The effect of acute and chronic exercise on hepatic lipid composition

Greater hepatic lipid saturation is associated with impaired glycaemic regulation in men with metabolic dysfunction-associated steatotic liver disease but is not altered by 6 weeks of exercise training

AIMS

To examine the impact of impaired glycaemic regulation (IGR) and exercise training on hepatic lipid composition in men with metabolic dysfunction-associated steatotic liver disease (MASLD).

MATERIALS AND METHODS

In Part A (cross-sectional design), 40 men with MASLD (liver proton density fat fraction [PDFF] ≥5.56%) were recruited to one of two groups: (1) normal glycaemic regulation (NGR) group (glycated haemoglobin [HbA1c] < 42 mmol∙mol-1 [<6.0%]; n = 14) or (2) IGR group (HbA1c ≥ 42 mmol∙mol-1 [≥6.0%]; n = 26). In Part B (randomized controlled trial design), participants in the IGR group were randomized to one of two 6-week interventions: (1) exercise training (EX; 70%-75% maximum heart rate; four sessions/week; n = 13) or (2) non-exercise control (CON; n = 13). Saturated (SI; primary outcome), unsaturated (UI) and polyunsaturated (PUI) hepatic lipid indices were determined using proton magnetic resonance spectroscopy. Additional secondary outcomes included liver PDFF, HbA1c, fasting plasma glucose (FPG), homeostatic model assessment of insulin resistance (HOMA-IR), peak oxygen uptake (VO2 peak), and plasma cytokeratin-18 (CK18) M65, among others.

RESULTS

In Part A, hepatic SI was higher and hepatic UI was lower in the IGR versus the NGR group (p = 0.038), and this hepatic lipid profile was associated with higher HbA1c levels, FPG levels, HOMA-IR and plasma CK18 M65 levels (rs ≥0.320). In Part B, hepatic lipid composition and liver PDFF were unchanged after EX versus CON (p ≥ 0.257), while FPG was reduced and VO2 peak was increased (p ≤ 0.030). ΔVO2 peak was inversely associated with Δhepatic SI (r = -0.433) and positively associated with Δhepatic UI and Δhepatic PUI (r ≥ 0.433).

CONCLUSIONS

Impaired glycaemic regulation in MASLD is characterized by greater hepatic lipid saturation; however, this composition is not altered by 6 weeks of moderate-intensity exercise training.

Exercise training adaptations in liver glycogen and glycerolipids require hepatic AMP-activated protein kinase in mice

Regular exercise elicits adaptations in glucose and lipid metabolism that allow the body to meet energy demands of subsequent exercise bouts more effectively and mitigate metabolic diseases including fatty liver. Energy discharged during the acute exercise bouts that comprise exercise training may be a catalyst for liver adaptations. During acute exercise, liver glycogenolysis and gluconeogenesis are accelerated to supply glucose to working muscle. Lower liver energy state imposed by gluconeogenesis and related pathways activates AMP-activated protein kinase (AMPK), which conserves ATP partly by promoting lipid oxidation. This study tested the hypothesis that AMPK is necessary for liver glucose and lipid adaptations to training. Liver-specific AMPKα1α2 knockout (AMPKα1α2fl/fl+AlbCre) mice and littermate controls (AMPKα1α2fl/fl) completed sedentary and exercise training protocols. Liver nutrient fluxes were quantified at rest or during acute exercise following training. Liver metabolites and molecular regulators of metabolism were assessed. Training increased liver glycogen in AMPKα1α2fl/fl mice, but not in AMPKα1α2fl/fl+AlbCre mice. The inability to increase glycogen led to lower glycogenolysis, glucose production, and circulating glucose during acute exercise in trained AMPKα1α2fl/fl+AlbCre mice. Deletion of AMPKα1α2 attenuated training-induced declines in liver diacylglycerides. In particular, training lowered the concentration of unsaturated and elongated fatty acids comprising diacylglycerides in AMPKα1α2fl/fl mice, but not in AMPKα1α2fl/fl+AlbCre mice. Training increased liver triacylglycerides and the desaturation and elongation of fatty acids in triacylglycerides of AMPKα1α2fl/fl+AlbCre mice. These lipid responses were independent of differences in tricarboxylic acid cycle fluxes. In conclusion, AMPK is required for liver training adaptations that are critical to glucose and lipid metabolism.NEW & NOTEWORTHY This study shows that the energy sensor and transducer, AMP-activated protein kinase (AMPK), is necessary for an exercise training-induced: 1) increase in liver glycogen that is necessary for accelerated glycogenolysis during exercise, 2) decrease in liver glycerolipids independent of tricarboxylic acid (TCA) cycle flux, and 3) decline in the desaturation and elongation of fatty acids comprising liver diacylglycerides. The mechanisms defined in these studies have implications for use of regular exercise or AMPK-activators in patients with fatty liver.