Myocardial steatosis across the spectrum of human health and disease

Abstract Preclinical data strongly suggest that myocardial steatosis leads to adverse cardiac remodelling and left ventricular dysfunction. Using 1H cardiac magnetic resonance spectroscopy, similar observations have been made across the spectrum of health and disease. The purpose of this brief review is to summarize these recent observations. We provide a brief overview of the determinants of myocardial triglyceride accumulation, summarize the current evidence that myocardial steatosis contributes to cardiac dysfunction, and identify opportunities for further research.

Moreover, myocardial triglyceride content is relatively stable within the small variations in substrate availability/utilization that may occur with day-to-day changes in dietary or exercise habits (Aengevaeren et al., 2020;Bucher et al., 2014;Ith et al., 2014;Smajis et al., 2020;van der Meer, Hammer et al., 2008).However, it is possible to shift the balance between lipid oxidation and storage, such that myocardial triglyceride content can change, even in healthy adults.For example, reducing circulating free fatty acids with acipimox (an anti-lipolytic agent) reduces myocardial triglyceride content (Figure 1b) (Lehto et al., 2012;Winhofer et al., 2015).In contrast, multiple lines of evidence highlight acute increases in myocardial triglyceride content in the healthy myocardium when substrate availability exceeds utilization (Figure 1c,d), such as during periods of severe caloric restriction and fasting (Hammer, van der Meer, Lamb, Schar et al., 2008;Oneglia et al., 2023;van der Meer et al., 2007), following aerobic exercise in a fasted state (Bilet et al., 2011), and during hyperinsulinaemiahyperglycaemia (Winhofer et al., 2012).Among these latter examples, substrate availability exceeds utilization in clear and close relation to nutritional state.Indeed, low nutrient intake depletes glycogen stores so that the body must instead rely on free fatty acids and ketones for energy, both of which increase their circulating concentrations via lipolysis of adipose tissue.The effect on myocardial triglyceride content can be seen in as little as 4 h post-exercise and 48-h of fasting (Bilet et al., 2011;Oneglia et al., 2023).

AGE AND SEX
Several studies suggest that myocardial triglyceride content increases with age (Petritsch et al., 2015;Sarma et al., 2013;van der Meer, Rijzewijk et al., 2008); however, the total sample size available to support this conclusion is relatively small, with very few reports measuring myocardial triglyceride content in individuals >65 years of age.Sex does not appear to influence myocardial triglyceride content in young, normal weight, healthy adults (Oneglia et al., 2023;Petritsch et al., 2016;Winhofer et al., 2012); however, the interaction between sex and metabolic dysfunction/disease has not been thoroughly investigated.Where available, sex as a biological variable is described in each of the following sections.

OBESITY AND DIABETES
Mounting evidence support a stepwise increase in myocardial triglyceride content with obesity, impaired glucose tolerance, and type

Highlights
• What is the topic of this review?
The role of myocardial steatosis on cardiac morphology and function.
• What advances does it highlight?
Myocardial steatosis contributes to adverse cardiac remodelling and left ventricular dysfunction.
However, obesity is rarely defined by a simple increase in circulating free fatty acids, but rather also involves varying degrees of insulin resistance.Therefore, abnormal substrate utilization, in addition to an obesity-related increase in substrate availability, augments myocardial triglyceride content among those at risk of developing T2D and those with diagnosed T2D (Figure 2b) (Gao et al., 2020;Graner et al., 2013;Levelt, Mahmod et al., 2016, Levelt, Pavlides et al., 2016;Muniyappa et al., 2015;Nyman et al., 2013;Rijzewijk et al., 2008).
While myocardial triglyceride content may be associated with ectopic fat deposition elsewhere in the body, including visceral adiposity (Gaborit, Kober et al., 2012;Graner et al., 2013;Iozzo et al., 2009;Liu et al., 2014;Rayner et al., 2018), pericardial fat content (Gaborit, Kober et al., 2012;Graner et al., 2013;Iozzo et al., 2009) and hepatic steatosis (Graner et al., 2013;Liu et al., 2014), these associations are not perfect, emphasizing the importance of measuring myocardial triglyceride content directly.Results from interventional trials further demonstrate this point, as summarized in Table 1.This is perhaps best exemplified following bariatric surgery, where myocardial triglyceride content remains elevated 6 months after surgery, despite a ∼20% reduction in body mass, and marked reductions in pericardial, visceral and subcutaneous fat (Gaborit, Jacquier et al., 2012;Hannukainen et al., 2018;van Schinkel et al., 2014).We interpret this dissociative pattern to reflect a shift in myocardial substrate utilization, whereby improvements in myocardial glucose utilization, without changes in myocardial fatty acid utilization, result in a preservation of myocardial triglyceride content.Myocardial steatosis decreases only when fatty acid utilization exceeds fatty acid supply, so that the heart may instead rely on stored triglycerides.This could also explain why the majority of the T2D literature finds no direct relationship between myocardial steatosis and whole body insulin sensitivity/resistance (Graner et al., 2013;Iozzo et al., 2009;Kosi-Trebotic et al., 2017;Krssak et al., 2011;Muniyappa et al., 2015;Winhofer, Krssak et al., 2014), and a number of glucose-lowering drug

Exercise intervention
In a relatively small sample, Iozzo et al. (2009) first reported that myocardial triglyceride content was lower in obese women than men but was comparable between sexes with impaired glucose tolerance or T2D, suggesting that women may be protected from obesityrelated increases in myocardial triglyceride content.Studies conducted since, however, suggest otherwise (Dong et al., 2023;Gaborit, Kober et al., 2012).Indeed, myocardial triglyceride content is still elevated in overweight and obese women compared to normal weight and over-weight men (Liu et al., 2014).Furthermore, sex does not influence the independent relationship between obesity and myocardial steatosis among adults across a range of obesity (Banerjee et al., 2015).As such, sex does not appear to influence myocardial triglyceride content in obesity.

HEART DISEASE
Myocardial triglyceride content varies across different forms of heart disease (Graner et al., 2014;Nakae et al., 2010;Sai et al., 2017).Myocardial ischaemia appears to promote the accumulation of triglycerides, likely related to a metabolic switch away from free fatty acid oxidation.Indeed, myocardial triglyceride content is elevated in patients with ischaemic coronary artery disease (CAD), but not patients with non-ischaemic CAD or non-CAD controls (Hannukainen et al., 2016).Moreover, myocardial steatosis was elevated in women with ischaemia but non-obstructed coronary arteries, and inversely related to LV diastolic function (Wei et al., 2016).Notably, in this latter cohort, triglyceride content was not elevated elsewhere in the body (Hannukainen et al., 2016;Wei et al., 2016), arguing against systemic metabolic disease and in support of the ischaemia hypothesis.Myocardial triglyceride content was also elevated in heart failure with preserved ejection fraction (Mahmod et al., 2018;Wu et al., 2020), but not heart failure with reduced ejection fraction (Wu et al., 2020) or end-stage heart failure (Chokshi et al., 2012).However, these latter two cohorts were primarily non-obese and non-diabetic, which may have influenced the results (Sharma et al., 2004).
Interventional studies aimed at regressing myocardial steatosis in heart disease are limited.In one investigation, 1 year of high-intensity exercise training, without dietary intervention, promoted positive cardiac and vascular remodelling in adults at risk for heart failure, but did not affect myocardial steatosis (Hearon et al., 2022).
Other investigations targeted myocardial lipid composition in heart failure, rather than total content per se, but research in this area is extremely limited (Chang et al., 2020;Liao et al., 2016).Hata et al.

HUMAN IMMUNODEFICIENCY VIRUS
The influence of human immunodeficiency virus (HIV) and associated highly active anti-retroviral therapy has received considerable attention with respect to myocardial steatosis and its influence of future cardiovascular disease risk.Indeed, myocardial triglyceride content is often elevated among individuals with HIV, and associated with worsening LV systolic and diastolic function (Holloway et al., 2013;Nelson et al., 2014;Shitole et al., 2023;Thiara et al., 2015;Toribio et al., 2019).Several mechanisms may contribute to an increase TA B L E 2 Myocardial triglyceride content in special populations.et al., 2017;Neilan et al., 2020;Toribio et al., 2019), body mass index (Lai et al., 2017;Neilan et al., 2020), ectopic body fat deposition (Chew et al., 2017;Diaz-Zamudio et al., 2015;Howard et al., 2016;Thiara et al., 2015) and even duration of anti-retroviral therapy (Lai et al., 2017;Nelson et al., 2014).Sex does not appear to influence this association, with myocardial triglyceride content elevated in both men and women with HIV.Studies aimed at regressing myocardial triglyceride content in HIV are actively underway (NCT02344290) (Grinspoon et al., 2019).

SPECIAL POPULATIONS
Investigations into other clinical populations not yet described are summarized in Table 2, ranging from valvular heart disease to metabolic disease (e.g., non-alcoholic fatty liver disease, generalized lipodystrophy, Cushing's syndrome).Where correlation analyses were performed, the accumulation of myocardial steatosis is often associated with adverse ventricular remodelling and/or cardiac dysfunction, except in non-alcoholic fatty liver disease.

CONCLUSION
Myocardial triglyceride content is determined by the (im)balance between free fatty acid substrate availability and utilization.Myocardial steatosis increases with obesity and insulin-resistance, is elevated in multiple disease states, and may be an important source of cardiac dysfunction and/or adverse remodelling.More work is still needed, however, to address remaining gaps in the literature, including the influence of age and sex, and the independent role myocardial steatosis plays in the development and progression of heart disease.

AUTHOR CONTRIBUTIONS
Conception or design of the work: Andrew P. Oneglia, Lidia S.

F I G U R E 1
Schematic representation of a cardiomyocyte under normal conditions (a), during systemic treatment with acipimox (b, a lipid lowering agent), during 48-h fasting (c), and in response to a whole-body hyperglycaemic-hyperinsulinaemic clamp (d).FA, free fatty acids; G-6-P, glucose-6-phosphate; GLUT, glucose transporter; TG, triglyceride.
2022 are the only group to investigate the longitudinal effects of myocardial steatosis on LV diastolic function.Using computed tomography, patients with no or mild coronary artery stenosis were grouped by the presence or absence of myocardial fat deposition at baseline.Echocardiography was then performed at baseline, 1-2 years and 2-3 years after the baseline assessment.When compared to patients without excess myocardial fat deposition, those with excess myocardial fat deposition had worse LV diastolic function at baseline and follow-up.Moreover, the decline in LV diastolic function with ageing appeared accelerated in patients with excess myocardial fat deposition.While these data support the negative effect of myocardial steatosis on LV function, a direct cause-and-effect relationship between the two is limited by the study's retrospective design.Therefore, future prospective, longitudinal studies are needed to confirm these results.

Szczepaniak,
Vlad G. Zaha, Michael D. Nelson .Drafting the work or revising it critically for important intellectual content: Andrew P. Oneglia, Lidia S. Szczepaniak, Vlad G. Zaha, Michael D. Nelson.All authors have read and approved the final version of this manuscript and agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.All persons designated as et al. (2013) 39 aortic stenosis patients vs. 20 matched controls • mTG ↑ in severe aortic stenosis • mTG ↓ in patients after aortic valve replacement to concentrations similar to controls BMI, body mass index; FA, fatty acid; FFA, free fatty acid; mTG, myocardial triglyceride content; PAH, pulmonary arterial hypertension; SAT, subcutaneous adipose tissue; TG, triglyceride; VAT, visceral adipose tissue. in myocardial triglyceride content with HIV, including age(Chew