Dietary Protein and the Risk of Mortality in Adults with Low Bone Mineral Density: A Longitudinal Study

Low bone mineral density (BMD) is common in older adults and is associated with increased mortality risk. Although higher protein intake is recommended for skeletal health, it remains unclear whether total protein intake or protein source is more relevant for mortality risk. We examined associations of dietary protein quantity and source with mortality risk among adults with low BMD. Data from 4148 adults aged ≥ 40 years with osteopenia or osteoporosis who participated in six National Health and Nutrition Examination Survey cycles from 2007 to 2008 through 2017-2018 were analyzed. Survey-weighted estimates indicated a mean age of 58.1 years, a female proportion of 65.0%, and mean total protein intake, animal protein density, and plant protein density of 1.04 g/kg/day, 18.5 g/1000 kcal, and 6.0 g/1000 kcal, respectively. Protein intake was assessed using two 24-hour dietary recalls. Total protein was expressed as g/kg/day, and animal and plant protein as energy-adjusted densities (g/1000 kcal). Mortality outcomes were ascertained via linkage to the National Death Index. Associations with all-cause and cause-specific mortality risk were estimated using survey-weighted Cox proportional hazards models. Cross-sectional associations with BMD were also evaluated. Over a median follow-up of 6.7 years, 578 deaths occurred. Total protein intake was not associated with the risk of all-cause mortality (HR per 0.1 g/kg/day: 1.00; 95% CI 0.96-1.05). Higher plant protein density was associated with a lower risk of all-cause mortality (HR per 10 g/1000 kcal: 0.72; 95% CI 0.55-0.95), whereas animal protein density was not (HR: 0.94; 95% CI 0.84-1.06). Plant protein density also showed evidence of an inverse association with the risk of cardiovascular mortality, but not with the risk of cancer mortality. Cross-sectionally, both animal and plant protein densities were positively associated with femoral neck BMD, whereas no association was observed for lumbar spine BMD. In adults with low BMD, mortality risk differed by protein source rather than total intake, with higher plant protein density associated with lower risk of all-cause mortality and showing evidence of an inverse association with the risk of cardiovascular mortality.