What is kelp
Kelp (Laminaria japonica or Laminaria japonica) is widely cultivated large seaweeds, belonging to the brown algae and classified in the order Laminariales, and are an important food source in many Asian cultures. Kelp is available worldwide in grocery stores as it is a key ingredient of Japanese miso soup. Kelp (Laminaria japonica) is used for food in daily life and also used in traditional medicine in China, with China being the largest producer 1). According to the “Compendium of Materia Medica” 2), kelp is cold, salty, has efficacy in clearing water, is soft, firm, dissipating and can dissolve phlegm 3), as well as alleviate edema, and eliminate carbuncle. Kelp belongs to the Phaeophyta Laminariaceae Laminaria, containing laminarin, ammonium alginate, mannitol, vitamins, amino acids and various normal and trace elements, with a variety of 40 active components 4). The variety of physiological functions of the kelp relate to the biological activity of polysaccharides which can improve the immunity function, anti-aging, anti-tumor 5), anti-atheroscloresis, anti-diabetics 6) and other such biological activity.
Table 1. Raw Kelp nutrition facts
|Nutrient||Unit||Value per 100 g|
|Total lipid (fat)||g||0.56|
|Carbohydrate, by difference||g||9.57|
|Fiber, total dietary||g||1.3|
|Vitamin C, total ascorbic acid||mg||3|
|Vitamin B-12, added||µg||0|
|Vitamin A, RAE||µg||6|
|Vitamin A, IU||IU||116|
|Lutein + zeaxanthin||µg||0|
|Vitamin E (alpha-tocopherol)||mg||0.87|
|Vitamin E, added||mg||0|
|Vitamin D (D2 + D3)||µg||0|
|Vitamin K (phylloquinone)||µg||66|
|Fatty acids, total saturated||g||0.247|
|Fatty acids, total monounsaturated||g||0.098|
|Fatty acids, total polyunsaturated||g||0.047|
|20:5 n-3 (EPA)||g||0.004|
|22:5 n-3 (DPA)||g||0|
|22:6 n-3 (DHA)||g||0|
|Fatty acids, total trans||g||0|
Table 2. Dried Kelp nutrition facts
|Nutrient||Unit||Value per 100 g|
|Total lipid (fat)||g||0|
|Carbohydrate, by difference||g||60|
|Fiber, total dietary||g||30|
|Vitamin C, total ascorbic acid||mg||24|
|Vitamin A, IU||IU||1000|
|Fatty acids, total saturated||g||0|
|Fatty acids, total trans||g||0|
- In the United States, recommended daily allowances (RDA) for iodine intake are 150 μg in adults, 220–250 μg in pregnant women, and 250–290 μg in breastfeeding women 9). The U.S. diet generally contains enough iodine to meet these needs, with common sources being iodized salt, dairy products, some breads, and seafood. During pregnancy and lactation, women require higher amounts of iodine for the developing fetus and infant. The American Thyroid Association recommends that women take a multivitamin containing 150 μg of iodine daily in the form of potassium iodide 10) during preconception, pregnancy, and lactation to meet these needs 11).
- The American Thyroid Association advises against the ingestion of iodine and kelp supplements containing in excess of 500 μg iodine daily for children and adults and during pregnancy and lactation 12). Long-term iodine intake in amounts greater than the tolerable upper limits should be closely monitored by a physician. There are only equivocal data supporting the benefit of iodine at higher doses than these, including a possible benefit for patients with fibrocystic breast disease 13). There is no known thyroid benefit of routine daily iodine doses in excess of the U.S. recommended daily allowances (RDA).
- Kelp is a rich source of iodine that may interfere with thyroid replacement therapies. Kelp is available as a dietary supplement marketed for thyroid support. Iodine content in kelp varies depending on harvest location and preparation. Its average iodine content has been estimated to be about 1,500 to 2,500 μg/g (microgram per gram) of dried preparation, that is i.e., 10 to 16.7 times the recommended daily allowance of 150 μg (microgram) 14), 15). In some individuals, the high iodine load from kelp supplements can result in thyroid dysfunction 16), 17). Iodine-induced thyrotoxicosis has been reported after consuming a kelp-containing tea 18).
- Herbal medicine, including kelp and kelp-containing dietary supplements, are not regulated by the FDA. In a study to evaluate the extent of arsenic contamination in commercially available kelp, nine samples were randomly obtained from local health food stores 19). Eight of the nine samples showed detectable levels of arsenic higher than the Food and Drug Administration tolerance level of 0.5 to 2 ppm for certain food products 20). Chronic arsenic toxicity may cause peripheral neuropathies, parasthesia, ataxia, cognitive deficits, fatigue, and muscular weakness 21). Gastrointestinal complaints include anorexia, hepatomegaly, jaundice, nausea, and vomiting. Skin afflictions may include erythema (skin redness), eczema, pigmentation (arsenic melanosis), diffuse alopecia (hairloss), keratosis (especially of palms and soles), scaling and desquamation, brittle nails, white lines or bands in the nails (Mees lines), and localized subcutaneous edema. White striae in the fingernails are consistent with a diagnosis of arsenical polyneuritis, even though urine and hair arsenic concentrations may be within normal limits 22).
- Past studies have shown that many herbal remedies are contaminated with potential toxicants including mercury and lead. To prevent more inadvertent poisonings, scientists and physicians recommend that manufacturers be required to prove safety before marketing their products.
Kelp is also available as a dietary supplement in the form of capsules, powder and teas. Kelp is a rich source of iodine that may interfere with thyroid replacement therapies. Kelp is available as a dietary supplement marketed for thyroid support. Iodine content in kelp varies depending on harvest location and preparation. Its average iodine content has been estimated to be about 1,500 to 2,500 μg/g (microgram per gram) of dried preparation, that is i.e., 10 to 16.7 times the recommended daily allowance of 150 μg (microgram) 23), 24).
In the United States, recommended daily allowances (RDA) for iodine intake are 150 μg in adults, 220–250 μg in pregnant women, and 250–290 μg in breastfeeding women 25). The U.S. diet generally contains enough iodine to meet these needs, with common sources being iodized salt, dairy products, some breads, and seafood. During pregnancy and lactation, women require higher amounts of iodine for the developing fetus and infant. The American Thyroid Association recommends that women take a multivitamin containing 150 μg of iodine daily in the form of potassium iodide 26) during preconception, pregnancy, and lactation to meet these needs 27).
The American Thyroid Association advises against the ingestion of iodine and kelp supplements containing in excess of 500 μg iodine daily for children and adults and during pregnancy and lactation 28). Long-term iodine intake in amounts greater than the tolerable upper limits should be closely monitored by a physician. There are only equivocal data supporting the benefit of iodine at higher doses than these, including a possible benefit for patients with fibrocystic breast disease 29). There is no known thyroid benefit of routine daily iodine doses in excess of the U.S. recommended daily allowances (RDA).
In some individuals, the high iodine load from kelp supplements can result in thyroid dysfunction 30), 31). Iodine-induced thyrotoxicosis has been reported after consuming a kelp-containing tea 32). In that case, the patient suffered from multinodular goiter in an endemic area of moderate iodine deficiency, that turned toxic after ingestion of the iodine-rich kelp and hyperthyroidism did not resolve spontaneously following discontinuation of the kelp-containing tea. In another case, a person with no evidence of pre-existing or underlying thyroid disease consumed for 10 days a daily dose of about 1800 μg iodine. Hyperthyroidism developed shortly, followed by overt hypothyroidism 33).
The normal thyroid gland of human adults secretes about 50 μg iodine daily, an amount the gland can take up in a dietary iodine intake between 100 and 150 mg per day 34). Iodine excess may be tolerated or may induce thyroid disease with hypothyroidism or hyperthyroidism with or without goiter and autoimmunity. Most individuals with a normal thyroid gland tolerate large iodine excess, maintaining thyroid hormones within the normal range, although serum free T4 and free T3 can usually be moderately reduced, TSH increased and a small goiter can develop 35).
The thyroid gland reacts to excess iodine intakes by different mechanisms the most important of which are the Wolff-Chaikoff effect, the down expression of the sodium–iodide symporter, and the block of hormone secretion from stores. The thyroid gland accumulates iodide from plasma against a concentration gradient up to 1:80. This iodide accumulation is made possible by the sodium–iodide symporter that transports two sodium cations and one iodide anion across the basal cell membrane from the exterior into the interior of the cell.5 TSH strictly regulates sodium–iodide symporter expression so that in iodine deﬁciency, TSH increases and in turn TSH gene expression is increased 36). In 1948, Wolff and Chaikoff reported that elevated plasma iodide levels was followed by a decreased organic binding of iodide in the thyroid 37). This effect (acute Wolff-Chaikoff effect) was of short duration and escape occurred in approximately 2 days, in the presence of continued high plasma iodide concentrations. The mechanism responsible for the acute Wolff-Chaikoff effect remains unknown and has been hypothesised to be caused by organic iodocompounds formed within the thyroid 38).
Following the escape from the acute Wolff-Chaikoff effect, the biosynthesis of sodium–iodide symporter can be shut down by a TSH-independent mechanism. Chronic excess iodide in rat thyroid cells had no effect on sodium–iodide symporter gene expression, whereas decreased sodium–iodide symporter protein at a post-transcriptional level 39). In rats in vivo, excess iodide administration decreased both sodium–iodide symporter mRNA and protein expression, by a mechanism that is at least in part, transcriptional 40). This mechanism has been proposed to account for the escape from the acute Wolff-Chaikoff effect. Excess iodine also blocks secretion of stored preformed hormone, as demonstrated in dog thyroid slices 41). Large quantity of iodide can also have cytotoxic effects as shown in normal human thyroid cells in vitro where it induces apoptosis, through a mechanism involving generation of free radicals 42). The normal thyroid gland adapts to iodine excess by these mechanisms so that most individuals tolerate a chronic excess of iodide without clinical symptoms.
Sometimes these mechanisms fail and iodine excess causes overt clinical hyperthyroidism or hypothyroidism. In some countries, iodine supplementation through iodised salt resulted in a signiﬁcant number of cases of severe hyperthyroidism 43). Hyperthyroidism is most commonly encountered in regions of chronic iodine deficiency, in participants with long-standing nodular goiter and in hidden Graves’ disease or toxic adenoma silent because of iodine deficiency.
Because kelp is a nutritional supplement and not a drug, the U.S. Food and Drug Administration (FDA) does not require manufacturers to demonstrate safety or efficacy. Herbal medicine, including kelp and kelp-containing dietary supplements are being used by an increasing number of people 44). In a study to evaluate the extent of arsenic contamination in commercially available kelp, nine samples were randomly obtained from local health food stores 45). Eight of the nine samples showed detectable levels of arsenic higher than the Food and Drug Administration tolerance level of 0.5 to 2 ppm for certain food products 46). None of the kelp supplements contained information regarding the possibility of contamination with arsenic or other heavy metals. Manifestations of chronic arsenic ingestion depend on both the intensity and duration of exposure.
Arsenic occurs naturally in some soils, and can contaminate bodies of water. The metalloid concentrates in fish that eat arsenic-rich algae and can also be found in plants that absorb it from the soil or water in which they are grown. Human exposure typically comes from diet, contaminated drinking water, or occupational exposures, as in smelters; people ingest an average of 40 μg per day.
Chronic arsenic toxicity may cause peripheral neuropathies, parasthesia, ataxia, cognitive deficits, fatigue, and muscular weakness 47). Gastrointestinal complaints include anorexia, hepatomegaly, jaundice, nausea, and vomiting. Skin afflictions may include erythema (skin redness), eczema, pigmentation (arsenic melanosis), diffuse alopecia (hairloss), keratosis (especially of palms and soles), scaling and desquamation, brittle nails, white lines or bands in the nails (Mees lines), and localized subcutaneous edema. White striae in the fingernails are consistent with a diagnosis of arsenical polyneuritis, even though urine and hair arsenic concentrations may be within normal limits 48).
Now researchers at the University of California, Davis, report the case of a 54 year old woman who received toxic doses of arsenic from kelp supplements 49). The patient had started taking kelp to treat minor memory loss and fatigue. She initially took the dose recommended on the bottle, then doubled it when her symptoms failed to improve. She took kelp for one year, during which her fatigue worsened to the point that she had to switch from full- to part-time work. She also experienced rash, diarrhea, vomiting, severe headaches, and hair loss. Three weeks after she abandoned kelp, the woman resumed full-time work. Her urine arsenic concentration dropped more than a third in two months and was undetectable after another two months. Eventually all her symptoms resolved.
Kelp vs Seaweed
Kelps are large brown algae seaweeds that make up the order Laminariales. Kelp grows in “underwater forests” (kelp forests) in shallow oceans, and is thought to have appeared in the Miocene, 23 to 5 million years ago 50). Kelp require nutrient-rich water with temperatures between 6 and 14 °C (43 and 57 °F). Kelps are known for their high growth rate—the genera Macrocystis and Nereocystis can grow as fast as half a metre a day, ultimately reaching 30 to 80 meters (100 to 260 ft) 51).
Through the 19th century, the word “kelp” was closely associated with seaweeds that could be burned to obtain soda ash (primarily sodium carbonate). The seaweeds used included species from both the orders Laminariales and Fucales. The word “kelp” was also used directly to refer to these processed ashes.
“Seaweed” is a colloquial term and lacks a formal definition 52). A seaweed may belong to one of several groups of multicellular algae: the red algae, green algae, and brown algae. As these three groups do not have a common multicellular ancestor, the seaweed are in a polyphyletic group. In addition, some tuft-forming blue-green algae (Cyanobacteria) are sometimes considered to be seaweed.
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