What does your liver do for your metabolism, and how do you repair a fatty liver?

The science, in full

Your liver is the central processing plant of your metabolism. Almost everything you eat and drink passes through it first: blood from your gut travels straight to the liver before it reaches the rest of your body, so the liver gets first choice over the nutrients that arrive and decides what to store, release, rebuild or break down. It handles sugars, fats and proteins, produces bile for digestion, makes clotting and transport proteins, and clears alcohol, waste products and many medicines from your blood. No other organ juggles so many chemical tasks at once.

Its role in blood sugar is especially elegant. After a meal, when glucose floods in, the liver takes up the surplus and stores it as glycogen, a compact sugar reserve. Between meals and overnight, when nothing is coming in, it does the reverse: it breaks that glycogen back down and, when the store runs low, it manufactures fresh glucose from scratch through a process called gluconeogenesis. This quiet back and forth is why your blood sugar stays remarkably steady through the night and between meals, without you ever noticing.

The liver is just as busy with fat. It assembles fats into transport particles, produces cholesterol and bile, and can turn a surplus of carbohydrate into fat itself through a pathway called de novo lipogenesis. In a healthy liver this is a modest background activity. The trouble begins when the factory is asked to process more energy, day after day, than the body can use or store elsewhere.

When fat starts to build up inside the liver cells themselves, the result is a fatty liver. In 2023 an international consensus renamed this condition from non-alcoholic fatty liver disease (NAFLD) to metabolic dysfunction-associated steatotic liver disease, or MASLD, to make clear that it travels together with the rest of metabolic health rather than being a separate, purely alcohol-related problem. It is strikingly common: worldwide, roughly a quarter to a third of adults have some degree of fatty liver, and the figure rises further in people with obesity or type 2 diabetes. Most of them have no symptoms and no idea.

How does the fat get there? The main driver is a lasting surplus of energy in a body that has become resistant to insulin. When fat cells, especially around the belly, are full and inflamed, they spill fatty acids into the blood that the liver takes up. At the same time the liver keeps making new fat from sugar. Added sugar plays a particular role here: fructose, the sweet half of table sugar, is processed almost entirely in the liver and is a strong stimulus for de novo lipogenesis. Controlled human studies show that diets high in fructose and refined carbohydrate raise liver fat and lipogenesis, and that saturated fat can burden the liver even more than sugar in the short term. The picture is one of overload, not a single villain.

A fatty liver is not only a passive store, it actively worsens metabolism. Fat inside liver cells makes the liver itself resistant to insulin, so it fails to switch off its overnight glucose production. That is one reason people with a fatty liver often wake up with a raised fasting blood sugar. The relationship runs both ways: insulin resistance drives liver fat, and liver fat deepens insulin resistance, a loop that helps type 2 diabetes take hold.

This is why doctors increasingly read a fatty liver as an early warning light rather than a footnote. Large pooled analyses show that people with a fatty liver have roughly double the risk of developing type 2 diabetes in the following years, and their risk of heart and vessel disease is raised as well. MASLD tends to travel in company with a bigger waist, higher blood pressure, higher blood fats and a blood sugar that is starting to slip, the familiar cluster of metabolic dysfunction.

The encouraging part is how responsive the liver is. It is one of the most regenerative organs in the body, and its fat is among the first things to shift when metabolism improves. In a classic study, a modest weight reduction of only a few kilos already reversed liver fat, restored the liver's sensitivity to insulin and brought fasting blood sugar back towards normal in people with type 2 diabetes. Researchers describe a personal fat threshold: each person has an individual level of fat, much of it in and around the liver and pancreas, above which metabolism tips over, and dropping back below that threshold can let it recover.

The size of the effect tracks the effort in a fairly orderly way. In people with the inflamed form of fatty liver (steatohepatitis), losing around 5 per cent of body weight measurably reduces liver fat, losing roughly 7 to 10 per cent can calm the inflammation, and larger losses can even allow early scarring to regress. Exercise adds its own contribution: pooled trials show that regular activity lowers liver fat even when the scales barely move, so movement is worthwhile in its own right, not only as a route to weight loss.

None of this means every fatty liver behaves the same. Genes influence how readily fat accumulates and how far it progresses, and in a minority the condition advances over years to serious scarring (fibrosis and cirrhosis) that needs medical care. The honest message is hopeful but not a promise: for most people, cutting back on sugary drinks and refined carbohydrate, moving more and losing some weight can genuinely undo much of the damage, often within months. If your liver values are raised, if you have been told you have a fatty liver, or if you are weighing up medication or a diagnosis, discuss it with your own doctor, who can look at your full picture.

Putting this into practice. Give your overworked liver less to store by easing off the two things that load it most, sugar and alcohol. Try several alcohol-free days a week, or a stretch with none at all, and see how you feel. Cut back on sugary drinks, sweets and fast carbohydrates, and lean towards vegetables, protein and wholefoods instead. Add regular movement you will actually repeat, a daily walk counts, since exercise and gradual weight loss both help the liver shed fat. Take it step by step rather than all at once. If you have a known fatty liver or abnormal blood values, follow them up with your own doctor.