Why does fat accumulate in the liver?

Metabolic dysfunction-associated steatotic liver disease (MASLD) is the hepatic expression of a multisystem disturbance in fuel storage and handling. In children and adolescents with obesity, the central pathway is usually not dietary fat simply being deposited directly in the liver. Rather, adipose-tissue dysfunction, insulin resistance, increased substrate delivery and altered hepatic lipid handling interact until lipid input exceeds the liver's capacity for oxidation and export. [1,4,6]

The liver normally helps maintain fuel balance by taking up fatty acids from the circulation, oxidising them for energy, re-esterifying them into triglyceride and exporting triglyceride in very-low-density lipoprotein (VLDL). Hepatic steatosis develops when fatty-acid uptake and de novo lipogenesis together exceed fatty-acid oxidation and VLDL export. [3,6]

In simplified form:

Adipose-derived fatty acids + hepatic de novo lipogenesis > beta-oxidation + VLDL export

Adipose tissue is the principal upstream buffer

Healthy adipose tissue is an active endocrine and storage organ that buffers energy away from ectopic sites. As adipocytes enlarge and adipose tissue becomes insulin resistant, insulin suppresses lipolysis less effectively. More non-esterified fatty acids (NEFA) are therefore released into the circulation and taken up by the liver. This adipose-to-liver fatty-acid flux is a major source of hepatic lipid in MASLD. [3,4]

The process does not require an elevated fasting insulin concentration. Insulin resistance is tissue-specific and dynamic, while a fasting insulin measurement is an imprecise snapshot influenced by secretion, hepatic clearance, puberty and assay variability. A child can therefore have substantial adipose and hepatic metabolic dysfunction, and MASLD, despite a fasting insulin result within the local reference interval. [2]

The liver also makes new fat

Hepatic de novo lipogenesis converts carbohydrate-derived substrate into fatty acids. Hyperinsulinaemia promotes lipogenic signalling, and carbohydrate availability also activates substrate-responsive pathways. Hepatic insulin resistance is partly pathway-selective: insulin may become less effective at suppressing hepatic glucose production while lipogenic signalling remains active. Consequently, insulin resistance and hyperinsulinaemia can coexist with continued or increased hepatic fat synthesis. [3,4]

This should not be reduced to a single nutrient or behaviour. Total substrate flux, adipose storage capacity, dietary pattern, physical activity, sleep, genetics, developmental stage and the wider social environment all modify susceptibility. [1,6]

Mitochondrial compensation can become overload

Hepatic mitochondrial beta-oxidation may initially increase in response to greater fatty-acid delivery. With persistent substrate excess, however, oxidative capacity may be exceeded and mitochondrial efficiency may deteriorate. VLDL secretion can also increase, but may remain insufficient to match fatty-acid uptake and synthesis. The resulting imbalance favours retention of triglyceride within hepatocyte lipid droplets. [3,6]

Triglyceride storage is not synonymous with lipotoxicity. Esterifying fatty acids into triglyceride can initially protect hepatocytes by sequestering excess lipid in a relatively inert form. Cellular stress is more closely linked to the accumulation of less safely stored lipid species, including saturated fatty acids, diacylglycerols, ceramides and free cholesterol. These can impair insulin signalling and contribute to mitochondrial dysfunction, oxidative stress, endoplasmic-reticulum stress and hepatocellular injury. [4,6]

From steatosis to possible MASH and fibrosis

MASLD describes a spectrum rather than an inevitable linear progression. In some children, persistent lipotoxic and oxidative stress activates Kupffer cells and recruits other inflammatory cells. Continued injury may activate hepatic stellate cells and extracellular-matrix deposition, leading to fibrosis. Metabolic dysfunction-associated steatohepatitis (MASH) denotes steatosis with hepatocellular injury and inflammation, with or without fibrosis. [4,5]

Progression along this route is not inevitable. Nevertheless, clinically important inflammation or fibrosis may already be present at recognition, and duration since diagnosis does not reliably indicate duration or severity of disease. [1,2]

Interpreting ALAT/ALT

Alanine aminotransferase (ALAT/ALT) is primarily a marker of hepatocellular injury, not a direct measure of hepatic synthetic function or liver-fat content. Injured hepatocytes can release more ALAT into the circulation, making a raised value a useful warning signal in the appropriate clinical setting. [5]

Its limitations should remain explicit [5]:

MASLD can be present when ALAT is within the reference interval.
ALAT concentration does not reliably stage steatosis, inflammation or fibrosis.
A falling ALAT may support improvement but does not prove complete histological resolution.
Raised ALAT is not specific to MASLD; alternative and coexisting causes of liver injury require consideration.

Why early recognition matters in children

The paediatric liver has substantial capacity to recover, particularly when disease is recognised before advanced fibrosis has developed. Reducing the underlying metabolic load can lower hepatic fat and ALAT and allow hepatocellular metabolism to move towards healthier function. However, describing recently diagnosed paediatric MASLD as universally or completely reversible would be too absolute: the condition may have preceded diagnosis for years, fibrosis may already be present, and individual response varies. [1,2,5]

The clinically useful message is therefore hopeful but qualified: early liver-fat accumulation in children can often be reversed, and early recognition creates an opportunity to reduce metabolic strain before lasting injury develops.

Clinical perspective

MASLD should be framed for families as a physiological consequence of metabolic overload, not as evidence of poor choices or lack of effort. Management belongs within comprehensive, family-centred obesity care and should address the child's overall health, context and treatment needs rather than focusing narrowly on body weight or a single laboratory result.

An isolated fasting insulin result should neither establish nor exclude the relevant physiology. Similarly, ALAT should be interpreted as one component of assessment rather than as a binary MASLD test.

Clinical pearls

Think of paediatric MASLD as a multisystem disorder involving adipose tissue, liver, skeletal muscle and insulin physiology.
The central lipid-balance equation is increased NEFA uptake and de novo lipogenesis relative to beta-oxidation and VLDL export.
Normal fasting insulin does not exclude adipose or hepatic insulin resistance.
Hepatic triglyceride is a marker of excess lipid flux and can initially be partly protective; lipotoxic intermediates are more directly linked to cellular injury.
ALAT is an injury marker, not a direct measurement of liver function, liver fat or fibrosis.
Progression to MASH and fibrosis is possible but not inevitable.
Early paediatric disease often has substantial potential for improvement, particularly before advanced fibrosis.

Key messages

Dysfunctional adipose storage increases fatty-acid delivery to the liver.
The liver also generates fat through de novo lipogenesis.
Mitochondrial oxidation and VLDL export may fail to keep pace with lipid input.
Persistent lipid overload can produce lipotoxic, oxidative and inflammatory hepatocellular stress.
ALAT may provide an early warning, but normal ALAT does not rule out MASLD.
Early recognition offers an opportunity to reduce metabolic overload and help the liver find its balance again.

References

Schwimmer JB, Biddinger SB, Ibrahim SH. MASLD in children: integrating epidemiological trends with mechanistic and translational advances. Journal of Clinical Investigation. 2025;135(13):e186422. doi:10.1172/JCI186422.
Segev N, Mouzaki M. Updates on paediatric MASLD: insights from an endocrine lens. Diabetologia. 2026;69(11):3121–3130. doi:10.1007/s00125-026-06724-3.
Steinberg GR, Valvano CM, De Nardo W, Watt MJ. Integrative metabolism in MASLD and MASH: Pathophysiology and emerging mechanisms. Journal of Hepatology. 2025;83(2):584–595. doi:10.1016/j.jhep.2025.02.033.
Gastaldelli A, Scoditti E, Macedo MP. Metabolic drivers of MASLD and MASH: from hormonal imbalance to fibrosis. Diabetologia. 2026;69(11):3088–3104. doi:10.1007/s00125-026-06774-7.
European Association for the Study of the Liver (EASL), European Association for the Study of Diabetes (EASD), European Association for the Study of Obesity (EASO). EASL-EASD-EASO Clinical Practice Guidelines on the Management of Metabolic Dysfunction-Associated Steatotic Liver Disease (MASLD). Obesity Facts. 2024;17(4):374–444. doi:10.1159/000539371.
Capuccio S, Cocuzza C, Di Marco GL, et al. Metabolic Dysfunction-Associated Steatotic Liver Disease (MASLD): From Steatosis to Systemic Metabolic Failure: Classical Pathophysiology, Emerging Systemic Mechanisms and Modifiable Lifestyle Determinants. Nutrients. 2026;18(14):2316. doi:10.3390/nu18142316.

Scope: This HCP companion is aligned with the approved family-facing source text and final infographic. It is a pathophysiology and interpretation brief, not a screening, diagnostic or treatment guideline.