As explained below, there is now convincing evidence that this has been caused by the consumption of diets that have a high content of sugar and carbohydrates. This in turn stimulates the pancreas to produce large quantities of insulin.
Insulin has many different functions but one of the most important is to control the level of glucose in the blood, most of which is derived from the sugar and starch present in the diet.
Essentially, it does this by pushing the glucose to the liver where it is converted into fat, which is then moved into the body cells for storage.
If there is excessive glucose, then there will be loads of fat all over the body.
Hence there will be fatty liver disease and high content of fat in the blood. This means that when a blood sample is tested for triglycerides (TG) (fat), a relatively high value will be reported.
It is therefore no great surprise that the storage of fat is why a person becomes overweight/obese.