Discusses how the incompleteness of wheat protein with regards to its lysine may be exploited in designing effectively low-protein meals.
<— [previous chapter] — [contents] — [next chapter] —>Lysine and Methionine
The dietary requirement for protein is more precisely a requirement for the amino acids found in protein that the adult body cannot synthesize on its own. These so called essential amino acids are isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Tyrosine can be made from phenylalanine andvice versa. Likewise cysteine from methionine.
A protein source must provide these essential amino acids in the same proportions as in the proteins being synthesized in order to be 100% utilized. If say lysine were in only half its required proportion then a maximum of only 50% of the dietary protein could end up as body protein. Protein synthesis allows no substitutions. The rest would have to be converted to carbohydrate by removing the nitrogens and burned for energy.
Normally this matters little since the standard western industrial diet provides many times the amount of amino acids required for the body’s protein synthesis needs. Lysine in rice protein, for example, is present in only 64% of the proportion needed for human protein synthesis. We then say itsavailabilityis 64%. Yet if you met your calorie requirements with rice alone, you would still get 256% of your lysine needs.
The goal of protein cycling, however, is to minimize protein availability during restriction, not to maximize it. We see from the food table that 100g of rice provides 130 calories of which 8% come from protein. 8% may be a little too high to trigger the autophagy we desire. For a meal where rice was the only protein source, however, we reduce the 8% to (8%*64%)=5% since lysine limits availability to 64% in normal rice. Add a 100 calorie 0g protein pat of butter and the effective percentage drops to below 4%, well below the level where autophagy must occur.
As for other low-protein calorie dense foods, isoleucine is limiting in white potatoes at 60% and lysine is limiting in wheat at 45%. Modern corn has often been genetically enhanced to produce more lysine and tryptophan so exploiting the limiting amino acids in corn can no longer be done reliably. The same may soon apply to wheat as well. And sometimes lysine is added to flour or bread to ‘enrich’ it.
Lysine has another property we can exploit. When proteins are heated, especially in the presence of sugars, some of the lysines in the protein chemically react and become permanently unavailable. When bread is toasted, it is this process that is thought to produce the characteristic browning and aroma.
From thefood tablewe see that unenriched white bread has a protein calorie percentage of 11%. Since lysine is limiting in wheat at 45%, the effective percentage is 11%*45%=5%. Slice it thin and toast it dark and the percentage may perhaps drop to 4%. Add a 100 calorie 0g protein pat of butter and the effective percentage drops to well below the magic 5%.
In some studies, the benefits of protein restriction to longevity have been traced to reduced total intake of methionine11 (or tryptophan is a few studies). It is hypothesized that methionine is particularly susceptible to oxidation and the production of ROS that damage cells89. Though protein cycling would not necessarily reduce total methionine intake, this issue does cloud interpretation of intermittent fasting studies on which protein cycling theory leans.
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