Effects of Dietary Protein Source and Amount on Bone Quantity and Body Composition following a High‐Protein Weight‐Loss Diet in a Rat Model of Postmenopausal Obesity
Authors: Wright Christian S , Main Russell P , Campbell Wayne W 
Published: November 2023

Obesity is common amongst post‐menopausal women and diet‐induced weight loss is an effective strategy for improving body composition and metabolic health. Though a high‐protein weight loss diet (>30% of energy from protein; HP) is shown to decrease fat mass and preserve fat‐free mass, the impact of a HP weight loss diet on bone quantity is both unclear and controversial. Results show either an exacerbation or an attenuation of weight‐loss induced bone loss with higher total protein intakes, suggesting an underlying discrepancy among clinical studies. Recent evidence suggests that these conflicting results may stem from differences in the predominant source of dietary protein. Different whole food sources of dietary protein possess innate nutritive and bioactive compounds, which when in conjunction with increased protein intake may differentially influence bone. Research from our lab showed that a HP meat‐based weight loss diet decreased total body bone mineral density (BMD) in postmenopausal women in comparison to a normal protein (NP) lacto‐vegetarian diet. In contrast, previous studies have shown that a HP dairy‐based weight loss diet attenuated and inhibited weight‐loss induced bone loss in postmenopausal women. Therefore, the objective of this study was to investigate the differential effects of protein source (freeze‐dried lean beef vs. milk protein isolate vs. soy protein isolate vs. casein) and protein amount (NP, 15% energy vs. HP, 35% energy) in a 12‐week energy‐restricted diet (40% restriction) on BMD, bone mineral content (BMC), and body composition in obese, ovariectomized 32‐week old female Sprague‐Dawley rats. We hypothesized that the HP beef‐based diet would decrease bone quantity in comparison to the HP milk‐based and HP soy‐based weight loss diets. Lunar PIXImus densitometer and peripheral quantitative computed tomography were utilized to assess changes in bone quantity (lumbar spine and right femur) and body composition (total fat, muscle area and density), respectively. The freeze‐dried lean beef powder was created in‐house, while the four energy‐restricted diets (HP beef‐based, HP milk‐based, HP soy‐based, & NP casein‐based) were formulated by ResearchDiets Inc. Predominate source and amount of dietary protein did not influence changes in bone quantity following the 12‐week energy restricted diet. Differential effects were observed for body composition as the HP milk‐based diet lost more fat mass than the HP soy‐based diet (p=0.031) and the NP casein‐based diet lost more body weight than the HP beef‐based (p=0.022) and the HP soy‐based diets (p=0.02). Overall, all dietary groups showed decreases in lumbar spine BMD (p<0.001) and BMC (p<0.001), right femur BMD (p=0.002), body weight (p<0.001), muscle area (p=0.014), and total fat (p<0.001) with increases in muscle density (p<0.001) following weight loss. In summary, neither predominate source nor amount of dietary protein influenced changes in bone quantity during energy restriction. Numerous sources of dietary protein can be utilized in a HP weight loss diet to improve body composition without negatively influence bone quantity in a model of postmenopausal obesity.Support or Funding InformationSupported by Women's Global Health Institute's Mildred Elizabeth Edmundson Research Grant