Soy Protein vs. Whey Protein: Which Builds Muscle Better?

observational-study PMID: 16948480

Quick Summary: This study looked at whether taking soy protein or whey protein after workouts helps young adults build muscle and get stronger. The results showed that both types of protein, when combined with exercise, helped people gain muscle and strength compared to not taking any protein.

Does Soy Protein Build Muscle?

Yes! This research shows that soy protein can help build muscle, just like whey protein. Both types of protein were equally effective in helping people gain lean muscle mass and increase their strength when combined with resistance training.

Study Details

Who was studied: 27 healthy young adults (ages 18-35) who weren't used to lifting weights.

How long: The study lasted for 6 weeks.

What they took:

One group took whey protein (1.2 grams per kilogram of body weight)
Another group took soy protein (1.2 grams per kilogram of body weight)
A third group took a placebo (a powder that didn't contain protein)

What This Means For You

Soy is a good option: If you're looking to build muscle and prefer a plant-based protein, soy protein is a great choice. This study shows it works just as well as whey protein for young adults starting a workout program.

Protein helps, but it's not everything: While protein helped, the study found the benefits were "minimal." This means that while protein is helpful, the most important thing is still the exercise itself and a good overall diet.

Consider your goals: If you're just starting out, soy protein can be a good choice. If you're more experienced, you might want to consider higher protein doses.

Study Limitations

Small study: The study only included a small number of people, so the results might not apply to everyone.

Short time frame: The study only lasted 6 weeks. Longer studies might show different results.

Not for everyone: The study was done on people who weren't used to working out. The results might be different for people who are already experienced lifters or older adults.

Other ingredients: The protein powders also contained other ingredients like sugar, which could have affected the results.

Key Findings

Both whey and soy protein supplementation (1.2 g/kg/day) combined with resistance training significantly increased lean tissue mass and strength in young adults compared to placebo. No differences were observed between whey and soy protein groups, and neither protein affected myofibrillar protein catabolism (urinary 3-methylhistidine). The authors concluded that protein supplementation provides "minimal beneficial effects" over resistance training alone in this population.

Study Design

This was a double-blind, randomized, placebo-controlled trial (RCT) lasting 6 weeks. 27 untrained healthy adults (18 female, 9 male; age 18–35 years) were assigned to whey protein (n=9), soy protein (n=9), or placebo (n=9). All groups underwent identical resistance training protocols. Outcomes were measured pre- and post-intervention using dual-energy X-ray absorptiometry (DEXA) for lean mass, 1-repetition maximum (1-RM) tests for strength, and urinary 3-methylhistidine for protein catabolism.

Dosage & Administration

Participants consumed 1.2 g/kg body mass of whey or soy protein daily, mixed with 0.3 g/kg sucrose powder. The placebo group received 1.2 g/kg maltodextrin + 0.3 g/kg sucrose. Supplements were provided as powders, presumably mixed with water or food, though exact timing (e.g., pre/post-workout) was not specified.

Results & Efficacy

Lean Tissue Mass: Both protein groups showed statistically significant increases compared to placebo (p < 0.05), though effect sizes were not quantified.
Strength: 1-RM bench press and hack squat improved significantly in whey and soy groups vs. placebo (p < 0.05). No inter-group differences between whey and soy.
Myofibrillar Catabolism: Urinary 3-methylhistidine concentrations did not differ between groups (p > 0.05), indicating no impact on protein breakdown.
Statistical Significance: All primary outcomes reported p-values < 0.05 for protein vs. placebo comparisons.

Limitations

Small Sample Size: Only 9 participants per group, limiting power to detect subtle differences.
Short Duration: 6 weeks may be insufficient to assess long-term effects or divergence between protein types.
Untrained Population: Findings may not apply to trained individuals or older adults.
Sucrose Co-administration: The addition of sucrose (a carbohydrate) could confound results, as carbs independently support muscle recovery.
Lack of Effect Size Data: Statistical significance was reported, but magnitude of benefits (e.g., kg gained in lean mass) was not quantified.

Clinical Relevance

For young adults initiating resistance training, supplementing with either whey or soy protein (1.2 g/kg/day) may yield small but measurable gains in lean mass and strength compared to carbohydrate-only placebo. Soy protein appears equally effective as whey in this context, offering a plant-based alternative. However, the "minimal" benefits suggest that protein supplementation may play a secondary role to training intensity and overall dietary intake in untrained individuals. Users should consider higher protein doses (e.g., 1.6–2.2 g/kg/day in later guidelines) for more pronounced effects.

Note: This study predates recent advancements in protein research and may not reflect current recommendations for optimal dosing or timing.