Review of Scientific Research on Edible Bird's Nest

Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University

Shun Wan CHAN

Abstract: Edible bird's nest is one of the widely used health foods in Chinese communities. The market generated by it is increasing because it exhibits a lot of beneficial effects to human beings. For its high market value, fake edible bird's nest and misleading promotional description emerged. This article briefly reviews the scientific research on authentication technologies and pharmacological effects of edible bird's nest. It is expected that people's knowledge on edible bird's nest could be enhanced.
摘要：燕窩在華人社會是其中一種非常普及的健康食品。一直以來，人們都相信燕窩有多種不同之效用，因此燕窩 市場不斷在擴展。由於燕窩的價值絕不便宜，故此假燕窩和一些帶有誤導性的推銷描述不斷在市場上出現。本文將簡略 回顧一些有關燕窩辨別真偽的技術與藥理效用之科學研究，希望藉此能加深社會對燕窩的認識。

Background
Swiftlets (collocaliini) are tiny insectivorous birds that are distributed from the Indian Ocean , through Southeast Asia and North Australia to the Pacific[1]. Amongst various species of swiftlets in the genus of Collocalia, only the nests of four species habituating in the Southeast Asian region have commercial value because of human consumption. They are Collocalia fuciphaga, Collocalia germanis, Collocalia maxima and Collocalia unicolor[2]. Collocalia species, average 6.5g in weight, have glossy plumage[1]. Their nests are constructed with salivary glue, a cementing substance, and may incorporate other materials such as vegetation or feathers. It takes about 20 days to finish the nest. The edible bird's nest ( 燕窩 in Chinese) making up with purely salivary glue are much more expensive than those incorporating with other materials (see Figure 1 in p.40).

In Hong Kong and in Chinese societies throughout the world, traditional Chinese medicine (TCM) is commonly used to treat diseases and enhance health. It is believed that TCM herbs have wide-ranging effects for enhancing health, lowering risk of diseases and promoting life span [3,4,5]. Being one of the TCMs, edible bird's nest is believed to have health enhancing effects such as anti-ageing, growth promoting and immunoenhancing properties. In fact, the medicinal use of edible bird's nest can be traced back to 17th century[6]. However, edible bird's nest is different from most of the TCMs. It is not only a medicine to make people healthy but also a pleasant food. Traditionally, it is double boiled with rocky sugar to make a delicacy known as "bird's nest soup".

Although the size of Collocalia is small, the market generated by it is tremendous. The estimated market of edible bird's nest in 2004 is worth about HK$ 3 billions in Hong Kong . The local market is the world's largest consumer of edible bird's nest[2,7]. The annual percentage increase of the local market is in doubledigit[7]. It may be due to the fact that Hong Kong people are more and more concerned about their health and the status of TCM has risen after a series of government policies on TCM. Moreover, a dual nature of edible bird's nest, that could be treated as medicine or/and food, may play a role. In the past, people could only buy dried edible bird's nests. For the advancement in food technology, large variety of edible bird's nest related products emerge to the market. They are readily to serve products. No cooking process is required. Amongst those new products, most of them are still in the traditional form as bird's nest soup, such as instant bird's nest in different concentrations. Some instant bird's nest may also supplement with other TCMs. Apart from the traditional form, there is a trend of using edible bird's nest extract as one of the chief ingredients of the products. These products focus mainly on the medicinal use of edible bird's nest. However, some of them may exaggerate the therapeutic use of edible bird's nest. For the limited supply and high price of edible bird's nest, it is not uncommon to hear reports of fake edible bird's nest in the market. The imitation substitute commonly used is the edible plant-exude, gum karaya or sterculia[8]. Recently, there are reports of fake edible bird's nest made from fishes' skin, mushroom or algae in China [9]. Therefore, it is a great concern on authenticating the genuineness of edible bird's nest. In this review, the author will briefly summarize some advanced technologies in authenticating edible bird's nest. The medicinal benefits of edible bird's nest with contemporary scientific evidence will also be given.

Authenticity of Edible Bird's Nest
The first comprehensive report on authentication of edible bird's nest can be traced back to the early 1990s. Sam et al., (1991) demonstrated the possibility to use scanning electron microscopy, energy dispersive X-ray microanalysis, flame atomic emission spectroscopy, inductively coupled plasma-atomic emission spectroscopy, ultraviolet-visible spectroscopy and other physico-chemical techniques to ascertain the authenticity of edible bird's nest[8]. Since they only made a comparison with some imitation bird's nest with substances from plant origins, it limited the generalizing ability of the techniques to other imitation materials. In addition, they relied on sophisticated equipment. It is hard for commercial testing laboratories to carry out. Recently, a research team in China has developed a simple but accurate and reliable spectrophotometry method to determine edible bird's nest content[10]. It could also be used to differentiate genuine edible bird's nest with saliva, pig's skin and Tremella fucifomis[10]. The method is based on the reaction between N-acetylneuramic acid and ninhydrin in acid solution. The method evaluates the internal content of N-acetylneuramic acid, a nine-carbon sugars, which is one of the major components in edible bird's nest.

Nutritional Content and Medicinal Use
Edible bird's nest contains mainly carbohydrates, amino acids and mineral salts. The major ingredients of edible bird's nest are glycoproteins[11]. Amongst the carbohydrates in edible bird's nest, sialic acid (9%) is the major one. It was found that exogenous source of sialic acid may contribute to neurological and intellectual advantages in infants[12]. However, the nutritional and biological mechanisms of sialic acid in human body are still under investigation. The other major carbohydrates include 7.2% galactosamine, 5.3% glucosamine, 16.9% galactose and 0.7% fucose[11].

Amino acids and mineral salts are also important components in edible bird's nest. Three non-essential amino acids (aspartic acid, glutamic acid, praline) and two essential amino acids (threonine and valine) can be found[11]. They could facilitate normal body functions such as repairing and immunity. Edible bird's nest is rich in mineral salts. It contains high content of sodium and calcium. It is because the source of edible bird's nest is derived from saliva Collocalia inhabiting mainly in limestone caves. In addition, low levels of magnesium, zinc, manganese and iron are also detected in edible bird's nest[8].

In spite of the long history of using edible bird's nest for medicinal purposes, there are not many scientific researches related to the therapeutic use of it in literature. The first scientific evidence was given by Ng et al. (1986) in Hong Kong. Edible bird's nest aqueous extract was found to potentiate mitogenic response of human peripheral blood monocytes to stimulation with proliferative agents, Concanavalin A and Phytohemagglutinin A[13]. It suggested that edible bird's nest might possess immunoenhancing effect by aiding cell division of immune cells.
One year later, other scientific evidence was published by Kong et al. They demonstrated an epidermal growth factor (EGF)-like activity in aqueous extract of edible bird's nest that stimulated the DNA synthesis in 3T3 fibroblast in a dose dependent manner in vitro[6]. EGF is a 6,000 Da polypeptide hormone produced by glands of the gastrointestinal tract, namely the salivary and Brunner's glands. It appears to play a crucial role in major normal cellular processes such as proliferation, differentiation and development[14]. It may offer a rationale for the medicinal use of edible bird's nest in ageing resistance. Since the receptor for EGF is highly expressed in a number of solid tumors, including breast, head-and-neck, non-small-cell lung, renal, ovarian and colon cancer[15], people are worried about a possibility to induce tumor progression and to resist chemotherapy/radiation treatment in tumor cells; in consequence, suggest that cancer patients should avoid edible bird's nest. In fact, there is no evidence supporting this suggestion. Currently we have evaluated the effects of aqueous extract of edible bird's nest on the viability on two human cancer cell lines, human breast cancer MCF-7 (ATCC HTB-22) and human liver cancer HepG2 (ATCC HB-8065). There was no observable effect on cell viability when comparing with the control group (unpublished data).

In 1994, a research team in China, evaluated the pharmacological effects of edible bird's nest and pearl powder containing formulation. The formulation was demonstrated to have immunoenhancing effects by elevating DNA synthesis of T-lymphacytes and circulating immunoglobulin M content in mice. In addition, the formulation also showed ageing retardation by increasing the level of superoxide dimutase[16]. However, the study did not explore whether the effects came from either edible bird's nest, pearl powder or both.

Further Studies
Edible bird's nest has been used for several hundred years. Its usage is based mainly on historical, anecdotal and observational reports of its benefit. Scientific evidence for its efficacy is still limited. The putative health benefits such as resisting ageing and improving immunity of edible bird's nest may be linked, at least in part, to EGF-like activity and mitogenic factor. However, there may be other mechanisms involved. In addition, there may be additive, synergistic or antagonistic effects between different components of edible bird's nest. Work is needed to establish health-related effects of edible bird's nest, for example, through assessing biomarker response, isolating and identifying the active components and investigating their possible interaction. In Hong Kong, many people take edible bird's nest regularly. It is worth establishing epidemiological study to measure relationship of consistent use of edible bird's nest and its putative beneficial effects in human beings. For the large market of edible bird's nest, developing a systematic method to identify the sources of edible bird's nest and check the authenticity of its sample is, undoubtedly, indispensable.