吃褐色海藻可能预防黑色素瘤

Melanoma is a relatively rare skin cancer in most of the countries of the world, accounting for about 2 % of all cancers, and is responsible for 1 % of cancer deaths (Global Burden of Disease Cancer 2015). It has been called the most aggressive of all human cancers (Chin et al. 1998). Globally, an estimated 272,000 people are diagnosed with melanoma (Global Burden of Disease Cancer 2015; World Health Organization 2016). However, the incidence rates vary profoundly. In Japan, less than one person per 100,000 is diagnosed with melanoma, compared with Australia where current estimates for women are 30/100,000 and for men are 40/100,000 (Queensland Government 2015). Incidence rates have increased in many countries, but not in Japan. Five-year survival rates for people with melanoma depend on the stage of the disease at the time of diagnosis. If treated early, there is a high rate of cure, but for patients with late stage disease the success rate for therapies, even the new immunotherapies, is about 15–20 % at 5 years (American Cancer Society 2016). The development of treatment resistance suggests that much remains to be understood about how melanoma can remain dormant for years, avoiding detection by the body’s immune system.

Melanoma has been reported in dogs, horses, pigs, birds, and fish (Sweet et al. 2012; Williams et al. 2012). It also seems to be part of our human heritage, although found most commonly among fair-skinned people with red hair. The oldest record of melanoma dates back 2400 years ago to a mummy in Peru (Rebecca et al. 2012). About the same time in Greece, Hippocrates described melanoma when he wrote about “fatal black tumors with metastasis” (Chin et al. 1998). Generally, the modern risk factors include fair skin, light-colored eyes, red hair, inability to tan, intermittent exposure to strong sunlight, especially blistering sunburns during childhood, and family history of melanoma (World Health Organization 2016). In the last 100 years, the incidence of melanoma has dramatically increased. This has been largely attributed to changing attitudes towards having tanned skin. Coco Chanel, a French fashion designer and cultural icon in the 1920s, popularized tanned skin as glamorous and it became associated with having leisure time associated with wealth and high social status (Erdmann et al. 2013). Generally, this image of a healthy tan has persisted, particularly where people no longer do outside manual labor, such as farming. The use of tanning beds during cold dark winters continues despite recent scientific evidence that their use increases the risk of developing melanoma (Lazovich et al. 2010). People in Asia have a very different attitude towards tanning, and in general, avoid sun exposure. Pale skin is preferred which can be seen in the high demand for cosmetics that lighten skin (Quah et al. 2014).

Although much has been discovered about the genetic basis for melanoma, a cure for this cancer remains elusive. The development of melanoma is a complex multistage process, including a three-step initiation–promotion–progression system mediated by several cellular, biochemical, and molecular alterations (Rocha et al. 2007). Melanocytes are derived from pluripotent neural crest stem cells and are responsible for melanin production in specialized melanosomes. The pigment produced leads to variations in color of skin, hair, feathers, eyes, and scales in all vertebrates (Mort et al. 2015). Melanocytes and their development is modulated by the KIT (CD117) and microphthalmia-associated transcription factor (MITF), and is involved in pigmentation and cell death, which also plays a major role in the development and differentiation of melanoma, migration of melanoma cells, and drug resistance (Yajima et al. 2011; Lau et al. 2015). MITF is also involved in regulating the amount of melanin produced by melanocytes, thereby a prime target for treatments that aim to lighten skin.

Sunburns may be a major factor in reducing melanoma risk, however evidence has accumulated that diet may also play a role. Melanoma risk is positively associated with obesity as is a progression in diet-induced obesity (Chen et al. 2013; De Pergola and Silvestris 2013). People in Japan have a fraction of the rate of obesity (3.2 %) as that reported in high melanoma countries such as the USA (30.6 %), Australia (21.7 %), and New Zealand (20.9 %) (Organization for Cooperation and Development 2005). Diet choices have fueled this increase in obesity, with larger portion sizes, higher caloric intake, and increased intake of refined carbohydrates and fats (Tokudome et al. 2000). However, studies of high-fat diets have revealed contradictory data. A healthy low-fat American diet seems to be protective, but a change from a high-fat diet to a low-fat (<20 %) diet with increased fruit and vegetables and grain was associated with an increased risk of melanoma in a large study of nurses in the USA (Gamba et al. 2013). A study in Australia, where the incidence of melanoma is the highest in the world, likewise found suggestive evidence that a high-fat diet was not associated with high risk of melanoma and might even be protective (Granger et al. 2006).

The range of food eaten may be important. The Japanese diet is noted for its wide assortment of vegetables, fish, shellfish, and drinking tea, and thus is similar to eating a traditional Mediterranean diet, which is associated with about a 40 % decrease in the risk of melanoma (Fortes et al. 2008). This protective effect of a healthy diet was also observed in a second study, although the specific effects of a Mediterranean diet were limited to women under the age of 50 (Malagoli et al. 2015). There have been no studies of diet and melanoma in a Japanese population.

Several studies have previously evaluated the use of specific diets to treat melanoma patients. One retrospective comparative analysis reported a six-fold increase in five-year survival rates of melanoma patients treated with the Gerson diet (low sodium, high potassium, lactovegetarian diet that emphasizes fresh vegetables, fruit juices, and vitamin supplements). However, the flawed methodology limits the reliability of this study, but the results are intriguing (Hildenbrand et al. 1995).

Due to the increasing incidence and elusive cure for melanoma, there has been increasing interest in natural compounds that may hold promise (Chinembiri et al. 2014). Collins and his colleagues have reviewed the potential uses of seaweed extracts in a range of modern life diseases, but again, it does not include the consumption of whole seaweeds in a whole diet (Collins et al. 2016). Mohamed and colleagues have reviewed the health benefits of seaweed as a sustainable functional food for use in complementary and alternative therapies (Mohamed et al. 2012).

Dietary seaweed is one of the many differences between Western diets and a traditional Japanese diet. Given its unusual distribution as a culinary favorite, it is easy to speculate that including seaweed might make a difference. On average people in Japan eat 14.3 g day⁻¹ of seaweeds (Fukuda et al. 2006). The actual amount of seaweed is difficult to estimate since it is consumed as a hot water extract in soup, as a flavoring for other foods (dashi), as a condiment, as a side dish, and occasionally as part of an entrée. It is also commonly eaten as a snack. Currently, dietary seaweed is increasing worldwide, and it has been called the “superfood” by several prominent chefs including the British chef Jamie Oliver in his most recent book (Oliver 2015).

Several difficulties are associated with the study of seaweeds against any human disease. First, there are about 30,000 seaweeds, although only five Konbu (Saccharina japonica), Wakame (Undaria pinnatifida), Hijiki (Sargassum fusiforme), Nori (Pyropia tenera), and Mozuku (Nemacystus decipiens) are commonly eaten in Japan (Fukuda et al. 2006). Additionally, of the 267 studies using seaweeds in cancer studies (indexed by PubMed, 5 August 2016), many have used either a proprietary blend or rarely eaten seaweed, perhaps in hopes of identifying a patentable extract.