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Endocrinology Vol. 149, No. 7 3722-3728
Copyright © 2008 by The Endocrine Society

Efficacy of Ghrelin as a Therapeutic Approach for Age-Related Physiological Changes

Hiroyuki Ariyasu, Hiroshi Iwakura, Go Yamada, Kazuwa Nakao, Kenji Kangawa and Takashi Akamizu

Ghrelin Research Project, Translational Research Center (H.A., H.I., T.A.), Kyoto University Hospital, and Department of Endocrinology and Metabolism (G.Y., K.N.), Kyoto University Graduate School of Medicine, Kyoto 606-8507, Japan; and Department of Biochemistry (K.K.), National Cardiovascular Center Research Institute, Osaka 565-8565, Japan

Address all correspondence and requests for reprints to: Hiroyuki Ariyasu, Ghrelin Research Project, Translational Research Center, Kyoto University Hospital, Kyoto 606-8507, Japan. E-mail: ariyasu{at}kuhp.kyoto-u.ac.jp.


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Aging is associated with decreases in food intake and GH secretion, termed the anorexia of aging and somatopause, respectively. The mechanisms underlying these phenomena are not fully understood. Although many approaches have attempted to improve these age-related physiological changes, none have achieved satisfactory results. Ghrelin, a 28-amino-acid acylated peptide, was identified as an endogenous ligand for the GH secretagogue receptor. Ghrelin stimulates GH secretion and food intake in animals and humans. Previous studies have demonstrated that the mean plasma concentrations of ghrelin in normal-weight elderly people were lower than those in younger people. We hypothesized that ghrelin administration might improve the metabolic and physiological changes that accompany the anorexia of aging and somatopause. First, 75-wk-old mice fasted for 72 h, after which they resumed feeding with sc administration of ghrelin (360 µg/kg) twice daily for 4 d. Multiple administrations of ghrelin after a 72-h fast increased food intake and hastened body weight recovery with a high lean body mass ratio. Next, 50-wk-old mice were sc injected with rat ghrelin (40 µg/kg) twice weekly from 50–80 wk of age. Long-term administration of ghrelin kept aged mice with low body weight and low adiposity. These results suggest that ghrelin might be a novel approach for the therapy of age-related metabolic and physiological changes.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
AGING IS ASSOCIATED with progressive decreases in food intake (FI), termed the anorexia of aging (1, 2, 3). The physiological causes of the anorexia of aging are largely unknown and likely multifactorial (1, 2). One of the key factors of the anorexia of aging appears to be loss of appetite (4, 5). In comparison with healthy young people, elderly people feel less hunger when fasting and earlier satiety after initiating a meal (5, 6, 7). This insensitivity to the signs of appetite can lead to unintentional weight loss and undernutrition in response to acute and chronic illness, resulting in the increasing morbidity and mortality seen in elderly people (1, 8). Our understanding of the control of feeding has increased markedly in recent years. Animal and human studies have examined the roles of orexinergic and anorexinergic hormones, such as neuropeptide Y, orexin, ghrelin, CRH, and cholesystokinin (9, 10, 11, 12, 13, 14). Of these, ghrelin has received great interest as a potential therapeutic agent for the anorexia of aging.

Ghrelin, an acylated peptide of 28 amino acids, was identified as an endogenous ligand for the GH secretagogue receptor (15). The major site of endogenous production of ghrelin is the stomach; this peptide is also expressed in the hypothalamus (16, 17, 18, 19). Administration of ghrelin stimulates GH secretion and FI in both animals and humans (15, 16, 20, 21, 22, 23). Plasma ghrelin levels are regulated by acute feeding states. They rise during fasting and are rapidly suppressed after feeding (12, 17, 24, 25). Ghrelin secretion is also regulated by chronic feeding states. Plasma ghrelin levels are elevated in food-restricted animals and patients with anorexia nervosa and are reduced in obese subjects (12, 17, 24, 25, 26, 27). These data suggest a role for ghrelin in energy homeostasis. Previous studies have demonstrated that plasma concentrations of ghrelin in normal-weight elderly individuals were lower than those in younger people (28, 29, 30). GH responses to ghrelin administration in elderly people are also lower than those seen in young people (31). It has been speculated that aging is associated with reduced production of ghrelin or attenuation of endogenous ghrelin signaling (32).

Aging is associated with decreases in lean body mass (LBM) and increases in relative fat mass (33, 34). These changes can result in alternations of blood lipid profiles, which favor the development of vascular disease. Decreases in GH secretion are also seen in elderly people (35, 36), termed somatopause, which may contribute to these metabolic and physiological changes. Although the mechanisms inducing somatopause and leading to changes in body composition are not fully understood, clinical studies have attempted GH replacement in elderly persons. Such treatment has only occasionally been effective in increasing muscle mass and strength in elderly subjects (37, 38), and adverse effects, such as glucose intolerance and fluid retention, occurred frequently. Chronic administration of the ghrelin mimetic MK-677 to elderly people, however, restored pulse amplitude of episodic GH secretion and serum IGF-I levels to those seen in young adults (39) and increased bone mineral density (BMD) at the femoral neck with few adverse effects (40). These findings implicated a hypothesis that administration of ghrelin might safely restore intrinsic GH secretion and improve age-related metabolic and physiological changes.

Using an animal model of aging, we performed three examinations to evaluate the effectiveness of ghrelin on age-related metabolic and physiological changes. Using an experimental model of physical stress with a 72-h fast established by Wolden-Hanson and colleagues (41, 42, 43), we evaluated the influences of aging on FI. Second, we assessed the efficacy of ghrelin on the anorexia of aging; we investigated whether multiple ghrelin administrations can increase FI and hasten the recovery of body weight (BW) and body composition after the physical stressor of a 72-h fast. Third, we assessed the efficacy of long-term ghrelin administration to reverse the age-related metabolic and physiological changes; we investigated whether twice-weekly ghrelin from 50–80 wk of age improved the blood metabolic parameters and body composition.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Animals
All procedures using experimental animals were approved by the Kyoto University Graduate School of Medicine committee on animal research. Procedures were performed in accordance with the principles and guidelines established by that committee.

Male C57BL/6 mice were purchased from Japan CLEA (Tokyo, Japan). Animals were housed in air-conditioned animal quarters with lights on from 0800–2000 h. Except where noted, mice were allowed ad libitum access to water and standard rat chow (CE-2, 352 kcal/100 g; Japan CLEA).

Experiment 1: influence of aging on recovery of BW after a short-term fast (72-h fast)
Four age groups (n = 10 per age group) of mice (10, 25, 50, and 75 wk old) were analyzed. To evaluate the baseline characteristics of these mice, we measured BW and monitored daily FI for seven consecutive days. After a 7-d baseline period, food was withheld for 72 h. After ad libitum feeding was resumed, BW and FI were assessed daily for 5 d. Mice were allowed ad libitum access to water throughout the experiment.

Experiment 2: effect of ghrelin on recovery of BW after a short-term fast in aged mice (75 wk old)
Rat ghrelin was purchased from the Peptide Institute, Inc. (Osaka, Japan). After a 7-d baseline period, food was withheld from 75-wk-old mice (n = 30) for 72 h. Ad libitum FI was then resumed for 8 d (d 0–7). For the first half of the refeeding period (d 0–3), half of the animals (n = 15) were sc injected twice daily with rat ghrelin (360 µg/kg) at 0900 and 1800 h (ghrelin group), whereas the other half were injected with saline (saline group). BW and FI were measured daily in both groups. During the latter half of the refeeding period (d 4–7), BW and FI were measured in the absence of ghrelin or saline injections. Using computed tomography (CT) (laboratory CT; Lacita, Aloka, Japan), we examined the body compositions of 75-wk-old mice before the 72-h fast (d –3), after the 72-h fast (d 0), and after ghrelin or saline injection (d 4).

Experiment 3: effects of a long-term ghrelin injection on BW and body composition in aged mice (50–80 wk old)
The aim of this experiment was to investigate whether a long-term administration with low-dose ghrelin, which stimulates GH secretion without increase in cumulative FI, may lead to increase in LBM and decrease in fat mass via lipolytic and anabolic effects of elevated GH. We selected a dose and frequency on the basis of three points: 1) a dose that can stimulate GH secretion. 2) a dose that doesn’t affect daily FI, and 3) a frequency that can avoid desensitization of GH in response to ghrelin

Because we have previously reported that sc administration of ghrelin at 40 µg/kg stimulated GH secretion and increased FI in 8-wk-old mice (21), we evaluated whether this dose of ghrelin could stimulate GH secretion and FI in 50-wk-old mice. Fifty-week-old mice were sc injected with rat ghrelin (40 µg/kg) or saline at 1000 h under ad libitum feeding conditions (n = 8 per group). Blood was collected from the tail veins of mice 15 min after injection. Fifty-week-old mice were sc injected with rat ghrelin (40, 120, and 360 µg/kg) or saline at 1000 h under ad libitum feeding conditions; daily FI was then measured. Previous reports have demonstrated that continuous administration of ghrelin with osmotic mini pump desensitizes GH secretion in response to ghrelin (16). To evaluate whether repeated administration of ghrelin desensitizes GH secretion in response to ghrelin, 20 mice were divided into two groups; half of them were sc injected with ghrelin at a dose of 40 µg/kg daily for 15 consecutive days, whereas the other half were injected with ghrelin at the same dose on d 1, 5, 8, 12, and 15 (twice weekly). On d 1 and 15, serum GH levels 15 min after ghrelin injection were measured. Fifty-week-old mice were also sc injected with rat ghrelin (40 µg/kg) at 0900 h twice weekly. Weekly FI was then measured.

Mice were scheduled sc administration of ghrelin twice weekly (Monday and Thursday). Fifty-week-old mice were also examined (n = 40). Half of the animals (n = 20) were sc injected with rat ghrelin (40 µg/kg) at 0900 h twice weekly (Monday and Thursday) from 50–80 wk of age, whereas the other half were injected with saline. BWs of both groups were measured weekly. CT was used to measure body composition before and after the experiment (50 and 80 wk old). After experimentation, blood samples were collected from the tail veins of mice at 1000 h under ad libitum feeding conditions for the measurement of blood glucose, serum insulin, triglycerides, total cholesterol, GH, and IGF-I levels. Blood glucose and serum insulin levels were also measured after an overnight fast (1700–0900 h).

Measurement of metabolic parameters and GH/IGF-I axis
Blood glucose was measured using a reflectance glucometer (One Touch II; Lifescan, Milpitas, CA). Serum total protein (Pierce, Rockford, IL), albumin (Albumin-E test; Wako, Osaka, Japan), triglycerides (Triglyceride-E test; Wako), total cholesterol (Amplex Red Cholesterol Assay Kit; Molecular Probes, Eugene, OR), serum GH levels (EIA kit; SPI-BIO, Bonde, France), and serum IGF-I levels (EIA kits; Diagnostic Systems Laboratories Inc., Webster, TX) were measured according to the manufacturer’s instructions. Serum was isolated by centrifugation and stored at –20 C until assayed.

Statistical analysis
Results are expressed as the means ± SEM. The statistical significance of the differences in mean values was assessed by two-way ANOVA or Student’s t test as appropriate. P values of <0.05 were considered to be statistically significant.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Experiment 1: recovery of BW after a short-term fast was delayed in aged mice
We assessed the changes in BW and FI of aged mice after a 72-h fast and after refeeding (Fig. 1Go). The initial BWs of 10-, 25-, 50-, and 75-wk-old mice were 24.7 ± 0.3, 31.1 ± 1.0, 33.2 ± 1.3, and 34.9 ± 1.4 g, respectively. The average daily FI adjusted for BW declined markedly with age (P < 0.01). Those of 10-, 25-, 50-, and 75-wk-old mice were 136 ± 2.7, 106.1 ± 5.2, 104.5 ± 2.8, and 93.7 ± 6.3 mg/day·g BW, respectively. With fasting, younger mice lost more weight than older mice, both in absolute values and as percentages of BW before fasting. Ten-week-old mice lost 5.9 ± 0.2 g (23.9 ± 0.7% of prefasting BW), and 25-wk-old mice lost 5.9 ± 0.2 g (18.9 ± 0.6% of prefasting BW), whereas 50- and 75-wk-old mice lost 4.8 ± 0.3 g (14.5 ± 0.9% of prefasting BW) and 4.6 ± 0.2 g (13.1 ± 0.6% of prefasting BW), respectively. Despite a disproportionately increased weight loss, the recovery of BW in 10-wk-old mice during refeeding was more rapid (on d 3) than that seen in all other age groups; the recovery of BW in 25-, 50-, and 75-wk-old mice was delayed. These animals did not recover to their average prefasting BW during the first 5 d of refeeding, reaching 98.6 ± 0.9, 98.0 ± 0.5, and 96.5 ± 0.9% of prefasting BW for 25-, 50-, and 75-wk-old mice, respectively. The difference in BW on d 5 between 10-wk-old mice and 25-, 50-, and 75-wk-old mice was significant (P < 0.001 for each group). The FI of 10-, 25-, 50-, and 75-wk-old mice in the first day of refeeding increased by 144 ± 5.7, 141.1 ± 6.6, 118.3 ± 3.2, and 116.2 ± 5.7% of prefasting FI, respectively. After adjustment for BW, the overall FI in 75-wk-old mice was significantly lower than that seen for 10-wk-old mice during the refeeding period (P < 0.01).


Figure 1
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  		FIG. 1. A, Influence of aging on the recovery of BW after a 72-h fast. The 10-, 25-, 50-, and 75-wk-old mice fasted for 72 h, after which they resumed feeding. Data shown are the means ± SEM. Changes of BW are expressed as a percentage of prefasting weight. #, P < 0.001 (10- vs. 25-, 50-, and 75-wk-old mice). B, Daily FI for the first 5 d of the refeeding period. Data are expressed as a percentage of the prefasting FI. *, P < 0.05, **, P < 0.01 (vs. 10-wk-old mice on the same day).

 
Experiment 2: multiple ghrelin injections restored BW loss after a short-term fast in aged mice
We monitored the changes in BW during experiment 2 (Fig. 2AGo). During the first half of the refeeding period (ghrelin or saline treatment period), BW recovery was more rapid in the ghrelin-treated subgroup in comparison with that seen in the saline-treated subgroup (P < 0.01). BW recovered fully by d 3 in the ghrelin-treated subgroup (100.6 ± 0.6% of prefasting BW), whereas that of the saline-treated subgroup was 97.9 ± 0.6% of prefasting BW on d 3. After discontinuing the ghrelin injections, however, BW in the ghrelin subgroup decreased to levels equal to those of the saline subgroup. On d 7, the average BW of the ghrelin- and saline-treated subgroups were 97.8 ± 1.1 and 97.6 ± 1.0% of prefasting levels, respectively.


Figure 2
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  		FIG. 2. Effect of repeated ghrelin administration on the recovery of BW and FI after a 72-h fast in aged mice. Seventy-five-week-old mice fasted for 72 h, after which they resumed feeding while being administered saline or ghrelin (360 µg/kg twice daily for 4 d). Data shown are the means ± SEM. Changes in BW (expressed as a percentage of the baseline BW) (A), daily FI (grams per day) (B), and CT-assessed body composition (expressed as a percentage of baseline fat mass or LBM) (C) were assessed in ghrelin-treated animals. *, P < 0.05; **, P < 0.005; #, P < 0.001 (vs. saline subgroup on the same day).

 
We evaluated daily FI during the refeeding period (Fig. 2BGo). During the first half of the refeeding period (ghrelin or saline injection period, d 0–4), daily FI increased in the ghrelin-treated subgroup in comparison with that seen in the saline-treated subgroup. The cumulative FI in the ghrelin-treated subgroup was 17.6 ± 0.6 g, whereas it was 15.7 ± 0.4 g in the saline-treated subgroup (P < 0.005). Daily FI during the latter half of the refeeding period (no-injection period, d 4–7), however, was similar between both groups. During this period, cumulative FI values in ghrelin and saline subgroups were 11.0 ± 0.5 g and 11.2 ± 0.6 g, respectively, which were not significantly different.

CT was used to measure body composition (Fig. 2CGo). As expected, adiposity and LBM decreased significantly after a 72-h fast (76.5 ± 2.2 and 81.1 ± 1.1% of prefasting levels, P < 0.001, respectively). After 4 d of refeeding and treatment, adiposity in the ghrelin-treated subgroup recovered to 83.9 ± 2.3% of prefasting levels and 86.6 ± 0.6% in the saline-treated subgroup; there were no significant differences between these values. LBM in the ghrelin-treated subgroup increased, surpassing the prefasting levels (105.1 ± 0.7%). This was significantly different (P < 0.05) from the saline-treated subgroup that returned only to prefasting levels (99.3 ± 2.2%).

Experiment 3: a long-term ghrelin injection decreased fat mass in aged mice
GH secretions of 50-wk-old mice were stimulated by sc ghrelin administration with a dose of 40 µg/kg. Fifteen minutes after ghrelin administration, serum GH levels were significantly higher than those seen after saline injection, 28.1 ± 7.6 and 8.4 ± 0.6 ng/ml, respectively (P < 0.05). Administrations of ghrelin with a dose of 40 µg/kg did not affect daily FI, 3.62 ± 0.18 g and 3.55 ± 0.25 g, respectively. Although those of 120 µg/kg or 360 µg/kg increased daily FI: 116.2 and 125.8% compared with control mice, respectively.

Repeated administration of ghrelin markedly attenuated the GH response to injected ghrelin; serum GH levels on d 1 and 15 were 20.4 ± 4.6 ng/ml and 5.8 ± 0.4 ng/ml, respectively (decrease to 28.6%) (P < 0.05). Although administration of ghrelin twice weekly did not attenuate the GH response to ghrelin; serum GH levels on d 1 and 15 were 21.6 ± 4.3 ng/ml and 22.3 ± 5.3 ng/ml, respectively. When mice were administrated ghrelin with a dose of 40 µg/kg twice weekly, weekly cumulative FI was equivalent for the saline-treated subgroups, 243.3 ± 2.5 g and 24.5 ± 2.3 g, respectively.

We followed the changes in BW from 50 to 80 wk old (Fig. 3AGo). The initial BWs of the saline- and ghrelin-treated subgroups were 33.2 ± 0.4 g and 32.8 ± 0.5 g, respectively. There were no significant differences between these values. Body weight increased gradually in both groups. Ghrelin-treated mice exhibited significantly lighter BW than the saline-injected mice throughout the course of the experiment (P < 0.01). By the end of the experiment, the average BW of the ghrelin-treated subgroup was 34.9 ± 0.9 g, whereas that of the saline-treated subgroup was 37.7 ± 1.0 g.


Figure 3
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  		FIG. 3. Effect of long-term ghrelin administration on BW and body composition in aged mice. Fifty-week-old mice were injected with ghrelin (40 µg/kg twice weekly) for 30 wk (50–80 wk old). Data shown are the means ± SEM. Changes in BW (A) and body composition (B), as measured by CT (expressed as a percentage of the fat mass, LBM, or BMD of 50-wk-old animals), were measured in 80-wk-old mice. *, P < 0.05; **, P < 0.005 (vs. saline subgroup).

 
Body composition was assessed by CT before (50 wk old) and after (80 wk old) experimentation (Fig. 3BGo). As expected, we observed a significant effect of age on body composition. In the saline-treated subgroup, adiposity increased, whereas LBM and BMD decreased significantly with increasing age to 80 wk old (116.6 ± 8.4, 96.1 ± 1.6, and 92.9 ± 3.9% that seen for 50-wk-old animals, respectively) (P < 0.01). At the end of the experiment when animals were 80 wk old, LBM and BMD were similar for both the saline- and ghrelin-treated subgroups (saline vs. ghrelin subgroup, LBM was 96.1 ± 1.7 vs. 95.7 ± 2.1% of 50-wk-old mice, and BMD was 92.9 ± 3.9 vs. 91.2 ± 3.0% of 50-wk-old mice). Adiposity, however, decreased significantly in the ghrelin-treated subgroup (saline vs. ghrelin subgroup, 116.6 ± 8.4 vs. 102.2 ± 5.7% of 50-wk-old mice, P < 0.05).

Blood glucose (ad libitum), serum insulin (ad libitum), triglyceride, total cholesterol, GH, and IGF-I levels did not differ between the saline- and ghrelin-treated subgroups. Blood glucose measured after an overnight fast was significantly lower in ghrelin-treated animals than in the saline subgroup (P < 0.05). Serum insulin levels after an overnight fast were undetectable in both groups (Table 1Go).


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  		TABLE 1. Blood glucose (ad libitum or overnight fast), serum insulin (ad libitum or overnight fast), triglyceride, total cholesterol, GH, and IGF-I levels at the end of experiment 3 (80-wk-old mice)

 

   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
Many studies have demonstrated that as people age, they eat less food (1, 2). Elderly people have an impaired ability to recover BW fully after physical and mental stresses, such as acute and chronic illnesses, surgery, or bereavement (1, 8). Animal models of aging also reflect the decline in FI observed in humans both spontaneously and in response to disturbances in feeding (43, 45, 46). A spontaneous decline in FI with aging was also observed in C57BL6 mice in this study. Wolden-Hanson and colleagues (41, 42, 43) reported that, in comparison with young animals, aging male Brown Norway rats fail to regulate BW in response to the metabolic stressor of a 72-h fast and fail to increase FI appropriately after the fast. In accordance with those results, we observed that aging male C57BL6 mice recover BW more slowly than younger mice in response to the stressor of a 72-h fast. Thus, C57BL6 mice can serve as an animal model for the anorexia of aging.

As shown in Fig. 2Go, ghrelin treatment increased both BW gain and FI in 75-wk-old mice after the physical stress of a 72-h fast in comparison with untreated age-matched controls. The time course of BW recovery in ghrelin-treated 75-wk-old mice resembled that of 10-wk-old animals. CT measurement of body composition revealed that the improvement in BW resulted from increased LBM, which might include changes in body water, in ghrelin-treated mice. Yukawa and colleagues (47) demonstrated that ghrelin treatment prevented BW loss after surgery without increasing visceral fat mass in aging animals. Recent publications suggested that endogenous ghrelin may play a role in obesity and exogenous ghrelin-induced weight gain due to increased fat mass (23, 48, 49, 50). Most of these studies were performed under healthy conditions. On the other hand, under unhealthy conditions, it is reported that ghrelin administration to the patient with functional dyspepsia and chronic heart failure increased FI and LBM without fat accumulation (51, 52). As is often the case with an inpatient, when decreased BW recovers to the baseline level after physical and mental stress, such as acute and chronic illness and surgery, a nonfat component, which is necessary to maintain a normal physical function, recovers first, and then fat mass accumulates. We therefore think that the result of experiment 2, where ghrelin administration hastened the recovery of BW after 72-h fasting without fat accumulation, were not inconsistent with the reported effects of ghrelin. Involuntary weight loss in which loss of muscle predominates may predispose the elderly to muscle weakness and protein energy malnutrition, leading to increased risk for extended hospitalization, mortality, and morbidity (53). Several drugs have been suggested for the treatment of the anorexia of aging, such as cyproheptadine (an antihistaminergic, antiserotonergic drug), GH, ornithine oxoglutarate, and anabolic steroids. None, however, have been established in the management of weight loss in elderly people (1). This study and previous findings support a possible role for ghrelin as a means to prevent weight loss after acute and chronic illness, surgery, or bereavement in elderly patients.

As suggested by Toshinai et al. (46) and Sun et al. (54), sc administration of ghrelin at 40 µg/kg increased serum GH levels and did not affect daily FI in 50-wk-old mice. In addition, administration of ghrelin with a dose of 40 µg/kg twice weekly did not attenuate the GH response to ghrelin, whereas repeated administration of ghrelin markedly attenuated the GH response to injected ghrelin. These results strongly suggest that some intervals are needed to maintain the stimulating effect of ghrelin on GH secretion and support our regimen. The stimulating effect of ghrelin on GH secretion persisted at 80 wk of age (the end of experiment 3, data not shown).

In this study, long-term administration of ghrelin maintained low adiposity in aged mice without impairing glucose tolerance or lipid metabolism. Serum insulin levels and ad libitum and fasting blood glucose levels in ghrelin-treated mice tended to be lower than those seen in saline-treated mice. It is reported that peripheral ghrelin administration increased BW and cumulative FI and reduced insulin secretion (23, 55). This BW gain mainly results from increased fat mass and might be caused by decreased energy expenditure in addition to increased FI (50). Previous reports using mice showed that the effects of ghrelin on fat deposition were obtained by ghrelin administration at relatively high doses (50, 56). In this study, aged mice were administered low-dose ghrelin, which stimulates GH secretion without increase in cumulative FI. Discrepancy between previous reports and our result could be explained by differences in the doses and frequencies of ghrelin administration. Indeed, serum GH levels were increased by sc administration of ghrelin at a dose of 40 µg/kg, but daily and weekly FI were not increased in this study. Ghrelin-induced GH secretion may contribute to low adiposity. In contrast to our expectations, long-term administration of ghrelin did not increase either LBM or BMD. Clinical studies in elderly humans have indicated that MK-677 can reactivate the GH/IGF-I axis, increasing serum IGF-I levels, improving lean body composition (57), increasing BMD (40), and restoring lower-extremity function (44). In this study, serum IGF-I levels in ghrelin-treated mice were similar to those seen in saline-treated mice. The lack of change in IGF-I is likely why no increases in LBM or BMD were observed. Different results might have been obtained with ghrelin administration at higher doses and frequencies. In that case, however, fat mass would likely also have increased. Additional studies will be needed to investigate the optimal dose and frequency of ghrelin administration to improve the age-related metabolic and physiological changes.

In conclusion, we demonstrated that repeated ghrelin administration increased FI and hastened the recovery of BW after a short-term physical stress. In addition, long-term administration of ghrelin maintained low adiposity in aged mice. Multiple approaches have attempted to improve such age-related metabolic and physiological changes. None of these, however, have yet achieved satisfactory results. Our results suggest that ghrelin may be a candidate therapeutic approach to combat such age-related metabolic and physiological changes.


   Acknowledgments
 
We thank Ms. Ishimoto, Ms. Takehisa, Ms. Fukuda, and Ms. Shiraiwa for their excellent technical assistance.


   Footnotes
 
This study was supported by funds from the Ministry of Education, Culture, Sports, Science and Technology of Japan, and the Ministry of Health, Labor and Welfare of Japan.

Disclosure Statement: The authors have nothing to disclose.

First Published Online March 27, 2008

Abbreviations: BMD, Bone mineral density; BW, body weight; CT, computed tomography; FI, food intake; LBM, lean body mass.

Received December 4, 2007.

Accepted for publication March 19, 2008.


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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Taste, Smell, and Anorexia in the Aged: What Can We Do To Help the Patients? An Initial Primer
Gerontology, December 1, 2008; 13(2): 40 - 64.
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