Deficient means there are lower levels of certain amino acids, but they're not missing entirely. It wouldn't make any biological sense for plants to be missing amino acids. If you read the actual cited source, you will find this:

> Usual dietary combinations of plant proteins are complete; specific plant proteins may be low in specific amino acids

> There is no evidence that amino acid imbalances per se are important; possible imbalances can be created by inappropriate amino acid supplementation, but this is not a practical problem.

Beriberi is a nutritional deficit in vitamin B1, not protein, which is ironically easily remedied with potatoes.

Oh, come on - you don't really think so, do you? How many people do you know with diabetes as a result of eating potatoes or mangoes? Check out Starch Solution by John McDougall, if you're being serious. Here's a small tidbit on it - https://www.drmcdougall.com/misc/2006nl/july/glycemic.htm - the guy literally uses potatoes and other starchy vegetables to treat diabetes.

> there is little evidence that total carbohydrate intake is associated with the development of type 2 diabetes. Rather, a stronger association has been observed between total fat and saturated fat intake and type 2 diabetes

http://care.diabetesjournals.org/content/27/9/2266.full

> The available data support the idea that consumption of diets high in total carbohydrate does not adversely affect insulin sensitivity compared with high fat diets. Animal data suggest that simple sugars, in particular fructose, have adverse effects on insulin action, but adverse effects have not been shown conclusively in humans.

http://www.ncbi.nlm.nih.gov/pubmed/11584106