The red algal Porphyra sp. is thought to be the single most important seaweed genus for providing food globally (Madlener 1977, Indergaard 1983, Kenicer et al. 2000). It is a common and conspicuous component of rocky intertidal habitats in temperate marine waters worldwide and is widely utilized by humans for its nutritional values nearly universally where it is found (Mumford and Miura 1988). In some cultures it was used incidentally or only in times of need, in others its use reached a high degree of sophistication (Mumford and Miura 1988). Today, it is one of the most valued seaweeds with an annual value of over US$1.8 billion (Sahoo 2000).
The three classes of seaweed (macroalgae) can be distinguished according to the nature of their pigments; brown seaweed (Phaeophyta), red seaweed (Rhodophyra) and green seaweed (Chlorophyra) (Darcy-Vrillon 1993). The taxonomic groups that the genus Porphyra belongs to are: Kingdom – Protista, Phylum – Rhodophyta, Class – Rhodophyceae, sub-Class – Bangiophycidae, Order – Bangiales, Family – Bangiaceae (Chen 1999). Globally, nearly 139 species of Porphyra have been reported, including 28 species from Japan, 32 from the North Atlantic coasts of Europe and America, 26 from the Pacific coasts of Canada and the USA, and 27 species reported from the Indian coast (Yoshida et al. 1997, Lindstrom and Cole 1992, Lindstrom and Fredriqc 2003).
Species of Porphyra are identified under a variety of terms, some very general and some more specific. It is popularly known as nori in Japan, kin in Korea and ziicai in China. In Hawaii, Porphyra is known as limu or luau; in Scotland it is known as slack; in Ireland it is known as slacke, sloke, slouk, sloukaon, and sloukaum (Williams 1979). In North America and Europe it has been termed seaweed (with many First Nation translations), sluckus, marine algae, marine flora, benthonic algae, fixed algae, attached algae, red algae, red seaweed, laver, purple laver, purple seaweed, and purple algae (Turner 2003, Williams 1979). The breadth of terminology in such a large variety of cultures indicates that this genus is important worldwide.
Why is Porphyra use so widespread?
Seaweeds in general are known to be highly nutritious and Porphyra is probably one of the healthiest foods on our planet (Druehl 2000, Indergaard 1983, Turner 2003). Food gatherers from many different cultures have discovered that Porphyra tastes good, is nutritious, and is also a flavor enhancer for other foods (Mumford and Miura 1988).
Research into the value of Porphyra as food and medicine provides undeniable evidence of its health benefits. Porphyra is a food free of many of the sugars and fats implicated with the health problems with other diets (Mumford and Miura 1988). It is nutritionally valuable fresh or dried, or as an ingredient in a wide variety of prepared foods (Robledo and Plegrin 1997). Surprisingly, the process in which nori is processed prevents breakdown of its values, thus nutrient quality of dried Porphyra is almost as high as fresh Porphyra (Noda 1993).
Mumford and Miura (1988) state that its food value lies primarily in providing essential vitamins and minerals, such as B, C, and iodine. However, according to Indergaard (1983), the highest food value of Porphyra lies in its high protein content (25 – 35 % dry weight), as well as providing a substantially high level of vitamins, and mineral salts, especially iodine. Regardless of that debate, it is widely accepted to be beneficial in the human diet. The positive effects of specific carbohydrates, proteins, vitamins, minerals, and fatty acids of Porphyra have been studied extensively.
Edible seaweeds are rich in non-starch polysaccharides (dietary fibre) (Lahaye 1991, Wong and Cheung 2000, Gudiel-Urbano and Goni 2002). Upon examination of the digestibility of the dietary fibre of Porphyra, it was found that rats fed Porphyra experienced a metabolic adaptation after 12 days that increased bacterial utilization of the red algae polysaccharides (Gudiel-Urbano and Goni 2002). This adaptation involves an induction of bacterial enzyme systems capable of degrading and metabolizing red algal polysaccharides (Gudiel-Urbano and Goni 2002). It was determined that the nori diet contained more soluble fibre and somewhat more metabolizable energy than expected (Gudiel-Urbano and Goni 2002). Once a mammal can digest Porphyra, it has been demonstrated that there are a number of positive effects of its specific dietary fibre contents.
Benefits are realized from the sulfated galactan, porphyran, which is similar to agar, and occurs in relatively large quantities in Porphyra (Noda 1993, Zhang et al. 2003). This is a water-soluble dietary fiber that has functional activities such as an anti-blood coagulant, and shows anti-hypercholesterolemia, anti-tumor activity, and anti-oxidizing activity. Administration of porphyran demonstrated anti-tumor activity by preventing 1,2- dimethyl hydrazine induced intestinal carcinogenesis (Yamamoto et al. 1987), chemically induced mammary tumors (Noda 1993), and Meth-A fibrosarcoma (Noda et al. 1990). Additionally, porphyran has been reported to have antioxidant activities (Zhang et al. 2003). Evidence implicates that aging is associated with a decrease in antioxidant status and that age-dependent increases in lipid peroxidation are a consequence of diminished antioxidant protection (Zhang et al. 2003). Porphyran was successful in inhibiting lipid peroxidation in aging mice (Zhang et al. 2003).
Porphyra contains nearly 17 types of free amino acids, and the protein content of Porphyra sp. is comparable with that of high-protein plant foods such as soybean (Norziah and Ching 2000, Sanchez-Machado et al. 2004). Furthermore, the digestible proportion (69 – 75%) of the protein and carbohydrates is very high (Goni et al. 2002, Turner 2003).
The levels of taurine (>1%), a sulphated amino acid, are notable as this compound aids enterheptic circulation of bile acid, thus preventing gallstones through controlling blood cholesterol levels (Noda 1993, Tsuji et al. 1981, Tsuji et al. 1983).
Seaweed mineral content is higher than that of land plants and animal products (Ito and Hori 1989, Ortega-calvo et al. 1993). Bocanegra et al. (2003) found that the addition of algae to the diet did not significantly change total food intake or body weight gain of growing rats but did increase consumption of several minerals.
Porphyra is rich in iron (Shaw and Liu 2000) and zinc (Nisizawa et al. 1987). Haemoglobin regeneration assays demonstrated that the amount of available iron from Porphyra (0.28 mg to 0.91 mg Fe/g dry matter) was comparable to many iron-fortified foods (Amine and Hegsted 1974). Porphyra also contains relatively high contents of essential trace elements including copper (Cu), manganese (Mn), and selenium (Se) (Noda 1993).
Porphyra contains vitamins A (as beta-carotene), B12, other B-group vitamins, and vitamin C. One sheet of high-grade nori contains 27% of the recommended daily allowance of vitamin A as beta-carotene (Mumford and Miura 1988). With the rise in popularity of diets that reduce the risk of cancer and the recent discovery that beta-carotene is effective in lowering the risk of a wide variety of cancers, the use of Porphyra could become more widespread (Mumford and Miura 1988).
Nori can contain up to 12.0 – 68.8 ug/100g of dry weigh of vitamin B12, and this is roughly comparable to that in animal viscera (Noda 1993). Only two or three sheets of nori (3g) are sufficient to satisfy the recommended daily intake of vitamin B12 for the adult human (2 ug/day) (Watanabe et al. 1999). Porphyra also contains relatively high levels of choline and inositol (Noda 1993). Both are included in the vitamin B complex, and they play important roles in the utilization of fats and cholesterol in the body and in nourishing brain cells (Mindel 1981).
Finally, gram for gram, Porphyra contains 1.5 times more vitamin C than found in oranges (Noda 1993).
The fatty acids of seaweeds generally have linear chains, an even number of carbon atoms, and one or more double bonds (Shameel 1990). Although seaweeds are not a conventional source of energy, their polyunsaturated fatty acids contents can be as high as those of terrestrial vegetables (Darcy-Vrillon 1993). Lipids comprise 2-3% of dry weight of Porphyra (Noda 1993). Porphyra contains the essential fatty acids C18:2ω3 (linolenic acid), C20:4ω6 (arachidonic acid) and C20:5ω3 (eicosapentaenoic acid – EPA) (Sanchez-Machado et al. 2004). Interestingly, EPA has been recognized as being effective in preventing arteriosclerosis (Dyerberg et al. 1978). Furthermore, the ω6/ω3 ratio, which is recommended should be no higher than 10 in the diet as a whole, was at most 1.32 in Porphyra. Therefore, Porphyra may be of use for the reduction of ω6/ω3 ratio (Mahan and Escott-Stump 2000).
This brief report touches upon the many benefits that consuming Porphyra may have concerning human health. The nutritional benefits provided by Porphyra are widely known globally. From the long list of cultures (in the introduction) that have given special significance to Porphyra, it is conclusive that people have eaten it all over the world, not only in Japan as may be conceived by the majority of North Americans.
Sushi is becoming more and more popular, and people are beginning to prepare it at home. Seaweed salads are almost vogue. People are breaking the barriers of prejudice against eating seaweeds, but it’s still a long road ahead for Porphyra to be a part of the most people’s daily diet intake. When that happens, we’ll all look young and beautiful forever! Well…maybe fewer wrinkles.
So can you eat 6 – 9 grams (2 – 3 sheets) of nori per day? I sure can.
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