Fasting hyperglycaemia in T2D results from increased rates of hepatic gluconeogenesis and EGP and from hepatic insulin resistance, characterized by reduced ability of insulin to suppress this process38,39,40,41. This may be because of direct IR-mediated cell-autonomous or indirect effects (substrate availability, allosteric regulation or redox status)42 (Fig. 1b). Recent studies showed that these indirect effects probably result from insulin action on WAT and mainly account for acute suppression of gluconeogenesis and EGP during postprandial hyperinsulinaemia14. Consistent with a minor role for direct hepatic effects of insulin, rodent models with altered hepatic insulin signalling exhibit relatively normal glucose tolerance and compensatory hyperinsulinaemia, with reduced hepatic glycogen synthesis as the only indication of disrupted insulin signalling14,43,44,45,46,47.

Direct assessment of glycogen synthesis by 13C magnetic resonance spectroscopy demonstrated lower rates of postprandial and insulin-regulated hepatic glycogen synthesis in people with T2D38,39. The higher half-maximal effective concentration and lower maximum effect of insulin on hepatic glycogen synthesis39 indicate impaired IR activation with subsequent posttranslational modifications of the glycogen synthetic machinery and transcriptional regulation of glucokinase (Fig. 1b). Whereas other insulin effects, such as transcriptional DNL activation via sterol receptor enhancing binding protein-1c (SREBP1c), would be expected to be blunted, hepatic insulin resistance is generally associated with increased hepatic TAG and NAFLD. Accordingly, it has been proposed that only the FOXO1-dependent, but not the SREBP1c-dependent branch of insulin signalling, is defective, suggesting selective hepatic insulin resistance48. This hypothesis relies on the assumption that DNL is the major source of hepatic TAG and on experiments showing different roles of insulin receptor substrate (IRS)-1 and IRS-2, substrate-specific AKT phosphorylation or intrinsic pathway sensitivities to insulin. Conversely, NEFA re-esterification probably accounts for the majority of hepatic lipogenesis and very low-density lipoprotein (VLDL) secretion49,50,51. Decreased insulin-stimulated hepatic IR kinase activity suggests a common proximal abnormality in T2D52. Furthermore, DNL upregulation is not dependent exclusively on IR kinase activity, but can also occur through activation of carbohydrate receptor enhancing binding protein (ChREBP)53, mTORC1–SREBP1c54 and fructose-stimulated pathways55 (Fig. 1b). A recent study found that fatty acid esterification to TAG is mostly dependent on NEFA delivery to the liver and independent of hepatic insulin signalling16. This alternative hypothesis also explains the development of NAFLD through increased NEFA flux derived from increased lipolysis by insulin-resistant WAT.

In addition to caloric overload, macronutrients exert specific effects by modulating enteroendocrine secretion and, in turn, pancreatic islet and brain function before reaching the splanchnic bed to directly stimulate insulin secretion and entering the liver. Only around 33% of dietary carbohydrates enter the liver, and dietary fat is considered to amount to only 10–20% of the hepatic fatty acid pool49. Nevertheless, macronutrients can deliver substrates for the hepatic acetyl-CoA pool, which allosterically stimulates gluconeogenesis or activates nutrient-sensitive pathways (ChREBP, mTORC and SREBP) to collectively stimulate the transcriptional DNL program. Elevated hepatic acyl-CoA favours production of sn-1,2-DAG, sphingolipids and TAG. In obese humans with NAFLD, the sn-1,2-DAG–PKCε pathway tightly correlates with hepatic insulin resistance56,57,58,59,60, whereas ceramide–JUN N-terminal kinase (JNK) correlates more with hepatic oxidative stress and inflammation58,61,62 (Fig. 1b). In this context, lowering cellular ceramide by ablating dihydroceramide desaturase 1 increased mitochondrial oxygen flux and improved steatosis and glucose metabolism in insulin-resistant mice63. Conversely, mitochondrial C16:0 ceramide, generated by overexpression of ceramide synthase 6 (CerS6), interacts with mitochondrial fission factor (MFF) to promote mitochondrial fragmentation, insulin resistance and steatosis64. Silencing of MFF prevented CerS6-dependent metabolic abnormalities despite elevated C16:0 ceramide. This suggests that the effects of ceramides on insulin-stimulated glucose metabolism might result indirectly from impaired mitochondrial function with lower fatty acid oxidation, giving rise to other metabolites, for example, sn-1,2-DAG or acetyl-CoA, rather than from direct ceramide interference with insulin signalling. Recent studies indicate a critical role of molecular compartmentation of sn-1,2-DAGs, specifically in the plasma membrane, in inducing nPKC translocation and insulin resistance. Mice treated with CGI-58 antisense oligonucleotide exhibit elevated hepatic TAG and DAG in lipid droplets, are protected from lipid-induced hepatic insulin resistance and show reductions in plasma membrane DAG and PKCε translocation65.

Alvarez-Hernandez et al. monitored the earliest diet-induced metabolic alterations by examining the effect of a single oral saturated fat load in healthy humans66. This study revealed that saturated fat simultaneously induces insulin resistance in liver, skeletal muscle and WAT, and is associated with 70% higher rates of hepatic gluconeogenesis and 20% lower rates of net hepatic glycogenolysis. Similar studies in mice found upregulated expression of toll-like receptor (TLR) and inflammatory pathways, which might contribute to progression of NAFLD, including non-alcoholic steatohepatitis (NASH)66. Of note, chronic overfeeding also increased levels of intestine-derived endotoxins promoting TLR4-induced cytokine release by Kupffer cells67,68. Other intestinal functions also affect glycaemia and diabetes risk: integrin β7-knockout mice, which lack natural small-intestinal intraepithelial T lymphocytes, are metabolically hyperactive and resistant to obesity and diabetes69. Finally, dietary habits may affect the gut microbiota, modulating intestinal metabolite release and insulin sensitivity70. Humans with T2D and NAFLD show distinct metagenomic signatures along with increased branched-chain amino acids71,72 and decreased short-chain NEFA73, which may affect body weight and metabolism.

In summary, overnutrition and WAT dysfunction lead to increased WAT lipolysis, which promotes insulin-independent hepatic lipogenesis resulting in increased ectopic lipid deposition and increased hepatic gluconeogenesis owing to increased increased acetyl-CoA stimulation of pyruvate carboxylase as well as increased glycerol conversion to glucose. This mechanism obviates the previously reported need to invoke selective hepatic insulin resistance to explain the discordance of increased hepatic lipogenesis occurring simultaneously with increased gluconeogenesis48 (Fig. 1b). This is in line with recent studies showing that weight loss caused by very-low caloric diets rapidly normalizes hepatic steatosis and insulin resistance in liver, but not intramyocellular lipid content or muscle insulin resistance in individuals with T2D3,11,74.