If you want to make foie gras, you feed ducks and geese copious quantities of grains, such as corn and wheat.
The carbohydrate-rich diet causes fat deposition in the liver via processes such as de novo lipogenesis, the conversion of carbohydrates to triglycerides. Ducks and geese are particularly good at this, since they store plentiful fats in the liver to draw from during sustained periods of not eating during annual migration.
Modern humans are trying awfully hard to create their own version of foie gras-yielding livers. While nobody is shoving a tube down our gullets, the modern lifestyle of grotesque carbohydrate overconsumption, like soft drinks, chips, pretzels, crackers, and--yes--"healthy whole grains" causes fat accumulation in the human liver.
Over the past few years, there has been an explosion of non-alcoholic fatty liver disease and non-alcoholic steatosis, two forms of liver disease that result from excess fat deposition. The situation gets so bad in some people that it progresses to cirrhosis, i.e., a hard, poorly-functioning liver that paints a very ugly health picture. The end-result is identical to that experienced by longstanding alcoholics.
While Hannibal Lecter might celebrate the proliferation of human fatty livers with a glass of claret, fatty liver disease is an entirely preventable condition. All it requires is not eating the foods that create it in the first place.