Muscle and liver protein metabolism in rats fed raw or heat-treated pea seeds

Abstract

Raw or extruded pea (Pisum sativum, cv. Ballet) diets with or without supplementary amino acids were fed for 15 days to young growing rats and the effects on tissue weights, liver and muscle protein metabolism and hormone levels monitored. Body weight gain, liver and gastrocnemius muscle weights and protein contents were reduced and some key hormones altered when rats were fed unsupplemented raw pea diets. This appeared to be a result of amino acid deficiencies in the diet, the action of antinutritional factors and the refractory nature of the reserve proteins and other seed components. However, this did not in itself improve the nutritional performance of the rats due to the overriding effects of the amino acid deficiencies in the pea diets. After supplementation, extruded peas supported much higher rates of growth and skeletal muscle deposition than did supplemented raw peas. Despite this, the weight gains remained less than achieved on a high quality control diet. Protein synthesis and degradation rates in skeletal muscles and total protein contents were similar to control values. The lower growth rate did not appear to be due to impaired deposition of skeletal muscle. Deposition of other body components, possibly lipids, may have been lowered by supplemented extruded pea diets. Liver protein levels were reduced in rats fed supplemented raw peas and blood corticosterone was elevated. In conclusion, extrusion treatment of peas in combination with amino acid supplementation appeared to abolish the negative effects of peas on skeletal muscle deposition.

Introduction

Legumes and among them peas, play an important role in the traditional diets of many regions throughout the world. Pea seeds are low in fat and are excellent sources of protein, dietary fiber and a variety of micronutrients and bioactive compounds [1]. This nutrient composition meets the requirements of the prudent diet, recommended for the prevention and treatment of chronic degenerative Western diseases [2]. Recent studies have shown that legume consumption can have beneficial effects by lowering the incidence of certain cancers [3], [4], protecting against osteoporosis [5], lowering serum cholesterol [6] or reducing body lipid accumulation [7] among others. However, there are a number of components in legumes that may exert a negative effect on the nutritional quality of the proteins. Among these factors are protease inhibitors and lectins [8]. Protease inhibitors exert their antinutritional effect by reducing or preventing digestion of nutrients and possibly impairing body metabolism, growth and health [9]. Lectin, by virtue of its ability to bind to carbohydrate receptors of gut epithelial cells, not only interferes with nutrient absorption but may also be taken up systemically and affect hormone balance and lipid and muscle metabolism [10].

As a protein source, most legumes do not meet the sulfur amino acid requirements of humans and some animal species. This could therefore compromise the efficiency with which their proteins can be utilized and their health-promoting value since sulfur amino acids have an important role in antioxidant defense and immune systems and influence of genetic function [11]. This deficiency could be overcome by supplementing legume-based diets with sulfur amino acids or plant materials rich in sulfur amino acids.

In recent years, extrusion cooking has been widely applied in the production of nutritious foods. It can be described as a high temperature, low liquid, and short time processing technique [12], [13]. This thermal processing procedure, which denatures and inactivates antinutritional factors and increases protein quality and availability, has become one of the most popular processes developed by the food and feed industry [14]. Extrusion treatment of dry beans is relatively new and provides an economically and convenient alternative to cooking and canning [15], [16].

The aims of the present study were to investigate in feeding trials with rats the effects of diets based on raw or extruded peas with or without supplementary amino acids on: 1) organ growth, 2) the fractional rates of protein synthesis (k_s), degradation (k_d), and accretion (k_g) in liver and hindlimb muscles and 3) blood hormones.