Resistance exercise is a popular training method, approved by major medical groups including the American Heart Association, the American College of Sports Medicine 12 to increase muscle mass and improve blood lipid profiles. It is common for males to consume commercial protein supplements and use high intensity resistance training to develop "muscle bulk" for reasons of physical appearance, competition, and/or strength gains. Sedentary individuals may also participate in resistance training to improve physical appearance, but many initiate weight lifting programs with the goal of improving overall health and fitness. It is well documented that lean muscle mass plays a significant role in determining basal metabolic rate and, thus, daily energy expenditure 3. Creating and maintaining sites of ATP turnover and enhancing metabolic expenditure through resistance training can help prevent an age-associated decline in metabolic rate and undesirable gains in fat mass 245. A high percentage of body fat is associated with hyperlipidemia, a known cardiovascular disease (CVD) risk factor 3. Given that the relative risk of CVD for physically inactive individuals versus active individuals is 1.5–2.4 and that 60% of U.S. adults do not participate in regular physical activity 6, the benefit of resistance exercise in reducing CVD risk is widely recognized and is supported by all major health organizations 27. Promoting the benefits and encouraging participation in this low-cost activity could help prevent CVD and other behavior-driven chronic diseases, and may provide significant cost-savings to an over-burdened health care system.

Amino acid availability is an important regulator of muscle protein metabolism during resistance training exercise 8. Muscle net protein balance must be positive (greater muscle protein synthesis than breakdown) to experience an increase in muscle mass, which occurs only when sufficient amino acids are available in the intracellular pool. Whey and soy are both high quality sources of protein and popular supplements in the exercise community. It has been suggested soy supplementation may reduce CVD risk, a benefit that consumption of whey protein does not provide. Both proteins are easily digestible and have similar absorption kinetics 9, but some controversy exists whether soy will support skeletal muscle protein accretion in response to resistance training as effectively as whey. Phillips et al 10 reported that whey was superior to soy in stimulating amino acid uptake during a resistance training program. More recently Anthony et al 9 observed similar protein synthesis rates in exercised skeletal muscle in rats who ingested either whey or soy protein. In addition, several human studies observed no differences in either strength gains or increases in lean mass in resistance trained subjects who supplemented their diets with either soy or whey 10111213.

While supplementation with whey protein is popular with weight lifting enthusiasts, mainly to promote gains in muscle size, supplementing with soy protein is not as common. But, because of its potential to improve blood lipid profiles 141516 soy consumption may be more appealing to a sub-set of exercisers – those at moderate or high risk for CVD. Soy's non-essential amino acid content favors post-prandial production of glucagon, which, as opposed to insulin, down-regulates lipogenic enzymes and lowers cholesterol synthesis 17. Soy also has a number of other physiologically active compounds with cholesterol-lowering properties such as isoflavones, fiber, and phytoestrogens 14151819. Soy protein supplementation in combination with resistance training may promote protein (or muscle) anabolism to the same extent as supplementation with whey protein 122021; so the plant-based protein's potential to lower blood lipids and reduce oxidative stress may provide a simple, cost-effective means of reducing cardiovascular disease risk.

The purpose of this study was to document changes in strength, body composition, and blood lipid profiles in sedentary, overweight, hypercholesterolemic male subjects who participated in a 12-week resistance training program and who supplemented their usual diets with either whey or soy protein versus placebo. It was hypothesized that: 1) subjects receiving either protein supplement would have equivalent gains in both strength and lean body mass and these gains would be greater than the placebo group; 2) Subjects receiving the soy supplementation would have a significant reduction in fasting blood lipid levels versus the whey and placebo groups.

Over the course of the study, three subjects dropped out because of the inability to schedule training sessions between employment demands and outside interests. One individual ceased participation due to relocation. Twenty-eight subjects completed the study and were included in the final statistical analysis.

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 252627. 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 2829. 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 5814. 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 12330, 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 2029. 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 4691219. 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 16183132. Coupled with our findings and those of others 91219 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), 23and 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 16313233. 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% 3536, 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 1432, to enhanced bile acid secretion 32, increasing LDL receptor activity, or to enhancement of thyroxine and thyroid-stimulating hormone 1432.

The observation that serum triglycerides showed no significant changes over the 12 weeks of the study is consistent with previous studies 3738. 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 183334.

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.

Our findings add to the growing evidence that resistance training is beneficial for reducing cardiovascular risk. Our results suggest that protein supplementation is not necessary for strength or body composition changes in overweight men consuming a diet with an adequate supply of amino acids to meet the needs for stimulation of muscle protein synthesis during resistance exercise. Resistance exercise training alone increases muscle mass and improves body composition measures in sedentary, overweight men. Soy based protein supplements appear to be as effective as animal-based protein to support strength gains. Our results also suggest that soy protein supplementation during resistance training warrants further study in larger samples over longer periods of time since previous work has shown that regular soy consumption improves lipid profiles and the insulin-to-glucagon ratio and lowers oxidative stress 3161731323334.

The authors declare that they have no competing interests.

CD and HB developed the study hypothesis, research design, data collection, analysis, and manuscript preparation. PH participated in research design, data interpretation and manuscript preparation. JL participated in subject screening, interviews and manuscript preparation. RB participated in blood collection technique, analysis and interpretation of results. All authors read and approved the final manuscript.