G. Sarwar Gilani

Impact of Antinutritional Factors in Food Proteins on the Digestibility of Protein and the Bioavailability of Amino Acids and on Protein Quality

Dietary antinutritional factors have been reported to adversely affect the digestibility of protein, bioavailability of amino acids and protein quality of foods. Published data on these negative effects of major dietary antinutritional factors are summarized in this manuscript. Digestibility and the quality of mixed diets in developing countries are considerably lower than of those in developed regions. For example, the digestibility of protein in traditional diets from developing countries such as India, Guatemala and Brazil is considerably lower compared to that of protein in typical North American diets (54-78 versus 88-94 %). Poor digestibility of protein in the diets of developing countries, which are based on less refined cereals and grain legumes as major sources of protein, is due to the presence of less digestible protein fractions, high levels of insoluble fibre, and/or high concentrations of antinutritional factors present endogenously or formed during processing. Examples of naturally occurring antinutritional factors include glucosinolates in mustard and canola protein products, trypsin inhibitors and haemagglutinins in legumes, tannins in legumes and cereals, gossypol in cottonseed protein products, and uricogenic nucleobases in yeast protein products. Heat/alkaline treatments of protein products may yield Maillard reaction compounds, oxidized forms of sulphur amino acids, D-amino acids and lysinoalanine (LAL, an unnatural nephrotoxic amino acid derivative). Among common food and feed protein products, soyabeans are the most concentrated source of trypsin inhibitors. The presence of high levels of dietary trypsin inhibitors from soyabeans, kidney beans or other grain legumes have been reported to cause substantial reductions in protein and amino acid digestibility (up to 50 %) and protein quality (up to 100 %) in rats and/or pigs. Similarly, the presence of high levels of tannins in sorghum and other cereals, fababean and other grain legumes can cause significant reductions (up to 23 %) in protein and amino acid digestibility in rats, poultry, and pigs. Normally encountered levels of phytates in cereals and legumes can reduce protein and amino acid digestibility by up to 10 %. D-amino acids and LAL formed during alkaline/heat treatment of lactalbumin, casein, soya protein or wheat protein are poorly digestible (less than 40 %), and their presence can reduce protein digestibility by up to 28 % in rats and pigs, and can cause a drastic reduction (100 %) in protein quality, as measured by rat growth methods. The adverse effects of antinutritional factors on protein digestibility and protein quality have been reported to be more pronounced in elderly rats (20-months old) compared to young (5-weeks old) rats, suggesting the use of old rats as a model for assessing the protein digestibility of products intended for the elderly.

Effects of dietary protein and fat on cholesterol and fat metabolism in rats

Abstract The influence of two fat sources (soybean oil and a 4:1 mixture of coconut oil and soybean oil) fed at three different levels (5, 10 and 20% by weight) and two protein sources (casein and gelatin supplemented with limiting amino acids) on cholesterol and fat metabolism in rats was determined. Fat or protein type had a significant ( P P de novo fat synthesis and cholesterol kinetics. De novo fat synthesis was highest in animals fed gelatin-soybean oil diets. De novo cholesterol synthesis followed the same patterns observed for fat synthesis. Since overall serum cholesterol levels were lower in rats fed gelatin-soybean oil diets, cholesterol clearance rates must have been higher to compensate for the increased synthesis observed in rats fed these diets.

Background on international activities on protein quality assessment of foods.

The subject of protein quality assessment of foods and diets was addressed at the Codex Committee on Vegetable Proteins (1982-1989), FAO/WHO (1989, 2001) and WHO/FAO (2002) expert reviews. These international developments are summarized in this manuscript. In 1989, a Joint FAO/WHO Expert Consultation on Protein Quality Evaluation reviewed knowledge of protein quality assessment of foods, and specifically evaluated amino acid score corrected for protein digestibility, the method recommended by the Codex Committee on Vegetable Proteins. The report of the Consultation published in 1991 concluded that the Protein Digestibility-corrected Amino Acid Score (PDCAAS) method was the most suitable approach for routine evaluation of protein quality for humans. The Consultation recognized that the amino acid scoring pattern proposed by FAO/WHO/UNU (1985) for preschool children was at that time the most suitable pattern for calculating PDCAAS for all ages except infants in which case the amino acid composition of human milk was recommended to be the basis of the scoring pattern. The rat balance method was considered as the most suitable practical method for predicting protein digestibility by humans. Since its adoption by FAO/WHO (1991), the PDCAAS method has been criticised for a number of reasons. The FAO/WHO (2001) Working Group on analytical issues related to protein quality assessed the validity of criticisms of the PDCAAS method. While recognizing a distinct regulatory use of protein quality data, the Working Group recommended that the PDCAAS method may be inappropriate for the routine prediction of protein quality of novel and sole source foods which contain high levels of anti nutritional factors; and that for regulatory purposes, the method should be revised to permit values of >100 for high quality proteins. In evaluating the recommendations of the Working Group, the WHO/FAO (2002) Expert Consultation on Protein and Amino Acid Requirements endorsed the PDCAAS method with minor modifications to the calculation method but also raised several issues. These included the calculation of scoring patterns; prediction of amino acid digestibility by faecal and ileal methods; reduced bioavailability of lysine in processed proteins; truncation of the amino acid score and consequent PDCAAS value; protein digestibility as a first limiting factor in determining the overall available dietary nitrogen; and the calculation of amino acid score for a dietary protein mixture. These concerns were considered particularly important in relation to the regulatory aspects of protein quality of foods, and their resolution was urgently recommended through a new separate expert review.

Impact of dietary protein on lipid metabolism in hamsters is source-dependent and associated with changes in hepatic gene expression.

This study tested the hypothesis that protein source is a factor determining the impact of the diet on lipid metabolism in hamsters. Twenty-eight hamsters of similar body weight were assigned for a period of 8 weeks to one of the following four diets (seven per group) containing either 20 % (w/w) casein (CAS), beef protein (BF), wheat gluten (WG) or soya protein (SOY). The fat composition of the diet was the same (15.5 % w/w) in all groups and provided SFA, MUFA and PUFA representative of the average Canadian diet. After an overnight fast, blood and liver were collected for the measurement of serum lipids, fatty acid composition of liver phospholipids and mRNA levels of selected genes involved in lipid metabolism. WG resulted in lower total cholesterol, HDL-cholesterol and non-HDL-cholesterol but, along with SOY, in higher mRNA levels of cholesterol 7 alpha-hydroxylase and LDL receptor. Furthermore, both WG and SOY resulted in lower 18 : 3n-3, 20 : 4n-6, total n-6 PUFA, 18 : 1n-9 and total MUFA, but higher 22 : 6n-3, total n-3 PUFA, 22 : 6n-3/18 : 3n-3 and 22 : 5n-3/18 : 3n-3 ratios in liver phospholipids, and higher hepatic Delta6-desaturase mRNA levels. These results show that the impact of dietary protein on lipid metabolism is source-dependent and associated with changes in mRNA abundances of key hepatic enzymes and receptors.

Effects of supplemental cystine or methionine on growth and lifespan of stroke-prone spontaneously hypertensive rats

Stroke-prone spontaneously hypertensive (SHRSP) rats are considered a suitable model for studying the effects of dietary and other environmental factors on human essential hypertension and haemorrhagic stroke. To investigate the suitability of a control diet for this strain of rats, we studied the effects of supplementing casein and soya protein isolate (SPI) with two sulphur amino acids (methionine and cystine) on the growth and lifespan of SHRSP rats. The source of dietary protein and the type of supplemental sulphur amino acid had significant (P < 0.05) effects on food intake and weight gain measured after 31 d of the feeding study, while only the type of supplemental sulphur amino acid had significant effects on mean survival times and the survival rates. On average, the casein groups had higher food intake and weight gain compared with the SPI groups. The methionine-supplemented groups had lower food intake but higher weight gain than the cystine-supplemented groups. Similarly, the methionine-supplemented groups had higher mean survival times and survival rates compared with the cystine-supplemented groups. The data would suggest that a control diet based on cystine-supplemented casein (as recommended for normal healthy rats by the American Institute of Nutrition), may not meet the sulphur amino acid requirements for SHRSP rats, and that the methionine-supplemented casein would be an appropriate control diet for this animal model.

Effects of folate, vitamin B-12 and vitamin B-6 supplementation on methionine-induced hyperhomocysteinemia in rats

An excessive intake of dietary methionine increases plasma total homocysteine (tHcy, an independent risk factor for premature cardiovascular disease) by enhancing the synthesis of homocysteine. Information on the influence of excess dietary vitamins involved in homocysteine metabolism on the methionine-induced hyperhomocysteinemia is, however, limited. Thus, a six-week study was conducted to determine the influence of excess folic acid, vitamin B-12 and vitamin B-6 on the methionine-induced hyperhomocysteinemia in rats. Supplementation of the casein control diet with 10 and 20 g/kg L-Met increased plasma tHcy to 2.0 and 8.0 times control, respectively. The hyperhomocysteinemia caused by the addition of 10 g/kg L-Met to the control diet, was completely counteracted by extra folic acid or three vitamins combined (folic acid, 2 mg/kg; vitamin B-12, 25 μg/kg; plus vitamin B-6, 6 mg/kg) but the addition of extra vitamin B-12 or vitamin B-6 alone had no effect on plasma tHcy. Similarly, extra dietary folic acid or the three vitamins combined caused substantial reduction in plasma tHcy of rats fed the control diet supplemented with 20 g/kg L-Met but addition of vitamin B-12 or vitamin B-6 alone exacerbated plasma tHcy.