Nutritional strategies to improve muscle quality during ageing

Increased lifespan is a great achievement of humankind, and opens new opportunities for the life of ageing human beings, in part conditional on health and independence. Health and independence are related to physical performance which is often compromised in older adults, to some extent due to age-related decreases in muscle mass and muscle strength decrease. During ageing, muscle strength decreases at a faster rate than muscle mass, indicating a loss in muscle quality. Muscle quality is defined as muscle strength per unit of muscle mass and is determined by factors such as specific muscle fibre atrophy, mitochondrial functioning, neuromuscular functioning and degree of fat infiltration. Targeting the quality of muscle tissue is a promising strategy to improve muscle quality during ageing.

An active lifestyle seems to prevent losses in muscle quality to some extent, whereby exercise programs can improve markers of muscle quality. Not many intervention trials have aimed to improve muscle quality via nutrition. However, metabolic aspects play a role in muscle quality, and many nutrients act on pathways that are related to physiological determinants of muscle quality. Together, this gives reasons to pursue the identification of nutritional strategies to target muscle quality. Therefore, in this thesis, we aimed to identify novel nutritional strategies to improve muscle quality and physical performance in older adults.

In Chapter 2, we investigated the reliability and validity of handheld dynamometry (HHD) for the assessment of leg strength. Leg strength assessment is proposed to be a better alternative to the often used handgrip strength measurement. However, the current advised equipment to measure leg strength is static and costly, while handgrip strength can be measured with an inexpensive and small device. The results of Chapter 2 show that the HHD is a valid and reliable tool to assess changes in leg strength in older adults or to rank older adults on strength level, indicating that HHD can be used as an alternative to handgrip strength assessment. The results of Chapter 2 are important for the development of leg strength measurements for strength monitoring in older adults.

Chapter 3 and Chapter 4 describe the results of two clinical trials aimed at improving physical performance in older adults. In Chapter 3, an intensive exercise and nutrition intervention augmented muscle mass (+5%), strength (+13%), and physical performance (10-20%) of older adults already after four weeks. The findings are relevant and important because short-term interventions like these are urgently needed for the future of prehabilitation. Prehabilitation is a discipline aimed at training patients before planned surgery so that patients lose less muscle mass and strength during hospitalisation.

In Chapter 4, a novel medical nutrition product was tested against standard medical nutrition in older adults with (or at risk of) undernutrition. Without a concurrent exercise regimen, this novel medical nutrition treatment appeared to be able to improve walking performance and mitochondrial functioning in this vulnerable group. The improvement in walking performance is of clinical relevance, as it relates to lower chances of future falls, hospitalisation and mortality. Being able to improve physical performance without a concurrent exercise program is an important finding for the field of nutrition and ageing, suggesting that besides exercise interventions, there is a potential for nutritional interventions as well.

In Chapter 5 and Chapter 6, we aimed to identify new nutritional targets to improve muscle quality in older adults. Chapter 5 was a hypothesis-driven approach in which we tested the associations between dietary intake of vitamin B3, B6, B12, folate and measures of physical performance in healthy older adults. Chapter 6 was a data-driven approach, in which we tested for associations between all nutrients and measures of physical performance in healthy, frail or malnourished older adults. Chapter 5 identified an association between the intake of vitamin B6 and chair rise test in the full population (3% improvement per mg of vitamin B6) and an association between intake of vitamin B6 and handgrip strength in participants with low physical activity (also a 3% improvement per mg of vitamin B6), but not in those with normal or high physical activity. Chapter 6 also identified an association between vitamin B6 and chair rise test and total physical performance score. Chapter 6 additionally identified associations between intakes of vegetable protein, vitamin B12 and folate with measures of physical performance. Chapter 6 made use of Copula Graphical Modelling, a new methodology with many potential applications for the field of nutritional epidemiology.

This thesis adds important knowledge on how to measure muscle quality, which strategies are effective and which nutritional strategies could be further explored in the future. Advancing the integration of muscle quality measurements in the field of nutrition and ageing research can lead to important advances in the quest to find strategies that improve physical performance in older adults. With improved physical performance, older adults are more likely to stay mentally, socially, and physically active. This in turn helps them to age healthily and happily.