Featured article in the August, 2016 Issue of Nutrition Research Update; written by Christian Wright, Doctoral Candidate, Department of Nutrition Science Laboratory of Nutrition, Fitness, and Aging Purdue University
It goes without saying that our nation currently faces a serious obesity crisis. Nearly half of the United States has an obesity prevalence greater than 30% and not a single state in the U.S. shows a prevalence less than 20% (Fig. 1). This pervasiveness of obesity has led to a dramatic spike in cardiovascular disease and type II diabetes cases, which has ultimately decreased the quality of life and life expectancy for many Americans (1). One solution to this nationwide problem is weight loss, particularly diet-induced weight loss (2). Even a 5% reduction in body weight is shown to improve fasting blood lipid and glucose concentrations while decreasing the risk of all-cause mortality (3, 4). Indeed, weight loss is beneficial and is needed to combat our on-going battle with obesity. However, the loss of body mass without considering changes in body composition is irresponsible. Though beneficial for metabolic health, weight loss is shown to decrease bone mass (5) which could, in turn, increase the risk of osteoporosis and skeletal fracture.
Bone is a metabolically active organ serving many functional roles in the body including mechanical support and protection of vital organs, both of which are heavily influenced by bone mass. Bone mineral density (BMD) is a clinical measurement of bone mass and is closely tied to total body mass (6). Many factors contribute to this close relationship, however, it is the mechanical load placed on bone via body mass, particularly lean mass (7), that induces bone formation and closely ties these two together (8). Weight loss reduces this mechanical load place on bone and encourages the loss of BMD (9). This is particularly detrimental in overweight/obese older adults (10) who inherently have lower bone mass and less lean mass (11).
In order to reduce the negative complications of weight loss, dietary strategies that preserve both bone and lean mass should be prioritized, particularly in older adults. Moderately high-protein (1.0 to 1.2 g protein/kg/day or 22% to 29% of total energy consumed) to high-protein (>1.2 g protein/kg/day or >30% of total energy consumed) weight loss diets are shown to beneficially affect soft tissue by preferentially decreasing fat mass while preserving lean mass (12). Can a high protein weight loss diet (HPWL) also preserve bone mass?
Evidence from randomized controlled trials suggest that a HPWL can indeed maintain bone mass if not attenuate weight-loss induced bone loss. Thorpe MP and colleagues showed that a one year HPWL (1.4 g protein/kg/day) maintained total body BMD in obese middle-aged adults (N=130, 45-48 years, ~31 BMI) versus a normal protein diet (0.8 g protein/kg/day) (Fig. 2) (13). Additionally, following a one-year study in post-menopausal women (58.0 ± 4.4 years; BMI 32.1 ± 4.6 kg/m2), Sukumar D and colleagues showed that a HPWL (24% energy from protein) attenuated decreases in BMD and showed lower markers of bone resorption (Fig. 3)(14).
Widely used for their desirable weight-loss and health-related outcomes, recent evidence suggests that HPWL could help preserve bone mass during weight loss and prevent the increased risk of osteoporosis and skeletal fracture. Increasing the consumption of protein-rich foods such as low-fat dairy, lean meats, and/or eggs during energy restriction not only provides a high quality source of dietary protein but bioactives potentially beneficial for bone (15). When advocating weight loss, clinicians must consider the appropriate dietary strategy to optimize all health-related outcomes.