MASLD (Metabolic dysfunction-associated steatotic liver disease) is a condition where fat builds up in the liver. It is the most common cause of liver disease worldwide. In some people, the fat can irritate the liver (inflammation) and cause damage. This is a more serious condition called MASH (Metabolic dysfunction-associated steatohepatitis). People with MASH more at risk of liver cirrhosis (advanced scarring in the liver) and liver cancer. It is not fully understood why MASLD becomes MASH, or why this happens in some people but not in others. However, it is known that our diet plays a role. Research shows a diet high in a type of sugar called fructose might make MASLD worse. Fructose is found in fruit, honey and table sugar, and lots of processed food and drinks. The body deals with fructose differently to other sugars, which is why fructose may be a problem. Although scientists have studied the effects of fructose in healthy people, no studies so far have included people with MASH, so it is not known if fructose might make the condition worse. To answer this question, the researchers will conduct a four-week randomised, double-blind study to compare the effects of fructose with another sugar called glucose in 36 people with MASH, 18 people with 'simple' MASLD, and 18 controls without liver disease. Participants will follow a low-sugar diet and, after 14 days on this diet, they will add either a glucose or fructose supplement for another 14 days. Participants will attend 3 study visits, where blood, urine, stool, and saliva samples will be taken. The main question is whether fructose causes more inflammation in people with MASH compared to those with MASLD, or people without liver disease. The researchers will also investigate how fructose affects liver fat content, the gut microbiota, and other processes relevant to MASLD/MASH.