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The effects of low energy diets on protein metabolism in terms of the metabolic pool, active protein pool, and active and inactive protein synthesis rates were studied using [15N]glycine in five obese patients (percentage of ideal body weight, 120-190%). For 10 days, the patients were given a control diet containing 2,000 kcal of energy and 80 g of protein. For the next 2 weeks, they were given Diet A with 1,100 kcal of energy and 70 g of protein, and for the last 2 weeks given Diet B with 1,100 kcal of energy and 50 g of protein. During the Diet A period, the active protein pool and the active and inactive protein synthesis rates were about the same as during the control diet period, although the metabolic pool tended to be slightly smaller than during the control diet period. During the Diet B period, the metabolic pool, active protein pool, and active protein synthesis rate were all significantly different from the values during the control diet period. The results suggest that protein metabolism in obese patients is not maintained with less than 70 g of protein daily when energy intake was restricted to 1,100 kcal/day.

The effect of restricted diets on protein metabolism was studied in obese rats (obesity had been induced by ad libitum feeding of a diet containing 30% fat and 25% casein). The obese rats were fed on one of three restricted diets, each containing 5% fat, for 2 weeks (restricted feeding groups); a high-protein diet (HPD, 50% casein), a standard-protein diet (SPD, 25% casein), or a low-protein diet (LPD, 5% casein). The food intake was restricted to 5 g per day per rat. On the eleventh day, the rats were given [15N]glycine orally, and 4 days later, they were killed. The restricted feeding groups all showed similar weight losses (about 100 g), 2 weeks after the start of the restricted diet. The 15N distribution in whole body was measured and results were compared with those of control rats given 5%- or 30%-fat diet ad libitum. The whole-body distribution of 15N in the HPD group was similar to that in the rats fed ad libitum although the diet intake was restricted. The results suggested that the amount of protein in a restricted diet is important for maintenance of protein metabolism in obese rats.

Choroid plexus in experimentally induced hydrocephalic dogs according to the Wisniewski's method was examined by an electron microscope. Epithelial cell of the choroid plexus appeared in various deformation and formed a marked dilatation of extracellular spaces between apposed two cell membranes and at the basal interdigitation. Dilatation of the extracellular space was more pronounced at the stromal side of the epithelial layer. This finding might be due to anatomical specificities of apposed epithelial cell membranes and not only indicate a disturbance of the CSF secretion. According to the dilatation of extracellular space, apposed two epithelial cell membranes separated each other, however, at sites of intercellular junctions, adjacent cell membranes remained closely apposed. The dilated extracellular space at the basal site continued to the dilated intercellular space, while, no continuity was recognized between the dilated intercellular space and ventricular lumen. These extracellular speces appeared low electron density and showed little specific structure. No particular change was recognized in organelles in the epithelial cell, except increased pinocytotic vesicles in number. Stroma of the choroid plexus appeared edematous, that was, electron density was low and a few cellular and stromal elements were found. Capillary vessels showed almost normal structure. In hydrocephalic dogs also in normal one, Lanthanum infused into lateral ventricle precipitated on the surface of microvilli of choroid plexus epithelial cell. The Lanthanum, entered through ventricular end of intercellular space, was blocked to penetrate beyond the intercellular junction (zonula occludens) and any bit of the tracer was not found in the dilated extracellular space. The tracer was found neither in epithelial cell nor in stroma. These findings of tracer-study may indicate that the CSF absorption through the choroid plexus is not increased in chronic hydrocephalus. Consequently, the ultrastructural changes of the choroid plexus in chronic hydrocephalus, that is, enlarged extracellualr space, increased pinocytotic vesicles and edematous stroma, are thought to suggest a disturbance of CSF secretion from the choroid plexus.

The effect of nitrogen intake on nitrogen balance was studied in six obese patients receiving low energy diets. They were given a control diet containing 2,000 kcal of energy and 80 g of protein for the first ten days. Then they were given Diet A with 1,100 kcal of energy and 70 g of protein for the next 2 weeks, followed by Diet B with 1,100 kcal of energy and 50 g of protein for 2 weeks. The relationship between nitrogen intake (X, mg/kg) and nitrogen balance (Y, mg/kg) during the low energy diet periods was statistically significant, with Y = 0.388X-60.32 (SD = 17.71, r = +0.67, n = 11, p less than 0.05). The nitrogen and protein requirements were estimated from this equation to be 201.1 mg/kg and 1.26 g/kg, respectively. In our experiment, the nitrogen balance in obese patients was well maintained although total energy was reduced to 1,100 kcal/day in Diet A. It is suggested that protein quantity in the diets should be taken into account when a low energy diet is used for the treatment of obesity.

The effects of low protein intake on protein metabolism, including the size of pools and the protein synthesis rates, were studied by use of [15N]glycine in Papua New Guinea highlanders. Studies were made on 9 men between October and December in 1982. In experiment 1, two subjects were given a protein-free diet (PFD) containing 49.1 kcal/kg of energy. In experiment 2, subjects were given a sweet-potato diet (SPD) containing 45.4 kcal/kg of energy and 0.507 g/kg of protein for 8 days, and then were given a low-protein sweet-potato diet (LPSPD) containing 50.0 kcal/kg of energy and 0.265 g/kg of protein. During the SPD period, the sizes of the metabolic and active protein pools (mean +/- SD) were 270 +/- 134 mgN/kg and 362 +/- 107 mgN/kg, respectively, and the rates of active and inactive protein synthesis were 463 +/- 161 mgN/kg/day and 299 +/- 38 mgN/kg/day, respectively. During the LPSPD period, the sizes of the metabolic pool and active protein pool were 131 +/- 64 mgN/kg and 378 +/- 106 mgN/kg, respectively, and the rates of active and inactive protein synthesis were 490 +/- 206 mgN/kg/day and 280 +/- 26 mgN/kg/day, respectively. The protein metabolism in the LPSPD showed no significant difference from the SPD. The results suggest that, when the energy levels were approximately the same, protein metabolism in Papua New Guinea highlanders was maintained in spite of the decrease in protein intake.