Energy intake at the intersection of the clock and ageing

Abstract

Calorie restriction and timed dietary intake are two approaches known to increase lifespan or delay age-associated diseases. New studies reveal the importance of the ‘how much’ and ‘when’ of dietary intake in ageing modulation and collectively demonstrate how protection of the internal clock by diet can delay the ageing process.

Ageing is associated with a gradual decline of circadian rhythmicity, the 24-h biological oscillations that drive daily rhythms in physiology and behaviour. This association raises questions as to whether pharmacological or behavioural mechanisms that increase circadian robustness can slow the ageing process. Feeding restriction is one mechanism by which to augment internal rhythms. Time-restricted high-fat diet feeding, even during the active phase, can prevent diet-induced obesity and associated co-morbidities^1,2. Although caloric restriction has been proven to promote longevity in a variety of species³, the extent to which the 24-h biological clock is involved is less clear. This year, several studies added new insights into the multifaceted mechanisms by which calorie restriction might delay disease progression and ageing, revealing that circadian rhythms in behaviour and cellular metabolism contribute strongly to its anti-ageing effect.

A new study by Hodge et al.⁴ reveals that dietary restriction can prevent age-associated visual senescence, which controls lifespan in Drosophila (Fig. 1a). Using flies under a high-yeast (5%; ad libitum) diet or a low-yeast (0.5%; dietary restriction) diet, the authors analysed oscillatory gene expression in each feeding group, subtracting out oscillations that were potentially light-driven but not clock-driven. Surprisingly, over double the number of genes oscillated in expression in flies maintained on the dietary restriction paradigm. Though phototransduction gene expression was oscillatory in all flies, the rhythms were of greater amplitude in dietary restricted flies compared with those on an ad libitum diet. Because gene expression was initially obtained from the whole body, investigators studied the Drosophila clock gene (CLK) in photoreceptor cells specifically. Loss of CLK accelerated visual impairments and shortened lifespan compared with the wild-type, but overexpression of CLK protected from deficits in age-associated photoreceptor function in the context of ad libitum feeding. The protective mechanisms involve CLK-mediated suppression of genes involved in phototoxic cell stress, with CLK-activated genes responsive to dietary restriction also required for dietary restriction-responsive delays in visual senescence and lifespan extension. This study underscores the importance of dietary restriction on tissue-specific and cell-specific clock function, which can contribute to the anti-ageing effects of dietary restriction.

Fig. 1|. Dietary restriction can delay ageing in a circadian clock-dependent manner.

a, Tissue-specific clocks respond to dietary restriction to delay ageing in Drosophila⁴. b, Dietary restriction regulates rhythmic ketogenesis⁵, known to promote tissue-specific delays in disease progression. c, Calorie restriction preserves insulin sensitivity throughout ageing, and active-phase energy intake is necessary for maximal lifespan extension under calorie restriction⁷. βOHB, 3-β-hydroxybutyrate.

A second study⁵ addresses the effect of calorie restriction on rhythmic ketogenesis, a process known to produce protective metabolites for a variety of tissues throughout ageing (Fig. 1b)⁶. Ketone bodies provide fuel for the body under conditions of low levels of glucose and can have anti-epileptic and neuroprotective properties at physiological levels. This study used nocturnal rodents (mice) that were fed a fullcalorie diet or a calorie-restricted meal (70% of required calories) once per day, two hours after ‘lights off’. Importantly, studies using similar calorie restriction paradigms have demonstrated that mice eat their entire dietary allotment within an approximately two-hour window^7,8. Calorie restriction produced a pronounced circadian induction of the primary ketone body metabolite 3-β-hydroxybutyrate (βOHB). Controlling for potential masking effects of light in this study, rhythms of βOHB could be detected under ‘free running’ (constant dark) conditions.

To demonstrate the extent to which the observed rhythms in calorie restriction-induced ketogenesis were dependent on the circadian system, total body knockout mice of the cryptochrome 1 and 2 (Cry1 and Cry2) genes were used. Interestingly, although Cry1^−/− and Cry2^−/− mice responded to calorie restriction similarly to wild-type mice in terms of body weight loss and lowering of levels of glucose, circadian profiles of blood levels of βOHB were significantly altered, particularly in the rest phase, and were independent of any changes in feeding pattern. Thus, calorie restriction imparts a circadian clock-dependent regulation of ketogenesis.

Finally, using an innovative study design, Acosta-Rodriguez and colleagues interrogated ageing effects of caloric intake versus time of day of feeding⁷ (Fig. 1c). Remarkably, increases in lifespan in calorie-restricted mice were profound, ranging from 10–35% compared to the ad libitum-fed group. Feeding groups consisted of the following: first, ad libitum (non-calorie restricted); second, ad libitum calorie restriction, administered at active phase onset; third, ad libitum calorie restriction, administered at resting phase onset; fourth, calorie restriction, administered in equal increments during the active phase; fifth, calorie restriction, administered in equal increments during the rest phase; and finally, calorie restriction administered equally throughout the 24-h period, so as to eliminate fasting and abolish temporal feeding rhythms.

All calorie restriction groups showed increased longevity compared with ad libitum-fed mice; the calorie restricted group provided pellets equally across 24 h, showing the smallest lifespan extension (10.5%). Evenly distributed, active phase calorie restriction resulted in the greatest degree of lifespan extension, along with the calorie restricted group fed ad libitum at the beginning of the active phase, suggesting that even a twelve-hour rest phase fast was sufficient for maximum age lengthening of calorie-restricted groups compared in this study with calorie restriction. Importantly, lifespan extension differences in calorie restricted groups compared with the ad libitum feeding group occurred despite similar changes in body weight and adipose tissue mass (although all calorie-restricted mice showed reduced weight and adipose tissue mass compared with mice on an ad libitum diet.) Although circulating levels of insulin were similar in young mice on an ad libitum or calorie restricted diet, levels of glucose were lower in the calorie restriction groups, suggesting increased insulin sensitivity compared with mice on an ad libitum diet. In addition, although all aged calorie-restricted mice showed lower levels of insulin than ad libitum mice, aged calorie-restricted mice showed elevations in levels of glucose compared with their young counterparts, similar to ad libitum groups. Thus, calorie restriction protects against insulin resistance across lifespan. Interestingly, comparing age-associated changes in gene expression across all feeding conditions, hepatic circadian rhythms of aged calorie-restricted mice fed during the active phase were most similar to those in young mice, and the greatest difference in rhythmic gene expression between young and old mice was observed under ad libitum feeding.

Two benefits of calorie restriction in these studies include the protective effects of calorie restriction on insulin sensitivity and the inhibition of inflammation during ageing. Insulin sensitivity protects from obesity-associated cardiovascular disease, increasing lipid storage in adipose tissue and preventing its spill over into insulin-sensitive tissues, including the liver and muscle. Levels of insulin affect circadian rhythms, directly regulating transcription factors of the circadian clock using insulin receptor-mediated signalling pathways⁹. Rhythmic insulin release and insulin-like growth factor 1 receptor signalling also alter circadian timekeeping in vivo, in part by driving synthesis of the circadian PERIOD proteins¹⁰. Regarding the anti-inflammatory response, pan-neuronal loss of CLK in the Drosophila study resulted in elevated immune response and shortened lifespan compared with wild-type controls⁴. Similarly, Acosta-Rodriguez et al. reveal that in aged mice on an ad libitum diet, upregulated genes were significantly related to inflammation and the immune response, including factors involved in immunosenescence.

Collectively, these studies underscore the extensive interplay between dietary intake and circadian robustness and take us closer to understanding how our dietary behaviours might promote healthy ageing at the level of individual tissues by maintaining circadian robustness. Importantly, these studies reveal that calorie restriction-induced delay of age-associated diseases involving clock function are likely multi-faceted and tissue specific.

Key Advances.

• Dietary restriction results in a clock-dependent improvement in photoreceptor activity and lifespan in Drosophila⁴.

• Caloric restriction induces oscillation of the ketone body, 3-β-hydroxybutyrate, which is both neuroprotective and anti-inflammatory⁵.

• Time administration of calorie restriction to the active phase promotes insulin sensitivity during ageing and provides maximal lifespan extension, compared to ad libitum feeding or calorie restriction during the resting phase⁷.