Scientific Name(s): Juglans nigra L., Juglans regia L.
Common Name(s): American walnut, Black walnut, Caucasian walnut, Circassian walnut, English walnut, European walnut, Persian walnut
There are approximately 15 species of Juglans walnuts. "Walnut" refers to several varieties, most commonly the English walnut (J. regia) and the black walnut (J. nigra). Walnut trees have short trunks with round-topped crowns and can grow up to 45 m in height. The black walnut is native to the deciduous forests of the eastern United States (central Mississippi and Appalachian regions) and Canada. Walnut tree leaves are compound and between 15 and 30 cm long. The male flowers are long, drooping catkins, while the female flowers are short spikes. Walnut trees self-pollinate and cross-pollinate. J. regia is native to Asia but is now cultivated in France and other parts of Europe, North Africa, North America, and East Asia.1 The wood is valued for its rich beauty and is used to make furniture, cabinets, and gun stocks. The black walnut fruit is an elongated drupe containing a 4-ribbed edible nut within a thick, hard, black shell that is smaller than the English walnut.1, 2
Walnuts have been found in prehistoric deposits in Europe dating from the Iron Age and are mentioned in Old Testament references to King Solomon's nut garden. The genus name Juglans comes from the Latin Jovis glans, meaning "nut of Jupiter" or "nut of the Gods." Many legends have been associated with the walnut; the ancient Greeks and Romans regarded them as symbols of fertility. In the Middle Ages, walnuts were thought to ward off witchcraft, the evil eye, and epileptic fits because of the belief that evil spirits lurked in the walnut branches.3
Historically, walnut oil was prescribed for colic, to soothe intestines, and to relieve diarrhea and hemorrhoids.3, 4, 5 Further folk uses include treating rickets, frostbite, and glandular disturbances, and as an astringent, tonic restorative, and disinfectant.6, 7 Some cultures use walnut bark for cleaning the teeth, possibly improving oral hygiene because it increases the pH of saliva.8 Walnut may possess anti-helminthic activity.7 It is considered helpful in inflammatory conditions, including rheumatoid arthritis, and certain skin disorders. Blisters, ulcers, itchy scalp/dandruff, sunburn, and perspiration are some of the conditions treated with various walnut preparations.6
The traditional black walnut herbal medicine is extracted from the black, tarry, sticky part in the outermost hull, and has been used in skin conditions, including eczema, pruritus, psoriasis, warts, and parasitic skin conditions. Treatment of eye irritations and styes are other uses for black walnut. Extract of black walnut was used to dye the hair, skin, and clothing.3, 6, 7 As a food, black walnut is commonly used in baked goods, candies, and frozen foods.3, 9
Walnuts are a common food source and are used in cooking and baking.3, 9 Walnut extract is an old-fashioned hair dye and also has been used to darken (stain) the skin.6, 9 Walnut shell flour has been used as a carrier for insecticides, filler for building materials, and stuffing in toys.9
Walnuts contain 3% to 4% water, 60% oil, and 15% to 20% protein, bearing approximately 700 calories per 100 g.9 The mineral content includes iron and zinc (approximately 3 mg per 100 g each), sodium (2 mg per 100 g), selenium (19 mcg per 100 g), calcium, magnesium, potassium, copper, and phosphorus.7, 9, 10 Vitamins E and C also are found in walnut.5, 11
Walnuts differ from other nuts in that they predominantly contain the polyunsaturated fatty acids alpha linolenic (n-3) acid and linoleic (n-6) acid, rather than mono-unsaturated fatty acids.12, 13 L-arginine (a precursor to nitric oxide) is also found in significant amounts in walnuts.12
The chief known chemical constituent in walnut is juglone (5-hydroxy-1,4-naphthagulone). Also present are alpha-hydrojuglone (1,4,5-trihydroxynaphthalene) and its glycoside beta-hydrojuglone, along with caffeic acid, ellagic acid, hyperin, and kaempferol, and the tannins galloylglucose and ellagitannins.6, 9 In one report, 45 volatile compounds were isolated from whole green walnuts.14 Gamma lactones are present in walnut oils.15
Uses and Pharmacology
Lipid profile and cardiovascular effects
Research reveals no animal data regarding the use of black walnuts for cardiovascular effect.
Clinical trials conducted in healthy adults12, 18, 19, 20, 21, 22 and in patients with type 2 diabetes23, 24 and metabolic syndrome25, 26, 27 have been critically reviewed.13, 28 Studies in patients with heart disease are lacking.28
The majority of studies show a reduction of total cholesterol and low-density lipoprotein–cholesterol to cardioprotective levels. Effects on high-density lipoprotein–cholesterol are inconsistent, as are those for triglycerides. The diminishing positive effects observed may be a consequence of increased fat intake with higher dosages used (and lesser effects due to lower dosages).13, 28
The role of walnut in atherosclerosis is unclear. Improved endothelial function has been demonstrated, possibly due to alpha linolenic acid or L-arginine content.12, 29 In another study, walnut activated the nuclear transcription factor identified in human atherosclerotic plaques in healthy men.30 Assessment of data from 6,705 participants without baseline atrial fibrillation in the PREDIMED trial revealed a significant relevant reduction in risk of atrial fibrillation (38%) with the Mediterranean diet supplemented with extravirgin olive oil (50 g/day or more) but not with the Mediterranean diet supplemented with nuts (almonds, hazelnuts, walnuts).31 There is general agreement that no gain in body weight results from the addition of walnuts to the diet.19, 23, 28
A small study (n=36) attempted to investigate the difference between the 2 species with respect to cardiovascular benefit of dietary supplementation. Effect on endothelial function was reported to be absent in participants fed black walnut in comparison to the English variant.32
Black walnut leaf was evaluated for its antioxidant activity. Radical scavenging and antiradical-generating effects were demonstrated.33, 34 For this reason, black walnut has been proposed as a candidate for chemotherapy because of the toxic nature of juglone and plumbagin; however, studies to support this are lacking.35, 36
Apoptosis and necrosis effects have been demonstrated in cancer cells with extracts of black walnut. Juglone and plumbagin, the yellow quinone pigments of black walnut, were shown to decrease cell viability and cell death.36, 37
Results of studies investigating the effects of walnuts on oxidative stress and apoprotein response are variable.25, 28, 30 Research reveals no clinical data regarding the use of black walnuts for antioxidant activity.
Walnut extract was demonstrated to inhibit and defibrillize amyloid beta protein—a feature of the amyloid plaque seen in the brains of patients with Alzheimer disease.38
A study from China found borneol-walnut oil to be superior to neomycin in treating otitis media without any toxic effects.39 The constituent juglone has been demonstrated to possess antimicrobial and antifungal effects.8
As a component of medical nutrition therapy for patients with types 1 or 2 diabetes, the American Diabetes Association Standards of Care (2014) recommend an increase in foods containing alpha-linolenic acid based on beneficial effects observed on lipoprotein profiles, heart disease prevention, and overall positive health in patients with diabetes (moderate-quality evidence). Likewise, as a component of medical nutrition therapy for patients with type 2 diabetes, the American Diabetes Association Standards of Care (2014) recommend higher quality dietary fat intake, as an alternative to decreased fat intake, by replacing saturated and/or trans fats with mono- and poly-unsaturated fatty acids in the diet. This Mediterranean-style approach to eating may improve glycemic control and cardiovascular disease risk factors (moderate-quality evidence).40
Walnut leaves have been approved by the German Commission E for external application for excessive perspiration and skin inflammation.43
Pregnancy / Lactation
White walnut is generally recognized as safe when used as food. The possibility of in utero sensitization has been debated.44
None well documented. Walnut interferes with the absorption of iron.28
Allergies to nuts are common in the United States (an estimated 1%)47 with walnut and other tree nut allergy considered to be second only to peanuts (a legume) in anaphylactic reactions. However, cross-reactivity to the proteins from tree nuts among peanut-allergic people is considered low. A co-allergy is probably the cause of allergic reactions among atopic individuals.44, 47 Cross-reactivity between walnut and peach lipid transfer protein allergen has been noted.48, 49, 50
Fatalities from anaphylaxis to walnuts have been recorded.48
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