4. Discussion
muscle mass, muscle degeneration, fatigue, age-related impairments, sarcopenia, MPS, knee osteoarthritis, Nagai, MPB, reduced mobility, muscle hypertrophy, malnutrition
MUSCLE DEGENERATION, ADVERSE EVENTS, SARCOPENIA, KNEE OSTEOARTHRITIS, MALNUTRITION, TYPE 2 DIABETES
Cross-sectional studies have found an association between low BMI and frailty [We used the Locomo Scan-II device to measure knee extension strength. The knee extension strength has been measured using an isokinetic machine as the gold standard method, but such machines are large, immovable, require a skilled operator, and are not suitable for quick measurements on a large group of subjects. Oomori et al. reported that the knee extension strength measured using the Locomo Scan-II prototype showed a significant correlation with the measurements obtained using an isokinetic dynamometer (Biodex system 3; Biodex Medical System Inc., Shirley, New York, NY, USA) in a sample of 1016 patients with knee osteoarthritis thus confirming its utility [Knee extension strength is widely used in clinical practice as a representative measure of lower limb support during walking, and it is reportedly closely related to walking independence and walking speed [In this study, we measured knee extension strength using the Locomo Scan-II device as a tool to assess the degree of age-related impairments that increase the risk of physical frailty and the effectiveness of interventions in mitigating these impairments. The average knee extension strength in the study subjects was 327 N for men and 277 N for women in their 60s, and 300 N for men and 262 N for women in their 70s. In a study by Narumi et al. that measured knee extension strength using the Locomo Scan in healthy individuals, the average values were 471 N for men and 405 N for women in their 60s, and 385 N for men and 340 N for women in their 70s [In this study, grip strength was also measured. Since walking was prescribed as a moderate-intensity exercise in this trial, there was no direct stimulation to the upper limb muscles through exercise. However, ghrelin which is secreted from the lower limb muscles due to walking or from the stomach because of MCT intake may have a positive impact on the muscles related to grip strength through the stimulated secretion of IGF-1. The MCT intake group showed a significant increase in grip strength compared to the baseline. Although there was no statistically significant difference in grip strength between the MCT intake group and the control group, it is speculated that MCT intake promoted the secretion of ghrelin in the subjects’ bodies, leading to an increased secretion of IGF-1, which in turn resulted in increased grip strength compared to before MCT intake. Grip strength is related to knee extension strength [In this study, body muscle mass was measured in healthy subjects and no significant differences were found between the groups. Similar results were reported in a previous study in which severely frail subjects with sarcopenia were fed the same amount of fats and oils with the same MCFA composition as the high octanoic acid food group in this study. Abe et al. found that after 3 months of consumption of 6 g of MCT and 1.2 g of L-leucine or 20 μg of cholecalciferol or 6 g of MCT per day by residents of a special nursing home who required helper assistance, knee extension time (a measure of muscle endurance) and the number of leg openings and closings over 10 s (a measure of muscle strength) improved significantly compared to controls. Although they improved significantly, there were no differences in the calculated AMA or leg circumference (a measure of muscle mass) between the study groups [The balance between muscle protein synthesis (MPS) and muscle protein breakdown (MPB) determines the net rate of muscle growth [The effects of MCT intake, which are speculated to be the underlying mechanisms for the observed increase in muscle strength in this study, have been reported in previous studies. For instance, Ishizawa et al. compared rats fed a high-fat diet with either LCT or MCT for 4 weeks to rats fed a standard diet. They found that MCT intake resulted in increased mitochondrial citrate synthase enzyme activity in all types of skeletal muscles examined (triceps brachii, superficial and deep portions of the gastrocnemius, and soleus muscle) compared to the control group fed a standard diet (There have been several reports on the effects of combining exercise with food components other than MCT on muscle mass, muscle strength, and muscle function in healthy older adults. Nagai et al. reported the results of a randomized controlled trial (RCT) with resistance training, in which 36 healthy men and women aged 71 to 76 years assigned to either a control group (In these interventions, unlike MCT, there were differences in muscle mass compared to the control groups. As a result, it is suggested that muscle mass was maintained. One possible reason for the higher muscle mass is the potential effect of these food components on stimulating muscle protein synthesis. For instance, when maslinic acid was administered in the feed of rainbow trout for 225 days, a significant increase in white muscle weight and protein synthesis rate was observed in the trout compared to the control group [In this study, the results showed that while there was no significant improvement in muscle mass, there was a significant improvement in muscle strength after 12 weeks of concurrent intake of MCT along with moderate-intensity exercise. Additionally, it is generally believed that muscle strength does not improve with training below moderate intensity. However, in this study, the control group that consumed LCT showed an increase in muscle strength after 12 weeks, and the MCT group demonstrated a significantly greater increase in muscle strength compared to the LCT group.Resistance training has been considered the most effective exercise therapy for maintaining and improving muscle mass in patients with sarcopenia. Typically, strength training with a load setting of 60–70% of one-repetition maximum (1RM) has been recommended for the purpose of muscle hypertrophy and strength enhancement [Recently, it has become evident that muscle hypertrophy can be induced by repeatedly performing long-duration low-intensity exercise to the point of fatigue [We investigated the possibility of maintaining muscle mass and strength with a combination of dietary and exercise interventions, even with lower intensity exercise. As a result, lower limb strength increased, but it did not lead to an increase in muscle mass. This could be attributed to factors such as the intensity or frequency of the exercise employed in this study, or the duration of the intervention, which may have been insufficient compared to previous reports that demonstrated changes in muscle mass with exercise loads below moderate intensity. On the other hand, even in the control group that consumed LCT, knee extension strength increased compared to the pre-intervention measurements. In middle-aged and older individuals who are at high risk of future frailty due to lack of exercise or a sedentary lifestyle, it is believed that, if they are healthy, resolving the issue of physical inactivity through exercise can lead to the maintenance of muscle strength and reduction in frailty risk. The results suggest that MCT intake has the potential to further enhance the preventive effects against frailty.In this study, we also examined the effects of different quantities and compositions of MCFAs in MCT. Regarding the observed changes and trends in knee extension strength and grip strength in this study, we investigated the differences in the composition of MCFAs in MCT consumed. However, no significant differences were found between the group consuming MCT with a high dose of octanoic acid (6 g/day) and the group consuming MCT with decanoic acid. The daily intake of MCFAs in each group was as follows: for the decanoic acid group, octanoic acid ranged from 1.40 to 1.95 g and decanoic acid ranged from 3.47 to 4.00 g; for the high-dose octanoic acid group, octanoic acid ranged from 3.72 to 4.14 g and decanoic acid ranged from 1.06 to 1.46 g; and for the low-dose octanoic acid group, octanoic acid ranged from 1.24 to 1.38 g and decanoic acid ranged from 0.352 to 0.487 g. The high-dose octanoic acid group and low-dose octanoic acid group predominantly contained octanoic acid, while the decanoic acid group contained decanoic acid as the major fatty acid. Although all are MCFAs, they reportedly have different physiological effects. In studies using in vitro or type 2 diabetes model animals, a mixture of octanoic acid and decanoic acid has shown potential to protect islet β-cell function from impairment [With regard to energy supply in skeletal muscles, it has been suggested that octanoic acid increases the supply of energy sources to muscles through ketone body production in the liver, while decanoic acid has the potential to increase mitochondrial biogenesis and energy production in skeletal muscles through the peroxisome proliferator-activated receptor (PPAR), which is a fatty acid receptor. Fukazawa et al. reported an increased protein expression of OXCT (a ketone body-utilizing enzyme) in the epitrochlearis muscle of rats fed a ketogenic high-fat diet containing octanoic acid-dominant MCT, similar to the fat intake of the high-dose octanoic acid food intake group in this study [In this study, although the changes in knee extension muscle strength were similar among the two groups that consumed 6 g of MCT and the group that consumed 2 g of low-dose octanoic acid food, the changes in grip strength were different. The maximum left-hand grip strength in the 6 g intake group showed no significant difference compared to the control group, but a significant increase was observed compared to the baseline. On the other hand, the maximum left-hand grip strength in the low-dose octanoic acid food intake group did not show significant changes compared to the baseline throughout the intake period. Tsujino et al. reported that ingestion of 2 g of MCT, predominantly composed of octanoic acid, for 2 weeks in 29 healthy individuals with an average age of 50.3 years (17 males and 12 females) resulted in an increased rate of fatty acid oxidation during daily activities [As mentioned above, MCT did not induce changes in muscle mass in this study, and previous evidence has also not observed favorable changes in muscle mass or lean body mass with MCT intake, suggesting a limited effect on skeletal muscle protein metabolism. However, there have been reports of potential effects on liver protein metabolism. Administration of MCT to protein–energy malnutrition (PEM) model animals fed a low-protein diet resulted in enhanced protein synthesis pathways in the liver and increased liver albumin levels. As a result, serum albumin levels were also higher compared to the LCT intake group [Recently, Hirabayashi et al. induced sarcopenia in model animals via low-protein diet restriction and administration of MCT-containing food during the recovery period. As a result, increased phosphorylation of rpS6 in the liver and tibialis anterior muscle was observed, along with a higher muscle fiber protein content and cross-sectional area compared to the group that received an LCT-containing diet during the recovery period [In this study, the TUG was conducted to measure walking ability. TUG is highly correlated with lower limb strength, balance, walking ability, and ADL, and it reportedly has good reliability and validity [The results of the TUG also reflect balance function during walking. In this study, although no significant difference was observed between the groups in terms of static balance function measurement, the effects of MCT on balance function during walking have been reported. Mutoh et al. reported that healthy older adults with an average age of 69.8 years who consumed 18 g of MCT per day for 3 months showed improved balance during walking compared to the control group [No adverse events attributable to the ingestion of the test foods were observed in this study. However, adverse events related to both single-dose intake [In this study, walking was imposed as the exercise load, which is relatively easy to engage in. However, it is worth noting that there are many older adults who have impairments in their lower limbs. The effects of MCT intake on muscle strength in such individuals, when combined with different forms of exercise load, remain unknown. To clarify the effects of MCT on age-related muscle degeneration and reduced mobility in middle-aged and older individuals, a large-scale intervention study may be necessary. In addition, the subjects’ residential areas were limited. This study was conducted with healthy middle-aged and older adults living at home in urban areas. In rural areas, compared to urban areas, there tends to be a smaller number of destinations (such as restaurants and retail stores), and fewer opportunities for older adults to engage in group activities, hobbies, sports, and so on. Furthermore, public transportation in rural areas of Japan has fewer services and is less convenient. As a result, individuals in rural areas may have fewer opportunities for outdoor activities. Social isolation and confinement at home are considered one of the most important risk factors associated with functional decline and the development of mobility and instrumental activities of daily living (IADL) impairments [