The principle findings of this study were that 12 weeks of resistance exercise training significantly increased muscle strength and fat free mass and significantly decreased waist-to-hip ratio, percent body fat, and total serum cholesterol in overweight, hyperlipidemic men. All groups had an equal reduction in total cholesterol, although the ratio of LDL cholesterol to HDL cholesterol tended to improve more in the soy group. These results provide further support for a structured resistance training program to improve strength and the cardiovascular risk profile of sedentary, overweight adult men desiring to improve their overall health.

Although no significant differences were observed among groups in total cholesterol and HDL-C after 12 weeks of resistance training, the soy group showed a tendency to improve both TC:HDL-C and LDL-C:HDL-C. These values were 2.5 and 2.0 times those of the whey group, respectively. These ratios are important variables in the prediction of CVD risk [25–27]. HDL-C levels are inversely related to CVD risk because HDL-C inhibits LDL oxidation (central to the initiation and progression of atherosclerosis) and reverses cholesterol transport [28, 29]. Though all experimental groups demonstrated an equal reduction in total cholesterol, it may be relevant that ratios of LDL cholesterol to HDL cholesterol improved more in the soy group.

Regional distribution of fat is an important risk factor for cardiovascular disease with central (abdominal) fat deposits posing higher risk [2]; therefore our finding of a reduction in waist to hip ratio is of significant importance. The average reductions in waist and hip circumferences were 1.4 inches and 1 inch, respectively. These reductions are not likely the result of dietary changes as there were no significant changes in total calories, total fat or body weight over the course of the 12-week study. This finding supports previous studies that show resistance training decreases abdominal adiposity and reduces the waist-to-hip ratio, although total body weight changes may be small [5, 8, 14]. Banz et al. [1] and Ibanez et al. [14] demonstrated a significant reduction in waist-to-hip ratio and total body fat after subjects were placed on 10 and 16 weeks of resistance exercise sessions, respectively. Campbell et al. [30] also saw significant reductions in percent body fat and fat mass and a significant increase in fat free mass after 12 weeks of resistance training with subjects either on a low protein diet (0.8 g/kg/day) or on a higher protein diet (1.62 g/kg/day) diet. Our findings agree with these studies in that major changes in body weight or BMI were not observed, despite significant reductions in fat mass and adiposity. Body weight and BMI typically do not change because of concomitant increases in muscle mass and reductions in fat mass. These results, combined with others, demonstrate the limitations inherent in using changes in BMI and body weight to track the benefits of weight management programs.

Also consistent with previous studies [1, 23, 30], we demonstrated a significant accretion in muscle mass in a relatively short time. The ability to maintain or increase lean body mass, especially given the progressive decline in muscle mass that normally accompanies aging, is an important contributor to lowering cardiovascular disease risk [20, 29]. While the use of whey supplementation to support muscle hypertrophy has been the topic of many studies, the ability of soy protein to support lean body mass gains is controversial [4, 6, 9, 12, 19]. We were most interested, though in the potential for soy to have an added benefit for groups at risk for cardiovascular disease. Several studies have shown that soy reduces serum lipid concentrations [16, 18, 31, 32]. Coupled with our findings and those of others [9, 12, 19] the combination of resistance training and dietary manipulation, as part of long-term lifestyle change, may reduce risk factors for cardiovascular disease by lowering body fat stores, increasing fat free mass (an important determinant of metabolic rate), [2, 3]and improving blood lipid levels.

The absence of between-group differences in strength gains between an animal-based protein supplement (whey) and vegetable-based protein supplement (soy) agrees with other studies examining the relationship between different protein sources and improved strength with resistance training. Phillips et al [10], in a study of young, healthy men completing 12 weeks of resistance training, found no significant differences in strength gains between a milk-supplemented group, a soy protein-containing group, and an energy control group. Haub et al [13] examined different protein sources in combination with 12 weeks of resistance training in older men. Their subjects displayed increased strength, with no differences between those who consumed a meat-containing diet (57% of the protein source) versus a vegetable (soy)-based diet (53% of the protein source). Strength gains were similar among all groups in our study, indicating that adequate protein rather than the protein source is important in sustaining a positive nitrogen balance for muscle accretion to occur. It should be noted that guiding subjects in all groups to consume as close to 1.2 g/kg/day of protein was to rule out confounding variables such as an excess of protein in one or more comparisons groups (i.e. the supplemented groups). While this was the intent, it can't be ruled out that this may have brought all groups to the threshold needed to gain lean body mass on a resistance training program.

The finding of a significant decrease in total serum cholesterol but no change in LDL-C, HDL-C or triglycerides and no difference among groups is surprising. The benefits of soy supplementation on improving lipid profiles are well documented [16, 31–33]. Zhan et al [32] completed a meta-analysis on 23 randomized controlled trials investigating the effects of soy protein containing isoflavones on lipid profiles. The average study length in this review was 10.5 weeks. They concluded that soy protein with isoflavones significantly reduces total cholesterol, LDL cholesterol and triglycerides and the magnitude of the effect was related to the level and duration of supplement intake, to the sex of the subjects and to initial serum lipid concentrations. Anderson et al [18] also concluded that the effects of soy on lipid profiles is most pronounced in hyercholesterolemic subjects when isoflavones in the soy supplement ranged from 40 mg/day to greater than 80 mg/day. The soy supplement in our study contained 56.2 mg of isoflavones in the aglycone form. In a recent meta-analysis of 41 randomized trials with an average study length of 10 weeks, Reynolds et al [34] found that soy supplementation was associated with a significant reduction in total cholesterol, LDL cholesterol, and triglycerides (-5.26 mg/dl, -4.25 mg/dl, -6.26 mg/dl respectively) and a significant increase in HDL cholesterol (0.77 mg/dl). In a 2006 review, Torres et al [33] suggested that soy consumption reduces the clinical and biochemical abnormalities in lipid disorder-related diseases. In contrast, a study by Ma et al [35], in which subjects consumed a milk protein supplement or a soy protein supplement, found no treatment effect on lipid profiles. The length of that particular study was five weeks, which may not have been long enough to observe an effect on serum lipid levels. It was surprising that our subjects did not have a greater improvement in serum lipids with the soy supplementation after 12 weeks. A possible explanation may be individual differences in the intestinal absorption of isoflavones. Equol is a byproduct of the bio-transformation of the isoflavone diadzein by microflora in the large intestine and is a potent antioxidant [36]. Equol is not produced in the same amount in all people in response to soy consumption. It is estimated that the range of persons in the general population that are classified as "equol producers" is 14–70% [35, 36], which could contribute to the variability of the effect of soy on serum lipids. The mechanisms responsible for the isoflavone-effect on lipid profiles are not currently known but may be due to their biological similarity to estrogens and estrogen-receptor-dependent genes [14, 32], to enhanced bile acid secretion [32], increasing LDL receptor activity, or to enhancement of thyroxine and thyroid-stimulating hormone [14, 32].

The observation that serum triglycerides showed no significant changes over the 12 weeks of the study is consistent with previous studies [37, 38]. But, subjects in the soy group exhibited a trend toward reduction (lowered by 8.6% – versus a reduction in the whey group of 4.2% and an increase in the control group of 16.2%). This trend suggests that an intervention extending beyond 12 weeks may result in significant changes. Indeed, other studies have reported a beneficial effect of soy consumption alone on serum triglycerides [18, 33, 34].

We attempted to eliminate diet changes other than inclusion of assigned supplements. The percent of calories derived from fat decreased significantly (p < 0.05) due to the increase in energy from protein and carbohydrates in spite of no change in total energy intake. It cannot be ruled-out that the dietary fat content played a role in improved lipid profiles but its role would be minor, at best, in view of the fact that total energy and grams of fat did not change significantly. The percent of energy from protein was expected to increase in the whey and soy supplemented groups. The reasons for the increased energy from protein in the placebo group and for energy derived from carbohydrates in all groups are unknown. Community-living subjects may have naturally chosen to alter their food choices and/or lifestyle based on their enthusiasm of improved health from participation in the study.

Study limitations

We may not have observed significant changes in body composition and lipid profiles among the different protein supplements because of a type II error and it may be that a longer (>12 weeks) training period is required to show significant changes in body composition and in lipid ratios such as TC:HDL-C and LDL-C:HDL-C. Meta-analysis by Zhan et al [32] confirmed that improvements in HDL cholesterol with soy protein supplementation were only observed in studies > 12 weeks in duration. In addition, a diet intervention (for example, limiting daily fat calories to <25%) in combination with the resistance training may have shown more dramatic results in body composition and lipid profile changes. Another limitation that may have affected the outcome of the study was the difference in initial waist:hip. After randomized enrolment it was observed the soy group had significantly higher waist:hip than the other two groups. It may be that the effect of soy was diminished because of this discrepancy. It should be noted that individuals in the placebo group did modify their diet and this included an increased percentage of energy from protein and carbohydrate sources and a decrease percent of calories from fat sources. The results of training could also be due in part to these diet changes, however; the changes in percent of energy sources as noted in the placebo group do not typically result in such dramatic increases in strength gains.