Scientific Name(s): Corylus americana Walter (American hazel)., Corylus avellana L. (common hazel)., Corylus colurna L. (Turkish hazel).
Common Name(s): Dukkah, Filbert, Hazelnut
The genus Corylus includes at least 11 species of hazel trees and hybrids also exist.1 The trees grow from 3 to 8 m in height and were probably introduced into North America from Europe. Turkey is the leading producer of hazelnuts in the world.2 The deciduous trees bear large (6 to 12 cm) rounded leaves with soft hairs on both surfaces and a doubly serrate margin. The tree flowers in early spring before the leaves develop, and the nuts hang in clusters of 1 to 5. A "husk" encloses 75% of the yellow-brown, spherical/oval nut, leaving a scar at the base of the hazelnut kernel.1
In addition to consumption of the nuts as a source of protein, the leaves of the hazel tree have been used in folk medicine to relieve hemorrhoidal symptoms and varicose veins caused by purported vasoconstrictor properties.3
The minerals of the nut include calcium, copper, iron, magnesium, manganese, phosphorus, selenium, and zinc; vitamins include alpha-tocopherol, ascorbic acid, folates, niacin, riboflavin, thiamin, and vitamin K as phylloquinone; fatty and amino acids include alanine, arginine, aspartic acid, glutamic acid, and leucine.2, 4, 5
Tocopherol composition remains fairly consistent across geographic distribution and harvesting period.6 The hazelnut is the best dietary nut source of alpha-tocopherol, containing 2 to 3 times more than olive oil.7
The kernels contain about 60% seed oil2, 7 which is derived by cold pressing. The oil contains saturated fatty acids (7%), mono- (78%) and polyunsaturated fatty acids (10%), and phytosterols (0.1%).5, 8 Composition of the oil varies with geographic origin8; however, roasting the nuts prior to compressing appears to have little or no effect on oil composition.2, 9
Oleic, linoleic, linolenic, palmitoleic, and steric fatty acids are found in the oil.2, 7, 10 Beta-sitosterol, campesterol, avenasterol, and clerosterol sterols have been identified, and their concentrations are dependent on geographic origin and harvesting period.2, 7, 8
Uses and Pharmacology
Hazelnuts are a dietary source of protein, unsaturated fats, minerals and micronutrients, phytochemicals, alpha-tocopherol, and fiber; they are considered a good source of tocopherol when used to meet the recommended daily allowance for vitamin E.4, 7, 12
Rabbits fed a high-cholesterol diet and concurrent hazelnut oil maintained their baseline lipid profiles. No effect of hazelnut oil on the lipid profile was found in the control rabbits fed a normal diet. Hazelnut oil was also protective against atherosclerotic plaque formation upon histological examination.13
Results from a small stud revealed that hazelnut supplementation of 1 g/kg body weight/day showed reduced low-density lipoprotein and total cholesterol and increased high-density lipoprotein and triglycerides.12 Researchers have demonstrated improved lipid profiles (increased HDL and decreased LDL-C and total-C:HDL-C ratio) with hazelnut supplementation.14, 15
Trials and epidemiological studies have found the consumption of nuts to be protective against cardiac morbidity and mortality. The Physicians' Health Study, a large prospective cohort study of United States physicians, associated dietary nut intake with a reduced risk of sudden cardiac death (47% lower risk for those who consumed nuts 2 or more times a week) but did not find it protective against nonfatal myocardial infarction.16
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).17
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).18
Antioxidant activity and radical scavenging properties of the kernel and green leaf/flower portions of the hazel tree have been demonstrated.11 In doxorubicin-induced cataracts in rats, the administration of hazelnut was protective in lower-dose doxorubicin; hazelnut did not prevent cataracts at higher doses of doxorubicin. Because doxorubicin causes oxidative stress to cells, the anticataract effect is considered to be due to the antioxidant action of vitamin E and other elements found in the hazelnut (eg, copper, iron, selenium, zinc).19
An antiprostaglandin/anti-inflammatory action has been described.20
There are inadequate data to support dosing for therapeutic effect. One human trial used hazelnuts 1 g/kg body weight/day.12 The recommended daily amount of 15 mg vitamin E would be contained in approximately 100 g of nuts.5
Pregnancy / Lactation
Information regarding safety and efficacy during pregnancy and lactation is lacking.
Allergies to nuts are common in the United States (estimated 1%)23; however, cross-reactivity to the proteins from tree nuts (hazelnut) among people with peanut (legume) allergies is considered low. Rather, a co-allergy is probably the cause of allergic reactions among these atopic individuals. Cross-reactivity between hazelnut and Brazil nut has been documented; similar proteins are found in the nuts of these botanically distinct trees.24, 25
Anaphylaxis, urticaria, vomiting, and wheezing have been reported in response to hazelnut consumption. Immunoglobulin E (mast cell) and T-cell (cytokine) responses are described. Because sensitization is required, the possibility of these responses occurring in utero or via breast milk has been postulated, but no high-quality studies exist to support this theory.24 Oral desensitization to hazelnut may offer partial protection.23
The use of antiulcer drugs is another possible method of sensitization to hazelnut. Hazelnut allergens are not resistant to gastric and pancreatic acid digestion, and the use of famotidine, omeprazole, ranitidine, and sucralfate in animal and human experiments displayed some evidence of this effect.26
Placental transfer of fatty acids has been documented, but no teratogenicity studies specific to hazelnut have been found.10
A review by the Cosmetic Ingredient Review Expert Panel concluded there are insufficient data to support safety of hazelnut oil in cosmetic products.10
Aflatoxins are considered carcinogenic and are possible contaminants of hazelnuts.10
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