A Diet Containing Soy Protein Isolate is Not Detrimental to Skeletal Development in Spontaneously Hyperphagic, Adolescent Male OLETF Rats

Abstract

Obesity and its associated metabolic dysfunction are associated with increased fracture risk in children and adolescents, suggesting excess adiposity negatively impacts skeletal development. Although the consumption of milk‐based proteins has been recommended for normal skeletal development, soy‐based proteins have emerged as popular plant‐based dairy alternatives. However, the effect of soy protein on bone health during adolescent skeletal development has not been studied. The purpose of this study was to examine the effects of isocaloric diets containing milk protein isolate (MPI), soy protein isolate (SPI), or a 50/50 combination of the two (MIX) on metabolic health indicators and bone outcomes in adolescent, hyperphagic, male Otsuka Long Evans Tokushima Fatty (OLETF) rats. At 4 weeks of age, rats were randomized to three treatment groups (MPI, SPI, and MIX) and provided ad libitum access to the diets until 20 weeks of age, at which time animals were fasted overnight, euthanized, and blood and hind limbs collected. Fasting glucose, insulin, lipids, and bone formation (N‐terminal propeptide of type 1 procollagen, P1NP) and resorption (C‐terminal telopeptide of type I collagen, CTx) markers were measured using commercially available ELISA kits. Tibial trabecular microarchitecture and cortical geometry were measured by micro‐computed tomography (μCT); and, biomechanical properties were measured by torsional loading to failure. Femoral calcium and phosphorus content were measured using Inductive Coupled Plasma‐Optical Emission Spectroscopy. One‐way ANOVA was used to test for significant differences among groups for metabolic outcomes and bone turnover markers. One‐way ANCOVA, with final body weight as a covariate, was used to determine differences among groups for all bone outcomes. At 20 weeks of age, there were no group differences in body weight or body fat. Animals in all groups were insulin resistant based on fasting glucose and insulin. MPI reduced fasting insulin compared to MIX and SPI. SPI reduced total serum cholesterol compared to MPI and MIX. P1NP was greater in MIX than MPI, but CTx was not different among groups. Trabecular spacing was reduced in SPI compared to MIX, while cortical geometry and biomechanical properties were not different among groups. MPI had higher calcium and phosphorus content than MIX. In conclusion, these results suggest that a diet containing soy protein is not detrimental to skeletal development in spontaneously hyperphagic, adolescent male OLETFs rats, and may benefit trabecular microarchitecture. In addition, these results suggest that a diet containing a combination of milk‐ and plant‐based proteins might promote bone formation during skeletal growth.Support or Funding InformationSupport: DuPont Nutrition & Health, NIH R01DK088940 (JPT) and VHA‐CDA2 IK2BX001299 (RSR).