Abdominal obesity, in particular abdominal subcutaneous and visceral fat, are known to be independent predictors of the metabolic risk factors that are antecedents for cardiovascular disease and type 2 diabetes (7,8,14,18). This suggests that treatments designed to reduce obesity and related co-morbidities would ideally be associated with substantial reductions in abdominal fat. As exercise is a common feature of obesity reduction programs, it is important to clarify whether exercise-induced weight loss is associated with corresponding reductions in abdominal obesity. Indeed, the intent of this review was to determine whether exercise-induced weight loss is associated with corresponding reductions in abdominal obesity measured by external anthropometry (e.g., waist circumference) and/or abdominal subcutaneous and visceral fat measured using imaging techniques.
The format of this review follows the guidelines set forth in the recent National Institutes of Health, National Heart, Lung, and Blood Institute (NHLBI) document (17). As such the Current Knowledge section is comprised of a series of evidence statements followed by a brief rationale for each. Following each evidence statement is an evidence category that, for the most part, is consistent with the criteria established by the Expert Panel (17). The single exception is that in this review, randomized, controlled trials (RCT) less than 4 months in duration were weighted similarly to longer term trials. The rationale for this position is provided within the review.
To consider whether exercise-induced weight loss is associated with concomitant reductions in abdominal obesity a Medline search (1966–1998) was performed using appropriate keywords. The reference lists of those studies identified were then reviewed for additional studies. Appropriate studies were identified using the following inclusion criteria:
- 1. The subjects participating in the exercise group had to consume an isocaloric diet for the duration of the study, thereby ensuring that the negative energy balance observed (e.g., significant weight loss and/or body composition change) was induced by the increase in physical activity.
- 2. Measurements of whole-body and regional fat distribution were obtained, and the authors reported whether abdominal fat (measured either by waist circumference or imaging techniques) was preferentially reduced.
- 3. The subjects had a body mass index (BMI) greater than 30 kg·m−2.
The initial literature search could not identify any study that met all criteria. Thus, it is not possible to consider whether abdominal fat is preferentially reduced when the energy deficit is induced by regular exercise.
A second review was performed using the following selection criteria:
- 1. All subjects in the exercise group were instructed not to change their diet (eating) habits, and thus in theory a negative energy balance would be induced by an increase in exercise.
- 2. Measurements of whole-body and regional fat distribution were obtained, and the authors reported whether abdominal fat was preferentially reduced.
Five RCT (3,11,16,24,26) and nine nonrandomized studies (2,4,5,9,10,13,15,23,25) met the first of these criteria. However, without exception these studies were not designed to determine whether there was a preferential reduction in abdominal fat. Because of the paucity of data we included two RCT wherein the subjects in the exercise group were instructed to maintain weight (6,12).
CURRENT STATUS OF KNOWLEDGE
Evidence Statement: Physical activity, in the absence of weight loss, is not associated with reductions in abdominal obesity as measured by waist circumference or waist-hip-ratio. (Evidence Category B).
Seven RCT consider whether exercise has an influence on abdominal obesity as measured by waist circumference and/or waist-hip-ratio (Table 1). For five studies the subjects in the exercise group were instructed not to change their dietary (energy consumption) habits (3,11,16,24,26). Despite this instruction, in all but one study the addition of exercise did not result in body weight or fat loss, nor was abdominal obesity reduced. In contrast to these observations, waist circumference decreased by 2 cm within the single study that reported a significant exercise-induced weight loss (11). In summary, the evidence from these studies suggest that physical activity without weight or fat loss is not associated with a reduction in abdominal obesity (Table 1).
A similar trend is observed for the nonrandomized studies (Table 2). Whereas waist circumference is unchanged for those studies reporting no loss in weight or fat (5,10,13), a modest reduction is observed in response to weight loss of about 2 kg (2,15,23). A caveat in interpretation of those studies reporting an exercise-induced decrease in abdominal obesity is that waist circumference was measured at the level of the umbilicus. Waist circumference measured at this level is not recommended for use in intervention studies since the umbilical level may change with weight loss (19,21,22).
From the studies in Tables 1 and 2 come the following summary observations:
- 1. There are no RCT wherein it is clear that physical activity alone has induced a negative energy balance.
- 2. Based on limited RCT evidence, exercise without weight loss is not associated with a reduction in abdominal obesity measured by waist circumference.
- 3. Limited evidence from nonrandomized or controlled studies suggests that a modest reduction in waist circumference is observed in response to weight loss.
- 4. Without exception, these studies were not designed to determine whether abdominal obesity was preferentially reduced.
Evidence Statement: Physical activity with or without weight loss is associated with reductions in visceral and abdominal subcutaneous adipose tissue. (Evidence Category C).
Absent from the literature are RCT that consider whether visceral or abdominal subcutaneous fat is preferentially mobilized when the energy deficit is induced by regular physical activity. However, we are aware of a single RCT wherein the effects of exercise alone on visceral and subcutaneous fat per se were considered (Table 3). Mourier et al. (16) observed large reductions in both visceral (∼48%) and abdominal subcutaneous adipose tissue (∼18%) in response to moderate exercise performed three times per week for 8 wk in men and women with type 2 diabetes. In that study no change in body weight was observed (Table 3).
Four nonrandomized studies considered the effects of exercise alone on visceral and subcutaneous fat (Table 3). Table 3 reveals that exercise with (4,9,23) or without weight loss (23,25) is associated with reductions in both visceral and abdominal subcutaneous fat. These studies were not designed to determine whether visceral fat was preferentially reduced in comparison with total fat.
The majority of the studies reviewed did not monitor energy intake carefully, nor did they prescribe an exercise program that resulted in a meaningful energy expenditure. For example, in the study by Mourier et al. (16) the regimen prescribed included stationary cycling three times per week for approximately 30 min. This represents a very modest energy expenditure and no doubt at least partially explains the absence of weight loss. In contrast, Bouchard et al. (4) rigorously controlled energy intake through the prescription of an isocaloric diet for the duration of a 3-month study (Table 3). In that study the seven twin pairs exercised on a stationary bicycle 6 d·wk−1 such that 1000 kcal were expended each day. In response to this program the authors observed that an exercise-induced 5 kg reduction in body weight was associated with a 36% reduction in visceral adiposity. Although this study did not include a control group, to our knowledge it is the only study wherein the effects of exercise-induced weight loss on visceral adiposity are reported.
Consistent with evidence from RCT, several studies in Table 3 report exercise-induced reductions in visceral and subcutaneous fat without a corresponding change in waist circumference. In fact this is true for the study that reported the greatest reduction in visceral fat (16). As the magnitude of the reduction in visceral and subcutaneous fat is greater than the error generally attributed to repeated measurements of abdominal fat by either computerized tomography or magnetic resonance imaging (1,20), it is assumed that the reductions observed in visceral fat are real and not the consequence of technical error. Thus it is unclear why a substantial reduction in abdominal adipose tissue is observed without a corresponding reduction in waist circumference. These observations are troubling, however, as they suggest that the influence of exercise on abdominal obesity measured by external anthropometry may provide misleading results.
Issues and limitations.
Absent from the literature are RCT in which the primary objective is to determine the influence of exercise-induced weight loss on total and regional fat distribution. Accordingly, the potential influence that age, race, gender, and exercise modality or intensity may have on exercise-induced changes in fat distribution is unknown. Moreover, for those studies that have attempted to define the exercise-induced effects on regional adiposity, energy intake and expenditure are poorly controlled. In addition, we are unaware of any study that reports whether abdominal fat is preferentially reduced in comparison with total adiposity. Thus the following questions remain unanswered:
- 1. Is exercise-induced weight loss associated with a reduction in abdominal obesity (visceral and/or subcutaneous) that is greater than exercise alone (e.g., no weight loss)?
- 2. Is there a preferential reduction in abdominal obesity?
- 3. What is the impact of varying degrees of exercise-induced weight loss on abdominal adiposity? Are reductions in abdominal obesity linearly related to the magnitude of weight loss (i.e., 7 vs 15%)?
- 4. Is equivalent diet- or exercise-induced weight loss associated with a reduction in abdominal obesity (visceral and/or subcutaneous), and if so, are there treatment differences?
Shorter trials be used to determine the influence of exercise-induced weight loss on abdominal fat.
Assurance that exercise is uniquely responsible for inducing a negative energy balance requires tight control of energy intake. This implies that attempts must be made to ensure the consumption of an isocaloric diet throughout the course of the study. This is of particular concern when the magnitude of the energy expenditure induced by exercise is relatively small. Indeed, for most of the studies reviewed, in particular the RCT, it is likely that small variations in energy intake compensated for the increased energy expended during exercise. As a consequence little change is observed in either body weight or total and regional fat. Tight control of energy intake is not easily accomplished for long-term trials (e.g., 1 yr). It is suggested, therefore, that shorter term trials (e.g., 4 months) may be more feasible with respect to controlling both energy intake and expenditure. In other words, shorter trials permit more rigorous control of energy intake and would also facilitate the prescription of more extensive daily exercise regimens.
Increase the energy expenditure induced by the exercise treatment.
For the most part researchers have employed exercise regimens that are characteristically associated with improvements in cardiovascular fitness (e.g., peak V̇O2) but not necessarily associated with significant energy expenditure and thus meaningful weight loss. If the objective is to determine the influence of regular exercise on obesity and its co-morbidities, it is suggested that the exercise program be designed with the priority to increase energy expenditure. In this way low intensity exercise (e.g., 50–60% of maximum heart rate (brisk walking)) should be prescribed on an almost daily basis for longer durations (e.g., ∼60 min).
Use a bony landmark (e.g., floating rib) to locate waist circumference measurement.
That waist circumference did not change in two of the three studies wherein substantial reductions in visceral and subcutaneous fat was observed is a concern. The use of a bony landmark to guide the measurement of waist circumference in intervention will help assure that technical issues do not explain this phenomenon. It has been reported that reductions in waist circumference measured at the level of the last rib (floating rib) follow corresponding reductions in visceral fat (22).
Acquire total and regional body composition measurements.
Determination of whether abdominal fat is preferentially reduced in response to exercise requires obtaining both whole body and regional measurements of fat distribution. Although not generally feasible, these measurements ideally would be obtained using either imaging or duel energy x-ray absorptiometry techniques. Acquisition of whole body and regional measurements of fat distribution would permit the researcher to identify the separate contribution of abdominal fat verses, for example, gluteal femoral or lower body fat. Traditionally, determination of differences in regional obesity with weight loss have entailed comparisons of abdominal versus whole body adiposity. This analysis is confounded by the inclusion of abdominal obesity in the whole body measurement.
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