Summary The fundamental questions of what represents a macronutritionally balanced diet and how this maintains health and longevity remain unanswered. Here, the Geometric Framework, a state-space nutritional modeling method, was used to measure interactive effects of dietary energy, protein, fat, and carbohydrate on food intake, cardiometabolic phenotype, and longevity in mice fed one of 25 diets ad libitum. Food intake was regulated primarily by protein and carbohydrate content. Longevity and health were optimized when protein was replaced with carbohydrate to limit compensatory feeding for protein and suppress protein intake. These consequences are associated with hepatic mammalian target of rapamycin (mTOR) activation and mitochondrial function and, in turn, related to circulating branched-chain amino acids and glucose. Calorie restriction achieved by high-protein diets or dietary dilution had no beneficial effects on lifespan. The results suggest that longevity can be extended in ad libitum-fed animals by manipulating the ratio of macronutrients to inhibit mTOR activation.

Elevated branched-chain amino acids (BCAAs) are associated with obesity and insulin resistance. How long-term dietary BCAAs impact late-life health and lifespan is unknown. Here, we show that when dietary BCAAs are varied against a fixed, isocaloric macronutrient background, long-term exposure to high BCAA diets leads to hyperphagia, obesity and reduced lifespan. These effects are not due to elevated BCAA per se or hepatic mammalian target of rapamycin activation, but instead are due to a shift in the relative quantity of dietary BCAAs and other amino acids, notably tryptophan and threonine. Increasing the ratio of BCAAs to these amino acids results in hyperphagia and is associated with central serotonin depletion. Preventing hyperphagia by calorie restriction or pair-feeding averts the health costs of a high-BCAA diet. Our data highlight a role for amino acid quality in energy balance and show that health costs of chronic high BCAA intakes need not be due to intrinsic toxicity but instead are a consequence of hyperphagia driven by amino acid imbalance.

In invertebrates, reproductive output and lifespan are profoundly impacted by dietary macronutrient balance, with these traits achieving their maxima on different diet compositions, giving the appearance of a resource-based tradeoff between reproduction and longevity. For the first time in a mammal, to our knowledge, we evaluate the effects of dietary protein (P), carbohydrate (C), fat (F), and energy (E) on lifespan and reproductive function in aging male and female mice. We show that, as in invertebrates, the balance of macronutrients has marked and largely opposing effects on reproductive and longevity outcomes. Mice were provided ad libitum access to one of 25 diets differing in P, C, F, and E content, with reproductive outcomes assessed at 15 months. An optimal balance of macronutrients exists for reproductive function, which, for most measures, differs from the diets that optimize lifespan, and this response differs with sex. Maximal longevity was achieved on diets containing a P:C ratio of 1:13 in males and 1:11 for females. Diets that optimized testes mass and epididymal sperm counts (indicators of gamete production) contained a higher P:C ratio (1:1) than those that maximized lifespan. In females, uterine mass (an indicator of estrogenic activity) was also greatest on high P:C diets (1:1) whereas ovarian follicle number was greatest on P:C 3:1 associated with high-F intakes. By contrast, estrous cycling was more likely in mice on lower P:C (1:8), and the number of corpora lutea, indicative of recent ovulations, was greatest on P:C similar to those supporting greatest longevity (1:11).aging | macronutrients | lifespan | reproduction | nutrition N utrition profoundly influences reproduction and lifespan. Traditionally, dietary restriction has been the central focus of most research, with numerous studies showing that caloric restriction can improve age-related health and prolong lifespan across a wide range of taxa ranging from yeasts to humans (1-6). The extension of lifespan under caloric restriction has been explained by resource-mediated tradeoffs between reproduction (age of sexual maturity, number of offspring, and parental investment) and longevity (senescence and lifespan), with these outcomes titrated for maximum overall reproductive success under given levels of resource availability (7,8). However, the view that there is a simple constitutive tradeoff between reproduction and longevity has been challenged by accumulating experimental evidence from invertebrate models, beginning with experiments in Caenorhabditis elegans, demonstrating that lifespan in long-lived mutants was not affected by the ablation of germ cell and somatic gonad precursors, indicating that neither germ cells nor progeny production was directly responsible for increased longevity in the worm (9). Later, it was found that manipulating the insulin/insulin-like growth factor-1 (IGF1) pathway in adult worms can extend lifespan without the loss of fecundity (10). In beetles, the increased risk of death associated with high egg pr...

Highlights d Mice adaptively alter metabolism and food choice during protein restriction d The liver hormone FGF21 is robustly increased by protein restriction d Metabolic responses to protein restriction require FGF21 signaling in the brain d Brain FGF21 also mediates adaptive changes in macronutrient selection

Diet influences health and patterns of disease in populations. How different diets do this and why outcomes of diets vary between individuals are complex and involve interaction with the gut microbiome. A major challenge for predicting health outcomes of the host-microbiome dynamic is reconciling the effects of different aspects of diet (food composition or intake rate) on the system. Here we show that microbial community assembly is fundamentally shaped by a dichotomy in bacterial strategies to access nitrogen in the gut environment. Consequently, the pattern of dietary protein intake constrains the host-microbiome dynamic in ways that are common to a very broad range of diet manipulation strategies. These insights offer a mechanism for the impact of high protein intake on metabolic health and form the basis for a general theory of the impact of different diet strategies on host-microbiome outcomes.

Fibroblast growth factor 21 (FGF21) is the first known endocrine signal activated by protein restriction. Although FGF21 is robustly elevated in low-protein environments, increased FGF21 is also seen in various other contexts such as fasting, overfeeding, ketogenic diets, and high-carbohydrate diets, leaving its nutritional context and physiological role unresolved and controversial. Here, we use the Geometric Framework, a nutritional modeling platform, to help reconcile these apparently conflicting findings in mice confined to one of 25 diets that varied in protein, carbohydrate, and fat content. We show that FGF21 was elevated under low protein intakes and maximally when low protein was coupled with high carbohydrate intakes. Our results explain how elevation of FGF21 occurs both under starvation and hyperphagia, and show that the metabolic outcomes associated with elevated FGF21 depend on the nutritional context, differing according to whether the animal is in a state of under- or overfeeding.

Summary Both caloric restriction (CR) and low protein, high carbohydrate (LPHC) ad libitum-fed diets increase lifespan and improve metabolic parameters such as insulin, glucose and blood lipids. Severe CR, however, is unsustainable for most people; therefore, it is important to determine whether manipulating macronutrient ratios in ad libitum-fed conditions can generate similar health outcomes. We present the results of a short-term (8 week) dietary manipulation on metabolic outcomes in mice. We compared three diets varying in protein to carbohydrate ratio under both CR and ad libitum conditions. Ad libitum LPHC diets delivered similar benefits to CR in terms of levels of insulin, glucose, lipids and HOMA, despite increased energy intake. CR on LPHC diets did not provide additional benefits relative to ad libitum LPHC. We show that LPHC diets under ad libitum-fed conditions generate the metabolic benefits of CR without a 40% reduction in total caloric intake.

Most research on nutritional effects on aging has focussed on the impact of manipulating single dietary factors such as total calorie intake or each of the macronutrients individually. More recent studies using a nutritional geometric approach called the Geometric Framework have facilitated an understanding of how aging is influenced across a landscape of diets that vary orthogonally in macronutrient and total energy content. Such studies have been performed using ad libitum feeding regimes, thus taking into account compensatory feeding responses that are inevitable in a non-constrained environment. Geometric Framework studies on insects and mice have revealed that diets low in protein and high in carbohydrates generate longest lifespans in ad libitum-fed animals while low total energy intake (caloric restriction by dietary dilution) has minimal effect. These conclusions are supported indirectly by observational studies in humans and a heterogeneous group of other types of interventional studies in insects and rodents. Due to compensatory feeding for protein dilution, low-protein, high-carbohydrate diets are often associated with increased food intake and body fat, a phenomenon called protein leverage. This could potentially be mitigated by supplementing these diets with interventions that influence body weight through physical activity and ambient temperature.