In an attempt to clarify the nutritional enigma of the healthy strong physique of Papua New Guinea (PNG) highlanders who have a protein-deficient diet mainly composed of sweet potato, a pilot study was performed as follows with 10 volunteers of PNG highlanders, and with 8 Japanese controls, one group of whom took an experimental protein-deficient diet in Japan.
1. In 4 groups of subjects, i.e. adults of PNG highlanders, their children, Japanese controls who were having standard Japanese food (SPD), and those who were having a low protein diet (0.5g/kg) for 2 weeks (LPD), urea metabolism was investigated after oral administration of 15N urea. By tracing the cumulative excretion of ¹⁵N in urine successively for about 10 days after 15N urea administration, it was found that children of PNG highlanders can retain a large amount of ¹⁵N in the body, Japanese controls of LPD fairly well, PNG adult slightly and the Japanese controls of SPD the least of the four groups. It was demonstrated that the ¹⁵N atom% excess in the plasma protein of PNG adults, children, and Japanese control of LPD is maintained in the range of 0.02-0.05% fluctuating for 10 days after 15N urea administration. On the other hand, 15N atom% excess in plasma protein of Japanese control of SPD was within the scope of error (0.01%). ¹⁵N atom% excess in the lysine fraction of the hydrolysate of plasma protein was found in the range of 0.01-0.05% in a large number of cases of PNG subjects, and Japanese control of LPD, while it was not significantly detectable in Japanese controls of SPD. ¹⁵N atom% excess in essential and nonessential amino acids of the hydrolysate was significantly detectable in most cases of PNG subjects and Japanese controls of LPD, while not in the case of Japanese controls of SPD.
From discussions on the above findings it is presumed that 15N urea may be utilized in PNG highlanders and Japanese controls of LPD to produce amino acids, especially lysine in the intestine where the bacterial species are changed by a long-continued protein-deficient diet from those of Japanese controls of SPD. The possibility of urea recycling was thus verified.