4.2.1. Muscle Disuse Atrophy

Skeletal muscle injuries account for over 40% of all sports-related injuries, with a two times higher risk in male than female athletes [233]. Fatigue fractures are the most common bone injuries in athletes (0.7% to 20% of all injuries), especially in women with reduced energy availability [218], and often occur during periods of high-volume and high-intensity training that are characterized by excessive and rapid increases in training and competition load [234]. Fatigue fractures emerge from prolonged mechanical overuse, so that bone's capacity to repair microdamage through the remodelling process is overwhelmed by the emergence of new bone microdamage sites, which ultimately results in bone material fatigue [94]. Thus, the factors that predispose to fatigue fractures include the intensity, frequency and duration of loading exercises, as well as short recovery periods between loading cycles [235]. Based on the current epidemiological evidence, the masters athlete's age, per se, does not increase the prevalence and risk of injury within competition [236], although little is known about injuries during training, including the incidence of fatigue fractures. It is rather that inadequate energy intake and/or deficits in muscle strength, flexibility or aerobic fitness increases the risk of sustained sports injuries.

Muscle disuse due to prolonged best rest (e.g., hospitalization, recovery from surgery), limb immobilization or reduced PA due to illness result in rapid muscle atrophy ( ∼0.5% of muscle mass per day of immobilization) and deconditioning of muscle tissue [237,238]. Furthermore, short-term (5 days) muscle disuse has been demonstrated to lower postabsorptive and post-prandial MPS rates and to induce anabolic resistance to protein ingestion in healthy young adults [239]. Thus, muscle disuse represents a unique metabolic and nutritional challenge for the young but even more so for the masters athlete as energy and macronutrient requirements are altered considerably. Although robust data on the consequences of these changing metabolic demands and the efficacy of nutritional interventions in injured athletes are still lacking, models of muscle disuse (i.e., bed-rest, step reduction) have revealed insights into the likelihood for deleterious metabolic adaptations that occur during a short-term reduction in PA with increased sedentary behavior [2,240,241]. A recent study in young adults with a habitually active lifestyle (>10,000 steps/day) provided direct evidence of a number of unfavourable adaptations to body composition with loss of lean mass ( ∼0.3 kg) and accretion of abdominal and liver fat, with development of whole-body insulin resistance, after 2 weeks of physical inactivity (>80% step reduction) [2]. Older adults with lower baseline lean mass and slower rate of recovery following physical inactivity may be even more prone to these acute periods of muscle disuse when compared to young people [2,241].

#### 4.2.2. Role of Nutrition for Injury Recovery

While sports nutrition has typically focused on augmenting performance and adaptations, far less attention has been given to nutrition for the injured athlete. Injury, per se, results in significant stress response and increases energy expenditure by 15% to 50%, depending on the type and severity of the injury. During the first stage following an injury, an inflammatory response is initiated accompanied by an increase in catabolic hormones (i.e., cortisol and catecholamines) and a decrease in anabolic hormones (i.e., GH, testosterone), resulting in a catabolic environment that can lead to a sudden and large loss of lean body mass [242].

Nutritional strategies have been proposed to help improve recovery from exerciseinduced injuries involving immobilization and/or reduced activity [243–247]. One of the key considerations during the injury is to ensure that sufficient energy is consumed to prevent excessive muscle disuse atrophy and to support repair, without significantly increasing body fat. Hence, identifying energy needs during injury via indirect calorimetry or estimated using predictive equations is an important first step to maintain the caloric balance. Thereby, total daily energy expenditure can be calculated as: resting metabolic rate x stress factor (bone fracture, minor surgery = 1.2) × activity coefficient (sedentary = 1.2, lightly active = 1.4) [245]. Especially in the older athlete, where the muscle could develop anabolic resistance, it is crucial to ensure a higher protein intake for repair. The recommendation is 1.6–2.5 g/kg body mass, evenly spread across the day, every 3–4 h around a rehabilitation session, and before sleep, in amounts of 20–35 g, which contain high amounts of leucine (2.5–3 g), and additionally casein prior to sleep [246,248,249]. Other nutrients, such as creatine monohydrate (10 g/day for 2 weeks), fish oil-derived omega-3 fatty acids (4 g/day), and ß-hydroxy-ß-methylbutyrate (3 g/day) have been proposed as beneficial for the treatment of injury [248]. However, although dietary-supplement strategies may be useful if caloric intake and appetite is reduced, nutritional considerations to promote injury recovery should be explored in a food first approach, rather than a reliance on supplements [243].

#### *4.3. Immune Function and Risk of Infection*

Age-related declines of both the innate as well as the adaptive immune system contribute to the increased susceptibility of older individuals to acute and chronic infections, autoimmune diseases, and systemic inflammatory diseases. While a lack of PA, decreased muscle mass, and poor nutritional status facilitate immunosenescence (namely, a decline of naïve T cells and the CD4/CD8 T-cell ratio, an increase in memory/effector T-cells and senescent/exhausted T-cells) and inflammaging (characterized by elevated levels of IL-6, TNFα, and IL-1β), moderate exercise training positively affects the composition of the T cell compartment, the function of various leukocyte subpopulations, and counteracts

hallmarks of immunosenescence [250]. Thus, maintaining regular PA throughout life helps to maintain function of the immune system during aging, which could play a role in preventing infection. A recent study in masters cyclists aged 55 to 80 years found that compared with inactive older adults, the cyclists showed reduced evidence of a decline in thymic output and inflammaging [251]. Masters cyclists showed higher serum levels of the thymo-protective cytokine IL-7 and lower IL-6, which promotes thymic atrophy. In addition, maintaining high levels of aerobic fitness during aging may help prevent the accumulation of senescent T-cells [252]. While moderate exercise reduces the risk of illness [253], prolonged intense exercise is associated with a transient depression of immune function and can lead to immune impairment in athletes associated with an increased susceptibility to infections (mainly of the upper respiratory tract) [254]. There has been much interest in how dietary strategies can improve immunity in athletes. So far, there is limited evidence that the dietary practices of athletes such as low energy or carbohydrate availability suppress immunity. Athletes are recommended to follow a balanced diet to avoid a nutrient deficiencies required for proper immune function [255]. A metabolomics approach offered by Nieman and Mitmesser [191] highlights the potential impact of high carbohydratepolyphenol food sources to counter post-exercise inflammation and to enhance oxidative and anti-viral capacity, with reduced upper respiratory illness (URI) rates [256]. Another intervention by which the immunomodulatory effects of high-intensity training might be countered is by manipulating dietary protein intake. For example, a reduced incidence of URI was observed in elite cyclists undertaking 2 weeks of high-intensity exercise training while consuming a high protein diet (3 g/kg body mass/day) [257]. The authors concluded that it is possible that immune surveillance might be maintained during heavy training by consuming a high protein diet. The new theoretical perspective offered by Walsh et al. [258] sharpens the focus on tolerogenic nutritional supplements (e.g., probiotics, vitamin C and vitamin D) shown to reduce the burden of infection in athletes at a non-damaging level. In the present section, we focus on vitamin D and probiotics, with a particular interest in the gu<sup>t</sup> microbiota because of the close relationship between the microbiome and the immune system.
