No Soy for Your (Grand-)Parents! Soy Blunts Strength Gains in 12-Week Resistance Training Study - Avg. Gains Approx. 30% Lower Than in Both Dairy and No Protein Group!

In view of  the anti-strength effects observed in the study at hand: It's almost sarcastic that many of the proteins that are specifically marketed to older men and women contain soy protein.

While most of you are probably interested in studies on young, resistance trained individuals, studies in older individuals is - and even I have to admit that - significantly more relevant. Why? Well, the world's population is aging, with the number of people over 60 yr expected to more than double from 841 million in 2012 to more than 2 billion in 2050. That's a development that brings profound implications for many aspects of life.

One of the most visible signs of aging is the loss of skeletal muscle mass and strength, which leads to decrements in physical function and may predispose to disability (Kooman. 2009). As Thomson et al. point out in their latest paper it is thus "important to develop strategies that promote maintenance of lean tissue mass, strength and physical function in older adults [...] to reduce disability into older age." (Thomson. 2015).

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One of the ways to do that, i.e. to reduce disability into older age, is to combine resistant training with an increased protein intake. Both the optimal amount of protein and the optimal source are yet still debated. According, the

"aim of [Thomson et al.'s] study was to evaluate whether, when protein intake was at least 20 g at each meal, the consumption of a eucaloric high protein diet rich in dairy protein would provide greater increases in muscle strength, lean mass and physical function compared with either an isocaloric diet representative of the typical Australian dietary protein intake (i.e. ∼1.1 g/kg/d [10]) or an isocaloric diet high in non-dairy (i.e. soy) protein in older adults undertaking a program of resistance training" (Thomson. 2015).

To this ends, 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 from either...

• 

  Table 1: Average dietary intake during the 12 week study period (Thomson. 2015).

  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. And the results were... well, at least for vegans, surprising:

Figure 1: Relative changes in muscle strength values at baseline (Week 0) and after the intervention (Week 12) for participants compliant with the study protocol (Thomson. 2015).

As you can see in Figure 1, the soy protein group (HP-s) saw significantly less pronounced strength gains compared to the high dairy (HP-D) and even the regular protein group (UP | HP-D 92.1 ± 40.8%, HP-S 63.0 ± 23.8%,UP 92.3 ± 35.4%; P = 0.002 treatment effect). A difference that occured despite the fact that the resistance training programs were identical, i.e.

• 

  Figure 2: CONSORT diagram (HP-D, high dairy protein diet; HP-S, high soy protein diet; UP, usual protein diet | Thomson. 2015).

  progressive overload, whole body resistance training program 
• three days per week on non consecutive days
• five exercises per formed on weight stack pin loaded machines
• leg press, chest press, knee extension, lat pull down and leg curl, as well as seated bent knee hip flexions. 
• 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) 
• resistance was maintained until participants could perform three sets of 12 repetitions; 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
• for the seated bent knee hip fl exions 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
• all exercise programs were performed under supervision of trainers

and irrespective of the fact that the protein intake was higher in both, the 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) group.

Figure 3: A previous study by Kraemer et al. (2013) shows that compared to whey protein soy protein blunts the post-workout increase in testosterone and decrease in cortisol. In the long run this could explain why we don't see the same increases in muscle strength in the soy vs. whey and control group in the Thomson study. 

So far the bad news: The only good news is that the effects on lean mass, physical function and mental health scores as well as the decrease in fat mass was identical for all three training groups.

Against that background it's however even more difficult to explain the differential effects of increased soy vs. increased dairy and no increase in protein intake. From previous studies we know that the ingestion of soy protein promotes smaller increases in muscle protein synthesis and muscle protein accretion compared with the ingestion of dairy protein (Phillips. 2009). This has been attributed to the less anabolic amino acid profile. Obviously this couldn't be the reason in the study at hand; ... I mean, in that case the low protein group would have had to have seen the lowest strength gains. As Thomson et al. point out "it is more likely that the attenuation of the strength increase in the HP-S group was due to some [direct] effect of the soy inhibiting the increase in strength", of which they speculate that they may be a results of the estrogenic properties of the low amount of isofla- vones in soy proteins (Barnes. 2004) | Comment!

Edit: Since you've asked me to speculate, here is a speculation. Another potential explanation for the difference is that soy is significantly less insulinogenic than whey (Veldhorst. 2009). This would also suggest lower PWO IGF-1 levels and could contribute to (a) the lower strength gains in the soy group and (b) the increased strength gains in the no protein group which consumed more carbs and had higher insulin and probably IGF-1 levels. In addition it would suggest that this effect is more pronounced in the elderly, because their IGF-1 levels plummet (Breese. 1991).

References:

• Barnes, Stephen. "Soy isoflavones—phytoestrogens and what else?." The Journal of nutrition 134.5 (2004): 1225S-1228S.
• Breese, Charles R., Rhonda L. Ingram, and William E. Sonntag. "Influence of age and long-term dietary restriction on plasma insulin-like growth factor-1 (IGF-1), IGF-1 gene expression, and IGF-1 binding proteins." Journal of gerontology 46.5 (1991): B180-B187.
• Koopman, René, and Luc JC van Loon. "Aging, exercise, and muscle protein metabolism." Journal of Applied Physiology 106.6 (2009): 2040-2048.
• Kraemer, William J., et al. "The effects of soy and whey protein supplementation on acute hormonal responses to resistance exercise in men." Journal of the American College of Nutrition 32.1 (2013): 66-74.
• Phillips, Stuart M., Jason E. Tang, and Daniel R. Moore. "The role of milk-and soy-based protein in support of muscle protein synthesis and muscle protein accretion in young and elderly persons." Journal of the American College of Nutrition 28.4 (2009): 343-354.
• Thomson, Rebecca L., et al. "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." Clinical Nutrition (2015).
• Veldhorst, Margriet AB, et al. "Dose-dependent satiating effect of whey relative to casein or soy." Physiology & behavior 96.4 (2009): 675-682.