Experts say the keto diet isn’t sustainable, so why is it so popular?

America is in the midst of a keto craze. The trendy diet — which bans carbs to make your body burn fat for fuel — has kicked Weight Watchers’ derri�re on the stock market, captured the endorsement of celebrities such as Kourtney Kardashian and Halle Berry, and deluged the internet with recipes and copious social media chatter about pounds lost. Now the popular diet even has a day named after it. The Vitamin Shoppe, which wants to sell you a ton of keto-based products, has named the first Sunday of this new decade “National Keto Day.” “What on Earth justifies granting a day to memorialize a fad diet?” said Dr. David Katz, founding director of the Yale...