Effects of ageing, a high-fat diet and physical exercise on skeletal muscle morphology

Age-related loss of skeletal muscle mass and strength can lead to reduced independence, quality of life and life expectancy, which may be exacerbated by an increased high-fat intake and a low physical exercise. This thesis investigated the effects of ageing, high-fat diet (HFD) and regular physical training on muscle morphology. Specifically, in study I, we compared intramyocellular lipid (IMCL) levels, capillarisation, fibre type and size, and oxidative capacity of fibres in locomotor (soleus and EDL) and respiratory (diaphragm) muscles in 20- (young-adult) and 79-week-old (early ageing) mice. Early ageing was characterised by an absence of muscle wasting in soleus, the EDL atrophied while the diaphragm hypertrophied without changes in the capillary numbers supplying a fibre, or their oxidative capacity. In study II, we studied the effects of a HFD on the morphology of the soleus, EDL and diaphragm in 20- and 79-week-old mice. Old mice were more susceptible to morphological alterations with a HFD compared to young mice. All fibre types showed similar adaptations in response to a HFD but they were muscle-specific with the EDL being least responsive. In study III, we assessed fibre type grouping in the vastus lateralis of athletes and nonathletes (19 - 85 years old) and evaluated to what extent any observed grouping, indicative of cycles of denervation and reinnervation following motor neuron loss, is more than expected from the fibre type composition of the muscle. Since regular physical exercise may stimulate fibre reinnervation, we hypothesised that master athletes have larger fibre groups than agematched non-athletes. An ‘enclosed fibre’ was any muscle fibre of a given type surrounded by fibres of the same type only. A ‘fibre group’ was defined as a group of fibres with at least one enclosed fibre. The prevalence of observed fibre type grouping was similar to that expected from the fibre type composition. No age-related effect on group size and group number in athletes or non-athletes was found. In conclusion, the current thesis described the morphological changes of CD-1 mouse skeletal muscles during ageing as muscle specific. Additionally, using the same mouse model, HFDinduced muscle morphological alterations depending on diet duration and age, varied between muscles. Moreover, the results of the current thesis do not show evidence for improved reinnervation of muscle fibres with regular physical training. Nevertheless, histological examination may not provide the full extent of ageing related motor unit remodelling