Richard M. Forbes’s research while affiliated with University of Illinois Urbana-Champaign and other places

Experiments were conducted to determine the effects of a dietary inadequacy of, and to estimate the dietary requirement for magnesium (Mg) in the growing Kitten. In Experiment 1, fourteen Kittens were fed a casein-based diet containing 50 mg/kg Mg without supplemental Mg or supplemented with 700 mg/kg Mg. Muscular weakness, hyperirritability, convulsions, reduced feed intake and poor growth were observed in kittens fed 50mg/kg Mg. Additionally, these animals exhibited reduced serum and bone Mg, and increased soft tissue calcium (Ca) concentrations. Graded levels of dietary Mg were fed in Experiment 2 (100, 400 and 700 mg/kg) and Experiment 3 (400, 550 and 700 mg/kg) to estimate the dietary requirement for Mg in the young kitten. Gross manifestations of a Mg deficiency were not observed. Serum Mg concentration declined within one week after kittens were fed 100mg/kg Mg, but were unchanged by feeding of 400, 550 or 700 mg/kg Mg. Calcium concentration was increased in aorta, liver and skeletal muscle, but not in kidney, and bone Mg concentration was reduced when kittens were fed 100 mg/kg Mg. No significant changes were observed in soft tissue Ca or bone Mg when kittens were fed diets supplemented to 400, 550 or 700 mg/kg Mg. We conclude that 400 mg/kg dietary Mg is adequate for growth, maintenance of blood and tissue Mg and prevention of calcification of soft tissues.

The extracutaneous tissues in pups fed a Zn-deficient diet were examined. Four pups were fed a Zn-deficient diet and 3 littermates were fed a Zn-adequate diet. After 5 weeks, the pups were euthanatized. Lesions found only in pups fed a Zn-deficient diet were located in the buccal mucosa, lymph nodes, spleen, and thymus. In the buccal mucosa, lesions consisted of irregular epithelial hyperplasia and disruption of epithelial cell layers. There was a marked absence of lymphocytes in the thymus and in T-cell areas of the lymph nodes and spleen.

The bioavailability of zinc from freeze-dried cooked beef was determined using log total tibia zinc and body weight gain as the response criteria. Control diets consisted of different levels of zinc carbonate added to an egg-white protein source. Experimental diets were made by substituting various levels of freeze-dried beef as the zinc source. All diets were isocaloric and isonitrogenous. Zinc in the control diets was utilized as effectively as zinc in the experimental diets. The relative biological value (RBV) of zinc (ZnCO3=100) in the experimental diet was 103 for 22-day weight gain, and 102 for total tibia zinc. These results indicate that zinc from cooked beef does not have an increased bioavailability over inorganic zinc added to an egg-white protein diet. Because a, large percentage of the total zinc in an animal is found in skeletal muscle, the content of zinc in two types of skeletal muscle was determined from animals fed different levels of dietary zinc. Animals consuming diets with zinc concentrations below their requirement had depressed growth rates; however, no significant differences were found in the zinc concentrations of either the soleus or plantaris muscle. The average zinc content of the soleus muscle (slow twitch oxidative fiber type) was 69 ppm and the plantaris muscle (fast twitch oxidative fiber type) was 15 ppm. These results indicate that the concentration of zinc in skeletal muscle is not significantly reduced in animals whose growth is restricted by low dietary zinc levels.

Young rats were fed diets containing 12 mg Zn/kg and varied levels of sodium phytate for 21-day ad libitum feeding periods. In experiment 1, Ca levels were 0.3, 0.5, 0.8 and 1.0%, and phytate:Zn molar ratio varied between 0 and 50. In experiment 2, Ca was maintained at 0.3%, Mg levels were 0.07, 0.22 and 0.37%, and phytate:Zn molar ratios were 0, 10, 20 and 30 at each Mg level. Major response criteria were body weight gain and tibia Zn accumulation. Weight gain was not influenced by Ca level in the absence of phytate or by phytate at 0.3% Ca; it was increasingly depressed as phytate was increased and by each increase in Ca in the presence of phytate. Total tibia Zn content was decreased at the highest Ca level in the absence of phytate; increasing the phytate progressively depressed tibia Zn at all Ca levels. Mg and phytate additions did not affect weight gain. Tibia Zn tended to be depressed by Mg and by phytate but these effects were significant only at the highest levels of the combined additions. These data corroborate and extend previously published findings on Ca and phytate effects on Zn utilization and show bone Zn accumulation to be a more sensitive criterion than weight gain in this connection. They also indicate that Mg exerts a less pronounced effect on Zn utilization in phytate-containing diets than does Ca.

The bioavailability of 65Zn from intrinsically and extrinsically labeled soy flour, acid-precipitated soy concentrate and neutralized soy concentrate was evaluated in rats. Weanling rats were fed marginally zinc-deficient diets, providing 8 ppm zinc from one of these three soy products, for 7 days. The rats then received a radioactively labeled test meal, identical in composition to the previous diet except that the soy product was either intrinsically or extrinsically labeled with 65Zn. After the test meal the rats were again fed diets the same as those consumed prior to the test meal. Whole-body retention of 65Zn at 24 hours and 12 days as well as 65Zn retained in tibias of rats given meals containing neutralized concentrate-based meals was significantly lower than for rats given meals containing the soy flour or acid-precipitated concentrate. In addition, retention of 65Zn from the extrinsically labeled acid-precipitated concentrate-based meal was significantly higher than from the same product intrinsically labeled. These findings confirm the results of previous feeding studies from which it was suggested that neutralization of soy protein concentrates reduces zinc bioavailability to the rat. In addition, the results are taken to suggest that experimental conditions may influence the validity of the extrinsic labeling technique for zinc.

A slope ratio assay was conducted with young albino rats to investigate the effect of maturity of soybean seeds on the bioavailability of their intrinsic zinc. Seeds (Williams) were harvested when well-developed but green (25% dry matter) and when mature (86% dry matter). The blanched seeds were wet-milled, freeze-dried and incorporated into 20% egg white protein diets at the expense of egg white to provide 20% protein diets containing equal calories and calcium but varying in zinc and phytate content. Zinc in the mature bean flour diets was 58% as available for weight gain and 57% as available for tibia zinc accumulation as was zinc in the green bean flour diets. The difference between zinc availabilities can be ascribed to the higher phytate content of the mature beans.

An assay was conducted with young albino rats to compare the bioavailability of zinc contained in calcium- and in magnesium-precipitated tofu to zinc supplied as the carbonate in egg white diets. Relative availability of zinc from both tofu preparations was 51% as measured by weight gain and 36-39% for log micrograms tibia zinc accumulation. In a separate experiment with zinc supplied at 9 mg/kg in all diets, the effect of increasing dietary calcium on weight gain and tibia zinc accumulation was tested. Performance of tofu-fed rats relative to zinc carbonate-fed rats at 0.4, 0.7 and 1.2% calcium was, respectively: 92, 67 and 48% for weight gain, and 70, 58 and 61 for tibia zinc content.

Six-week-old puppies fed a corn-soy based zinc-deficient diet developed lesions of parakeratosis, mild hyperkeratosis, alterations in germinal epithelium, erosions, ulcerations, vesiculation, alopecia, and inflammation of the skin. These changes were prominent in the skin of dependent regions, in areas of stretch and friction, and external contact. The epithelial lesions were reversible by adding zinc to the diet, with complete remission of external lesions by 6 weeks.