular basis, thus expending more calories to assist with abdominal and total fat loss and the prevention of weight regain.
More research is needed regarding the specific benefits of RT on the individual components of MetS. However, it is clear that RT, particularly when combined with aerobic exercise training, can help prevent and manage MetS.
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