Efficacy of silkworm (Bombyx mori L.) chrysalis oil as a lipid source in adult Wistar rats

Abstract

The effects of silkworm chrysalis oil, rich in n-3 α-linolenic acid (ALA), on lipid metabolism in Wistar rats were investigated. The rats were fed diets containing 7% soybean oil (control), silkworm chrysalis oil (SWO), or fish oil (FO) for 8 weeks. Plasma triglyceride and glucose levels were significantly lower in the SWO group after 8 weeks compared to the control and FO groups. The total cholesterol and blood urea nitrogen levels were higher in the control group than in the SWO and FO groups at 8 weeks post-consumption. However, aspartate amino transferase and alanine amino transferase levels were not significantly different among all groups. A higher arachidonic acid (AA) content was detected in the control group, while lower AA levels were observed with the increase in EPA and DHA in the SWO and FO groups. These results suggest that n-3 α-linolenic acid-rich silkworm chrysalis oil can improve hyperlipidaemia and hyperglycaemia.

Introduction

Dietary lipids play important roles in the maintenance of health and as a cause of disease. A number of studies and clinical investigations have focused on the metabolism of polyunsaturated fatty acids (PUFAs). Most of these studies place a considerable emphasis on the dietary intake of n-3 PUFAs, especially eicosapentaenoic acid (EPA, 20:5n-3) and docosahexaenoic acid (DHA, 22:6n-3), that are abundant in fish, shellfish, and sea mammals, and absent in land animals and plants (Din, Newby, & Flapan, 2004). The beneficial effects of these n-3 PUFAs on normal health and chronic disease, such as the regulation of lipid levels, cardiovascular disease, and immune function have been elucidated (Kris-Etherton et al., 2002, Hwang, 2000). In addition, n-3 PUFAs are essential for the normal growth and development of the brain and central nervous system, and may play an important role in the prevention and treatment of coronary artery disease, hypertension, diabetes, arthritis and cancer. DHA is a vital component of the phospholipids in cell membranes, especially in the brain and retina, and are necessary for their proper function. n-3 PUFAs also have anti-inflammatory, antithrombotic, antiarrhythmic, and vasodilatory properties, some of these effects being modulated through prostaglandins and leukotriene metabolism (Connor, 2000, Salem et al., 2001). It is also well known that n-3 highly unsaturated fatty acids (HUFA) are highly susceptible to oxidation. Several studies have shown that intake of oxidised oil can induce lipid peroxidation, which may affect the results negatively and even produce adverse effects (Engstrom et al., 2009, Kaasgaard et al., 1992).

Alpha-linolenic acid (ALA), the precursor of n-3 fatty acids, is an essential fatty acid found in high concentrations in certain plant oils, such as flaxseed, walnut, and canola oils. Silkworm chrysalis oil is a by-product obtained during the processing of rolling silk from cocoons. Chrysalis oil contains 40% total lipids with ALA as a prominent fatty acid, which can be converted to long chain n-3 PUFAs, and can therefore be used as a substitute for fish oil. The by-products of silkworm chrysalis are known for their nutritional value due to the presence of high protein and fat.

In some parts of Asia country, e.g. Thailand, China, Korea, India and Japan, the silkworm chrysalis is regarded as delicious food for human due to its nutritional value. Silkworm chrysalis constitutes 60% of dry cocoon weight. Hence, 23,334 tonnes of dry spent silkworm chrysalises is available in the country. Since chrysalises are reported to have 30% of fat and 50% of protein, around 7000 tonnes of fat are likely to be available from the chrysalises, as by products of the silk industry (Udayasekhara, 1994). The neutral lipid of silkworm chrysalis was considered to be a good source of ALA. This is based on numerous literature reports on fatty acid composition of neutral lipid, predominantly of silkworm chrysalis, Bombyx mori L. (Nakasone and Ito, 1967, Sridhara and Bhat, 1965).

A preliminary study in our laboratory found that silkworm chrysalis oil has a positive effect on reducing blood triglyceride (TG) levels in young Wistar rats. Contrary to the numerous studies on fish oil containing long-chain n-3 PUFAs, little is known about the effectiveness of ALA in lowering plasma lipid levels in adult rats (Agren et al., 1996, Harris, 1993, Kris-Etherton et al., 2002, Connor, 2000, Hwang, 2000). However, there have also been no reports describing the effects of silkworm chrysalis oil on lipid metabolism in land animals such as rats (Asano et al., 1994, Asano et al., 2001).

The aim of this study was to investigate the effects of dietary silkworm chrysalis oil, rich in ALA, on lipid metabolism in Wistar rats, and to estimate the nutritional value of silkworm chrysalis oil as a source of dietary lipids for adult Wistar rats.