Protein and muscle health during aging: benefits and concerns related to animal-based protein

It is estimated that by 2060, nearly one in four individuals in the United States (94 million) will be over the age of 65 (Vespa et al., 2018). A well-known consequence of aging is the loss of skeletal muscle mass and strength, which are often referred to as sarcopenia and dynapenia, respectively (Clark and Manini, 2008). Collectively, sarcopenia and dynapenia can alter energy metabolism and lead to the loss of physical function, ultimately manifesting as functional disability. Skeletal muscle wasting specifically, occurs at a rate of 1% to 2% per year beginning at age 50 yr and may significantly affect up to 45% of older adults (Baumgartner et al., 1998). For example, Figure 1 shows a magnetic resonance imaging axial scan of the left upper leg muscles in (Figure 1A) a younger, active male and (Figure 1B) an older, inactive male. When analyzed via color thresholding for various tissue types and concentrations, notice the dramatic decrease in muscular area (dark gray color) in the inactive, older adult, which is accompanied by an increase in lipid infiltration and fibrous tissue (white color) within the muscle area relative to the active, younger adult. Although this is not the same individual, it is recognized that nearly all humans will have a similar profile over time unless an intervention (physical activity, exercise, dietary, pharmacological) occurs to slow the progression of skeletal muscle loss and adipose tissue infiltration. As a result, sarcopenia may lead to a greater economic burden related to treatments associated with loss of muscle mass and physical function (Janssen et al., 2004). Open in a separate window Figure 1. Representative MRI images of the upper leg showing muscle area (dark gray), inter/intramuscular adipose/fibrous tissue (white area within and between muscles), and subcutaneous adipose tissue (white area around all the muscle groups) in (A) younger, active and (B) older, inactive male. Muscle area is increased in the younger, active male by 55% compared with the older, active male (82.21 cm2 vs. 52.87 cm2), whereas lipid/fibrous area is increased 7× in the older, inactive male compared with the younger, active male (11.50 cm2 vs. 1.64 cm2).