The suppressive effects of dietary protein restriction on the progression of renal impairment in OLETF rats

BACKGROUND

The suppressive effect of dietary protein restriction on the progression of diabetic nephropathy remains controversial. We investigated the effects of protein and energy restrictions on both albuminuria and morphology using diabetic-prone Otsuka Long-Evans Tokushima fatty (OLETF) rats.

METHODS

In this study, male OLETF rats were divided into two groups according to their energy intake. They were then further divided into three subgroups based on their amount of dietary protein, which ranged between 10% and 30% of their total intake. Urinary albumin excretion (UAE) was used as a marker of renal impairment, and body weight fasting (F) and postchallenge (P), blood glucose (BG) levels, and systolic blood pressure (SBP) were all measured during various experimental periods up to 28 weeks of age.

RESULTS

The OLETF rats fed with the high-calorie diet started to gain weight at 12 weeks, and their FBG and PBG were elevated at 22 weeks, while SBP did not differ between the two groups. In addition, UAE increased significantly in the rats fed with the high-calorie diet. However, the increasing rates of UAE with age were higher in the rats with a higher protein diet within the same energy groups. UAE correlated well with the amounts of dietary energy and protein at 16 and 28 weeks of age, while it correlated with both the FBG and PBG at only 28 weeks of age. A linear regression analysis, using the data obtained at 28 weeks, showed that the amount of protein intake and FBG explained 63.4% and 23.9% of the variation in UAE, respectively. Histological studies revealed that protein and energy restriction markedly reduced the sclerotic changes of the glomeruli.

CONCLUSION

Dietary protein restriction starting very early in the life of OLETF rats, in combination with energy restriction, clearly suppressed UAE and the typical morphological changes that otherwise occurred at around 16 weeks of age. This method also seemed to be more effective than energy restriction alone in slowing down any increase in UAE. The influence of BG levels on UAE was lower at an early age, while it became an increasingly important factor at later ages in the experimental rat model.

Obligatory nitrogen losses in pregnant rats with special reference to estimation of net protein utilization

For estimation of net protein utilization of dietary proteins during pregnancy, obligatory nitrogen losses were measured in protein-deficient rats in which pregnancy was maintained by administration of ovarian steroids. On shift from normal to protein-free diet, urinary nitrogen, expressed as mg/day or mg/100 g BW per day, decreased initially rapidly and then gradually during the first two weeks in both pregnant and nonpregnant rats. However, urinary endogenous nitrogen increased during the final week of pregnancy, whereas it continued to decrease in nonpregnant controls. The endogenous urinary nitrogen excretions during early-mid and late pregnancies were significantly higher in pregnant rats (666 mg/15 days and 234 mg/6 days, respectively) than in nonpregnant animals (585 mg/15 days and 153 mg/6 days, respectively), indicating pregnancy-induced protein hypercatabolism. The metabolic fecal nitrogen excretions in pregnant and nonpregnant rats were comparable. In pregnant rats, a protein-free diet resulted in decrease of basal energy expenditure, from 24 kcal/day on day 1 to about 15 kcal/day on days 16, 19 and 22 of pregnancy. Thus, the ratio of endogenous urinary nitrogen to basal energy expenditure increased in late pregnancy, indicating that "the law of a constant relationship of minimal nitrogen and energy output" is not applicable to the pregnant animals. We discuss which values for obligatory nitrogen loss should be used for estimating the net utilization efficiency of dietary proteins in pregnancy.

Prolonged fasting as conditioned by prior protein depletion: effect on urinary nitrogen excretion and whole-body protein turnover

To study the influence of previous dietary protein depletion on nitrogen (N) loss and protein turnover during a total fast, we measured plasma leucine kinetics and urinary N and 3-methylhistidine (3MH) excretion in obese and normal subjects. In one study, 10 moderately obese women fasted for 2 weeks after adaptation either to a normal maintenance intake of 80 g protein and 150% of estimated resting energy expenditure (control group), or to 10 days of a 950-kcal, 200-g carbohydrate, 4-g protein diet (depletion group), with measurement of postabsorptive (or fasting) plasma leucine turnover on the maintenance diet and after 3 and 10 days of fasting. As measured after 10 days of fasting, body N loss was blunted by 17% when preceded by the protein-deficient diet. Plasma leucine flux and oxidation of the control group increased in early fasting and decreased by 10 days, in accordance with previous reports. Results for the depletion group were similar in absolute magnitude, despite the preceding protein-deficient diet. In a second study of five normal men, leucine kinetics were measured on a maintenance diet, after 10 days of a protein-free diet, and after 3 days of fasting. After protein depletion, leucine flux decreased by 19% (P less than .05). After 3 days fasting, leucine flux was 16% higher than on the maintenance diet (P less than .05), but 44% higher than the value on the protein-free diet 3 days earlier (P less than .05).(ABSTRACT TRUNCATED AT 250 WORDS)

Urinary excretion of collagen metabolites in protein malnutrition

The effect of protein deficiency on the urinary excretion of hydroxyproline (total, nondialysable, dialysable and free fractions) and hydroxylysyl glycosides (glucosyl-galactosyl-hydroxylysine, glc-gal-hyl and galactosyl-hydroxylsine, gal-hyl) was investigated in female albino rats. In comparison to controls, the protein deficient animals were found to excrete significantly decreased amounts of urinary hydroxyproline fractions from the 7th day onwards. The excretion of total hydroxylysyl glycosides in urine parallels the excretion of hydroxyproline. The urinary output of both glc-gal-hyl and gal-hyl was also appreciably lower in deficient animals. The normal ratio of glc-gal-hyl/gal-hyl found in the urine of protein deficient animals suggests that there is a similar decreased turnover of collagen in both bone and skin.

[Xanthurenuria at different stages of liver lesion induced by protein and choline deficiency]

The parallelism between xanthurenuria and liver affections caused by protein and choline deficiency was studied. At the stages marking the development of lipohepatosis and fibrosis there occurred an intensive passage of xanthurenic acid. The intensity of xanthurenuria at the stage marking the appearance of hyperplastic nodes continued to gain strength. With progressive advance of pathological changes, which by the 9--12th months of the experiment reached the stage of a fully developed nodular cirrhosis xanthurenuria gradually stopped.

[Method of calculating endogenous protein losses by the human body]

Investigations conducted with participation of 30 examinees in whom the state of proteinic deficiency or protein fasting was provoked by different protein content in their diets a general regularity in the nature of passage of total nitrogen with urine, faces and summarily through other routes was educed and described mathematically. The endogenous excretion of total nitrogen with urine in young males was found to comprise 2.5 +/- 0.08, with feces--0.75 +/- 0.03 and summarily via other routes--0.59 +/- 0.36 g per day. The mentioned results correlated closely enough with corresponding figures in pertinent literature sources.

[Influence of riboflavin on xanthuria leveloping as a result of protein and choline deficiency]

In growing male-rats intensively excreting xanthuric acid under the effect of rations deficient in protein and choline the passage of riboflavin with urine was determined. A series of tests with deficient choline in conjunction with a well-marked protein shortage revealed an intensive passage of riboflavin. On the other hand, a series of tests with deficient choline against the background of a moderate protein shortage failed to show any difference in the passage of riboflavin by test and control anomals. A single administration of 200 gamma of riboflavin to rats helped bring down elevated concentrations of xanthuric acid in the urine.

The isolation and identification of piperidine-2-carboxylic acid in the urine of protein malnourished rats

1. A previously unidentified compound first observed in the serum of children with kwashiorkor has been isolated from the urine of experimental protein malnourished rats.

2. On mass spectroscopy and nuclear-magnetic-resonance spectroscopy the unknown compound appeared identical with authentic piperidine-2-carboxylic acid. This identity was confirmed by the behaviour of the unknown and authentic compounds on paper, ion-exchange, thin-layer or gas-liquid chromatography.

3. The significance of the accumulation of piperidine-2-carboxylic acid in the serum of children with kwashiorkor is discussed.