Lack of an association between insulin-like growth factor-I and body composition, muscle strength, physical performance or self-reported mobility among older persons with functional limitations

Current and investigational medications for the treatment of sarcopenia

Sarcopenia, defined as the loss of muscle mass and function, is a widely prevalent and severe condition in older adults. Since 2016, it is recognized as a disease. Strength exercise training and nutritional support are the frontline treatment of sarcopenia, with no drug currently approved for this indication. However, new therapeutic options are emerging. In this review, we evidenced that only very few trials have focused on sarcopenia/sarcopenic patients. Most drug trials were performed in different clinical older populations (e.g., men with hypogonadism, post-menopausal women at risk for osteoporosis), and their efficacy were tested separately on the components of sarcopenia (muscle mass, muscle strength and physical performances). Results from trials testing the effects of Testosterone, Selective Androgen Receptor Modulators (SARMs), Estrogen, Dehydroepiandrosterone (DHEA), Insulin-like Growth Factor-1 (IGF-1), Growth Hormone (GH), GH Secretagogue (GHS), drug targeting Myostatin and Activin receptor pathway, Vitamin D, Angiotensin Converting Enzyme inhibitors (ACEi) and Angiotensin Receptor Blockers (ARBs), or β-blockers, were compiled. Although some drugs have been effective in improving muscle mass and/or strength, this was not translated into clinically relevant improvements on physical performance. Finally, some promising molecules investigated in on-going clinical trials and in pre-clinical phase were summarized, including apelin and irisin.

The relationship between serum IGF-1, handgrip strength, physical performance and falls in elderly men and women

OBJECTIVE

Human aging is accompanied by a decrease in growth hormone secretion and serum insulin-like growth factor (IGF)-1 levels. Also, loss of muscle mass and strength and impairment of physical performance, ending in a state of frailty, are seen in elderly. We aimed to investigate whether handgrip strength, physical performance and recurrent falls are related to serum IGF-1 levels in community-dwelling elderly.

DESIGN

Observational cohort study (cross-sectional and prospective).

METHODS

We studied the association between IGF-1 and handgrip strength, physical performance and falls in participants of the Longitudinal Aging Study Amsterdam. A total of 1292 participants were included (633 men, 659 women). Serum IGF-1 levels were divided into quartiles (IGF-1-Q1 to IGF-1-Q4). Data on falls were collected prospectively for a period of 3 years. All analyses were stratified for age and physical activity and adjusted for relevant confounders.

RESULTS

Men with a low physical activity score in IGF-1-Q1 and IGF-1-Q2 of the younger age group had a lower handgrip strength compared to IGF-1-Q4. In younger more active males in IGF-1-Q2 physical performance was worse. Recurrent fallers were less prevalent in older, low active males with low IGF-1 levels. In females, recurrent fallers were more prevalent in older, more active females in IGF-1-Q2. IGF-1 quartile may predict changes in handgrip strength and physical performance in men and women.

CONCLUSIONS

Our results indicate that lower IGF-1 levels are associated with lower handgrip strength and worse physical performance, but less recurrent fallers especially in men. Associations were often more robust in IGF-1-Q2. Future studies on this topic are desirable.

Nonandrogenic Anabolic Hormones Predict Risk of Frailty: European Male Ageing Study Prospective Data

CONTEXT

Low levels of nonandrogenic anabolic hormones have been linked with frailty, but evidence is conflicting and prospective data are largely lacking.

OBJECTIVE

To determine associations between nonandrogenic anabolic hormones and prospective changes in frailty status.

DESIGN/SETTING

A 4.3-year prospective observational study of community-dwelling men participating in the European Male Ageing Study.

PARTICIPANTS

Men (n = 3369) aged 40 to 79 years from eight European centers.

MAIN OUTCOME MEASURES

Frailty status was determined using frailty phenotype (FP; n = 2114) and frailty index (FI; n = 2444).

ANALYSIS

Regression models assessed relationships between baseline levels of insulinlike growth factor 1 (IGF-1), its binding protein 3 (IGFBP-3), dehydroepiandrosterone sulfate (DHEA-S), 25-hydroxyvitamin D (25OHD), and parathyroid hormone (PTH), with changes in frailty status (worsening or improving frailty).

RESULTS

The risk of worsening FP and FI decreased with 1 standard deviation higher IGF-1, IGFBP-3, and 25OHD in models adjusted for age, body mass index, center, and baseline frailty [IGF-1: odds ratio (OR) for worsening FP, 0.82 (0.73, 0.93), percentage change in FI, -3.7% (-6.0, -1.5); IGFBP-3: 0.84 (0.75, 0.95), -4.2% (-6.4, -2.0); 25OHD: 0.84 (0.75, 0.95); -4.4%, (-6.7, -2.0)]. Relationships between IGF-1 and FI were attenuated after adjusting for IGFBP-3. Higher DHEA-S was associated with a lower risk of worsening FP only in men >70 years old [OR, 0.57 (0.35, 0.92)]. PTH was unrelated to change in frailty status.

CONCLUSIONS

These longitudinal data confirm the associations between nonandrogenic anabolic hormones and the changes in frailty status. Interventional studies are needed to establish causality and determine therapeutic implications.

Hormone replacement therapy and physical function in healthy older men. Time to talk hormones?

Improving physical function and mobility in a continuously expanding elderly population emerges as a high priority of medicine today. Muscle mass, strength/power, and maximal exercise capacity are major determinants of physical function, and all decline with aging. This contributes to the incidence of frailty and disability observed in older men. Furthermore, it facilitates the accumulation of body fat and development of insulin resistance. Muscle adaptation to exercise is strongly influenced by anabolic endocrine hormones and local load-sensitive autocrine/paracrine growth factors. GH, IGF-I, and testosterone (T) are directly involved in muscle adaptation to exercise because they promote muscle protein synthesis, whereas T and locally expressed IGF-I have been reported to activate muscle stem cells. Although exercise programs improve physical function, in the long-term most older men fail to comply. The GH/IGF-I axis and T levels decline markedly with aging, whereas accumulating evidence supports their indispensable role in maintaining physical function integrity. Several studies have reported that the administration of T improves lean body mass and maximal voluntary strength in healthy older men. On the other hand, most studies have shown that administration of GH alone failed to improve muscle strength despite amelioration of the detrimental somatic changes of aging. Both GH and T are anabolic agents that promote muscle protein synthesis and hypertrophy but work through separate mechanisms, and the combined administration of GH and T, albeit in only a few studies, has resulted in greater efficacy than either hormone alone. Although it is clear that this combined approach is effective, this review concludes that further studies are needed to assess the long-term efficacy and safety of combined hormone replacement therapy in older men before the medical rationale of prescribing hormone replacement therapy for combating the sarcopenia of aging can be established.

Aging phenotype and its relationship with IGF-I gene promoter polymorphisms in elderly people living in Catalonia

OBJECTIVES

Genetic variations in the Insulin/IGF-I genes pathway have been related to longevity, dementia, metabolic diseases and cancer. The purpose of the present study was to investigate the 192 bp allele of IGF-I gene promoter and its relationship with metabolic syndrome (MS) components, mental and nutritional state, muscle strength and functional capacity in an aged Spanish population.

DESIGN

Population-based study (Mataró Ageing Study), including 292 subjects (144 men and 148 women, mean age 77.0±5.4). Anthropometric variables, lipid profile, glucose and blood pressure (BP) were measured; mental state (MMSE), nutritional state (MNA) and Barthel scale were performed, and were correlated to the presence of the 192 bp allele of IGF-1 gene promoter polymorphisms.

RESULTS

MS (ATP-III criteria) was found in 49.5% (41.4% in men and 57.6% in women). The 192 bp allele of IGF-I gene promoter was distributed as: 41.9% homozygous, 44.3% heterozygous and 13.9% were non-carriers of this allele. A lower prevalence of metabolic syndrome was observed in homozygous (41.9% vs 54.9% in heterozygous+non-carriers, p=0.031). Mental state (MMSE), nutritional state (MNA) and Barthel scale were better in homozygous individuals compared to heterozygous and non-carriers (p=0.015, p=0.026 and 0.047, respectively). In men, MNA was better in homozygous with no differences in MMSE and Barthel scales. In homozygous women, BP was lower (p=0.009) and Barthel scale was better (p=0.05) with no differences in MMSE and MNA.

CONCLUSION

Homozygosity for the 192 bp allele of the IGF-I gene polymorphism suggests a healthier aging condition, with less prevalence of cardiometabolic disturbances, and better mental, nutritional and functional state.

Circulating IGF-I is associated with fitness and health outcomes in a population of 846 young healthy men

CONTEXT

Insulin-like growth factor-I (IGF-I) is thought to mediate many of the beneficial outcomes of physical activity. While IGF-I has previously been shown to be positively related with aerobic fitness, few studies have examined IGF-I relationships with other fitness and health parameters. The robustness of IGF-I as a biomarker of fitness and health has yet to be fully determined.

OBJECTIVE

To determine the association of circulating IGF-I with fitness, body composition and health parameters in young, healthy men.

DESIGN AND SUBJECTS

A cross-section of 846 young, healthy Finnish men (25±5 yr, 180±6 cm, 81±13 kg). Subjects were divided into quintiles of IGF-I concentrations (Q1: lowest; Q5: highest) for statistical evaluation.

MAIN OUTCOME MEASURES

Circulating IGF-I, physical fitness: peak aerobic capacity (VO(2) peak), maximal strength of leg and arm extensors, muscle endurance (sit-ups, push-ups, and repetitive squats) and health outcome parameters (total blood cholesterol, triglyceride, high-density lipoproteins (HDL), low-density lipoproteins (LDL), systolic and diastolic blood pressure, waist circumference, % body fat, and drinking, smoking and physical activity behavior).

RESULTS

Higher IGF-I was associated with higher VO(2) peak (Q1: 39±7 vs. Q5: 44±9 mL/kg/min), sit-ups (Q1: 35±10 vs. Q5: 41±10 repetitions), push-ups (Q1: 27±13 vs. Q5: 31±14 repetitions), repetitive squats (Q2: 42±10 vs. Q5: 45±8 repetitions), HDL (Q1: 1.5±0.4 vs. Q5: 1.53±0.3 mmol/L), and lower age (Q1: 28±6 vs. Q5: 23±2 yr), %BF (Q1: 20±7 vs. Q5: 16±6%BF), waist circumference (Q1: 89±11 vs. Q5: 84±9 cm), BMI (Q1: 25.6±4 vs. Q5: 24.3m(2)/kg), diastolic blood pressure (Q1: 78.5±9 vs. Q5: 75.4±8 mm Hg), cholesterol (Q1: 4.72±0.9 vs. Q5: 4.44±0.8 mmol/L) and smoking (Q1: 44% vs. Q5: 32%). No association was observed for IGF-I and maximal leg extension (Q1: 2982±927 vs. Q5: 2932±853 N) and bench press (Q1: 895±197 vs. Q5: 919±203 N) strength, fat-free mass (Q1: 64.6±8 vs. Q5: 66.6±7 KG), LDL (2.54±0.7 vs. Q5: 2.35±0.6 mmol/L), or triglycerides (Q1: 1.05±0.6 vs. Q5: 0.99±0.5 mmol/L).

CONCLUSION

IGF-I is positively associated with aerobic fitness and muscular endurance, but not with measures of muscle strength or FFM. IGF-I is positively associated with improved health and fitness outcomes in young, healthy men.

Serum insulin-like growth factor-I and body composition in community dwelling older people

OBJECTIVES

The decline in the growth hormone/insulin-like growth factor-I (GH/IGF-I) axis during normal aging might be involved in the changes in body composition associated with increasing age. We conducted a study to investigate serum IGF-I levels across different age categories and a possible association between serum IGF-I and measurements of body composition in older people.

DESIGN

A cross-sectional analysis of community dwelling older people, which participated in a large longitudinal cohort study (Longitudinal Aging Study Amsterdam).

SUBJECTS

1319 subjects, 644 men, mean age 75.6 +/- 6.6 years and 675 women, mean age 75.4 +/- 6.6 years.

MAIN OUTCOME MEASUREMENTS

IGF-I, body mass index (BMI), waist, waist-hip ratio (WHR), fat mass, lean body mass and total bone mineral density.

RESULTS

IGF-I levels were significantly lower in the highest age categories. BMI and biceps skinfold measurements were lower in the lowest IGF-I quartile in men aged > or =75.5 years. In men with a low total physical activity score (<131 min/day), BMI, WHR and skinfolds were significantly lower in the lowest IGF-I quartile. In women with a high total physical activity score (>174 min/day), WHR was lower in the lowest IGF-I quartiles.

CONCLUSION

In this large cohort of community dwelling older people, we observed lower serum IGF-I levels in the higher age categories. A low serum IGF-I was associated with significantly lower measurements of body composition, such as BMI, skinfolds and WHR. These results do not support previous findings that high IGF-I levels are favourable for a healthy body composition in community dwelling older people.

Bone mineral density and bone turnover in male masters athletes aged 40-64

We evaluated areal bone mineral density (aBMD), bone mineral content (BMC), and markers of bone turnover in male competitive masters athletes representing different training profile in the past and at present, aged 40-64 (14 endurance runners, and 12 speed-power athletes), and non-sport controls (n = 13). Dual-energy X-ray absorptiometry measurements of total body and regional aBMD, BMC and soft tissue composition were acquired. Serum concentrations of osteocalcin (OC), C-terminal crosslinking telopeptide of type I collagen (CTX), tumour necrosis factor-alpha (TNF-alpha), total testosterone (TT), free testosterone (FT) and insulin like growth factor-1 (IGF-1) were measured. Adjusted total and regional aBMD and BMC (covariates: body mass, body height and age) were significantly greater in all measured regions in speed-power athletes than in endurance athletes and control subjects, but adjusted aBMD and BMC values were not significantly different between endurance athletes and controls. No differences in bone formation (OC), bone resorption (CTX), and serum concentrations of TNF-alpha, TT, FT and IGF-1 were noted. This suggests that weight-bearing exercise in young age and the training continuation in later life may be an important contributor to the aBMD and BMC in the middle age and in the elderly. It seems also that training-related bone differences in men are not caused by present alterations in bone turn-over or somatotropic effects. However, conclusions must be drawn with caution due to a large variability of biochemical markers.

Influence of gender on serum growth hormone, insulin-like growth factor-I and its binding protein-3 during aging

PURPOSE

The serum concentrations of insulin-like growth factors-I (IGF-I), insulin-like growth factor binding protein-3 (IGFBP-3) and growth hormone (GH) are related to body composition, function and metabolism, and are influenced by the aging process. This study was to investigate the influence of gender on serum concentrations of IGF-I, IGFBP-3 and GH in middle and old age subjects.

MATERIALS AND METHODS

Sixty healthy volunteers (male 35, female 25, 36-70 years) were divided into < or = 50 and > 50 years groups, based on gender. Women > 50 years were post-menopause. IGF-I, IGFBP-3, and GH were determined by immunoradiometric assay.

RESULTS

IGF-I was shown to be negatively correlated with age (women r = -0.62, p < 0.001; men r = -0.38, p < 0.05), whereas there was no correlation between IGF-I and GH values. Women > 50 years showed a significant reduction in IGF-I values than women < or = 50 years (p < 0.01). Women > 50 years showed smaller IGF-I/IGFBP-3 molar ratios (0.177998 +/- 0.039404) than men of same age group (0.228326 +/- 0.050979, p < 0.01) and women < or = 50 years (0.247667 +/- 0.069411, p < 0.01). Age was shown to positively correlate with GH/IGF-I (r = 0.49, p < 0.05) and GH/IGFBP-3 ratios (r = 0.40, p < 0.05) in women.

CONCLUSIONS

The influence of aging on serum concentrations of IGF-I is more remarkable in women than in men. Menopause causes reduction of IGF-I/IGFBP-3 molar ratio. Women have the trend of progressive hypoactivity of GH to stimulate IGF-I and IGFBP-3 secretions with age.

Modulation of GH/IGF-1 axis: potential strategies to counteract sarcopenia in older adults

Aging is associated with progressive decline of skeletal muscle mass and function. This condition, termed sarcopenia, is associated with several adverse outcomes, including loss of autonomy and mortality. Due to the high prevalence of sarcopenia, a deeper understanding of its pathophysiology and possible remedies represents a high public health priority. Evidence suggests the existence of a relationship between declining growth hormone (GH) and insulin-like growth factor-1 (IGF-1) levels and age-related changes in body composition and physical function. Therefore, the age-dependent decline of GH and IGF-1 serum levels may promote frailty by contributing to the loss of muscle mass and strength. Preclinical studies showed that infusion of angiotensin II produced a marked reduction in body weight, accompanied by decreased serum and muscle levels of IGF-1. Conversely, overexpression of muscle-specific isoform of IGF-1 mitigates angiotensin II-induced muscle loss. Moreover, IGF-1 serum levels have been shown to increase following angiotensin converting enzyme inhibitors (ACEIs) treatment. Here we will review the most recent evidence regarding age-related changes of the GH/IGF-1 axis and its modulation by several interventions, including ACEIs which might represent a potential novel strategy to delay the onset and impede the progression of sarcopenia.

Body mass index, free insulin-like growth factor I, and physical function among older adults: results from the ilSIRENTE study

The aim of the present study was to evaluate the mediating role played by obesity on the relationship of free insulin-like growth factor I (IGF-I) and IGF-binding protein-3 (IGFBP-3) with muscle strength and physical performance. Data were from baseline evaluation of the ilSIRENTE Study. Muscle strength was measured by hand grip strength. Physical performance was assessed using the walking speed and the 0-3 Short Physical Performance Battery (SPPB) score. Based on its median value, free IGF-I was categorized in the following two groups: low IGF-I (IGF-I <0.65 ng/ml; n = 174) and high IGF-I (IGF-I > or =0.65 ng/ml; n = 175). Similarly, IGFBP-3 was categorized in the following two groups: low IGFBP-3 (IGFBP-3 <4,319.9 ng/ml; n = 174) and high IGFBP-3 (IGFBP-3 > or =4,319.9 ng/ml; n = 175). Body mass index (BMI) was categorized as follows: <25 kg/m(2) (n = 160), 25-29.9 kg/m(2) (n = 133), > or =30 kg/m(2) (n = 56). Mean age of the 349 participants was 85.8 yr, and 234 (67%) were women. After adjusting for potential confounders, no significant association of IGF-I and IGFBP-3 with study outcomes was observed. After the study sample was stratified by BMI groups, compared with participants with low IGF-I level, those with high IGF-I level had a significantly better grip strength [35.2 +/- 1.6 vs. 29.2 +/- 2.0 (SE) kg, P = 0.03], walking speed (0.55 +/- 0.04 vs. 0.40 +/- 0.04 m/s, P = 0.01), and SPPB score (1.9 +/- 0.1 vs. 1.5 +/- 0.1 m/s, P = 0.01) but only in the group with BMI > or =30 kg/m(2) and not in other BMI groups. A statistically significant interaction between BMI and IGF-I level was observed on all study outcomes. By contrast, no association was observed between IGFBP-3 and study outcomes, independently of BMI. In conclusion, high IGF-I level is associated with better physical function in older persons with obesity, but not in nonobese subjects.

Effects of aging on muscle fibre type and size

Aging has been associated with a loss of muscle mass that is referred to as 'sarcopenia'. This decrease in muscle tissue begins around the age of 50 years, but becomes more dramatic beyond the 60th year of life. Loss of muscle mass among the aged directly results in diminished muscle function. Decreased strength and power contribute to the high incidence of accidental falls observed among the elderly and can compromise quality of life. Moreover, sarcopenia has been linked to several chronic afflictions that are common among the aged, including osteoporosis, insulin resistance and arthritis. Loss of muscle fibre number is the principal cause of sarcopenia, although fibre atrophy--particularly among type II fibres--is also involved. Several physiological mechanisms have been implicated in the development of sarcopenia. Denervation results in the loss of motor units and thus, muscle fibres. A decrease in the production of anabolic hormones such as testosterone, growth hormone and insulin-like growth factor-1 impairs the capacity of skeletal muscle to incorporate amino acids and synthesise proteins. An increase in the release of catabolic agents, specifically interleukin-6, amplifies the rate of muscle wasting among the elderly. Given the demographic trends evident in most western societies, i.e. increased number of those considered aged, management interventions for sarcopenia must become a major goal of the healthcare profession.

Serum levels of insulin-like growth factor-I (IGF-I) and dehydroepiandrosterone sulfate (DHEA-S), and their relationships with serum interleukin-6, in the geriatric syndrome of frailty

BACKGROUND AND AIMS

The geriatric syndrome of frailty has been conceptualized as a loss of physiologic reserve associated with endocrine dysregulation and immune dysfunction. Our prior studies suggest that the frailty syndrome is associated with elevated serum IL-6 levels. In the present study, our aim is to evaluate the possible role of endocrine dysregulation and its relationship with serum IL-6 in the pathogenesis of this syndrome.

METHODS

Using a recently validated screening algorithm for frailty, we identified 18 frail and 33 non-frail community-dwelling older adults for inclusion in this study. Serum levels of insulin-like growth factor-I (IGF-I), DHEA-S, and IL-6 were measured by immunoassays. The inter-relationships among serum levels of IL-6, DHEA-S, and IGF-I were determined by linear regression analysis.

RESULTS

Age-adjusted serum levels of IGF-I (88+/-49 vs 122+/-47 [ng/mL], p<0.023) and DHEA-S (0.30+/-0.21 vs 0.53+/-0.25 [microg/mL], p=0.016) were significantly lower in frail vs non-frail individuals, respectively. There was a trend for IL-6 to be inversely correlated with IGF-I in the frail (r=-0.42; p=0.082) but not the non-frail group (r=0.12, p=0.521).

CONCLUSIONS

Frail subjects have lower levels of serum IGF-I and DHEA-S and higher levels of IL-6 than do non-frail, age-matched individuals. The trend toward an inverse correlation between IGF-I and IL-6 in the frail, but not the non-frail group, suggests potential interaction between endocrine and immune/cytokine dysregulation that requires further study in larger cohorts.

Chronic inflammation and the effect of IGF-I on muscle strength and power in older persons

Deregulation of the inflammatory response plays a major role in the age-related decline of physical performance. The causal pathway leading from inflammation to disability has not been fully clarified, but several researches suggest that interleukin-6 (IL-6) causes a reduction of physical performance in elderly through its effect on muscle function. In vitro studies demonstrated that IL-6 inhibits the secretion of insulin-like growth factor I (IGF-I) and its biological activity, suggesting that the negative effect of IL-6 on muscle function might be mediated through IGF-I. We evaluated the joint effect of IGF-I and IL-6 on muscle function in a population-based sample of 526 persons with a wide age range (20-102 yr). After adjusting for potential confounders, such as age, sex, body mass index, IL-6 receptor, and IL-6 promoter polymorphism, IL-6, IGF-I, and their interaction were significant predictors of handgrip and muscle power. In analyses stratified by IL-6 tertiles, IGF-I was an independent predictor of muscle function only in subjects in the lowest IL-6 tertile, suggesting that the effect of IGF-I on muscle function depends on IL-6 levels. This mechanism may explain why IL-6 is a strong risk factor for disability.

[Relation between physical activity, muscle function and IGF-1, testosterone and DHEAS concentrations in the elderly]

OBJECTIVE

Lower amounts of circulating anabolic hormones are thought to accelerate the age related decline in muscle mass and function. Replacement therapies are promising interventions but there are problems with these therapies. Thus alternative strategies should be developed. The age related changes in hormonal status may be probably influenced by exercise. The purpose of this study was: a) to confirm with other methods, more adapted for elderly people, the results of a previous study that has shown relationship between physical activity (PA) and quadriceps muscle function with dehydroepiandrosterone sulphate (DHEAS), insulin like growth factor-1 (IGF-1). Quadriceps muscle power (Pmax) is measured in this new work with a recently developed leg extensor machine and, b) to complete the results of the first study examining simultaneously the relationship between PA, Pmax and cardiorespiratory fitness (VO2max) with DHEAS, IGF-1 and testosterone in a group of healthy elderly people.

METHODS

Fifty independent, community dwelling elderly subjects (25 mens and 25 womens) aged from 66 to 84 volunteered to participate in the study. PA was evaluated by the questionnaire and expressed using two activity indices: mean habitual daily energy expenditure (MHDEE) and the daily energy expenditure corresponding to leisure time sports activities (Sports Activity). Pmax and optimal shortening velocity (vopt) were measured on a Ergopower dynamometer. The Pmax was expressed relative to body mass, Pmax/kg (W kg-1), and relative to the mass of the two quadriceps muscles, Pmax/Quadr (W.kgQuadr-1). VO2max has been measured during a maximal treadmill exercise.

RESULTS

In women, IGF-1 correlated significantly with MHDEE (r = 0.54, P = 0.004), Pmax/kg (r = 0.54, P = 0.004) and Pmax/Quadr (r = 0.46, P = 0.02), whereas DHEAS with MHDEE (r = 0.54, P = 0.004), Sports Activity (r = 0.65, P < 0.001), VO2max (r = 0.46, P = 0.02), Pmax/kg (r = 0.46, P = 0.02) and Pmax/Quadr (r = 0.55, P = 0.004). No such correlation was found in men.

CONCLUSION

These findings confirm that in healthy elderly women physical activity, cardiorespiratory fitness and quadriceps muscle function are similarly related to levels of circulating DHEAS and IGF-1 suggesting a favourable influence of exercise on anabolic hormonal production in the elderly.

Association of IGF-I levels with muscle strength and mobility in older women

The functional consequences of the age-associated decline in IGF-I are unknown. We hypothesized that low IGF-I levels in older women would be associated with poor muscle strength and mobility. We assessed this question in a population representative of the full spectrum of health in the community, obtaining serum IGF-I levels from women aged 70-79 yr, enrolled in the Women's Health and Aging Study I or II. Cross-sectional analyses were performed using 617 women with IGF-I levels drawn within 90 d of measurement of outcomes. After adjustment for age, there was an association between IGF-I and knee extensor strength (P = 0.004), but not anthropometry or other strength measures. We found a positive relationship between IGF-I levels and walking speed for IGF-I levels below 50 microg/liter (P < 0.001), but no relationship above this threshold. A decline in IGF-I level was associated with self-reported difficulty in mobility tasks. All findings were attenuated after multivariate adjustment. In summary, in a study population including frail and healthy older women, low IGF-I levels were associated with poor knee extensor muscle strength, slow walking speed, and self-reported difficulty with mobility tasks. These findings suggest a role for IGF-I in disability as well as a potential target population for interventions to raise IGF-I levels.

Insulin-like growth factor-I in muscle metabolism and myotherapies

The critical anabolic and trophic role of signaling by insulin-like growth factors (IGF) I and II via the type-I IGF receptor (IGF-IR) is reviewed throughout the life of skeletal myocytes. The proliferative effects of IGF-IR stimulation, both during embryogenesis and during satellite cell proliferation following denervation or muscle injury, are mediated primarily through activation of mitogen-activated protein kinases. Signaling through phosphatidylinositol 3-kinase is essential to muscle protein synthesis and glucose uptake and may contribute to the observed resilience of mature muscle to programmed cell death. Degeneration or inhibition of the GH--IGF-I axis by aging, cachexia, sepsis, diabetes, drugs, and disuse all enhance muscle catabolism, and opposition of these effects by IGF-I may form the basis of effective myotherapy.

Growth hormone administration and exercise effects on muscle fiber type and diameter in moderately frail older people

OBJECTIVE

Reduced muscle mass and strength are characteristic findings of growth hormone deficiency (GHD) and aging. We evaluated measures of muscle strength, muscle fiber type, and cross sectional area in response to treatment with recombinant human growth hormone (rhGH) with or without a structured resistance exercise program in frail older subjects.

DESIGN

Placebo-controlled, randomized, double blind trial.

SETTING

Outpatient clinical research center at an urban university-affiliated teaching hospital.

PARTICIPANTS

Thirty-one consenting older subjects (mean age 71.3 +/- 4.5 years) recruited as a subset of a larger project evaluating rhGH and exercise in older people, who underwent 62 quadricep-muscle biopsies.

INTERVENTION

Random assignment to a 6-month course of one of four protocols: rhGH administered subcutaneously daily at bedtime, rhGH and a structured resistance exercise program, structured resistance exercise with placebo injections, or placebo injections only.

MEASUREMENTS

Muscle biopsy specimens were obtained from the vastus lateralis muscle. Isokinetic dynamometry strength tests were used to monitor individual progress and to adjust the weights used in the exercise program. Serum insulin-like growth factor-I (IGF-I) was measured and body composition was measured using a Hologic QDR 1000W dual X-ray densitometer.

RESULTS

The administration of rhGH resulted in significant increase in circulating IGF-I levels in the individuals receiving rhGH treatment. Muscle strength increased significantly in both the rhGH/exercise (+55.6%, P =.0004) as well as the exercise alone (+47.8%, P =.0005) groups. There was a significant increase in the proportion of type 2 fibers between baseline and six months in the combined rhGH treated subjects versus those not receiving rhGH (P =.027).

CONCLUSIONS

Our results are encouraging in that they suggest an effect of growth hormone on a specific aging-correlated deficit. IGF-I was increased by administrating rhGH and muscle strength was increased by exercise. The administration of rhGH to frail older individuals in this study resulted in significant changes in the proportions of fiber types. Whether changes in fiber cross-sectional area or absolute number occur with long-term growth hormone administration requires further study.

Serum insulin-like growth factor-I (IGF-I) does not correlate positively with isometric strength, fatigue, and quality of life in post-polio syndrome

OBJECTIVES AND BACKGROUND

To determine if serum insulin-like growth factor-I (IGF-I) levels are associated with strength, body mass index (BMI), fatigue, or quality of life in post-poliomyelitis syndrome (PPS). PPS is likely due to a distal disintegration of enlarged post-polio motor units as a result of terminal axonal sprouting. Age-related decline in growth hormone and IGF-I (which support terminal axonal sprouts) is proposed as a contributing factor.

METHODS

As part of the North American Post-Poliomyelitis Pyridostigmine Study (NAPPS), baseline data on maximum voluntary isometric contraction (MVIC), BMI, subjective fatigue (fatigue severity scale, Hare fatigue symptom scale), health-related quality of life (short form health survey-36; SF-36), and serum IGF-I levels were gathered on 112 PPS patients. Pearson correlation coefficients were calculated to evaluate the association between serum IGF-I and MVIC in 12 muscles, BMI, two fatigue scales, and SF-36 scale scores.

RESULTS

There is a significant inverse correlation of IGF-I levels with MVIC in left ankle dorsiflexors (r=-0.30, P<0.01), and left and right knee extensors (r=-0.22, -0.25, P=<0.01, 0.01), but no significant correlations in other muscles. When men and women were evaluated separately, inverse correlations of IGF-I levels with MVIC were found only in men. IGF-I correlated inversely with BMI (r=-0.32, P=0006) and age (r=-0.32, P=0.0005). IGF-I did not correlate with the fatigue or SF-36 scales.

CONCLUSIONS

In this exploratory study, we found that contrary to our expectations, IGF-I did not correlate positively with strength. IGF-I correlated negatively with strength in several lower extremity muscles, BMI, and age. IGF-I is likely not an important factor in the pathogenesis of fatigue and in determining quality of life in PPS, but its role on strength should be studied further.

Reparameterizing the pattern mixture model for sensitivity analyses under informative dropout

Pattern mixture models are frequently used to analyze longitudinal data where missingness is induced by dropout. For measured responses, it is typical to model the complete data as a mixture of multivariate normal distributions, where mixing is done over the dropout distribution. Fully parameterized pattern mixture models are not identified by incomplete data; Little (1993, Journal of the American Statistical Association 88, 125-134) has characterized several identifying restrictions that can be used for model fitting. We propose a reparameterization of the pattern mixture model that allows investigation of sensitivity to assumptions about nonidentified parameters in both the mean and variance, allows consideration of a wide range of nonignorable missing-data mechanisms, and has intuitive appeal for eliciting plausible missing-data mechanisms. The parameterization makes clear an advantage of pattern mixture models over parametric selection models, namely that the missing-data mechanism can be varied without affecting the marginal distribution of the observed data. To illustrate the utility of the new parameterization, we analyze data from a recent clinical trial of growth hormone for maintaining muscle strength in the elderly. Dropout occurs at a high rate and is potentially informative. We undertake a detailed sensitivity analysis to understand the impact of missing-data assumptions on the inference about the effects of growth hormone on muscle strength.

Serum IGF-I is higher in gymnasts than runners and predicts bone and lean mass

INTRODUCTION

We examined the relationships between insulin-like growth factor I (IGF-I), its binding protein (IGFBP-3), body composition, and bone mineral density (BMD) in collegiate runners (N = 13), gymnasts (N = 10), and noncompetitive women (N = 10).

METHODS

Subjects were evaluated by dual-energy x-ray absorptiometry for body composition and BMD of the spine, hip, and whole body, fasting serum levels of IGF-I and IGFBP-3, and dietary intake. The ratio IGF-I/IGFBP-3 was calculated as a marker of IGF-I bioavailability.

RESULTS

In ANOVA, IGF-I and IGF-I/IGFBP-3 in athletes with oligomenorrhea and amenorrhea did not differ from eumenorrheic athletes; thus, values were pooled. Lean/height2 and bone mass at the hip and spine were higher in gymnasts than runners and controls. Total caloric intake was similar between groups. IGF-I and IGF-I/IGFBP-3 differed between groups with gymnasts having higher IGF-I values than runners (397+/-58 vs 288+/-73 ng x mL(-1), P < 0.001) and higher IGF-I/IGFBP-3 than controls and runners (0.065+/-0.009 vs 0.056+/-0.008 vs 0.045+/-0.009, P = 0.0001). In simple regression, IGF-I and IGF-/IGFBP-3 were related to lean/height2 and BMD of the lumbar spine and hip (P < 0.01-0.0001). IGF-I and IGF-I/IGFBP-3 were multicollinear; thus, the ratio was used in subsequent stepwise regression. Lean mass, corrected for body surface area (height2), independently predicted spine and trochanteric BMD (R2 = 0.26, 0.28, respectively), whereas IGF-I/IGFBP-3 and lean/height2 together contributed to 48% of the variance in femoral neck BMD.

CONCLUSION

We conclude that, in this group of young adult women, lower BMD in runners may be due, in part, to lower levels of IGF-I and the ratio of IGF-I-to-IGFBP-3 and that IGF-I may mediate the relationship between bone and lean mass.

Circulating IGF-I: New Perspectives for a New Century

Insulin-like growth factor I (IGF-I) is a ubiquitous endocrine, paracrine and autocrine polypeptide, which influences cell proliferation and differentiation in many tissues. Classically, IGF-I has been tied to growth hormone (GH) and has often been considered a surrogate marker of overall GH status. The advent of recombinant technology has made possible studies of GH and IGF-I for the treatment of chronic diseases (such as diabetes mellitus, osteoporosis and muscle atrophy) as well as to forestall the aging process. Examples of currently active areas of research include efforts to define the involvement of IGF-I physiology in bone remodeling, atherosclerosis and neoplasia. Recent epidemiological evidence suggests that individuals with IGF-I levels in the 'high normal' range have increased risk of common cancers relative to individuals with levels in the 'low normal' range. These findings have focused renewed attention on the genetic and non-genetic determinants of serum IGF-I levels. It is unlikely that the serum IGF-I level itself is related directly to risk of neoplasia, but it may serve as a surrogate for a variable that is important in epithelial cell carcinogenesis, such as rate of epithelial cell proliferation. We review relatively new data suggesting that adult serum IGF-I levels are under the control of heritable factors apart from GH. Such factors may influence tissue expression of IGF-I as well as serum IGF-I levels, and influence a number of clinically important outcomes, including bone density and risk of neoplasia. The concept that there is little physiological importance in the heterogeneity between individuals regarding IGF-I levels within the broad 'normal' range may require re-assessment.