What is kale
Kale or leaf cabbage refers to certain vegetable cultivars of the plant species Brassica oleracea. A kale plant has green or purple leaves and the central leaves do not form a head (as with headed cabbages). Kales are considered to be closer to wild cabbage than most domesticated forms of Brassica oleracea 1).
Recently, kale has become popular due to nutritive components beneficial for human health. Kale is rich in vitamin K, vitamin C, vitamin A, carotenoids lutein and zeaxanthin, vitamin B6, folate, fiber and manganese. Kale is also an important source of phytochemicals such as glucosinolates that trigger associated cancer-preventive activity 2). Glucosinolates are sulfur-containing compounds that are broken down into isothiocyanates and indole-3-carbinol after chewing, chopping, or cooking. In nature, glucosinolates act as a first-line defense for plants, protecting them from environmental and biological stresses (insects, fungi, drought conditions). These same substances are being researched for their proposed ability in humans to affect chronic conditions including certain types of cancer and heart disease. Laboratory studies have shown that isothiocyanates and indole-3-carbinol inhibit inflammatory processes, prevent the growth and spread of tumor cells, and protect healthy cells 3).
There is ever-increasing evidence that a higher consumption of Brassica vegetables, for example, broccoli, cabbage, kale, mustard greens, Brussels sprouts, and cauliflower, reduces the risk of several types of cancer 4), 5). The anticarcinogenic effect of these vegetables has been attributed to decomposition products of glucosinolates, indoles, and iso-thiocyanates 6), phytoalexins, and other antioxidants 7). Indole-3-carbinol, a natural component of Brassica vegetables 8), has an interesting anticarcinogenic potential, acting via different metabolic and hormonal pathways 9) and have been proved to reduce the incidence of tumors in reproductive organs 10) and the growth of human breast cancer cells 11).
Overall, to date, the most promising anticarcinogenic dietary compounds have been detected in cruciferous vegetables and further elucidation of their protective mechanisms and the identification of other active constituents may contribute to the development of highly health-supporting Brassica varieties 12). Extracts of the different species of the Brassicaceae family show antioxidant effects 13) and decrease oxidative damage 14), while the juice of some Brassica species has been proved to protect human hepatoma cells from the genotoxic effects of carcinogens 15).
However, observational studies that follow groups of people over time have sometimes suggested a protective effect of cruciferous vegetables on various cancers and cardiovascular health, but findings have not been consistent 16), 17), 18), 19). There are several possible reasons for this discrepancy. The use of different study designs and methods, as well as the way in which the vegetables were cooked can change the bioavailability of isothiocyanates and their effects on the disease process. Genes may also play a role, as some people metabolize isothiocyanates more efficiently than others 20). It is also possible that the amounts of cruciferous vegetables consumed by study populations have no important impact on disease risks. More research with larger and longer-term studies is needed.
Furthermore, compounds such as glucosinolates and phytates may also have a negative effect on human and animal health. For example, glucosinolates and glucosinolate by-products can be toxic and are responsible for the bitter, hot, and pungent flavors of Brassicaceae vegetables 21). Also, thiocyanates, isothiocyanates, and oxazolidine-2-thiones have been shown to be goitrogenic 22), and while Brassicaceae vegetables can be a good source of minerals, some antinutrients, such as phytates, can decrease their bioavailability 23). Goitrogens are naturally occurring substances that can block iodine from entering the thyroid gland. Iodine is a trace mineral needed by the body to make thyroid hormones that promote normal metabolism. A deficiency of iodine can lead to a condition called goiter, or enlargement of the thyroid. Healthy persons who eat enough iodine and metabolize iodine normally will not be affected by dietary goitrogens. However, if one has an underactive thyroid called hypothyroidism and cannot produce enough thyroid hormone, eating excess goitrogens, especially in raw form, may further suppress thyroid activity and increase the risk of goiter. Those who have hypothyroidism specifically due to an iodine deficiency are at greatest risk. A simple solution is to cook cruciferous vegetables, which deactivates the enzyme responsible for causing the goitrogenic effect 24). Including a wide variety of vegetables each week other than cruciferous will also protect against eating an excess amount of goitrogens.
Regardless, kale remains a highly nutritious food to include as part of a healthful dietary pattern. The 2015-2020 Dietary Guidelines for Americans recommend that adults eat a variety of vegetables as part of a healthy meal plan, and specifically at least 1½ cups of dark-green vegetables (including cruciferous) per week 25).
People who are placed on blood thinners or anticoagulant medication to prevent blood clots are sometimes concerned about eating kale and other green leafy vegetables that are rich in vitamin K. Vitamin K has a unique action that assists in clotting blood, and can interfere with the effects of some blood thinners. However, people taking these medicines can safely eat these vegetables with a general precaution: eating a relatively consistent amount from day to day can allow your physician to adjust the dose of medication to balance the dietary intake of vitamin K, and should not interfere with the anticoagulant medication’s effectiveness. For those who are on blood thinners or anticoagulant medications, it would be wise to check with their physician and possibly a clinical dietitian.
Kale nutrition facts
Raw kale is composed of 84% water, 9% carbohydrates, 4% protein, and 1% fat (Table 1). In a 100 gram serving, raw kale provides 49 calories. Like collards, it contains a large amount of vitamin K: several times the Daily Value (DV). It is a rich source (20% or more of the DV) of vitamin A, vitamin C, vitamin B6, folate, and manganese (see table “Kale, raw”). Kale is a good source (10–19% DV) of thiamin, riboflavin, pantothenic acid, vitamin E and several dietary minerals, including iron, calcium, potassium, and phosphorus (see Table 1. “Kale, raw”).
Boiling raw kale diminishes most of these nutrients except for vitamin K.
Table 1. Kale (raw) nutrition facts
|Nutrient||Unit||Value per 100 g|
|Total lipid (fat)||g||0.93|
|Carbohydrate, by difference||g||8.75|
|Fiber, total dietary||g||3.6|
|Vitamin C, total ascorbic acid||mg||120|
|Vitamin A, RAE||µg||500|
|Vitamin A, IU||IU||9990|
|Lutein + zeaxanthin||µg||8198|
|Vitamin E (alpha-tocopherol)||mg||1.54|
|Vitamin D (D2 + D3)||µg||0|
|Vitamin K (phylloquinone)||µg||704.8|
|Fatty acids, total saturated||g||0.091|
|Fatty acids, total monounsaturated||g||0.052|
|Fatty acids, total polyunsaturated||g||0.338|
|20:5 n-3 (EPA)||g||0|
|22:5 n-3 (DPA)||g||0|
|22:6 n-3 (DHA)||g||0|
|Fatty acids, total trans||g||0|
Kale health benefits
Brassicaceae vegetables have long been recognised for their nutritional attributes. Numerous epidemiological studies indicate that the consumption of Brassicaceae vegetables is linked to a reduced incidence of cancer and cardiovascular diseases 27).
Chinese kale (Brassica alboglabra Bailey) is an original Chinese vegetable belonging to the Brassicaceae family. It is widespread in China and Southeast Asia, and a growing popularity is the US. Generally, Chinese kale is consumed for its bolting stems as common edible parts, and the tender rosette leaves are also widely consumed as a leafy vegetable. Chinese kale exhibits a high nutritional value since it is a rich source of antioxidants and anticarcinogenic compounds, including vitamin C, glucosinolates and phenolic compounds 28). Vitamin C (l-ascorbic acid) is an important primary metabolite of plants, functioning as a powerful antioxidant, an enzyme cofactor, and a cell-signalling modulator in a wide array of crucial physiological processes 29).
Glucosinolates are a group of sulphur- and nitrogen-containing secondary metabolites that are mainly found in the Brassicaceae family. Glucosinolates are also responsible for the bitter acidic flavors of Brassicacea species 30) and the hydrolysis by-products of glucosinolates, such as isothiocyanates, nitriles, and thiocyanates, are responsible for the hot and pungent taste of the mustard that is often objected to by consumers 31). Many of these degradation products are volatile (Valette and others 2006) and also play an important role in the characteristic aroma or off-odor of Brassicacae (Miyazawa and others 2005). Epidemiological studies have shown that the hydrolysed products of glucosinolates could protect humans against different types of cancer 32), 33). These compounds have both positive and negative nutritional effects 34), appearing to posess anticarcinogenic properties, but also quite different toxicological effects 35). The effects of specific glucosinolate degradation products on individual organisms vary and are not always known. If used in excessive quantity, many of these compounds can be highly toxic 36). Glucosinolates and their concentrations vary among the different groups of Brassicaceae. In kale, the predominant glucosinolates were found to be sinigrin and glucobrassicin. Sulforaphane, the active hydrolysis compound of glucoraphanin, is a strong naturally occurring inducer of phase II enzymes that detoxify carcinogens 37). Phenolics have received considerable attention due to their potential human health-promoting effects, including antioxidant, anti-inflammatory, antimicrobial, antiallergic and anticarcinogenic capacities 38).
Goitrin (reduces the production of thyroid hormones such as thyroxine), a naturally occurring compound in cruciferous vegetables, can easily be nitrosated if in contact with nitrites in gastrointestinal conditions, yielding the mutagenic compound N-nitroso-oxazolidone, with loss of sulfur 39). Additionally, goitrin, which is a decomposition product of progoitrin, is known to be strongly goitrogenic (affect thyroid function by inhibiting synthesis of thyroid hormones, resulting in enlargement of the gland [goiter]), inhibiting the synthesis of thyroid hormones, thyroxine, and tri-iodine-thyronine by a selective binding of iodine that prevents iodine intake by the thyroid gland 40).
The other decomposion products of glucosinolates, are thiocyanates, isothiocyanates, and oxazolidine-2-thiones 41), and have also been shown to be goitrogenic.
Brassica vegetables contain high levels of vitamins, including carotenes, tocopherols, vitamin C, and folic acid (Table 1). It is a well-known fact that the carotenes, tocopherols, vitamin C have the potential to prevent and treat malignant and degenerative diseases 42). Kale (Brassica oleracea) extracts are protective against reactive oxygen species (ROS) presumably due to the presence of vitamin C, quercetin, kaempferol, lutein, zeaxanthin 43), α-tocopherol, γ-tocopherol, and β-carotene 44).
In some Brassica species, carotenoid content is 2-fold higher than in spinach 45). Lutein has also been isolated from extracts of fresh raw kale (Brassica oleracea var. Acephala) 46) and high levels of other carotenoids, mainly β-carotene, were also detected 46). Two other vegetables, Brussels sprouts and green cabbage, have been reported to contain significant amounts of trans-β-carotene and cis-β-carotene 47). Carotenoids present in dark green leafy vegetables might be involved in the prevention of several diseases related to oxidative stress 48).
The predominant tocopherol in kale is α-tocopherol or vitamin E 49). α-tocopherol (vitamin E) is a fat-soluble antioxidant that stops the production of reactive oxygen species (ROS) formed when fat undergoes oxidation. Antioxidants protect cells from the damaging effects of free radicals, which are molecules that contain an unshared electron. Free radicals damage cells and might contribute to the development of cardiovascular disease and cancer 50). Unshared electrons are highly energetic and react rapidly with oxygen to form reactive oxygen species (ROS). The body forms reactive oxygen species (ROS) endogenously when it converts food to energy, and antioxidants might protect cells from the damaging effects of reactive oxygen species (ROS). The body is also exposed to free radicals from environmental exposures, such as cigarette smoke, air pollution, and ultraviolet radiation from the sun. Reactive oxygen species (ROS) are part of signaling mechanisms among cells. Scientists are investigating whether, by limiting free-radical production and possibly through other mechanisms, vitamin E might help prevent or delay the chronic diseases associated with free radicals.
In addition to its activities as an antioxidant, vitamin E (α-tocopherol) is involved in immune function and, as shown primarily by in test tube studies of cells, cell signaling, regulation of gene expression, and other metabolic processes 51). Alpha-tocopherol inhibits the activity of protein kinase C, an enzyme involved in cell proliferation and differentiation in smooth muscle cells, platelets, and monocytes 52). Vitamin-E–replete endothelial cells lining the interior surface of blood vessels are better able to resist blood-cell components adhering to this surface. Vitamin E also increases the expression of two enzymes that suppress arachidonic acid metabolism, thereby increasing the release of prostacyclin from the endothelium, which, in turn, dilates blood vessels and inhibits platelet aggregation 53).
High levels of vitamin C (also known as L-ascorbic acid) is required for the biosynthesis of collagen, L-carnitine, and certain neurotransmitters; vitamin C is also involved in protein metabolism 54). Collagen is an essential component of connective tissue, which plays a vital role in wound healing. Vitamin C is also an important physiological antioxidant 55) and has been shown to regenerate other antioxidants within the body, including alpha-tocopherol (vitamin E) 56). Ongoing research is examining whether vitamin C, by limiting the damaging effects of free radicals through its antioxidant activity, might help prevent or delay the development of certain cancers, cardiovascular disease, and other diseases in which oxidative stress plays a causal role. In addition to its biosynthetic and antioxidant functions, vitamin C plays an important role in immune function 57) and improves the absorption of nonheme iron 58), the form of iron present in plant-based foods. Insufficient vitamin C intake causes scurvy, which is characterized by fatigue or lassitude, widespread connective tissue weakness, and capillary fragility 59).
Folic acid reduces the risk of neural tube defects and may be associated with the reduced risk of vascular disease and cancer 60), while low-folate intake has been identified as a main cause of anemia 61).
Brassica plants have been found to be rich in many minerals including calcium and iron 62). Among the green leafy vegetables, kale is an excellent source of minerals 63), accumulating high levels of P, S, Cl, Ca, Fe, Sr, and K (Table 1). Different Brassica vegetables such as cauliflower, bok choy (B. rapa) stems and leaves, broccoli (B. oleracea v. botrytis), and kale (B. oleracea v. acephala) are reported to have high mineral contents 64) (Table 1). Interestingly, all these Brassica vegetables exhibit excellent calcium bioavailability 65).
It is composed of nonstarch polysaccharides 66) and is an important constituent in Brassicaceae vegetables, contributing to prevention of colon cancer 67). In white cabbage (B. oleracea var. capitata) dietary fiber represents one-third of the total carbohydrate content, the other two-third being low-molecular-weight carbohydrates, including glucose (37%), uronic acid (32%), arabinose (12%), and galactose (8%). The dietary fiber content of 6 cultivars of white cabbage (B. oleracea var. capitata) was evaluated finding that of the average total dietary fiber of 241 mg/g of dry matter, approximately 25% was soluble.
Brassicaceae species contain a range of signaling and regulatory compounds known to be involved in general defense mechanisms activated by pathogen and herbivore attacks on plants 68). These include salicylic acid, ethylene, H2O2, and jasmonic acid (an acid-derived oxylipin) 69) and signal peptides, such as systemin 70). Some of these are bioactive compounds that exhibited anticancer activity in animals when added to experimental diets 71). In particular, jasmonic acid and its derivatives, which represent the best characterized class of signal compounds, mediating the defense responses to wounding and herbivore attack in Brassicaceae 72), have been proved to inhibit the proliferation of human prostate cancer cells, while not affecting normal human blood cells 73). Conversely, triindole, a major digestive product of indole-3-carbinol, has been proved to exhibit strong estrogenic activities increasing proliferation of estrogen-dependent breast tumor cells. Thus, the contribution of triindole to the cancer preventive or cancer-promoting effects of triindole remains to be established 74).
Phenylpropanoids, Flavonoids, and Tannins
Flavonoids, hydroxycinnamic acids, phenylpropanoids, and other minor compounds (Quercetin, Kaempferol, Apigenin and Lutein) are considered to be among the health promoting compounds in Brassicacae species 75). Plant polyphenols are multifunctional, having diverse biological activities apart from acting as reducing agents 76). Phenolics also contribute to the bitter, astringent, and unpleasant flavor of rapeseed, though the threshold of this unpleasant flavor is higher for individual phenolic compounds than for the mixture 77). In spite of this, they are considered to be beneficial and harmless components of rapeseed meal.
The contribution of Brassica vegetables to health improvement has generally been associated with their antioxidant capacity and, undoubtedly, phenolic compounds are the major antioxidants of Brassica vegetables 78). Phenolics is a generic term that refers to a large number of compounds that can be classified in groups, namely, phenolic acids, flavonoids, isoflavonoids, lignans, stilbenes, and complex phenolic polymers 79). These antioxidants have proved to be good for human health and also useful as food preservatives 80).
Flavonoids are one of the most common and widely distributed groups of plant phenolics. Over 5000 different flavonoids have been described to date and they are classified into at least 10 chemical groups. Among them, flavones, flavonols, flavanols, flavanones, anthocyanins, and isoflavones are particularly common in the human diet 81). As these compounds have interesting biological activities, these are being used in numerous medical treatments 82) connected to cancer-prevention 83) and cardiovascular system protection, including inhibition of oxidative damage 84). At higher doses, however, flavonoids may act as mutagens, pro-oxidants that generate free radicals, and as inhibitors of key enzymes involved in hormone metabolism 85).
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