Manipulating age-related metabolic flexibility
Dommerholt, Marleen
DOI:
10.33612/diss.172053834
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Publication date: 2021
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Dommerholt, M. (2021). Manipulating age-related metabolic flexibility: using pharmacological and dietary interventions. University of Groningen. https://doi.org/10.33612/diss.172053834
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Chapter
General discussion
While the worldwide average lifespan is increasing, the incidence of frailty and the development of related chronic diseases is elevated. The accumulation of age-related damage in cells affects metabolic flexibility leading to changes in whole-body energy metabolism. Hyperinsulinemia, increased adiposity, and muscle loss are some of the age-related changes contributing to a reduction in the quality of life. Poor metabolic health, caused by bad lifestyle habits such as an unhealthy diet and physical inactivity, also contributes to the progression of chronic metabolic diseases, meaning that the extended lifespan is rarely disease-free. In this thesis, we investigated the potential to increase metabolic health via modulation of age-related changes in metabolic flexibility using genetic-, pharmacological- and dietary interventions.
Metabolic flexibility as a mechanism to adapt to dietary changes at
an advanced age
In Chapter 1, we describe the metabolic alterations associated with ageing, such as white adipose tissue (WAT) redistribution, ectopic lipid accumulation, hyperinsulinemia, and muscle deterioration, that affect metabolic flexibility. While the exact mechanisms contributing to these metabolic alterations remain incompletely understood, this thesis aimed to characterized metabolic flexibility in aged mice. We investigated whether dietary interventions, such as caloric restriction (CR), protein restriction (PR), or high-fat diet (HFD), has a different effect on metabolic health in aged mice as compared to young animals.
First, it has been long known that insulin sensitivity is decreased during ageing, leading to compensatory hyperinsulinemia to maintain normal glycemia [1,2]. In Chapters 4&5, we confirmed the age-related hyperinsulinemia, but surprisingly discovered an improved glucose tolerance compared to young animals. Similarly, an HFD aggravates age-related hyperinsulinemia, as aged animals had to increase insulin levels up to 3.5 times higher during glucose administration compared to young mice, but it did not yet deteriorate glucose homeostasis (Chapter 4). Next, we investigated whether age-related hyperinsulinemia could be affected by PR, but in contrast to our hypothesis, we found no effects (Chapter 5). Glucose homeostasis and subsequent insulin secretion were not improved by a 3-month intervention of either low- or high protein intake. Interestingly, mice with a genetically reduced insulin dosage, that are protected from diet-induced hyperinsulinemia [3], still developed an age-related increase in insulin secretion (Chapter 6). While we describe an improvement of glucose metabolism by caloric restriction at all ages tested, these results appear to be independent of a reduction in fasting insulin levels. Together these results suggest that age-related hyperinsulinemia develops independent of insulin dosage, cannot be regulated by dietary interventions, but can deteriorate fast by an unhealthy diet.
Second, age-associated muscle decline is an important factor to reduce the quality of life in the elderly. It has been suggested that aged muscles require high dietary protein intake to maintain protein synthesis. While basal protein synthesis is maintained during ageing, sensitivity to detect amino acids declines and muscle protein synthesis after protein ingestion is reduced in aged individuals due to anabolic resistance involving insulin and mTOR signalling [4,5]. We determined an age-related decline in muscle
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strength but this was not affected by dietary protein intake (Chapter 5). At 18 monthsof age, a low protein diet was not detrimental to muscle strength and mitochondrial function, and a high protein diet did not prevent age-related decline. Previously, it has been shown that the decline of mitochondrial respiratory capacity can be prevented until 18 months of age by voluntary running, independent of the administered low- or high-fat diet [6]. Furthermore, muscle mitochondrial function of 18 months animals was not yet affected compared to young animals but decreased rapidly in response to 3 months HFD. Aged animals were incapable of increasing oxidative capacity in the muscle in response to nutrient excess, leading to lipid accumulation (Chapter 4). Muscle strength after high-fat feeding, measured by hanging wire or grip strength also deteriorated, but this could also be a secondary effect of increased weight gain. The insulin sensitivity, to which skeletal muscle is the greatest contributor, was already strongly reduced in aged animals compared to young and was further decreased upon a high-fat diet. Together, these results indicate that more research is necessary to find mechanisms to improve muscle function at an advanced age.
Third, age-related adipose tissue redistribution towards visceral WAT (vWAT), ectopic lipid accumulation and increased accumulation of senescent cells affect metabolic health at an advanced age [7]. Changes in sex hormones and subcutaneous WAT (scWAT) dysfunction have been shown to contribute to the relocation of lipids to vWAT, liver and muscle [8,9]. Ageing is associated with alterations in the immune cell environment of vWAT, subsequently causing adipose tissue dysfunction, insulin resistance, and ectopic lipid storage [10,11]. In Chapter 4, we observed that the aged animals have limited abilities to compensate when challenged with an HFD, leading to ectopic lipid accumulation in the muscle. We found that the expansion of scWAT was similar between young and aged animals, but the vWAT expanded only by 75% in aged animals while it was 3-fold in young animals. Based on these observations, we hypothesize that limited adipose tissue expansion due to age-related vWAT dysfunction resulted in ectopic lipid accumulation in the muscle after 3 months HFD. Furthermore, we indicate that the aged adipose tissue is still sensitive to changes in dietary protein intake and subsequent fibroblast growth factor 21 (FGF21) signalling, indicating metabolic flexibility. In Chapter 5, a low protein diet increased energy expenditure and stimulated the rejuvenation of scWAT by increasing the beige adipocyte population. Together, this thesis shows that a healthy dietary lifestyle can induce rejuvenation of adipose tissue, indicating the presence of metabolic flexibility at an advanced age.
Altogether, we conclude that healthy dietary interventions can improve metabolic health at an advanced age by stimulating metabolic flexibility, but that an unhealthy lifestyle is highly detrimental due to the loss of compensatory mechanisms. Together these results suggest that dietary composition is important during ageing. Whether these dietary interventions at advanced age also affect longevity remains to be investigated. The study of Hahn et al. demonstrated that a nutritional memory of aged adipose tissue prevented 21-months old mice from CR-induced longevity and metabolic remodelling of adipose tissue [12]. While differences in the experimental design, such as age at the start of the experiment, duration of the diet, or gender could explain these results, it remains unknown whether the improvements in longevity and
metabolic health are connected. The results in this thesis further suggest that diet composition is an important factor for the effectiveness of late-life dietary intervention strategies and age-specific recommendations might be necessary.
The complexity of IGF1 and FGF21 signalling in ageing and longevity
As discussed in Chapter 1, reduced growth hormone (GH) / insulin-like growth factor 1 (IGF1) / insulin signalling is an essential factor in longevity and healthy ageing. It has been suggested that dietary interventions promote metabolic health by affecting these metabolic targets, however, the role of these hormones in physiological ageing is paradoxical. While reduced IGF1-insulin signalling is an important aspect of increased longevity by caloric restriction, models of premature ageing also exhibited reduced IGF1 [13]. Analysis of significantly over-represented biological processes in both delayed and premature aging revealed highly similar profiles. Reduced expression of the GH-IGF1 pathway is found in natural ageing, accelerated ageing, and long-lived mouse models [13]. In an attempt to extend lifespan, the IGF1-insulin axis was reduced in order to limit cell growth [14]. However, in aged mice, this results in rapid and progressive decline in organ function, as dysfunctional or dying cells cannot be replaced [15]. Also in humans, secretion of IGF1 declines continuously after reaching adulthood, resulting in very low levels in aged individuals [16,17]. The total serum concentration of IGF1 in humans reaches a peak in young adults and then progressively declines, reaching a relatively steady level at the age of 75 years [18,19]. In humans, lower IGF1 concentrations have been associated with the development of age-related cognitive impairment and higher mortality [20], including all-cause mortality, cancer, cardiovascular diseases, chronic heart failure, and ischemic heart diseases [21–24]. Low protein intake has been associated with reduced IGF1 levels and improved disease incidence in the population below 65 years of age, but not in the older population [25]. However, the IGF1 axis is also associated with increased survival and better functional status in nonagenarians [17]. Low IGF1 predicts life expectancy in exceptionally long-lived individuals and even increased survival in elderly (95+) with a history of cancer [16]. These contradicting results suggest a complex role of IGF1 signalling during ageing which in healthy individuals can improve life, while others will be detrimentally affected by reduced IGF1 levels. It should therefore be further investigated how reduced IGF1 signalling can have divergent effects in elderly, and whether this will affect the efficacy of the response of dietary interventions.
Similarly, FGF21 has a paradoxical role in metabolic health and disease. FGF21 is produced by the liver and adipose tissues in response to prolonged fasting, thereby suppressing lipogenesis and stimulating hepatic fatty acid oxidation and ketogenesis [26,27]. FGF21 administration improves insulin sensitivity, hepatic triglyceride accumulation, and energy expenditure [28,29]. However, elevated FGF21 levels have also been ascribed during obesity, metabolic diseases and ageing, see Table 1 [30–42]. Plasma FGF21 levels have been reported to be significantly associated with body mass index, plasma triglyceride levels and insulin resistance [34,36]. In addition, the expression of βKlotho is downregulated in several pathophysiological conditions that display high FGF21 levels, such as in adipose tissue after an HFD,
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liver during non-alcoholic fatty liver disease, and pancreatic islets of diabetic ob/obmice [43]. Due to elevated FGF21 production by both liver and WAT, obesity has been classified as an FGF21-resistant state, but it remains unclear whether elevated FGF21 levels have functionality in obese individuals [44]. Classification of FGF21 resistance should therefore include the phosphorylation and expression of downstream signalling proteins (ERK, AMPK, Akt, Egr1, or cFos), as well as the physiological response to exogenous FGF21 both in vivo and in vitro [45]. However, studies investigating the state of ‘’FGF21 resistance’’ during obesity have found inconclusive results. While exogenous FGF21 administration in obese mice resulted in a strong reduction in the sensitivity of WAT and liver, measured by ERK phosphorylation and target gene mRNA expression (Egr1/cFos), the physiological responses to exogenous FGF21 like glucose-lowering and the reduction of body weight remain preserved in obese mice [46,47]. To the same extent, specific age-related changes in FGF21 signalling need to be better understood. Increased levels of FGF21 have been determined linearly across the age groups (from 20 to 80 years) in a healthy population [42]. However, centenarians display an interesting low level of FGF21 levels compared to elderly controls [33], and adipose explants from elderly humans respond similarly to FGF21 administration [32]. These results are in line with our findings in Chapter 5, in which aged animals were responsive to PR-induced elevation of plasma FGF21 and aged adipocytes could be stimulated to increase thermogenesis, indicating that ageing itself does not induce FGF21 resistance. However, the physiological relevance of elevated FGF21 levels remains elusive and it needs to be further studied whether chronically high FGF21 levels can interfere with the FGF21-induced effects of dietary interventions.
Table 1: FGF21 levels are associated with obesity, ageing, and metabolic diseases in humans. OBESITY Increased levels of FGF21 [30,31,34,35]
T2D Inconclusively increased by glucose intolerance [30,31,35–39] Inconclusively increased in T2D patients
Drug treatment of T2D patient does not affect FGF21 levels
T1D Decreased levels of FGF21 [39]
NAFLD/NASH Increased levels of FGF21, [41,48] most pronounced in NAFLD, not NASH
HYPERTENSION Increased levels of FGF21 [30]
AGEING Increasing levels of FGF21 over time [32,33,42] Reduced levels of FGF21 in centenarians
T2D; Type 2 Diabetes, T1D; Type 1 Diabetes, NAFLD; nonalcoholic fatty liver disease, NASH; nonalcoholic steatohepatitis.
The effect of weight loss on metabolic health
Changes in body weight and body composition are important aspects to take into account when using dietary interventions as a tool to study metabolic health. In this thesis, we investigated the effects of dietary interventions in aged mice, in which body weight loss, and especially lean mass deterioration, is not necessarily beneficial. It is therefore important to establish whether improved health by a dietary intervention depends on body weight. CR primarily interferes with the state of energy imbalance
by reducing energy intake and subsequently reducing body weight to juvenile levels. Normally, C57Bl/6 mice rapidly gain body weight during development by the expansion of lean mass, and body weight continues to increase after adolescence as a result of general overnutrition and low energy expenditure, resulting in overweight adult and aged mice [49]. However, since the effectiveness of CR varies significantly between different strains [50], it suggests that the effectiveness of CR correlates with the induced weight loss. For example, DBA/2 mice, which have a reduced weight gain over time due to a high rate of energy expenditure and therefore exhibit limited weight loss by CR, also appear to be unresponsive to the effects of CR on longevity [51–54]. This hypothesis is also in line with our findings in Chapter 6, in which we investigated the role of age in the effectiveness of CR. CR reduced body weight to a juvenile level, independent of age and its associated body weight. Our results indicate that there was a strong correlation between the reduction in body weight and the improvements in metabolic health at different ages. In addition, both short-term and a lifelong CR diet affected both body mass and metabolic health regardless of insulin gene dosage [55], suggesting that the duration of the diet and fasting insulin levels have limited effects on the metabolic improvements induced by CR. These effects have also been described in non-obese humans, in which long-term CR does not have a greater effect on metabolic health compared to short-term studies. In those studies, cardiovascular risk factors, adipokine secretion, and bone mineral density are not further improved after 12 months CR [56–59]. The weight loss (more specifically fat mass) declines between 12 and 24 months of CR, possibly due to a decline in the adherence to a CR diet. Together, these results suggest that changes in body weight and composition are the main drivers of metabolic changes by caloric restriction.
Body composition is another important factor when determining the efficiency of dietary interventions in the elderly. Especially at an advanced age, when immobility is complicated by medical or environmental factors, lean mass deterioration increases the risk of metabolic decline. In contrast, available research on protein restriction in young mice reported increased longevity and improved metabolic health, such as energy expenditure and glucose tolerance, together with a reduction in body weight and lean mass. A low protein diet of 5% or 7% induced a significant reduction in body weight and lean mass after 2 weeks [60,61]. Whether the metabolic effects of PR depend on changes in body weight or lean mass has not been investigated. However, the use of Fgf21 KO mice showed that PR-induced effects on energy expenditure and thermogenesis are FGF21 dependent. Furthermore, these mice also exhibit a different response in body weight and composition, in which the effects on body weight and lean mass are less severe compared to wild type mice [60,62–64]. Together, these studies suggest a synergy between the effects on body composition and energy expenditure. In contrast to these observations, we found no effect of protein restriction in aged mice on body weight or lean mass (Chapter 5). Since we observed an increase in energy expenditure and browning, this suggested that these effects are independent of body weight. As we didn’t observe an improvement in glucose homeostasis, we hypothesize that body composition is important for PR-induced glucose tolerance and insulin sensitivity. Together, these studies indicate that PR can improve metabolic
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health independent of weight loss and suggest distinct mechanisms between insulinresistance and energy expenditure. While further studies to determine this distinction are necessary, these observations indicate important differences between CR and PR in the treatment of obesity and its comorbidities.
Gender differences in WAT flexibility and effects on metabolic health
Metabolic research has traditionally been focused on using males. A smaller variability, due to the lack of hormonal fluctuations during the female reproductive cycle, has frequently been used as justification [65]. However, many gender-related differences can complicate the extrapolation of male-derived results of dietary restriction. Most appreciated of the gender differences are inherent differences in fat distribution and function. While women have a higher percentage of fat mass, which is mainly located subcutaneously (specifically around the hips and thighs), men have higher fat storage in visceral adipose tissue (belly) [65]. This distinction is important as the site of adipose tissue expansion in obesity is a major determinant of cardiometabolic disease risk. The high visceral adiposity in men is associated with an increased incidence of diabetes and atherosclerosis, while high subcutaneous adiposity, is associated with a reduced incidence of metabolic disease [66]. Subsequently, insulin resistance occurs more frequently in males than females in both mice and humans [65,67]. As a result of gender-specific distribution and function of adipose tissue, the progression of obesity occurs differently in males and females. Female mice gain weight at a slower pace, store lipids in scWAT rather than vWAT, and expand adipose tissue through hyperplasia rather than hypertrophy [68–71]. Altogether, gender differences contribute substantially to diet-induced obesity. In Chapter 4, we describe that the development of obesity in aged male mice is associated with impaired insulin sensitivity, ectopic lipid accumulation in the muscle, and the inability to induce beta-oxidation. However, as the adipose tissue expansion is different between males and females, it remains unclear whether aged female mice respond similarly. Distinct scWAT function could contribute to differences in insulin resistance and ectopic lipid accumulation in the muscle. Additional research is needed to determine whether a high-fat diet impacts muscle oxidation and lipid accumulation in aged female mice, independent of adipose tissue function.
Furthermore, the more obesogenic scWAT of aged females could be less sensitive to cues that promote browning of scWAT by protein restriction (Chapter 5). Unfortunately, only a few studies have investigated the effects induced by PR or methionine restriction (MR) in females, and report inconclusive findings between males and females on lean mass deterioration, adiposity, glucose tolerance, insulin sensitivity, and energy expenditure [72–75]. Gender-specific mechanisms to increase energy expenditure have been suggested as plasma FGF21 and uncoupling protein 1 (Ucp1) in scWAT are not induced by MR in females and the PR-induced magnitude of this response was diminished compared to males [72,73]. These results suggest that energy expenditure can be induced independently of the FGF21-UCP1 axis in females [73]. Furthermore, MR produced sexually dimorphic changes in body composition in young growing animals, while the effects on energy balance were comparable between sexes when the diet was initiated after reaching physical maturity [75]. Together these results
indicate the importance of the timing of dietary interventions to determine its effects in males and females. These findings are in line with our observations in Chapter 5, which highlight the importance of the timing of initiating a dietary intervention. We hypothesized that early intervention with a low protein diet affects the growth and body composition of mice, while these effects are absent when starting the diet after maturation. Together, these results indicate the importance of the life stage when applying the dietary intervention, and suggest important gender-specific differences.
Gender differences in IGF1-insulin signalling and effects on longevity
A gender-specific response to reduced IGF1-insulin signalling could also greatly contribute to differences induced by caloric restriction. It has been observed that extended longevity as a result of reduced nutrient-sensing is gender-biased [76,77]. While a range of pharmacological treatments to extend longevity is favoured in males, extended longevity by genetic models with IGF1-insulin deficiency is more pronounced in females, see Table 2 [78–100]. Only in
female and male Ames Dwarf and Snell Dwarf mice lifespan is affected equally. In line with these results, the mouse model used in Chapter 6 is also potentially affected by gender differences. Female Ins2+/- mice (on an Ins1 null background) were protected against high-fat diet-induced obesity, and extended longevity both on chow and HFD [83,101], while male Ins2+/- mice only showed highly variable results when challenged with an HFD [102]. In contrast, male Ins1+/- mice (on an Ins2 null background) were protected against obesity as a result of increased energy expenditure by immediate upregulation of mitochondrial oxidation [3,103], but no changes in metabolic health were observed in either chow-fed, high-fat diet-fed, or caloric restricted female Ins2-/- mice [3,55]. The
underlying mechanisms concerning these difference could be due to study design, timing or housing conditions, however, the impact of both insulin alleles in the regulation of insulin secretion between genders have not been systemically addressed.
Furthermore, differences in average lifespan exist between males and females, with women having a greater life expectancy at birth than men [76]. However, gender differences in animal models regarding
Table 2: Some models with reduced
GH-IGF1-insulin signalling show major gender-specific effects on longevity.
Males Females Ref. Snell dwarf ↑↑ ↑↑ [78] Ames dwarf ↑↑ ↑↑↑ [79] lit/lit ↑↑ ↑↑ [78] Ghr-/- ↑↑ ↑↑ [90] Ghrh-/- ↑ ↑↑ [94] Liver Igf1-/- ↓ − [95] Liver Igf1-/- − ↑ [96] Igf1r+/- ↑ ↑↑ [97] Igf1r+/- ↓ ↑ [98,99] brain Igf1r+/- ↑ ↑ [100] PAPP-A-/- ↑ ↑↑ [80] IR+/- ↑ − [81] FIRKO ↑ ↑ [82] Ins1-/-;Ins2+/- ? ↑ [83] IRS1+/- − ↑ [84] IRS1-/- − ↑↑ [84,85] IRS2+/- − − [84] IRS2+/- ↑ ↑ [86] IRS2-/- ↓↓ ↓ [84,87] brain IRS2+/- ↑ ↑ [86] p110a+/- ↑ − [88] mtor+/-;mist8+/- − ↑ [89] mtorΔ/Δ ↑ ↑ [91] TSC TG − ↑ [92] S6K-/- − ↑ [93]
− = no change, ↑ = small increase (<10%), ↑↑ = large increase (20-50%), ↑↑↑ = extra-large increase (>50%), ↓ = small reduction (<10%), ↓↓ = large reduction (20-50%)
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longevity are small and inconsistent. Depending on the species and strain, it has beenreported that males are the longer-lived sex, that there is no sex difference, or that females live longer [76]. However, as the average lifespan differs between males and females, the generic effects of caloric restriction cannot be assumed. Caloric restriction, defined as a 40% reduction in caloric intake, effectively extends lifespan and improves metabolic health, but the most optimal level of restriction may be strain- and gender-specific. A meta-analysis illustrated that the lifespan-extending effects of 40% CR in mice ranged between 4–27% [104]. In C57Bl/6, various levels of caloric restriction suggest that longevity in females is only extended by 20% CR, not 40%, while the beneficial effects on glucose homeostasis are not different between levels of CR and gender [105]. In contrast, we observed the expected rapid decrease in body weight and blood glucose levels after CR treatment, but found that it was more pronounced in females than in males (Chapter 6). Female mice showed a reduction in their islets insulin content due to CR, which was not found in males, suggesting that female Ins2−/− mice may be more susceptible to the effects of CR. Furthermore, the effects of CR on adipose tissue size seem stronger in females compared to males. Upregulation of genes involved in lipogenesis and thermogenesis in scWAT were more pronounced in females and brown adipose tissue (BAT) activity was still active in the female CR
Ins2−/− mice while it was diminished in males. Together, these results highlight the complexity of CR in the context of gender and suggest the uncoupling of healthspan and lifespan [105]. The molecular mechanisms underlying the sex differences in the metabolic responses to CR clearly require further study.
Translation of pharmacological and dietary intervention to humans
While this discussion describes several factors that confound the interpretation of data of dietary interventions in mouse models, species differences also need to be taken into account to predict the human outcome of these lifestyle changes or therapeutical interventions. This thesis described an important role of brown and white adipose tissue activation by protein restriction or FGF21 administration on metabolic health. However, it remains elusive whether FGF21 acts similarly in mice and men. In Chapter 2, we describe the clinical trials that are currently conducted with FGF21-analogues and highlight some differences in results compared to preclinical research. While very effective in reducing plasma TG and hepatic lipid accumulation, the glucose-lowering effects are not observed in NAFLD- and T2D patients [106]. These differences between humans and rodents are potentially explained by different mechanisms.
First, different body composition and adipose tissue function between mice and humans can lead to distinct effects. Many adipose tissue depots in mice are barely present in humans and vice versa, such as the human omentum or the murine epididymal fat [107]. Also, mice have relatively large amounts of BAT as they heavily rely on heat generation to maintain body temperature, while humans lose most of their BAT within a few years after birth [108]. Recently, the existence of genuine brown fat in adult humans has been proven by the detection of bona fide UCP1+ adipose tissue
in biopsies [109–111]. These UCP1+ cells in humans display molecular markers of
Pax7+Myf5+ precursor cells that give rise to classic BAT [107]. However, while the exact origin of these cells remains unknown, a specific CD81+ precursor population has
recently been discovered in the human subcutaneous adipose depot that correlates negatively with metabolic health [112,115,116]. Further studies need to determine how human brown/beige adipocytes are induced by dietary and pharmacological interventions. However, these relatively little amounts of BAT compared to rodents may account for the attenuated glucose-lowering response by FGF21 treatment observed in human clinical trials.
Second, FGF21-induced effects in mice and humans can be affected by species differences within FGFR and βKlotho expression. Previous results suggest that FGF21-induced effects in mice are dependent on βKlotho expression in the central nervous system to stimulate BAT and WAT [117]. However, while FGF-receptor 4 and βKlotho were detected in several regions of the murine brain, including the hypothalamus, mRNA encoding these receptors were not detected in cynomolgus monkey or human brain samples [118]. These species differences are not only important for FGF21 signalling, but possibly also affect the response of other FGF-based therapeutics. In this thesis, we have suggested species differences by fibroblast growth factor 1 (FGF1) signalling, independent of βKlotho (Chapter 3). As the therapeutic application of FGF1 is limited by its high mitogenic properties, several FGF1 analogues have been developed to reduce these effects [119–121]. While these compounds retain their metabolic activity in vivo, determined by glucose-lowering and antisteatotic effects in several mouse models, it remains to be determined whether species differences will affect the outcome. We determined the metabolic and mitogenic potential of these FGF1 analogues in a rat hepatocyte cell line to assess the potential as a NAFLD target, but observed an unexpected mitogenic potential of these FGF1 analogues that can potentially be explained by species differences. However, more research is needed to exclude tissue-specific or cell-specific effects.
Conclusion and further perspectives
Altogether, the research in this thesis highlights the perspective of dietary or pharmacological interventions in the context of ageing. While we know that ageing is associated with several processes that reduce metabolic flexibility, limited studies are available that study metabolic health in aged mice after dietary intervention. We found that the macronutrient composition of the diets is of vital importance to maintain insulin sensitivity, muscle strength, and adipose tissue function during ageing. Based on these findings, we conclude that healthy dietary interventions can improve metabolic health at an advanced age by stimulating metabolic flexibility, but that an unhealthy lifestyle is highly detrimental due to the loss of compensatory mechanisms. However, further studies are needed to investigate the metabolic impact of dietary interventions at different ages, with age-induced obesity taken into account. Ageing is a generic stochastic process, causing significant variation in body weight but also in other metabolic parameters such as hormone levels and incidence of metabolic diseases. Further research should therefore focus on various subcategories within the elderly to determine whether body weight and its associated metabolic status is a confounding
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factor in the efficiency of dietary interventions. This will potentially contribute to theimprovement of personalized dietary- or pharmacological interventions in the elderly, and further highlight age-related metabolic changes.
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