Vissa studier visar mycket riktigt att sojaprotein kan öka IGF-1, men samtidigt sänker sojaprotein testosteronnivåerna (1). Sen har också sojaprotein en mindre fördelaktig aminosyreprofil (färre essentiella aminosyror, EAA) än mjölkprotein (whey/kasein) med tanke på muskelbyggning (2). Studier som gjorts på djur med soja vs mjölkprotein visar också till lägre tillväxt hos individer uppfödda på soja (3).
Kort och gott: Vill du bygga muskler är sojaprotein inte ett bra val. Dock har sojaprotein visat sig kunna ha vissa fördelar för kraftigt överviktiga, och har också en del hälsomässiga fördelar bl.a. genom att det sänker testosteronnivåerna (höga testosteronnivåer förknippas med vissa typer av kreft).
1. "Conclusions: Soy protein powder decreases testosterone levels in healthy patients"
Clinical and biologic activity of soy protein powder (SPP) in healthy male volunteers: effect on testosterone and luteinizing hormone (LH)
Cancer Epidemiology Biomarkers & Prevention 16, 829-833, April 1, 2007
2. "Our findings revealed that even when balanced quantities of total protein and energy are consumed that milk proteins are more effective in stimulating amino acid uptake and net protein deposition in skeletal muscle after resistance exercise than are hydrolyzed soy proteins."
"What we observed was a greater whole-body lean mass gain in the milk versus the energy control group, but with no difference between the soy consuming group and either the milk or energy control group. Therefore, by comparison to simply consuming more energy as carbohydrate, milk proteins were more effective at supporting resistance exercise-induced lean mass gains. Similarly, the increase in muscle fiber (vastus lateralis) cross sectional area, while not statistically different, was greater in the milk consuming group than the other two groups"
Dietary Protein to Support Anabolism with Resistance Exercise in Young Men
Journal of the American College of Nutrition, Vol. 24, No. 2, 134S-139S (2005)
3. "Juvenile growing pigs were studied to explore whether a soy-based diet can induce persistent physiological alterations, especially in protein and energy metabolism, nutrient oxidation and redox homeostasis. In former studies we have shown that in juvenile pigs chronically fed protein diets based on either casein (CAS) or soy protein isolate (SPI), the SPI diet significantly decreases growth rate and increases oxidative stress responsiveness as compared to CAS. In addition, here we show that chronic feeding of SPI vs. CAS diet decreases whole body protein synthesis (WBPS) (p = 0.007) and hepatic gene expression associated with protein synthesis."
Studies on persisting effects of soy-based compared with amino acid-supplemented casein-based diet on protein metabolism and oxidative stress in juvenile pigs
Archives of Animal Nutrition, Volume 61, Issue 2 April 2007 , pages 75 - 89
