Mechanical signals, IGF-I gene splicing, and muscle adaptation.
Combining physiological and molecular biology methods made it possible to identify and characterize a local muscle growth/repair factor (MGF). Following resistance exercise, MGF “kick starts” muscle hypertrophy and is important in local tissue repair. Loss of muscle mass in old age and certain diseases is associated with an impaired ability to express MGF.
Physiology (Bethesda). 2005 Aug;20:232-8
Fatty acids as modulators of the immune response.
Research describing fatty acids as modulators of inflammation and immune responses abounds. Many of these studies have focused on one particular group of fatty acids, omega-3. The data from animal studies have shown that these fatty acids can have powerful anti-inflammatory and immunomodulatory activities in a wide array of diseases (e.g., autoimmunity, arthritis, and infection). However, the evidence from human trials is more equivocal. In this review, a historical framework for understanding how and why fatty acids may affect the immune system is provided. Second, highlights of two recent landmark reports from the Agency for Healthcare Research and Quality are presented. These reports critically evaluate the evidence from human clinical trials of omega-3 fatty acids and rheumatoid arthritis, asthma, and a few other immune-mediated diseases. Third, the data from human clinical trials investigating the impact of various bioactive fatty acids on ex vivo and in vivo immune response are reviewed. Limitations in experimental design and immune assays commonly used are discussed. The discordance between expectation and evidence in this field has been a disappointment. Recommendations for improving both animal-based and human studies are provided.
Annu Rev Nutr. 2006;26:45-73
Clinical anticachexia treatments.
Cachexia involves progressive loss of adipose tissue and skeletal muscle mass and is common in a number of end-stage diseases. Cachexia causes weakness and immobility, reduces the quality of life of the patient, and eventually results in death. We reviewed the medical literature concentrating upon agents that have undergone clinical evaluation for the treatment of patients with cachexia. These agents are discussed, together with their mechanisms of action. Megestrol acetate, corticosteroids, eicosapentaenoic acid, and thalidomide have shown some success in the treatment of cachexia. beta-hydroxy-beta-methylbutyrate, cyclooxygenase inhibitors, adenosine 5’-triphosphate, and growth hormone are undergoing clinical evaluation. Appetite stimulants such as cannabinoids and antiserotonic agents have been shown to be ineffective in preventing progressive weight loss in cachexia. Much of the success in the treatment of cachexia has come from agents capable of blocking protein degradation through the ubiquitin-proteasome proteolytic pathway. Muscle mass can be increased when such agents are combined with agents that stimulate protein synthesis. In order to develop new agents, more fundamental research is required on the cellular mechanisms governing protein synthesis and degradation in skeletal muscle in cachexia.
Nutr Clin Pract. 2006 Apr;21(2):168-74
Vitamin D in the aging musculoskeletal system: an authentic strength preserving hormone.
Until recently, vitamin D was only considered as one of the calciotrophic hormones without major significance in other metabolic processes in the body. Several recent findings have demonstrated that vitamin D plays also a role as a factor for cell differentiation, function and survival. Two organs, muscle and bone, are significantly affected by the presence, or absence, of vitamin D. In bone, vitamin D stimulates bone turnover while protecting osteoblasts of dying by apoptosis whereas in muscle vitamin D maintains the function of type II fibers preserving muscle strength and preventing falls. Furthermore, two major changes associated to aging: osteoporosis and sarcopenia, have been also linked to the development of frailty in elderly patients. In both cases vitamin D plays an important role since the low levels of this vitamin seen in senior people may be associated to a deficit in bone formation and muscle function. In this review, the interaction between vitamin D and the musculoskeletal components of frailty are considered from the basic mechanisms to the potential therapeutic approach. We expect that these new considerations about the importance of vitamin D in the elderly will stimulate an innovative approach to the problem of falls and fractures which constitutes a significant burden to public health budgets worldwide.
Mol Aspects Med. 2005 Jun;26(3):203-19
Expression of IGF-I splice variants in young and old human skeletal muscle after high resistance exercise.
The mRNA expression of two splice variants of the insulin-like growth factor-I (IGF-I) gene, IGF-IEa and mechano growth factor (MGF), were studied in human skeletal muscle. Subjects (eight young, aged 25-36 years, and seven elderly, aged 70-82 years) completed 10 sets of six repetitions of single legged knee extensor exercise at 80 % of their one repetition maximum. Muscle biopsy samples were obtained from the quadriceps muscle of both the control and exercised legs 2.5 h after completion of the exercise bout. Expression levels of the IGF-I mRNA transcripts were determined using real-time quantitative RT-PCR with specific primers. The resting levels of MGF were significantly (approximately 100-fold) lower than those of the IGF-IEa isoform. No difference was observed between the resting levels of the two isoforms between the two subject groups. High resistance exercise resulted in a significant increase in MGF mRNA in the young, but not in the elderly subjects. No changes in IGF-IEa mRNA levels were observed as a result of exercise in either group. The mRNA levels of the transcription factor MyoD were greater at rest in the older subjects (P < 0.05), but there was no significant effect of the exercise bout. Electrophoretic separation of myosin heavy chain (MHC) isoforms showed the older subjects to have a lower (P < 0.05) percentage of MHC-II isoforms than the young subjects. However, no association was observed between the composition of the muscle and changes in the IGF-I isoforms with exercise. The data from this study show an attenuated MGF response to high resistance exercise in the older subjects, indicative of age-related desensitivity to mechanical loading. The data in young subjects indicate that the MGF and IGF-IEa isoforms are differentially regulated in human skeletal muscle.
J Physiol. 2003 Feb 15;547(Pt 1):247-54
The effect of recombinant human growth hormone and resistance training on IGF-I mRNA expression in the muscles of elderly men.
The expression of two isoforms of insulin-like growth factor-I (IGF-I): mechano growth factor (MGF) and IGF-IEa were studied in muscle in response to growth hormone (GH) administration with and without resistance training in healthy elderly men. A third isoform, IGF-IEb was also investigated in response to resistance training only. The subjects (age 74 +/- 1 years, mean +/- S.E.M) were assigned to either resistance training with placebo, resistance training combined with GH administration or GH administration alone. Real-time quantitative RT-PCR was used to determine mRNA levels in biopsies from the vastus lateralis muscle at baseline, after 5 and 12 weeks in the three groups. GH administration did not change MGF mRNA at 5 weeks, but significantly increased IGF-IEa mRNA (237%). After 12 weeks, MGF mRNA was significantly increased (80%) compared to baseline. Five weeks of resistance training significantly increased the mRNA expression of MGF (163%), IGF-IEa (68%) and IGF-IEb (75%). No further changes were observed after 12 weeks. However, after 5 weeks of training combined with GH treatment, MGF mRNA increased significantly (456%) and IGF-IEa mRNA by (167%). No further significant changes were noted at 12 weeks. The data suggest that when mechanical loading in the form of resistance training is combined with GH, MGF mRNA levels are enhanced. This may reflect an overall up-regulation of transcription of the IGF-I gene prior to splicing.
J Physiol. 2004 Feb 15;555(Pt 1):231-40
Testosterone supplementation therapy for older men: potential benefits and risks.
Serum testosterone levels decline gradually and progressively with aging in men. Many manifestations associated with aging in men, including muscle atrophy and weakness, osteoporosis, reduced sexual functioning, and increased fat mass, are similar to changes associated with testosterone deficiency in young men. These similarities suggest that testosterone supplementation may prevent or reverse the effects of aging. A MEDLINE search was performed to identify studies of testosterone supplementation therapy in older men. A structured, qualitative review was performed of placebo-controlled trials that included men aged 60 and older and evaluated one or more physical, cognitive, affective, functional, or quality-of-life outcomes. Studies focusing on patients with severe systemic diseases and hormone deficiencies related to specific diseases were excluded. In healthy older men with low-normal to mildly decreased testosterone levels, testosterone supplementation increased lean body mass and decreased fat mass. Upper and lower body strength, functional performance, sexual functioning, and mood were improved or unchanged with testosterone replacement. Variable effects on cognitive function were reported, with improvements in some cognitive domains (e.g., spatial, working, and verbal memory). Testosterone supplementation improved exercise-induced coronary ischemia in men with coronary heart disease, whereas angina pectoris was improved or unchanged. In a few studies, men with low testosterone levels were more likely to experience improvements in lumbar bone mineral density, self-perceived functional status, libido, erectile function, and exercise-induced coronary ischemia with testosterone replacement than men with less marked testosterone deficiency. No major unfavorable effects on lipids were reported, but hematocrit and prostate specific antigen levels often increased. Based on these results, testosterone supplementation cannot be recommended at this time for older men with normal or low-normal testosterone levels and no clinical manifestations of hypogonadism. However, testosterone replacement may be warranted in older men with markedly decreased testosterone levels, regardless of symptoms, and in men with mildly decreased testosterone levels and symptoms or signs suggesting hypogonadism. The long-term safety and efficacy of testosterone supplementation remain uncertain. Establishment of evidence-based indications will depend on further demonstrations of favorable clinical outcomes and symptomatic, functional, and quality-of-life benefits in carefully performed, long-term, randomized, placebo-controlled clinical trials.
J Am Geriatr Soc. 2003 Jan;51(1):101-15
Creatine supplementation enhances isometric strength and body composition improvements following strength exercise training in older adults.
We sought to determine whether creatine monohydrate (CrM) supplementation would enhance the increases in strength and fat-free mass that develop during resistance exercise training in older adults. Twenty-eight healthy men and women over the age of 65 years participated in a whole-body resistance exercise program 3 days per week for 14 weeks. The study participants were randomly allocated, in a double-blind fashion, to receive either CrM (5 g/d + 2 g of dextrose; n = 14) or placebo (7 g of dextrose; n = 14). The primary outcome measurements included the following: total body mass, fat-free mass, one-repetition maximum strength for each body part, isometric knee extension, handgrip, and dorsiflexion strength, chair stand performance, 30-m walk test, 14-stair climb performance, muscle fiber type and area, and intramuscular total creatine. Fourteen weeks of resistance exercise training resulted in significant increases in all measurements of strength and functional tasks and muscle fiber area for both groups (p <.05). CrM supplementation resulted in significantly greater increases in fat-free mass and total body mass, as compared with placebo (p <.05). The CrM group also showed a greater increase in isometric knee extension strength in men and women, as compared with placebo (p <.05), and also greater gains in isometric dorsiflexion strength (p <.05), but in men only. There was a significant increase in intramuscular total creatine in the CrM group (p <.05). Finally, there were no significant side effects of treatment or exercise training. This study confirms that supervised heavy resistance exercise training can safely increase muscle strength and functional capacity in older adults. The addition of CrM supplementation to the exercise stimulus enhanced the increase in total and fat-free mass, and gains in several indices of isometric muscle strength.
J Gerontol A Biol Sci Med Sci. 2003 Jan;58(1):11-9
Sarcopenia of aging and its metabolic impact.
Sarcopenia contributes significantly to the morbidity, decrease in quality of life, and health care costs in the elderly. It is characterized by a decrease in muscle mass and strength, starting as early as the fourth decade of life in humans. Distinct muscle changes include a decrease in type 2 muscle fibers and a decrease in myosin heavy chains IIa and IIx mRNA levels. In addition, a decrease in whole body protein turnover, mixed muscle protein synthesis, myosin heavy chain synthesis, and mitochondrial protein synthesis have been reported. Different tissues and organs display different responses to aging, with more oxidative tissue generally having more age-related changes. Exercise has been shown to increase strength, aerobic capacity, and muscle protein synthesis, as well as to increase muscle mitochondrial enzyme activity in both young and older people; however, exercise does not reverse all age-related changes. The metabolic effects of sarcopenia include a decrease in resting metabolic rate secondary to decreased fat-free mass and decreased physical activity, leading to a higher prevalence of insulin resistance, type 2 diabetes mellitus, dyslipidemia, and hypertension. The way in which age-related changes in hormone levels affect muscle remains to be fully understood. The effect of replacing those hormones on sarcopenia has led to some conflicting results, but further investigations are ongoing.
Curr Top Dev Biol. 2005;68:123-48