Macronutrients and caloric intake in health and longevity

Introduction

Aging is one of the greatest societal challenges in the modern world. Lifestyle choices, improved technology and modern medicine have contributed to a rapidly growing aging population (Partridge 2014). While we live longer on average than our ancestors, increased lifespan is not without its drawbacks. The primary problem with living longer is that with increasing age comes a heightened risk of chronic diseases such as cancer, type II diabetes, stroke, dementia and cardiovascular disorders, leading to disability and related mortality (de Cabo and Le Couteur 2015; Fontana, et al. 2010; Piper, et al. 2011). Currently, the major focus of modern medicine is treating specific age-related diseases; but with a growing number of older people encumbered with multiple chronic conditions (Fontana, et al. 2014), this approach is problematic, e.g. leading to complications arising from multiple medications for different conditions (de Cabo and Le Couteur 2015). Rather than treat the symptoms of aging, a logical alternative approach would be to intervene in the aging process itself (Partridge 2014).

Interventions that slow the rate of aging and increase healthspan and lifespan have been of considerable interest over the past 80 years. While genetic and pharmaceutical interventions have been widely explored in laboratory models (Baur, et al. 2006; Kenyon, et al. 1993; Mitchell, et al. 2014), translating such approaches to humans is difficult (Fontana and Partridge 2015). Nutritional manipulations, however, have proven to be similarly robust across multiple animal models and humans, with profound impacts on reproduction, health and aging. The complex relationship between nutrition and age-related health is not fully understood, however. A growing body of evidence has pointed to dietary restriction as an important mediator of health and lifespan (Masoro 2000, 2003; Miller, et al. 2005; Piper et al. 2011). But what does dietary restriction actually mean? Throughout the literature, dietary restriction is often used interchangeably with caloric restriction (CR). Whereas dietary restriction can involve different feeding regimens such as intermittent fasting or alternate day feeding (Ingram and Roth 2015), CR refers more specifically to the reduction of total calorie intake by 20-50% without malnutrition (Masoro 2005; Weindruch, et al. 1986). Recent evidence, however, suggests that the balance of macronutrients, rather than total energy intake, plays a larger role in lifespan extension than previously attributed (Mair, et al. 2005; Solon-Biet, et al. 2014; Zimmerman, et al. 2003). Whether calories or specific nutrients affect aging is a critically important issue to resolve, with important implications for aging research (Simpson and Raubenheimer 2007). In this review, we discuss the current understanding and impact of both calories and macronutrients on health and lifespan based on studies in invertebrate and mammalian models, and highlight the use of nutritional geometry as a framework to help disentangle the complex relationship between diet and healthy aging.

Dietary restriction

There is widespread consensus in aging research that eating fewer calories results in a longer, healthier life. To date, CR has been the primary focus of most non-genetic nutritional interventions (Ingram, et al. 2004; Mattison, et al. 2003; Sinclair 2005). Yeasts, nematode worms, fruit flies, rodents and even non-human primates have been used as models for the study of CR and aging, suggesting that these effects must act via evolutionarily conserved mechanisms (Figure 1) (Lin, et al. 2000; Weindruch et al. 1986). Since the first account of the life-extending effects of CR in rats in the early 1930’s, there has been a substantial amount of research into the dietary basis of aging (McCay, et al. 1935). It was not until the early 1980s that the idea of CR as a viable model for aging and the study of age-related diseases really came to fruition (Masoro 1991; Masoro, et al. 1982; Walford, et al. 1992). Despite the great strides made towards understanding the mechanisms of CR, much still remains unknown. Initial work exploring CR as a robust nutritional intervention for aging began with yeasts, worms and flies. In yeast (Saccharomyces cerevisiae), CR is mediated by reduced glucose levels, extending both overall lifespan and replicative lifespan (Bonawitz, et al. 2007; Kaeberlein, et al. 2005; Powers, et al. 2006) and in the worm Caenorhabditis elegans and the fruit fly Drosophila melanogaster, some forms of food restriction via nutrient manipulation also successfully extended lifespan (Partridge, et al. 2005; Taormina and Mirisola 2014). Interestingly, the beneficial effect of CR does not appear to be universal. Although experiments in rodents produces a net beneficial effect to overall metabolic health in laboratory animals, (Bordone and Guarente 2005; Ingram et al. 2004; Masoro 2000) the effects on lifespan extension are highly dependent on various other factors such as strain and sex (Festing and Blackmore 1971; Liao, et al. 2010; Yuan, et al. 2009). 40% CR extends maximal lifespan in male B6D2F1 mice by 20% relative to ad libitum fed controls (Wolf, et al. 1995); however, whether this effect also extends to females remains to be seen. We do know that CR extends lifespan in genetically heterogeneous mice created from four inbred strains (BALB/c, C57BL/6, C3H, and DBA2), although more than 90% died of cancer which may not be representative of the human situation (Miller, et al. 2011). Translation into longer-lived mammals has continued to show conflicting results (Liao et al. 2010; Mattison, et al. 2012). Two long term studies in non-human primates were initiated in the early 1980s in order to address this question. Although both studies confirm the findings that CR delays the onset of age-associated diseases, CR monkeys from the National Institute on Aging (NIA) did not live longer than their ad libitum fed counterparts, which is in contrast to results obtained in the Wisconsin cohort (Colman, et al. 2009; Colman, et al. 2014; Mattison et al. 2012). These results were attributed to possible discrepancies in diet design and diet composition (Mattison et al. 2012). NIA monkeys were fed a diet rich in natural ingredients such as protein derived primarily from plant sources while the Wisconsin monkeys were fed a semi-purified diet with protein derived from lactalbumin (Ingram, et al. 1990; Ramsey, et al. 2000). Carbohydrate quality also differed between studies with the NIA diets containing significantly less sucrose than the Wisconsin study (Mattison et al. 2012).

Such studies highlight the question of whether CR per se is solely responsible for extended longevity or if particular macronutrients or a balance of macronutrients is more important (Table 1). While the effect of CR on human lifespan is yet to be determined, CR has been shown to improve several markers of health (Fontana et al. 2010; Heilbronn, et al. 2006). But despite these benefits, a central limitation is that compliance to lifetime CR is challenging in humans and the risk of missing essential nutrients can be detrimental to reproduction, bone structure and overall metabolic health (Fontana and Partridge 2015; Ingram and Roth 2015). Hence, dietary interventions involving ad libitum access to diets designed to prolong healthspan would be of greater utility than CR.

Calorie restriction or protein restriction?

Recent studies have suggested that the beneficial effects of CR on lifespan may be due to the reduced intake of specific dietary components such as proteins, rather than total energy intake (Mair et al. 2005; Pamplona and Barja 2006; Piper, et al. 2005; Zimmerman et al. 2003) with these effects acting largely through the same evolutionarily conserved signaling pathways (Figure 2). The restriction of protein intake, rather than energy, may offer a more feasible nutritional intervention in humans. Work by McCay as early as 1929 reported that a low protein diet extended the lifespan of trout (McCay, et al. 1929). Since then, it has been shown that the restriction of essential amino acids can increase lifespan in honeybees (Paoli, et al. 2014), and the restriction of particular amino acids, such as methionine, can extend lifespan in mice (Sun, et al. 2009) and rats (Orentreich, et al. 1993; Richie, et al. 1994), and lower the levels of serum IGF-I, insulin, glucose and thyroid hormone in (BALB/cJ × C57BL/6 J) F1 mice (Miller et al. 2005). Recently, the restriction of essential amino acids (Robertson, et al. 2015) and the sulfur amino acids methionine and cysteine (Robertson et al. 2015) have been shown to protect against hepatic ischemia reperfusion injury by preconditioning against oxidative stress, complications of cardiovascular surgery (Robertson et al. 2015) and mediating hydrogen sulfide production (H₂S) (Hine, et al. 2015). H₂S production under protein restriction exerts a hormetic response, acting on brain signaling and the vascular system to reduce blood pressure and trigger the same signaling response cascade observed in animals fed protein-restricted diets via activation of GCN2, eIF2α and ATF4 and repression of mTOR (Figure 2) (Hine et al. 2015; Robertson et al. 2015). Moreover, a meta-analysis of animal studies of caloric restriction and aging conclude that the restriction of protein, rather than caloric restriction, appeared to have the greatest effect on delaying aging (Nakagawa, et al. 2012). Data in humans indicate that reduced protein intake may become an important component of anticancer and anti-aging dietary interventions (Fontana, et al. 2008; Levine, et al. 2014).

Macronutrient balance

While both CR and protein restriction have been shown to impact aging, a fundamental limitation of these two one-variable-at-a-time approaches is that they cannot disentangle the interactive effects of nutrients and calories (Simpson, et al. 2015). Recent studies have begun to tackle these interactions and shown the importance of the balance of macronutrients on health and aging. Such evidence has been derived using the Geometric Framework for nutrition (GF; Simpson and Raubenheimer 2009, Simpson and Raubenheimer 2012). In the GF, nutrition is represented in an n-dimensional space, in which the components of n represent focal dietary components (e.g. macronutrients). Various phenotypic responses (e.g. lifespan) can be modelled onto this n-dimensional space, providing a detailed landscape of the effects of nutrition. Using this framework allows the use of nutritional geometry to simultaneously interpret the effects of energy, individual macronutrients (or other focal dietary components) and the interactions within and between nutrients (Piper et al. 2011; Simpson and Raubenheimer 2012). This framework has helped to resolve conflicting ideas about the nutritional determinants of health and aging, and to reconcile views on resource-mediated trade-offs between reproduction and longevity (Jensen, et al. 2015; Lee, et al. 2008; Solon-Biet, et al. 2015b; Tatar, et al. 2014).

Studies in both invertebrates and mice show that reproduction and longevity do not trade-off against one another; rather, these responses have different nutritional requirements. In the field cricket Teleogryllus commodus and fruit fly Drosophila melanogaster, the macronutrient blend that maximized lifespan was markedly different from diets which maximized reproductive variables (Jensen et al. 2015; Maklakov, et al. 2008). Maximal longevity occurred on low protein (P), high carbohydrate (C) diets in both males and females, while a higher P:C ratio was better for reproduction in females only. Consuming a low proportion of protein in the diet relative to carbohydrate, not total calories, extended lifespan in ad libitum-fed flies (Bruce, et al. 2013; Lee et al. 2008), while diets with a higher proportion of protein shortened lifespan but improved reproduction (Lee et al. 2008). This result has been replicated in several other insect species (Dussutour and Simpson 2009; Fanson, et al. 2009; Grandison, et al. 2009; Lee et al. 2008; Piper et al. 2011) and consistently indicates that the balance of macronutrients is the chief nutritional cue that directs metabolism towards longevity or reproduction (Wilder, et al. 2012). A recent study in mice showed that ad libitum low protein, high carbohydrate diets fed short-term improved several markers of health including insulin, HOMA, glucose tolerance and triglycerides to a level comparable to CR, but without at 40% reduction in total calorie intake (Solon-Biet, et al. 2015a). Long-term investigations in ad libitum-fed mice across 25 different diets varying in macronutrient composition support these findings, showing that latelife health and longevity were optimized not by reducing energy intake, but by low P:C diets (Solon-Biet et al. 2014). In an attempt to stabilize protein intake, mice displayed a compensatory increase in food intake on low protein diets, resulting in increased energy intake and greater adiposity, but experienced a significant increase in lifespan, improved blood pressure, lipid profiles, mitochondrial function, insulin sensitivity (Solon-Biet et al. 2014) and immune function (T and B cell populations) measured at 15 months (Le Couteur, et al. 2014). These health and longevity consequences were shown to be related to circulating branched chain amino acid (BCAA) levels, which, interestingly, were the only amino acids to be positively correlated to protein intake under chronic feeding conditions. BCAA levels were the lowest in mice on the low protein, high carbohydrate diets correlating to diet treatments that yielded the longest health and lifespan.

Reports about the role of BCAAs in aging and health are seemingly divergent. Some suggest that elevated BCCAs are harmful because they are linked with obesity and diabetes, while others suggest that BCAAs should be supplemented to increase mitochondrial biogenesis (D’Antona, et al. 2010). For example, in a major review, Newgard (2012) noted that human epidemiological studies and animal studies show that elevated BCAAs are associated with and predict diabetes, obesity and heart disease, while animal and cell studies show that BCAA supplementation increases activation of certain nutrient signaling pathways which are detrimental for aging (Chotechuang, et al. 2009). In another review, Valerio, et al. (2011) argue that BCAAs increase mitochondrial biogenesis and muscle function, thus BCAA supplementation should be considered as a treatment for older people. While the exact roles of BCAAs in health and lifespan are yet to be determined, evidence suggest that BCAAs may be an important mediator of key molecular pathways that link nutrition with aging.

Nutrient-sensing pathways

Nutrient-sensing pathways that mediate the effects of nutrition on health and aging have been explored in many experimental models (Chantranupong, et al. 2015; Fontana et al. 2010; Hubbard and Sinclair 2014). These include the evolutionarily conserved key regulators mTOR, AMPK, insulin/IGF-1 and sirtuins. Both calories and macronutrients influence these pathways which have evolved to respond to periods of famine by switching cells and organism from their focus on growth and reproduction, towards survival and resilience (Figure 3) (Kapahi, et al. 2010; Le Couteur, et al. 2012; Speakman and Mitchell 2011). Although there are at least four key nutrient-sensing pathways implicated in longevity, these interact and share many downstream targets that regulate cell processes involved in aging, including mitochondrial biogenesis, cellular metabolism, autophagy, DNA repair and expression, and translation.

Outlook

This review has focused primarily on the relationships between calories and macronutrients and their effects on health and aging. Although both CR and macronutrient balance have profound impacts on health and lifespan, it is important to note that other dietary regimens such as intermittent fasting and time-restricted feeding, also have beneficial effects in both mice and humans (Fontana and Partridge 2015; Mattson, et al. 2014). The fact that results of Solon-Biet et al (2014) show that limiting energy intake by dilution under ad libitum conditions has no benefit, yet under CR protocols it does, must suggest that it is not just the restriction per se that matters, but also the timing of intake (Simpson et al. 2015). Exactly how the complex network of nutrient signaling pathways interact to mediate the effects of various feeding regimens remains to be investigated. Although considerable research has gone into understanding these underlying mechanisms, none have yet studied it as a function of multiple nutrient dimensions. As highlighted in a recent review (Simpson et al. 2015), different nutritional interventions will have different effects on these pathways and understanding exactly how multiple nutrient dimensions affect these pathways can only be done using a framework that integrates these components simultaneously. The Geometric Framework is such a tool. Exactly how calories and macronutrients, and the interplay of both, influence these pathways is a fundamental question to resolve. A better understanding can have important implications for diet management, disease prevention and pharmaceutical interventions.