CHAPTER 1
General introduction
General Introduction
Age-related changes in muscle
The worldwide population of people aged 65 years and older is projected to be more than doubled from 2019 and become 1.5 billion by the year 2050 [1]. However, the healthspan (healthy, disease-free lifespan) has not increased with the same degree as the lifespan [2]. Key determinants of healthspan include muscle mass and muscle strength [3], which decline with increasing age [4]. After the fifth decade of life, muscle mass declines by 0.3%
to 0.5% per year [5] and muscle strength by 1.5% to 5.0% per year [6, 7]. With increasing age, muscle strength was reported to decline two to five times faster than muscle mass [4].
The term sarcopenia was introduced by Rosenberg in 1989 to describe “the lack of” (penia) “flesh” (sarx) [8], or better known as a lack of muscle. Sarcopenia is present when muscle mass and muscle strength are below a clinically relevant threshold [8].
Depending on the applied sarcopenia definitions [9-17], the prevalence of sarcopenia varies between 2% to 37% in community-dwelling older adults [18].
Although a worldwide consensus on the definition of sarcopenia has not been reached yet, sarcopenia is now recognised as a disease [19-21]. Sarcopenia is associated with physical dependence, falls, cognitive impairment, poor quality of life, hospitalisation and mortality [22-26]. The adverse health outcomes and the health economic burden [27] associated with sarcopenia make it essential to identify, prevent and treat sarcopenia.
Early identification of older adults at risk of sarcopenia and subsequent treatment may help prevent the loss of muscle mass and muscle strength and potentially extend healthspan along with lifespan.
Modifiable lifestyle factors such as physical activity and nutrition have been proposed to play a role in sarcopenia prevention and treatment [28-30]. For optimal sarcopenia prevention and treatment, more knowledge
has to be gathered to determine the role of energy expenditure and nutrition on the ageing muscle. Muscle mass and muscle strength, energy expenditure and nutrition are interrelated entities. A triangular relationship of these entities can be explained as follow: (1) Energy expenditure is mainly comprised of two components, the energy needs at rest (resting metabolic rate (RMR)) and the activity-induced energy expenditure (AEE) [31]. Muscle mass (metabolically active component of fat-free mass (FFM)) is one of the key determinants of RMR [32-34];
while muscle mass and muscle strength determine bodily movement (physical activity which contributes to AEE) [35]. In the opposite direction, physical activity, and thus the corresponding change in energy expenditure, is associated with muscle mass and muscle strength [36-38]. (2) The assessment of energy expenditure is needed to guide nutritional interventions with regard to the energy intake. Adequate energy intake is required to achieve an energy balance (i.e.
energy intake meet the energy requirement) to avoid an adaptive reduction of energy expenditure through a lowering of tissue metabolism and a reduction of bodily movement [39]. (3) Other nutrients, such as protein, also have the potential to prevent the decline in muscle mass and muscle strength [30, 40].
A better understanding of the triangular relationship between muscle mass and muscle strength, energy expenditure and nutrition could guide healthcare professionals to provide individualised nutritional interventions based on the nutritional needs, to optimise muscle health-related outcomes.
The overall aim of this thesis was to acquire knowledge about assessment and nourishment of the ageing muscle.
Assessing the ageing muscle
Why is it important to assess the ageing muscle?
Assessing the ageing muscle is important in
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order to identify older adults at risk of sarcopenia. Among the adverse health outcomes of sarcopenia, falls and fall-related injuries such as fractures are the leading cause of accidental deaths in the older adults [41, 42]. Some studies showed a positive association between sarcopenia with falls and fractures in older adults; while other studies did not find this association [43-48].
The inconsistent findings may be explained by the use of different sarcopenia definitions and prospective versus cross-sectional study designs. A systematic review showed evidence of an association between sarcopenia with various health outcomes including falls and fractures [22]. However, it focused on one sarcopenia definition and only included prospective studies [22].
Establishing the strength of the associations between sarcopenia with falls and fractures and examining whether this association is dependent on the study design, study population, sex, sarcopenia definition, continent, or study quality may facilitate recognition of the potential role of sarcopenia as a modifiable risk factor for falls and fractures. To date, a systematic review and meta-analysis of the association between sarcopenia with falls and fractures, including studies using different sarcopenia definitions and study designs, is not available yet.
Are healthcare professionals aware of sarcopenia?
For timely diagnosis and treatment of sarcopenia, knowledge amongst healthcare professionals is a prerequisite. A previous study showed a lack of knowledge regarding sarcopenia in Dutch healthcare professionals [49]. Therefore, raising awareness by utilising a professional development education may be an effective initial step in predisposing healthcare professionals to change their practice with regard to sarcopenia diagnosis and treatment. Since sarcopenia is a global public health problem [50], there is a need to examine the current knowledge and practice
of sarcopenia diagnosis and treatment among healthcare professionals in other countries and identify the possible barriers in diagnosing and treating sarcopenia in daily clinical practice.
How should muscle strength be assessed?
Consensus working groups have proposed to include measures of muscle strength as part of the diagnostic criteria for sarcopenia [13, 51, 52]. In an international survey among 255 healthcare professionals from 55 countries across 5 continents, 55% of them stated that they assessed muscle strength in their daily clinical practice, of which 66% of them assessed upper limb muscles strength, such as handgrip strength (HGS), compared with 7% to 24% (depending on the tools used) assessing lower limb muscles strength such as knee extension strength (KES) [53].
HGS measurement is widely used in clinical practice because the measurement is simple and the device is portable and inexpensive [54]. However, studies have shown that the decline of muscle strength with ageing is more prominent for the lower limb muscles compared to that of the upper limb [55-58].
It raised the question of whether HGS or KES can be an indicator of overall muscle strength and whether this depends on the population of different age and health status.
Although low HGS and low KES are independently associated with poor cognitive performance, functional limitations, impaired mobility and mortality [59-61], limited studies have compared the associations between clinical determinants with HGS and KES in the same individuals [62-65]. The Comprehensive Geriatric Assessment (CGA), a multi-dimensional, interdisciplinary diagnostic process, has been used in different healthcare settings to assess a range of health domains such as medical, psychological wellbeing, nutrition, cognition and physical function. Therefore, it is of interest to compare the associations between clinical determinants within
domains of the CGA with both HGS and KES in geriatric outpatients, a clinically relevant population. Recently (2018), the European Working Group on Sarcopenia in Older People (EWGSOP) has released an updated sarcopenia definition, which focuses primarily on low muscle strength as a key diagnostic measure of sarcopenia [14]. This further strengthens the need to compare the assessment method of muscle strength with regard to their clinical relevance.
Nourishing the ageing muscle
About one in five community-dwelling older adults have reported loss of appetite or a decrease in food intake [66]. A decrease in food intake may result in decreased intake of energy, protein and other nutrients [67]. To provide individualised nutritional interventions, a better understanding of the nutritional needs of the older adults is required. However, this data is lacking in geriatric outpatients.
Muscle is recognised as vital to bodily movement, which requires energy and thus nutrition [35]. Muscle also serves as a regulator for energy and protein metabolism throughout the body [68]. Glucose can be stored in muscle in the form of glycogen for metabolic energy functions to meet the energy requirement of an individual [69].
When glucose supplies are insufficient to meet the energy requirement, stored glucose is mobilised from liver, kidney and muscle as energy source (glycogenolysis). When these energy supplies are depleted, amino acids stored as protein in muscle will be broken down and used for energy production (gluconeogenesis) [36]. When amino acids are used for energy production, such metabolic shifts lead to loss of muscle mass.
With loss of muscle mass, energy and protein availability is lowered throughout the body [68]. As a first step to prevent the loss of muscle mass, adequate energy intake should be provided to meet the energy requirement, so that the muscle protein and their
constituent amino acids are spared as an energy source [68].
How can energy expenditure be determined?
RMR, the energy required to sustain normal body function and homeostasis, accounts for about 70% of the total energy expenditure [70]. Therefore, determining RMR is routinely the first step to determine an individual’s energy expenditure.
The gold standard to determine RMR is via a metabolic monitoring device, which measures gas exchange, including oxygen consumption (VO2) and carbon dioxide production (VCO2) [71, 72]. The Deltatrac device has been validated including comparison against mass spectrometry in mechanically ventilated patients [73, 74] and is considered as a gold standard metabolic monitoring device and a reference tool in various validation studies. However, the Deltatrac is no longer commercially available.
Objective measurement of RMR in clinical practice is difficult and impractical because most of the traditional metabolic monitoring devices are relatively costly, less portable and complicated to use. It raised the question of whether a portable metabolic monitoring device is accurate and reliable in measuring RMR compared with a traditional metabolic monitoring device that has been previously validated against the Deltatrac [75].
Apart from using metabolic monitoring devices to determine RMR, predictive equations have been developed and are frequently applied in clinical practice.
In the literature, 138 different equations have been published by 40 different authors. These equations are based on body weight, height, age, sex and body composition [76]. However, existing equations were mostly derived from young and healthy populations [77]. The applicability and accuracy of commonly used equations in older adults have been questioned because of the age-related changes in body composition and energy metabolism [70]. Studies comparing the
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objectively measured RMR with predicted RMR among healthy [78-81] and hospitalised [82, 83] older adults showed that predictive equations do not accurately estimate the RMR. It is still unclear which equation is the most appropriate for use in older adults and especially in geriatric outpatients, a population at higher risk of malnutrition [84]. Inappropriate use of equations may lead to an inaccurate estimation of energy requirements and the implementation of inappropriate nutritional interventions aiming to maintain optimal nutritional status. Among healthy older males and females, FFM and body mass index, respectively, were found to be significant determinants of RMR [85]. Similarly, identifying the key clinical determinants of RMR in geriatric outpatients may give insight regarding the factors that may need to be incorporated when developing or choosing the predictive equations of RMR specific for geriatric outpatients.
What nutrients are related to the ageing muscle?
Apart from energy intake, protein and antioxidant nutrients intake play an important role in preventing the decline in muscle measures, including muscle mass, muscle strength and muscle power [30, 40].
A longitudinal study of community-dwelling older adults showed that higher protein, iron, magnesium, phosphorus and zinc intakes at baseline were associated with increased muscle mass at 2.6 years follow-up; while no associations were found between nutrient intake and muscle strength [86]. Higher intakes of antioxidant nutrients (such as carotenoids, selenium, vitamin C) were associated with higher muscle mass, muscle strength and muscle power in community- dwelling older adults [86-88]. The influence of low nutrient intake and hence its role as a potential contributing factor of low muscle measures may be even more pronounced in geriatric outpatients, a vulnerable population
with high rates of multimorbidity [89]. More evidence is needed to establish the association of nutrient intake and muscle measures in this population.
Outline of this thesis
Figure 1 shows the thesis outline. To achieve the aim of this thesis, we have conducted a series of studies a) to explore the health outcomes of the ageing muscle (chapter 2);
b) to describe the knowledge, practice and barriers in diagnosing and treating sarcopenia among healthcare professionals (chapter 3); c) to assess the agreement of the assessment methods of muscle strength (chapter 4), and their associations with clinical outcomes (chapter 5); d) to explore the assessment methods (chapter 6) and clinical determinants of energy expenditure (chapter 7); and e) to investigate the potential role of nutrition in muscle measures (chapter 8-9). The final chapter (chapter 10) reflects on the main findings, practical implications and recommendations for future research.
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