Non-alcoholic fatty liver disease (NAFLD) is a condition exactly as the name suggests: it is accumulation of fat in the liver which is not due to alcohol intake. The increase in prevalence of this disease is astonishing: not long ago fatty liver was a condition associated with excessive and chronic alcohol intake. Now in most cases, the cause is not alcohol - but diet and physical activity. Even more astonishing - more and more children and teenagers are being diagnosed with fatty liver.
Fat in the liver is a problem because it interferes with the way the liver functions; and the liver plays a central role in many of the body’s most important processes. For example, the liver is involved in regulating glucose and fat metabolism, and too much fat in the liver is associated with insulin resistance (which can cause high blood glucose) and dyslipidaemia (high blood triglycerides and cholesterol). A fatty liver can also lead to more advanced liver diseases including cirrhosis and liver cancer.
So what is causing the rise in NAFLD? Unsurprisingly, it’s our unhealthy lifestyles which is the primary cause. The data on individual foods or nutrients is a bit more ambiguous (see below) but the evidence is clear that excessive calorie intake (i.e. consuming more energy in food and drink than we require) increases liver fat. Conversely, reducing energy intake and losing weight reduces liver fat.
Apart from weight loss, what other lifestyle factors could play a role? Some of the strongest evidence points towards sugar, but the key here is “dose” and whether the sugar is in solid or liquid form. Excessive sugar (25% of all calories; i.e. upwards of 125g of sugar; or >1L of a sugary drink like cola) leads to an increase in liver fat in people even if they don’t gain weight. Studies which have used lower concentrations of sugar or provide the sugar in solid as opposed to liquid form are less conclusive.
The type of sugar is important: sucrose (i.e. table sugar and the sugar in cola) appears to be detrimental due to the fructose part of it. Sucrose has one molecule of glucose and one of fructose. It is fructose and not glucose which raises liver fat independent of weight change. This is probably to do with how fructose is metabolised which is covered in this excellent article.
Others have proposed that reducing starch intake could reduce liver fat - but this has not been shown to be independent of weight loss. Giving extra calories - whether in the form of carbohydrate or fat- raises liver fat. However, there are studies which have lowered starch intake - with a concomitant rise in protein - which do lead to a reduction in liver fat even when weight does not change. This is important because there is an appreciable amount of work which shows that protein and amino acids could lead to a reduction in liver fat.
The type of fat may play a a role too. Giving extra calories in the form of saturated (palm oil) but not polyunsaturated fat (sunflower oil) raises liver fat. Supporting this finding, studies in which there is no weight change show that the vegetable fat lowers liver fat compared to butter. It is important to note here that the actual changes were quite small, though still clinically relevant.
You might wonder what role fibre plays in determining liver fat? Actually we don’t know because we don’t have good quality studies in humans to look at this. If it does have any effect it is likely that large intakes of fibre would be needed. And as usual with most dietary factors, any change pales in significance when you compare to the effect of weight loss itself.
To conclude, the best available evidence points towards maintaining a healthy weight. There is encouraging data pointing towards a higher protein intake being helpful for lowering liver fat but this needs further confirmation. Clinically, I always encourage as much plant-based protein intake as possible alongside some animal protein - and in a follow-up post I’ll explain the rationale for doing this.