Muscle strength gains during resistance exercise training are attenuated with soy compared with dairy or usual protein intake in older adults: A randomized controlled trial

Background & aims: Maintenance of muscle mass and strength into older age is critical to maintain health. The aim was to determine whether increased dairy or soy protein intake combined with resistance training enhanced strength gains in older adults.

Methods: 179 healthy older adults (age 61.5 ± 7.4 yrs, BMI 27.6 ± 3.6 kg/m2) performed resistance training three times per week for 12 weeks and were randomized to one of three eucaloric dietary treatments which delivered >20 g of protein at each main meal or immediately after resistance training: high dairy protein (HP-D, >1.2 g of protein/kg body weight/d; ∼27 g/d dairy protein); high soy protein (HP–S, >1.2 g of protein/kg body weight/d; ∼27 g/d soy protein); usual protein intake (UP, <1.2 g of protein/kg body weight/d). Muscle strength, body composition, physical function and quality of life were assessed at baseline and 12 weeks. Treatments effects were analyzed using two-way ANOVA.

Results: 83 participants completed the intervention per protocol (HP-D = 34, HP-S = 26, UP = 23). Protein intake was higher in HP-D and HP-S compared with UP (HP-D 1.41 ± 0.14 g/kg/d, HP-S 1.42 ± 0.61 g/kg/d, UP 1.10 ± 0.10 g/kg/d; P < 0.001 treatment effect). Strength increased less in HP-S compared with HP-D and UP (HP-D 92.1 ± 40.8%, HP-S 63.0 ± 23.8%,UP 92.3 ± 35.4%; P = 0.002 treatment effect). Lean mass, physical function and mental health scores increased and fat mass decreased (P ≤ 0.006), with no treatment effect (P > 0.06).

Conclusions: Increased soy protein intake attenuated gains in muscle strength during resistance training in older adults compared with increased intake of dairy protein or usual protein intake.


Alex’s Notes: A recent meta-analysis found that, in persons older than 50 years, the combination of resistance training with adequate amounts of protein (1.2g/kg) leads to a 38% greater increase in lean body mass and 33% greater increase in 1-RM strength when compared to older persons consuming less protein or not engaging in resistance training. However, there is reason to believe that the type of protein may also play a role. Dairy protein has been shown to elicit greater stimulation of muscle protein synthesis post-exercise and greater increases in lean mass when compared to soy protein in healthy young males. But older adults are not young healthy men. Thus, the current study sought to evaluate whether the consumption of a high-protein diet supplemented with dairy or soy proteins would differentially affect muscle strength, lean mass, and physical function in adults aged 50-79 years.

Ultimately, 83 participants completed all study aspects and were included for analysis. These men and women averaged 61 years of age and BMIs of 27-28. All were physically active and in good health, but none were engaged in structured exercise programs. Moreover, none were on hormone replacement therapies or attempting to lose weight. All underwent the same 12-week resistance training program consisting of a whole body workout three days per week on non-consecutive days: leg press, chest press, knee extension, lat pull down, and leg curl, as well as seated bent knee hip flexions.

“For the machine-based exercises the training load was progressive and started with one set of eight reps at a resistance equivalent to the participant's 8 repetition maximum (RM; maximum weight lifted for eight repetitions) and resistance was maintained until participants could perform three sets of 12 repetitions. The resistance was then increased so only eight repetitions could be performed again in the first set, and this resistance was maintained until participants could again perform three sets of 12 repetitions. This pattern of altering resistance and repetitions continued throughout the study period to ensure a progressive increase in training load. For the seated bent knee hip flexions participants gradually increased the number of repetitions and sets throughout the training period until they could perform two sets of 20 repetitions, and this was then maintained for the remainder of the study period.”

The only difference between the participants was their dietary group: a high dairy protein diet (HPD), a high soy protein diet (HPS) or a usual protein diet (UP).All diets were isocaloric, low-fat (30% fat, <8% saturated fat), aimed at maintaining energy balance, and provided 1.1 g/kg of body weight/d of protein, primarily from lean meat sources. Additionally, all diets had some foods provided to ensure compliance and protein intake in each group was spread evenly across each of three daily meals. This came out to be just over 25g per meal.

The only difference between the diets was a supplemental protein consumed by the HPD and HPS groups only, which was consumed after the resistance training program in addition to the meal that would normally be consumed at that time. On recovery days, the food was divided even across meals. In an effort to avoid the use of actual supplements and stick with foods, the HPD diet provided additional dairy-based protein in the form of a shake (475g reduced fat milk, 200g fat-free yogurt), the HPS diet provided additional soy protein in the form of a shake (300g reduced fat soy milk, 100g soy yogurt, and 20g soy protein powder). Energy was matched for UP by providing additional carbohydrate foods (500 ml orange juice, 13 g poly-joule and 2 Arnotts YoYo sweet biscuits). 

Okay, so if you skipped all that then start reading here

Basically, all diets were matched for energy with the UP consuming 1.1g/kg of lean meat protein. The HPD and HPS diets were the same except that they contained 1.4g/kg protein with the additional 0.3g/kg coming from dairy and soy foods, respectively.

Everybody wins! Seriously, all groups significantly increased strength on all exercises, as well as increased lean body mass, mental health score, and six minute walk tests distance, while decreasing weight and body fat over the 12-week intervention period. And that is awesome. Moreover, for mental health and body composition outcomes, there were no significant between-group differences.

However, the overall improvement in the 8-RM was greatest in the HPD (+92%) and UP (+92%) groups when compared to soy (+63%). This was despite no differences in the total training load lifted over the 12 weeks between the groups. These effects were most pronounced in the leg press, with the HPD improving by 136%, the UP by 135%, and the HPS by 65%, and there were no differences between diets for changes in chest press, knee extension, or leg curl 8RMs.

The lack of difference between the HPD and UP diets is interesting. It is possible that the distribution of protein intake at meals provided optimal stimulation of lean body mass growth and muscle strength even with the baseline protein intake of the UP diet. Moreover, the additional amount of protein in the HP groups was minor at ~27g/day. This had been shown previously, with no changes in muscle strength, lean body mass, or markers of muscle hypertrophy in older adults undergoing a 24-week resistance training program when a background protein intake of 1.1-1.2 g/kg was increased to 1.3-1.4 g/kg.

Regarding the soy… several studies have shown that soy protein is inferior to dairy proteins for promoting muscle protein synthesis and accretion. Yet, this fails to explain why the UP group would also fair better than the soy group. More likely, it was something about the soy protein that caused an attenuation of exercise adaptations. Soy foods not only contain soy protein, but also contain isoflavones, which exhibit estrogenic properties. If you are shaking your head in disbelief, then it is worth mentioning that 14 days of soy protein supplementation in resistance trained young men during training reduced serum testosterone concentrations in the first 30 min post-exercise compared with whey protein or a carbohydrate control.This may explain the attenuated increase in strength gains observed in the HPS group in the present study, but unfortunately blood samples were not collected so we cannot know for sure.

Bottom line

About 1.4 g/kg of protein is no better than 1.1 g/kg in otherwise healthy adults over the age of 50 years provided the protein is distributed evenly across 3 daily meals and that the adults are engaged in structured resistance training. However, consuming soy protein, even as an additional protein source, may inhibit the gainzzzzzzz.


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