OBJECTIVE: The primary goal of this study was to assess whether increases in fat-free mass (FFM) and decreases in total and percentage fat mass from 15 weeks of twice weekly supervised strength training would be maintained over 6 months of unsupervised exercise in a randomized controlled trial.
DESIGN: In all, 60 women aged 30–50 y, body mass index between 20 and 35 kg/m2, were randomized to control or treatment groups. The treatment group performed twice-weekly supervised strength training followed by 6 months of unsupervised training. Measurements at baseline, 15, and 39 weeks included body weight and body composition by dual-energy X-ray absorptiometry. Repeated measures regression was used to assess between-group differences for changes over time.
RESULTS: Almost 90% of prescribed exercise sessions were completed. The body composition treatment effects over 15 weeks were largely maintained over 6 months of unsupervised exercise. Over the total 39 weeks of strength training, the treatment group gained +0.89 kg more in FFM, lost −0.98 kg more in fat mass, and lost –1.63% more in percent body fat when compared to the control group. P-values for these between-group differences in 39-week changes were 0.009, 0.06, and 0.006, respectively. Strength training did not result in any significant weight loss or waist circumference attenuation. Adjustment for changes in energy intake and physical activity did not alter these results.
CONCLUSIONS: Twice-weekly strength training is behaviorally feasible for busy midlife women and the favorable body composition changes resulting from supervised strength training can be maintained over time. These findings lay the groundwork for determining the long-term health benefits of this behaviorally feasible exercise prescription, potentially including prevention of age-associated fat gains.
Obesity is epidemic in the US1 and quite recalcitrant to treatment. There is empirical evidence that it is possible to achieve and maintain weight loss long term.2 However, the majority of individuals treated in clinical settings regain most of the weight lost within 5 y after starting treatment.3 The difficulty in maintaining a weight loss long term has resulted in recommendations from several expert panels that adults who are normal weight to mildly obese (body mass index (BMI) up to 35 kg/m2) focus on weight maintenance rather than weight loss, particularly in the absence of comorbidities.4,5
The proximal cause of midlife weight gain is generally inadequate energy expenditure, given energy intake. The most common approach to altering this balance, for the purpose of weight control or weight loss, has been reduction of energy intake.5 A healthy diet is important to maintaining a healthy weight. However, national cohort data on physical activity and diet indicate a decline in physical activity throughout adulthood (at least until retirement age) with a stable (or slightly declining) energy intake.6,7
Observational epidemiologic evidence supports the use of increased physical activity in preventing age-associated weight and fat gains.8,9,10,11,12 For the most part, physical activity recommendations to treat or prevent overweight have focused on aerobic activities.8,9,13,14 Resistance or strength training may be a viable alternative to aerobic exercise for purposes of attenuating fat gain in adults. Changes in body shape are generally noticed between 12 and 20 weeks after starting strength training,15 and may provide motivation to continue at a time when the novelty of having started an exercise program is wearing off. Additionally, resting energy expenditure (REE) decreases with aging and this decrease is closely correlated to losses in fat-free mass (FFM).16,17,18,19,20 Loss of FFM could also contribute to age-associated fat gains by promoting an increasingly sedentary leisure and nonleisure activity pattern because the intensity of daily functional and leisure-time activities becomes relatively larger as FFM is lost through disuse. Without a systematic reduction of energy intake, age-associated loss of FFM, altered activity pattern, and reduced REE would contribute to fat gains.
Strength training increases FFM by 1–2 kg.15,21,22,23,24,25,26 If a twice-weekly strength training program is behaviorally feasible for a long period and if the expected FFM increases can be maintained, then strength training could prevent age-associated FFM losses, as well as associated REE and activity pattern changes. Over a period of years or decades, this could translate into real differences in daily energy expenditure, willingness to engage in functional and leisure activities, and age-associated fat gains.
Theoretically, a gain of 1 kg of FFM should result in an REE increase of approximately 88 kJ/kg.27 In actuality, strength training intervention studies in men and women, in a variety of participant age groups, have observed REE increases in the range of 117–913 kJ/kg increase of FFM.21,22,24,28,29,30,31 Few of these reported changes were statistically significant. Even if the small REE increases from a 1–2 kg increase in FFM are real, the day-to-day variability in REE may be too large to detect statistically significant changes. However, even without a change in REE, maintenance of FFM through midlife years may prevent age-associated fat gains by promoting maintenance of functional and leisure-time activity patterns. Recent evidence that strength is correlated with weight loss maintenance further supports the use of strength training for the prevention of age-associated fat gains.32 There is also animal model data to support the importance of FFM in suppressing body fat accumulation.33
Studies of the usefulness of strength training in the context of weight loss have had mixed results. Studies with more severe energy intake restriction have been unable to show FFM gains,34,35,36 while studies with less severe energy intake restriction have shown FFM gains with strength training even with moderate weight losses.37,38 Research to determine the amount of energy expenditure necessary to maintain weight after a recent weight loss2,39 addresses an important question. However, there may be a different physiologic response to efforts toward weight maintenance in individuals who have recently lost 10% or more of their body weight compared to individuals who have not recently lost weight.
The primary goals of this study were to assess whether weight-stable midlife women performing twice weekly supervised strength training for 15 weeks would increase and then maintain a higher FFM, as well as decrease their body fat and maintain a reduced total and percentage fat mass, over 6 months of unsupervised exercise, when compared to no-treatment controls. We chose fat and FFM changes as primary outcome variables in this study rather than body weight changes because of the confounding effects of the expected FFM increases on total body weight. We also evaluated training-induced changes in waist circumference, as well as upper and lower body strength.
Subjects and methods
Participant recruitment and retention
During December 1999 and January 2000, 60 women aged 30–50 y were recruited from the female faculty, staff, or students at the University of Minnesota. Women were screened for the following inclusion criteria: self-reported BMI between 20 and 35 kg/m2; no plans to leave the area for a month or more during the intervention; no recent or ongoing changes in hormonal status that might affect the outcome of the study (pregnant within the past 6 months, lactating within the past 2 months, planning to become pregnant during the study); if post-menopausal, taking hormone replacement therapy for at least the last 6 months and committed to continuing during the 9 months of the study; no body weight changes >10% over the past year; no uncontrolled hypertension (systolic ≤140, diastolic ≤90 mmHg); nonsmokers; no heart disease or other significant medical conditions, including diabetes mellitus and cancer within the past 5 y; no positive responses on the Physical Activity Readiness Questionnaire;40 no prescription medications expected to alter the results of the study (including cholesterol-lowering medications, psychiatric medications taken at dosages expected to affect weight, appetite suppressants, or thyroid medications); no conditions that might inhibit the ability to participate in strength training (including muscle injuries, orthopedic problems, motion-limiting osteoarthritis, or fibromyalgia); no health condition that affects metabolic rate (such as thyroid disease); sedentary to moderately physically active (up to three times a week of exercise and up to 5 METs intensity); no strength training during the past year; and no past strength training for two or more times a week for 6 months or longer ever in their lives. Blocked randomization was stratified by decade of age (30–39 vs 40–50 y), due to concern regarding menopausal effects on the outcomes of interest. There were four post-menopausal women in the study, two each in the treatment and control groups. Additional analyses excluding these women did not alter the results presented herein. A description of the 60 women participants in this study is given in Table 1.
Three women dropped out of the study: two women in the control group and one woman in the treatment group, all for personal reasons. In addition, one treatment group participant was diagnosed with Grave's disease 1 month prior to study completion; results for this participant are excluded from all analyses except those for Table 1. Results are presented in this paper for the remaining 56 women who successfully completed the study. There were no new injuries incurred in the treatment group beyond expected muscle strain and soreness. The study protocol was reviewed by and followed all regulations of the University of Minnesota Institutional Review Board for the protection of human subjects in research. Participants received $200 for successful study completion. Access to the exercise facility where the intervention took place was paid for by the study grant, but parking on the University of Minnesota campus was not paid for by the study.
All participants, regardless of group assignment, were asked to allow for seasonal changes in diet, but to avoid changes in their dietary habits for the purpose of weight change for the duration of the study. Participants who reported at baseline that they currently participated in some other form of exercise (most commonly 1–2 weekly walks) were asked to continue doing those activities, regardless of group assignment. The treatment group was enrolled in a 50-min strength training class held twice weekly for 15 weeks at the University Recreation Center. At each of these sessions, participants performed three sets each of nine common strength training exercises, with as much weight as they could lift for 8–10 repetitions. The nine strength training exercises included exercises performed on Cybex strength training equipment (squats, leg press, leg extension, seated leg curl, curl lat pulldowns) and free weight exercises (bench press, overhead press, bicep curls, and tricep extension). The protocol for increasing weight for each exercise was as follows: after two classes during which a participant lifted the same weight 10 times during each set, the weight was increased by the smallest possible increment. If the higher weight could be lifted at least eight times on the first set, and six times on the second set, additional set(s) were attempted with the higher weight. If the participant could not lift the higher weight at least eight times on the first set or six times on the second set, the weight was decreased back to that lifted in the previous class for the subsequent set(s). Participants were taught to record the weights and repetitions for each exercise; these logs were reviewed by the study staff every 2 weeks to assure that participants were increasing the weights according to the above-described protocol. Participant-to-exercise trainer ratio in the class was 4:1 and participants received a phone call after each absence to schedule a make-up session.
At the end of the 15-week class, participants were provided with a 6-month membership to the same exercise facility. They were instructed to continue doing at least two sets of all nine strength training exercises with at least the same or higher weight load lifted during the last week of the class. Participants were also allowed to continue with three sets per exercise. Approximately, 50% of the treatment group continued doing three sets. Additional analyses revealed no differences in effects for those who continued three sets vs those who did two sets during the 6 months of unsupervised exercise. Exercise session logs were maintained by participants and reviewed twice weekly by the study staff. If a participant did not record the first workout of the week by Wednesday afternoon or a second work out for the week by Friday afternoon, she received a supportive reminder call from research staff.
During the 15-week supervised intervention, 92% of prescribed exercise sessions were completed. During the following 6 months of unsupervised exercise, 89% of the prescribed exercise sessions were completed. The control group participated in measurements only.
Physical measurements were conducted in clinic visits at baseline, at the end of the 15-week supervised class (15 weeks), and at the end of the 6-month unsupervised intervention (39 weeks). Participants were asked to refrain from exercise for 48 h prior to each clinic visit.
Clinic visit 1 included circumference measurements and completion of several surveys. Waist circumference was measured at the level of the umbilicus, in duplicate; the mean was used for analysis. Participants were asked to complete the Typical Weekly Physical Activity Survey (TWPAS). The TWPAS was designed to assess multiple domains of physical activity in women.41 Reliability and validity assessment of this survey was conducted on 160 white, black, and Native American women. The intraclass correlation coefficient for repeated measures of total physical activity with the TWPAS was 0.51. Correlations of the total TWPAS physical activity score with physical activity record, percent body fat, maximal aerobic power, and accelerometer were 0.40, −0.09, 0.25, and 0.40, respectively. These correlations compare favorably with other well-accepted and commonly used physical activity surveys.42 The units for the TWPAS total physical activity score are MET minutes per day (MET min/day), which includes both duration and intensity components. At baseline, participants also completed a demographic survey including age, ethnicity, marital status, educational and employment history, and number of children living in their home.
During clinic visit 2, body weight and height measurements, and dual-energy X-ray absorptiometry (for body composition) were performed on participants between 06.30 h and 09.30 h, after a 12-h fast, and between 5 and 11 days after the start of menstrual flow for menstruating participants. Participants were asked to empty their bladder and change into a hospital gown prior to all measurements. Body mass was measured on a calibrated digital scale. Stature was assessed using a calibrated stadiometer. Body composition was measured on the Lunar Prodigy Dual-Energy X-ray Absorptiometer (DEXA) (software version 2.15, Lunar Radiation Corporation, Madison, WI, USA). Jensen et al43 found that duplicate scans showed a difference of 0.6±0.5% body fat using DEXA. Body fat measurement with DEXA has been shown to correlate with total body potassium counting, and total body water methods of measuring body composition, with correlations of 0.7–0.8.44 Body fat percentage is expressed as percentage of nonbone tissue that was fat.
Upper and lower body strength were assessed by one repetition maximum tests for the bench press and leg press in two additional clinic visits that occurred 48 h apart and were supervised by trained research staff. At 15 and 39 weeks, up to four women in each group failed to complete the bench press and/or leg press strength tests. Fear of exacerbating pre-existing injuries was the most common reason provided for not completing the strength tests. Participants were also asked to complete 4-day food records at baseline, 15 weeks, and 39 weeks after baseline. A registered dietitian entered these food records into the Nutrition Data System software (version 4.02_30, Minnesota Nutritional Coding Center, Minneapolis, MN, USA), which allowed for the determination of total energy intake and percent of energy from fat.
Statistical analyses were conducted with SAS version 6.12. Baseline characteristics were compared between groups by t-tests and χ2 tests. Repeated measures regression (PROC MIXED) was used to compare the between-group changes from baseline to 15 weeks, baseline to 39 weeks, and week 15 to 39 weeks. The ESTIMATE statement allowed for the testing of our specific hypotheses regarding the changes in body size variables. Because the hypotheses were proposed a priori, there is no need for adjustment for multiple comparisons. The same approach was used to assess changes in strength, dietary intake, and physical activity other than strength training. Adjustment for changes in energy intake and overall physical activity score did not alter the results presented. Repetition of these adjusted analyses after excluding participants with missing food records also did not alter the results presented.
Table 1 includes a description of all 60 women randomized to the control and treatment groups. The only between-group difference at baseline was for total energy intake, which was reported to be higher in the treatment than the control group. Most of the women had completed college and more than half had at least one child living at home. The health profile of these women was generally good, despite an average BMI over 25 kg/m2 in both groups. Of the 60 participants, three were African American, three Asian, and 54 Caucasian (10% minority participation).
Between-group differences for changes in variables associated with body size are presented in Table 2, unadjusted for energy intake changes or changes in physical activity outside the prescribed strength training intervention. There were modest but statistically significant improvements noted in FFM, fat mass, and percent body fat at 15 weeks that were largely maintained to 39 weeks. Comparisons of between-group changes from weeks 15 to 39 show that the between-group differences were maintained over the unsupervised portion of the intervention. There were two treatment group participants for whom 15-week DEXA data were not available; these women did not differ from the other treatment group women for any of the body composition variables. Means and standard error bars for percent body fat, fat mass, and FFM at each measurement time point are presented in Figure 1a–c according to treatment and control groups to illustrate the changes graphically.
Changes in upper and lower body strength, energy intake, percent energy intake from fat, and total physical activity levels are presented in Table 3. We noted significant increases in average individual changes in upper and lower body strength in the treatment women when compared to the control women. The between-group differences in upper and lower body strength also increase significantly during the 6 months of unsupervised exercise. Comparison of between-group changes in energy intake and physical activity between the two groups revealed no statistical differences. The changes in total energy intake indicate no tendency for the treatment group to increase energy intake in response to the intervention.
We have described an exercise intervention approach that is behaviorally feasible for midlife women. We suspect that our adherence rates were high at least in part because of the minimal time requirement for full intervention participation and the response to the objective time demands on the target population (midlife women). Most of the participants in this study were married, worked full time, and had at least one child living at home, indicating that they filled multiple life roles, each with competing time pressures.
The observed increases in FFM and strength over the initial 15 weeks of supervised exercise were comparable to results from other strength training intervention studies.21,22,23,24,25,26,29,30 However, this is the first randomized controlled study to report that FFM and strength increases from a supervised strength training program can be maintained over 6 months through an unsupervised program in midlife women. Taaffe et al30 reported ‘no further changes’ in body composition for older women who continued strength training to 52 weeks, after a 15-week supervised strength training intervention. However, no specific body composition values were provided and comparisons to control group women were not made at 52 weeks. Further, only women who chose to continue exercising were measured at 52 weeks (n=14, 58% of randomized exercise cohort at baseline).30
In the present study, waist circumference increased more, on average, in the control compared to the treatment group women over 15 and 39 weeks. However, the between-group difference in waist circumference never reached statistical significance (P=0.09 at 15 weeks, P=0.15 at 39 weeks). The waist circumference was not measured in the fasting state and there was no control for time of day, how recently participants ate or drank, or phase of menstrual cycle. Further, there is evidence that waist circumference is not sensitive enough to detect changes in visceral adipose tissue.45 Therefore, it is unclear whether this finding is a true reflection of the effects of strength training on abdominal obesity or the result of methodologic limitations. A recent review noted that of four completed randomized controlled trials that have assessed the effects of aerobic exercise on abdominal obesity, two demonstrated a significant loss in waist circumference or visceral adipose tissue in treatment compared to control participants.46 The same reviewer concluded, in a separate review, that there was insufficient evidence to determine whether aerobic exercise-induced weight loss resulted in preferential reductions of abdominal fat.47 Results from the present study do not support the conclusion that strength training, in the absence of weight loss, will result in preferential loss of abdominal fat. Future randomized controlled trials of strength training should include more accurate measures of abdominal fat to address this important research question.
Although the treatment group gained slightly more than one-quarter as much weight as the control group, the changes in both groups were small and were within the expected range of day-to-day variability for body weight. This underscores the difficulty in using body weight as a primary endpoint in prevention interventions.
The observed changes in body composition were too small to be clinically meaningful over 39 weeks. However, these data lay the groundwork for the testing of the hypothesis that twice-weekly strength training could prevent or attenuate age-associated fat gains over a period of years. We have demonstrated that the FFM gains from strength training can be maintained in an unsupervised exercise program that is behaviorally feasible. The next steps may be to determine whether this exercise prescription can be maintained over a period of years and whether this alters the pattern of functional and leisure-time activities in a manner that reduces the risk of significant fat gains. The magnitude of the day-to-day variability in REE makes it unlikely that we will ever demonstrate whether an FFM increase of 1–2 kg results in a statistically significant increase in REE.
The control group showed none of the expected increases in body fat over 9 months. This could be due to a screening effect. Volunteering for the study and/or repeated measurements of dietary intake and the other measurements conducted in the pilot study may have influenced behaviors enough to prevent the commonly observed gains in weight and body fat in our control group. Treatment and control group participants were drawn from a set of women who were relatively well educated and on the higher end of the general spectrum of socio-economic status. It is possible that this could explain the lack of expected increases in body fat or weight in the control group. It is also possible that recruiting women from a university environment, providing a fitness center membership located proximal to the participants' workplace and paying for successful completion of the study may have influenced the outcomes for the treatment group. It is important to repeat this study in a more diverse cohort of women to establish that these findings can be generalized to all women. It would also be of interest to determine the other health benefits and cost per participant in a community (nonuniversity) setting for this intervention.
Adjustment for energy intake and energy expenditure changes did not alter the findings in this study. However, the instruments used to measure these changes were crude in comparison with the quality of the measurements of body size changes. It is possible that some or all of the changes reported herein could be explained by eating or energy expenditure adjustments in reaction to participation in a strength training program.
In conclusion, we have described a twice-weekly strength training program that is behaviorally feasible in busy midlife women and demonstrated that the positive body composition changes associated with supervised strength training can be maintained over time. Given that strength training has been found to be efficacious for improving insulin sensitivity,48,49 preventing bone loss,20,25 and reducing risk factors for falls in older adults,50 the finding that a program of strength training is behaviorally feasible and that the body composition changes can be maintained over time lays the groundwork for testing the long-term efficacy of this behaviorally feasible exercise prescription for a variety of health benefits, potentially to include attenuation of age-associated body fat increases.
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This study was supported by a Minnesota Obesity Center pilot and feasibility grant (NIH Grant DK50456 from the National Institute of Diabetes and Digestive and Kidney Diseases) and the University of Minnesota GCRC (M01-RR00400). The investigators are grateful to the participants for their time and energy.
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Cite this article
Schmitz, K., Jensen, M., Kugler, K. et al. Strength training for obesity prevention in midlife women. Int J Obes 27, 326–333 (2003). https://doi.org/10.1038/sj.ijo.0802198
- resistance exercise
- body composition
- intervention studies
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