Theory: In some old "hunter/gatherer" parts of the human body, an all-potato diet scores: [X] plentiful; [X] nutritious; [X] highly non-perishable. So there is sure to be ample good food available for a long time, and no need for an extra-body-fat insurance policy.

It would be interesting to test whether other all-starchy-root-veggies diets had similar effects.