The Wall Street Journal reports the average price for avocados, butter, olive oil, and salmon have all increased as much as 60% since 2013. Meanwhile, the price of staples such as corn, soybeans, and wheat has remained more or less the same or dropped.

There are a number of factors at play.

Consumers across much of the world—not just in Western countries as has been in the past—are eating more food high in natural fats. It’s also not easy to ramp up food production to match growing demand.

Olives, for example, require adequate land and can only be grown under certain conditions. The WSJ also notes that recent droughts in California have posed an added challenge to growers. Almonds and avocados can also only be grown in particular areas and take several years to start producing fruit.

While butter consumption has increased—particularly consumption of European-style butter—manufacturers are reluctant to make enough to meet demand. The byproduct of butter is skim milk, which does not fetch high prices as, say, cheese or cream. Farmers are therefore producing less butter, leading to shortages of European-style butter and pushing prices up. The price of butter from New Zealand grew by 50% between 2012 and last year.

Environmental concerns are also at play in each of these industries, but it is perhaps most pronounced among salmon fisheries. The Norwegian and Scottish governments, for example, have limited the creation of new salmon farms due to farming difficulties and problems with sea lice, which can hurt the fish population.

Until the demand plateaus, don’t expect healthy fat food prices to drop any time soon.