You ‘heart’ what you eat!

This editorial refers to ‘Dietary carbohydrates restriction inhibits the development of cardiac hypertrophy and heart failure’ by M. Nakamura et al., pp. 2365–2376. The common say ‘You are what you eat’ amply echoed in the media highlights the insatiable appetite of the public for special diets to improve health, including cardiovascular health. It is generally accepted that diets rich in carbohydrates (e.g. in sodas) increase mortality1; conversely, adopting diets with low carbohydrates has a mixed effect altogether, raising question about the importance of the percentage composition of other nutrients on organ function. For example, low carbohydrates combined with high-fat increase ketogenesis in the liver,2 and ketone bodies (beta-hydroxybutyrate and aceto-acetate, principally) are likely to mediate indirect effects of such diet on the heart.3 The healthy heart is indeed characterized by a metabolic flexibility where a multitude of ATP-generating substrates can be used according to their availability and the hormonal status. Beside fatty acids and carbohydrates (glucose, pyruvate and lactate) which remain predominant, ketone bodies can be efficiently catabolized by the heart, especially under prolonged fasting. The failing heart can also use ketone bodies as an alternative fuel that helps maintain its function. Could a similar metabolic flexibility be used to influence cardiac remodelling in the face of hemodynamic stress? In their study,4 Nakamura and colleagues tested the influence of the differential composition of proteins (Pro-LC) vs. fat (Fat-LC) complementing a low carbohydrate (LC) diet on myocardial hypertrophy in a classical mouse model of transaortic constriction (TAC). A comparative group of mice were fed with normal carbohydrate (NC) content; all diets were isocaloric and mice fed ad libitum for 4 weeks. Fat-LC and NC mice gained weight, contrary to Pro-LC mice that apparently ate less, resulting in lower caloric intake. None of these diets influenced the cardiac phenotype under unstressed conditions after 4 weeks. After TAC however, Pro-LC- and Fat-LC-fed mice exhibited lower left ventricular hypertrophy (and cardiac myocyte size) compared with NC controls. This was paralleled with reduced left ventricular end-diastolic pressure (LVEDP; indicative of improved LV function) in the Fat-LC group only. Looking at potential hypertrophic signalling elements affected by these diets, the authors found that, compared with the NC group, Pro-LC and Fat-LC resulted in preserved (de-phosphorylated) glycogen synthase kinase 3 beta (GSK3beta) activity, expected to reduce hypertrophy, in myocardial extracts after TAC. Further supporting causality, the protective effect of Pro-LC (but not Fat-LC) diet was lost in mice with cardiac-specific deletion of GSK3beta. What, then, could explain the persisting protective effect of Fat-LC diet? Bearing in mind the ketogenic effect of high fat when combined with low carbohydrates, the authors confirmed that the Fat-LC (but not Pro-LC) diet increased beta-hydroxybutyrate levels in serum and heart tissue under basal conditions, but also restored these levels to normal after TAC. As beta-hydroxybutyrate is a precursor to acetyl-coA, a donor of acetyl groups, they looked at global protein acetylation in cardiac extracts and found that both Fat-NC and Pro-LC diets increased acetyl-Lysine levels in sham-operated mice; TAC also increased acetyl-Lysine levels per se, and this was enhanced by Fat-LC diet, but decreased by Pro-LC diet. In a more reductionist model of neonatal rat cardiac myocytes, ketone bodies both increased protein acetylation and (acetoacetate, more prominently) reduced myocyte hypertrophy in response to phenylephrine. Surprisingly, acetoacetate decreased GSK3beta activity; however, both ketone bodies suppressed another signalling pathway downstream the alpha1-adrenergic stimulus, i.e. mTOR. In support of causality again, the protective effect of ketones was reversed (albeit only in part) upon constitutive activation of mTOR with adenovirally transduced Rheb in cultured cardiac myocytes. These observations raise a number of mechanistic questions. Clearly, they point to effects of Pro-LC and Fat-LC on different signalling elements mediating hypertrophy (Figure 1). While both diets preserved cardiac GSK3beta activity, this seems critical for the Pro-LC diet only; ketone bodies formation likely mediates the additional effects of the Fat-LC diet (although not on GSK3beta activity, which was decreased by acetoacetate). Ketones are known to support the function of the failing heart,3 as mentioned above. In the present study, they activated AMP-activated protein kinase in cardiac myocytes, a known inhibitor of mTOR and myocardial hypertrophy5,6; ketones also up-regulate the endothelial isoform of nitric oxide synthase (eNOS), which could mediate mTORC1/mTOR inhibition through the cGMP/PKG1 phosphorylation of its inhibitor TSC2,7 although this was not tested here. Ketones’ effect on protein acetylation also raise the possibility of epigenetic effects through histone acetylation; again, this was not examined here, and many other (un)identified protein targets may be involved and could affect hypertrophy in a positive or negative manner. In line with this, ketone-mediated acetylation inhibits cardiac glucose uptake8 and its further use to generate ATP by glycolysis and/or oxidation. Inasmuch as a metabolic shift towards glucose use appears during (and may participate to) the development of heart failure, such inhibition of glucose metabolism by ketones but also more generally, by fatty acids could partly explain the protective action of Fat-LC diet. Also, the alternative glucose metabolism leading to protein O-GlcNAcylation increases during hypertrophy development; blocking this enhanced O-GlcNAcylation prevents hypertrophy development,6 so that examining the effect of Fat-LC diet on myocardial glucose metabolism and O-GlcNAcylation would clearly deserve further studies. Distinct direct and systemic effects of fat- or protein-enriched low carbohydrate diets on myocardial hypertrophy. Low carbohydrate diets complemented with high proteins inhibit cardiac myocyte hypertrophy by sustaining GSK3β activity, in addition to their effect to reduce calorie intake which, by itself, negatively regulates hypertrophy through still uncharacterized mechanisms. Complementing low carbohydrate diets with fat also negatively modulates hypertrophy through both direct and systemic effects on the heart, distinct from protein enrichment. Such fat enrichment may directly up-regulate cardiac intracellular acetylation and, among others, inhibit glucose uptake, thereby decreasing protein O-GlcNacylation commonly associated with the development of hypertrophy. Alternatively, high fat (combined with low carbohydrates) increases ketogenesis in the liver with subsequent delivery of plasma ketone bodies to the heart; ketone metabolism in cardiac myocytes, in turn, also increases acetylation; ketones also inhibit the NLRP3 inflammasome, which may indirectly attenuate myocardial remodelling; ketones also inhibit the pro-hypertrophic mTOR pathway, possibly through increased expression of the endothelial isoform of nitric oxide synthase (eNOS; expressed in endothelium and-not illustrated here-in cardiac myocytes) and activation of NO/cyclic-GMP/Protein Kinase G signalling (see text for details). Distinct direct and systemic effects of fat- or protein-enriched low carbohydrate diets on myocardial hypertrophy. Low carbohydrate diets complemented with high proteins inhibit cardiac myocyte hypertrophy by sustaining GSK3β activity, in addition to their effect to reduce calorie intake which, by itself, negatively regulates hypertrophy through still uncharacterized mechanisms. Complementing low carbohydrate diets with fat also negatively modulates hypertrophy through both direct and systemic effects on the heart, distinct from protein enrichment. Such fat enrichment may directly up-regulate cardiac intracellular acetylation and, among others, inhibit glucose uptake, thereby decreasing protein O-GlcNacylation commonly associated with the development of hypertrophy. Alternatively, high fat (combined with low carbohydrates) increases ketogenesis in the liver with subsequent delivery of plasma ketone bodies to the heart; ketone metabolism in cardiac myocytes, in turn, also increases acetylation; ketones also inhibit the NLRP3 inflammasome, which may indirectly attenuate myocardial remodelling; ketones also inhibit the pro-hypertrophic mTOR pathway, possibly through increased expression of the endothelial isoform of nitric oxide synthase (eNOS; expressed in endothelium and-not illustrated here-in cardiac myocytes) and activation of NO/cyclic-GMP/Protein Kinase G signalling (see text for details). Several caveats should also be considered. The amount of ketones fuelled to the heart probably needs to be fine-tuned for the appropriate beneficial effect. Like other tissues, the heart consumes ketones in proportion to its capacity for ketolysis supported by two key enzymes, 3-Hydroxybutyrate Dehydrogenase 1, coded by BDH1 and succinyl-CoA : 3-oxoacid CoA transferase or SCOT; coded by Oxct1. These enzymes were up-regulated by TAC in mice under NC, control diet; but BDH1 and SCOT proteins were down-regulated by Fat-LC, that on the other hand increased beta-hydroxybutyrate in serum and heart extracts. This is probably the result of a negative feedback on ketolytic enzyme expression (e.g. on SCOT), as high ketones are known to curtail ketolytic capacity.9 What, then, if ketones are not consumed as fuel for heart function? Do they exert effects on signalling independently of their metabolic effects? This remains an open question. It should also be noted that, when metabolized, ketones also inhibit fatty acid oxidation, which in some circumstances may adversely affect cardiac function. Among other potential effects, beta-hydroxybutyrate (but not acetoacetate) can inhibit the nucleotide-binding, leucine-rich family pyrin domain containing 3; or NLRP3 inflammasome10; this is unlikely to have affected the phenotype in the experiments reported here, because diets were initiated only 2 days after TAC, at a time when most of the acute inflammatory phenomena are expected to subside. This may also explain the lack of effect of any of the two diets on cardiac fibrosis, known to be initiated, in part, by inflammation. What should we eat, then, to protect our hearts? Fat? Ketones would only be produced if fat enrichment is combined with low carbohydrates; a high-fat diet will not be ketogenic per se. Then, what kind of fat? In the present study, mice were fed with saturated fats (from lard), which are notoriously bad for the cardiovascular system; nevertheless, as the authors pointed out, the apparent benefit from ketone bodies generated from the Fat-LC diet override the adverse effects of saturated fat, as well as of high-calorie intake known to be detrimental in mice submitted to TAC. Could the benefit have been even higher with polyunsaturated fat? This would be worth testing. Of interest, a constant and well-balanced increase in plasma ketones using an ingestible ketone ester diet seems equally promising.11 Low intake of calories, instead, would protect the heart. This in fact, more than protein enrichment, may have participated to the protection by the Pro-LC diet, less taken in by the mice and that made them loose weight. In the end, reducing carbohydrates is probably the key; compensating with fat may add the benefit of ketogenesis. We should remember, though, that all this was done in mice, with effects studied on a phenotype restricted to TAC-induced remodelling. Diet composition may greatly affect the intestinal microbiome, notoriously different across species and between disease states, with systemic release of metabolites and autacoids that are beginning to be identified12 and may directly or indirectly affect cardiac metabolism and function. Gender effects also add another level of complexity that was not examined here. Nevertheless, the present study by Nakamura et al. nicely illustrates the capacity of food nutrients to influence the cardiac phenotype. Their demonstration should be kept in mind for the interpretation of any future interventional study on mouse models, but also opens the door to explore tantalizing hypotheses on the effect of diet on cardiovascular health. Conflict of interest: none declared. L.B. and J.-L. B. are both Principal Investigators of the WELBIO Institute that supports their work. The opinions expressed in this article are not necessarily those of the Editors of Cardiovascular Research or of the European Society of Cardiology.