Comments and future directions arising from “The Impact of Dairy Protein Intake on Muscle Mass, Muscle Strength, and Physical Performance in Middle-Aged to Older Adults with or without Existing Sarcopenia”

Dear Editor:

We read with interest the systematic literature review by Hanach et al. (1) published in the January issue of Advances in Nutrition. We congratulate the authors for writing a high-quality review and meta-analysis that provides valuable insight into the benefits of dairy proteins in the aging population, and the prevention and management of sarcopenia. While we agree with their recommendations suggesting the need for larger scale clinical trials in this area, we also suggest that addressing pre-existing methodological gaps is essential, considering the growing number of recreational and competitive active older adults. This population represents individuals in the ≥5th decade of life that continued, returned, or initiated recreational or competitive sport and exercise activities, and are currently a substantially underrepresented population in the sarcopenia research literature. This significance, for example, is shown by the progressive increase in participation rates of active older adults in endurance-based events over the last 3 decades (2).

The WHO generally recommend that older adults should participate in at least 150 min/wk of light- [e.g., 3–5 metabolic equivalents (METs)] to moderate-intensity (e.g., 6–9 METs) physical activity or 75 min/wk of vigorous-intensity (e.g., >9 METs) physical activity (3). It is evident that active older adults can exercise >60 min/d recreationally to >4 h/d in ultraendurance competitive events (2, 3). In comparison to sedentary or low activity adhering older adult populations (e.g., community dwelling or institutionalized), recreational and competitive active older adults show that high levels of exercise adherence can effectively countermeasure against normal age-related muscle wasting and dysfunction (4). However, adults who undertake regular physical activity and exceed physical activity guidelines (e.g., endurance- and power-trained individuals) still show signs of decreasing muscle mass, strength, and performance with increasing age. For example, Pearson et al. (5) compared the muscle function of competitive masters weightlifters to age-matched untrained controls and found similar rates of decline in peak power (1.2% and 1.3%, respectively) and isometric strength (0.6% and 0.5%, respectively) with increasing age. This is supported by similar reductions in metabolic muscle function, as observed by Bagley et al. (6) who found a 7% to 14% decline in aerobic power (assessed through V̇O_2peak and relative fat oxidation, assessed by a single and 2-legged incremental bike test), per decade of life, in endurance- and power-trained masters athletes (aged 37–90 y). Such observations suggest that with increasing age there is still a decline in muscle function and strength, irrespective of exercise stress load. Despite active older adults still showing signs of sarcopenia progression, they represent an ideal population to study the aging process as they do not have the confounding factor of muscle disuse seen in sedentary or low activity individuals commonly recruited in experimental models (7).

It is proposed that no significant effects were reported in the systematic literature review with the use of dairy protein intake on performance (1), possibly because the performance measures used within the included studies [e.g., Short Physical Performance Battery (SPPB) and gait speed] may not be sensitive, and therefore, may not be suitable to measure performance in the population group that are considered active or even healthy. The SPPB assessment reportedly has a ceiling effect on a cohort of active older adults, whereby the majority (55%) of the physically active participants are reported to receive the top score of 12 points (8). Moreover, measurements such as gait speed are found to present a nonlinear relation between leg strength, resulting in a wide population variance (e.g., 22%) (9). The potential risk of outcome misinterpretation explains how small changes in physiological capacity produce large effects on performance in frail adults, whereas large changes in capacity have little or no effect on daily function in healthy adults (9). Therefore, the use of gait speed and SPPB in an active older cohort may not be sensitive enough to detect any clinically relevant change in response to exercise and/or nutritional interventions. Considering that in healthy older people, muscle power declines faster than strength, measuring power output may be a better predictor of certain functional activities (10). Evaluation techniques of muscle power output that have been validated and reliability checked include vertical jump on a force platform, unloaded leg extensor power, and isokinetic dynamometry (11). These measures have been incorporated into multiple studies of males and females, athletes and nonathletes, with correlations between vertical jumping power and chair stand-up power with age evident (12). Reid and Fielding's (11) synthesis of physical function in older adults concludes that muscle power rather than muscle strength better predicts functional performance in older adults. Hence, the inclusion of these methodological approaches in sarcopenia research, irrespective of whether they include dairy milk supplementation, will ensure that markers of physical performance align with all study cohorts. The inclusion of these measures will enable more discretion in the determination of sarcopenia status, overcoming the test sensitivity that may exist in the measurement techniques that currently predominate. Therefore, vertical jump, specific muscle power output, and/or isokinetic dynamometry should be considered in future studies assessing skeletal muscle performance in healthy older adult populations, especially if cohorts are described as “active”.

In addition, we note that some included studies provided participants with resistance training but did not assess potential confounding changes in cardiorespiratory fitness (CRF). Improvements in maximal (V̇O_2max) and submaximal (aerobic economy) oxygen consumption are seen as a result of regular aerobic training. However, growing evidence suggests that strength training may improve the metabolic efficiency of the muscle, which in turn increases aerobic capacity (13). This was observed in a study by Lovell et al. (13) that reported a significant, but modest, V̇O₂ improvement (+0.110 L/min; P < 0.05) in response to cycle exercise at 70% V̇O_2max in healthy older adult males who weight trained for 16 wk. Longitudinal evaluation of V̇O_2max has shown linear decline with age (14), in both male and female participants, irrespective of exercise stress status, with declines further influenced by fitness status (i.e., trained compared with untrained) (15). Moreover, there is also evidence that daily lifestyle determined CRF is positively correlated with V̇O_2peak (r =  0.597, P < 0.001), based on findings from a recent study that recruited a cohort of community-dwelling elderly men (16). Therefore, in longer intervention trials that include a resistance-training element, this may result in an increased physical activity adherence outside the exercise protocol (e.g., increased daily steps and daily activities), which is likely and/or will inadvertently affect cardiovascular adaptations and subsequently total aerobic capacity and aerobic efficiency. Controlling for change in CRF or measuring the activity levels along the experimental timeline is essential for accurate result interpretation. For example, an increase in CRF due to increased daily activity may inevitably influence muscle mass, power, strength, and performance outcomes, especially in proportion to baseline starting values in frail and sedentary muscle.

Overall, we concur with Hanach et al. (1) for the need for larger high-quality randomized clinical trials investigating the effects of dairy proteins on standardized outcomes (i.e., muscle mass, power, strength, and performance). However, as we have highlighted, an entirely unexplored area is the effect of dairy proteins and exercise on an active older cohort. We suggest that outcomes of future studies should include appropriate markers of physical performance and quality of life to match the exercise outputs of the active older adults.

Notes

The authors declared no funding and no financial support received for this letter.

Author disclosures: ZH, JP, and RJSC have received industry funding to conduct trials on the effect of milk products on the active aging population.