Potatoes and Protein

The Food and Nutrition Board of the National Academies of Science publishes the Dietary Reference intakes, which includes Estimated Average Requirements for Indispensible Amino Acids for Adults Aged 19 years or Older.

I decided to find out whether the average person could meet his or her IAA requirements eating a diet composed solely of white potatoes.  I used the USDA nutrient database to find the amino acid delivery of potatoes at 1880 kcal and 2350 kcal, approximate caloric requirements of a 120 pound woman or a 160 pound man, respectively.  I created the following table for the purpose:

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According to these FNB and USDA data, the average person can meet all indispensable amino acid requirements eating potatoes as his/her sole protein source.   No 'limiting amino acids' nor protein complementing required so long as caloric requirements are met.

Kon and Klein reported in 1927 on The Value of Whole Potato in Human Nutrition.  Two healthy adults obtained all of their protein and IAAs from potatoes for 167 days.  They maintained nitrogen balance.  They reported:

"The digestion was excellent throughout the experiment and both subjects felt very well. They did not tire of the uniform potato diet and there was no craving for change."

The potato provides only about 10% of calories as protein.  A mixed diet containing other plant foods providing higher proportions of protein, like green vegetables (20-40% of calories as protein), nuts (~25% of calories as protein), or legumes (~25% of calories as protein) will provide higher levels of the IAAs and total protein.

It seems that humans can obtain all of the protein they require from a food like the potato.  What does this tell us about ancestral nutrition?   

I decided to compare the IAA delivery of potatoes to that of 95% lean ground beef.  I compared the IAA delivery of amounts of beef and potatoes that provide comparable total protein, and created the following table to illustrate:

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Both the beef and the potato provide adequate amounts of total protein and IAAs. The much smaller portion of beef (200 g vs 2500 for the potatoes) provides larger doses of the IAAs, presumably constituting greater excesses of IAAs for the average individual.

The body will deaminate and oxidize these excesses of IAAs, increasing the amount of ammonia the liver must detoxify and sulfuric acid and urea the kidneys must excrete.

Since the beef supplies only about 14% of total energy requirements, the individual who fills in the other 86% of calories with whole foods that also provide protein will automatically consume a higher amount of IAAs than one who consumes a mix of plant foods with a far smaller amount of animal protein.

For those who have concerns about overconsuming methionine because some research suggests that reduced methionine consumption might increase longevity, I find it interesting to note that the ground beef provides about 15 times more methionine+cysteine per unit weight than the potatoes (about 9.5 mg per g cooked beef, versus 0.6 mg per g cooked potatoes).  

Let's say someone consumes the half-pound of beef (328 kcal) and gets the remainder of his required 2350 calories from potatoes (2022).  He would get 1906 + 0.86(1425) = 3132 mg of methionine+cysteine, compared to 1425 mg (less than half as much) if he ate only potatoes. 

I wonder if the body has an internal regulatory mechanism for amino acid consumption, which drives appetite to control total amino acid intake, such that if a person eats a diet rich in animal protein, the sytem drives the appetite toward attempting to fill the bulk of caloric requirements with low protein items like fat/oil, sugar, fruits, some very low protein tubers (e.g. cassava), or some refined starches?