Ground Nuts (Apios Americana)
Here is a plant I am growing as part of my high calorie garden. Around 36% of the raw root is carbohydrates but the tubers also have roughly three times the protein of potatoes. From what I can determine they have the highest protein content of any root crop.
Groundnut tubers are a good source of carbohydrates and contain between 13 and 17 percent protein by dry weight, or about three times more than potatoes or any other widely used vegetable root (Yanovsky and Kingsbury 1938, Watt and Merrill 1963).
The tubers are highly palatable with culinary characteristics of a potato, although the flavor can be somewhat nuttier than a potato and the texture can be finer. Studies in rats suggest that raw tubers should not be consumed because they contain protease inhibitors whereas cooking destroys the protease inhibitors rendering the tubers safe to eat. Tubers contain roughly three times the protein content of a potato (16.5% by dry weight), and the amino acid balance is good with the exception of cysteine and methionine. The fatty acid content of tubers is approximately 4.2% to 4.6% with linoleic fatty acids predominating. Thirty-six percent of the fresh weight of a tuber is carbohydrate (primarily starch). The tubers are also an excellent source of calcium and iron. Calcium content is 10-fold greater than a potato and iron is 2-fold greater than a potato, although vitamin C was considerably less than a potato. In addition, the tubers appear to have numerous health promoting factors. Hypertensive rats that were fed powdered tubers as 5% of their total diet experienced a 10% decrease in blood pressure and also a reduction in cholesterol and triglycerides. It has been shown that the tubers contain genistein and other isoflavones that have various health benefits, including an anticarcinogenic function against colon, prostate, and breast cancer.
While ground nut tubers are an amazing food, approximately 1% of the population can develop allergies to them. This allergy can hit the first time they eat the tubers or any time the sit down to a meal of them after that first time. There's no good way of telling in advance if a person might be allergic, but if someone has an extreme allergy to peanuts I would hesitate in giving them any part of the groundnut plant.
I have found some growing instructions at http://plants.usda.gov/plantguide/pdf/cs_apam.pdf
I know they are grown in Northern Japan as a commercial crop but I have little information of their cultivation in Japan.
I have been growing mine in 55 gallon tubs because I want to collect all the roots. I have been growing then in tubs next to tubes of Jerusalem Artichokes for around two years. The vines have grown into the Jerusalem Artichokes but seem to cause little problem. This year I have enough to experiment with so I am planting one large groundnut root cluster with four Jerusalem artichoke tubers. I will see how they grow together. I wish I had more to offer you on this plant but I am finding that there just is not a lot of information on the Internet about growing this plant. If my experiment works I hope to grow Jerusalem Artichokes and ground nuts together on a two year harvesting. Only issue I will have is grubs in fall. I will handle this with parasitic nematodes.