I only recall the guy that did it for 60 days but he had to eat 7-8 pounds of potatoes a day to hit minimum protein requirements. His health markers were also very misleading because he went into the experiment as an unhealthy person. His bloodwork showed improvement because he was only eating potatoes rather than junk. Potatoes don't have fat, so you're not getting any essential omegas and along with the B12, there is little to none of: A, D, E, K, B2, B7, selenium, calcium, zinc, and a couple essential aminos

Those deficiencies would lead to muscle wasting, loss of sight, hormone imbalances, weak bones, permanent nerve damage etc.

If you put someone who is in good health on a potato diet for a year, even with B12, they would likely be looking at multiple organ failure by the end of that year

So they're like whippets