Gaining

Olive oil. Has almost 3x the calories and total fat of heavy cream. 1,909 cal a cup.

Seriously, it's been a staple for bodybuilders for decades to hit that 10K calorie/day goal. Throw it in your shakes or drink it straight. Bodybuilders put olive/macadamia oil on everything: is actually really good for your health and does not tax your digestive system like cream.

Any high fat food can cause diarrhea due to fat not fully breaking down in the intestine. That said, olive oil is processed and absorbed far easier than dairy fats like heavy cream.

Olive oil does have a laxative effect similar to coffee if that's what you're referring to.