HerbalEGram: Volume 16, Issue 2, February 2019

Editor’s Note: Each month, HerbalEGram highlights a conventional food and briefly explores its history, traditional uses, nutritional profile, and modern medicinal research. We also feature a nutritious recipe for an easy-to-prepare dish with each article to encourage readers to experience the extensive benefits of these whole foods. With this series, we hope our readers will gain a new appreciation for the foods they see at the supermarket and frequently include in their diets.

We would like to acknowledge ABC Chief Science Officer Stefan Gafner, PhD, for his contributions to this project.

By Jenny Perez^a and Hannah Bauman^b

^a ABC Education Coordinator

^b HerbalGram Associate Editor

Overview

The genus Corylus consists of approximately 15 species of deciduous nut-bearing shrubs and trees in the birch family (Betulaceae).¹ Hazelnuts are acquired from C. avellana (common hazel), which is indigenous to Europe and western Asia, and C. cornuta (beaked hazel), which is native to the North American continent. Filberts are acquired from C. maxima (filbert tree), which is native to southeastern Europe.^1,2 The filbert tree grows to a height of 30 feet, while hazelnut species have a shrub-like growth habit and reach a height of 10 feet.^1,2

Corylus species have leaves that are alternate, serrate, obovate, and hairy. Hazel trees are monecious; the slender, greenish-yellow male catkins and smaller red-centered clusters of female flowers appear on the same tree in late winter.^1,2 The immature clusters ripen in late summer when their fuzzy outer husks begin to open and the hard, glossy hazelnut seeds fall to the ground.² The nut is oblong in shape, 1-4 cm (0.5 to 1.5 inches) long, and matures inside a partially or fully closed husk or involucre.^1,3 While sharing the same buttery sweet flavor, filberts are slightly larger than hazelnuts and have a fringed husk.²

Historical and Commercial Uses

Native to northern temperate climates, hazels were some of the first shrubby trees to migrate north over Europe after the last Ice Age.^1,4 Evidence from peat layers dated to 75000-5500 BCE indicate that hazel pollen grains outnumbered those of any other tree during that time period.⁴ Charred hazelnut shells were discovered in Mesolithic and Neolithic sites in northern Europe, which indicates that the nut was part of the residents’ diet. The filbert has been consumed in China for more than 5,000 years and was wild harvested by ancient Romans as food.²

The hazel tree has a rich mythological history and was thought to possess mystical powers.^2,4 The Roman god Mercury carried a winged wand made from hazel branches.⁴ Further, hazel was considered the wood of choice for divining rods, which were used to detect underground mineral ores.^2,4 For the Celts in sixth-century Britain, the hazel tree was considered a tree of knowledge and a potently protective plant.⁴ Hazel twigs were brought inside for protection against evil spirits on Midsummer’s Eve when the barrier between the natural and supernatural worlds was considered to be most fragile. Hazelnuts have also long been a symbol of fertility and were part of marriage feasts and celebrations.²

The common name “filbert” is derived from St. Philibert, a French abbot who died in 684. August 20, the saint’s feast day, coincides with the time when hazelnuts begin to ripen.⁴ In 1942, the American committee on horticultural nomenclature used the name “filbert” when referring to nuts from trees in the genus Corylus. However, “hazelnut” is the name used internationally.

Hazelnut species are valuable as hedgerow and ornamental plants, with their dangling catkins announcing the arrival of spring and decorative autumn foliage as winter approaches.¹ Corylus species are deep rooted and prefer well-drained soil and full sun for optimal nut production. Ideal climates are those that offer mild winters, warm springs, cool summers, and a nearby body of water to moderate temperatures.⁴

Hazelnuts are wild-harvested or cultivated commercially in temperate countries including Spain, France, Italy, and Turkey.³ Hazelnuts are the seventh leading nut crop globally, with an average annual production of 475,000 tons.^2,5 Harvesting occurs in autumn, and the hazelnuts are either sold fresh, with their husks intact, or dried.³ In Turkey, where hazelnuts have been cultivated for more than 2,500 years, annual production ranges from 350,000-600,000 tons, which accounts for approximately 70% of global commercial hazelnut production.² According to the 2016 Food and Agriculture Organization statistics, Turkey is the largest producer of hazelnuts followed by Italy, the United States, and Azerbaijan.⁵

Commercially grown hazelnuts are a popular dessert nut, often used roasted and salted in cookies, tortes, and confectionery, such as pralines.^3-5 Chocolate-hazelnut spreads, like Nutella, are popular, as is the hazelnut liqueur known as Frangelico.^2,4 Oil produced from common hazel is used not only in food products but also in soaps and perfumes.¹ Corylus trees are considered softwoods and yield a reddish white timber that is used for small items like tool handles and walking sticks.

Phytochemicals and Constituents

Hazelnuts are nutrient dense and contain significant amounts of protein, healthy fats, and fiber. They also are considered one of the richest sources of vitamin E (α-tocopherol) and folate (folic acid) of all tree nuts.^4,6

Although a 3.5-ounce serving of hazelnuts contains 582 calories, it also supplies approximately 16 grams of protein, 13.6 grams of fiber, and 54 grams of heart-healthy fats, of which 82% are monounsaturated fatty acids (MUFAs) and 11% are polyunsaturated fatty acids (PUFAs).² Additionally, a 3.5-ounce serving of hazelnuts provides 89% of the recommended daily intake (RDI) for vitamin E and provides an array of B vitamins including 53% of the RDI for thiamine (B₁), 49% of the RDI for pyridoxine (B₆), 29% of the RDI for niacin (B₃), and 19% of the RDI for riboflavin (B₂). Of all nuts, hazelnuts have the highest content of folate, which plays an important role in proper cell differentiation and a healthy circulatory system.^7,8

Hazelnuts are important sources of macro- and trace minerals. These include potassium, an vital mineral for healthy nervous systems and a normal heart rate; phosphorus, which is integral for bones and energy production; copper, which aids in red blood cell production, iron absorption and bone health; magnesium, which maintains healthy nerve and muscle function, and in conjunction with calcium, helps to keep a steady heartbeat; zinc, which is necessary for the function of at least 100 different enzymes, is involved in many physiological functions, most notably the maintenance of a healthy immune system; iron, which is essential for the production of blood; bone-fortifying calcium; and chromium, which is involved in lipid metabolism and works with insulin to regulate glucose levels in the body.^2,8

While hazelnuts are abundant sources of antioxidant vitamins (e.g., tocopherols, folates) and polyphenols (e.g., flavan-3-ols, proanthocyanidins, flavonols), research shows that most of these compounds are located in the cuticle, or papery husk, of the hazelnut and are lost when hazelnuts are peeled or roasted.⁹ Additionally, like other tree nuts, hazelnuts contain phytic acid, which impairs the absorption of many minerals inherently present in the hazelnut (including calcium, magnesium, zinc, copper, and iron) by forming mineral chelates that are not readily absorbed in the gastrointestinal (GI) tract.¹⁰ Evidence suggests that the biggest impact of phytic acid on human nutrition is the reduction of the bioavailability of zinc. Diets high in phytic acid can produce significant mineral deficiencies over time, resulting in conditions like rickets and osteoporosis.¹¹ Phytic acid also inhibits the action of important digestive enzymes including tyrosinase, trypsin, pepsin, lipase, and amylase in the GI tract.¹⁰

Individuals with healthy gut flora that include beneficial probiotic strains such as Lactobacillus naturally produce higher levels of phytase, an enzyme which helps reduce levels of phytic acid. Sprouting nuts and seeds activates phytase, reducing or even eliminating phytic acid.¹¹ In addition to sprouting, soaking or roasting nuts and seeds will reduce the overall negative impacts of phytic acid on nutrient absorption and enzyme inhibition.

The bark, shells, and leaves of C. avellana are being investigated as a potential source of taxol, a compound originally isolated from the bark of an evergreen tree known as the Pacific yew (Taxus baccata, Taxaceae) which is used to treat a wide variety of cancers.¹² Corylus species are the first angiosperms discovered to contain taxol and taxanes, which may provide a more sustainable, consistently reliable source of taxol and related anticancer compounds and ease the demand for slow-growing Taxus species. Using biotechnological production methods to grow in vitro cultures, Corylus species may be able to produce taxol more efficiently and at lower cost.

Modern Research and Potential Health Benefits

Evidence suggests that consuming mixed nuts is associated with a decreased risk of cardiovascular disease (CVD) .¹³ Due to its high MUFA content as well as the presence of amino acids such as arginine, the hazelnut shares many of the same cholesterol-lowering benefits as other tree nuts.² Comparatively, hazelnuts are the second most abundant source of MUFAs among nuts and are rich sources of vitamin E, phytosterols, polyphenols, folate and fiber, all of which provide numerous health benefits which may help preventing chronic diseases such as CVD, type 2 diabetes mellitus (DM2), and cancer.^13,14

Cardiovascular Benefits

Hazelnuts contain tocopherols and phytosterols, L-arginine, caffeic acid, epicatechin, and quercetin, all of which have potential anti-atherogenic effects.¹³ Tocopherols are forms of vitamin E, a fat-soluble antioxidant compound that stabilizes and protects cell membranes.⁷ Phytosterols, structurally similar to the body’s cholesterol, compete with dietary cholesterol for absorption and thus reduce overall cholesterol levels.⁹ Hazelnuts are rich sources of the amino acid arginine. Arginine is involved in nitric oxide synthesis which regulates vascular tone and blood pressure, reduces plaque formation in arteries and prevents low-density lipoprotein (LDL) oxidation.^9,13 The flavonoids caffeic acid, epicatechin, and quercetin each contribute to hazelnut’s antioxidant and anti-inflammatory benefits.¹³

Hazelnut-enriched diets are associated with lower LDL and total cholesterol levels. In a systematic review, nine clinical studies were analyzed for overall effects of hazelnut consumption on blood lipids and body weight. Participants (N = 425) aged 18 to 55 years completed hazelnut intervention diets lasting between 28-84 days with daily doses ranging from 29 to 69 grams, with an average of 45 grams daily.¹³ In all studies, there was consistent evidence that hazelnut-enriched diets lowered LDL concentrations while not significantly affecting high-density lipoprotein (HDL) cholesterol levels. In addition to hazelnut’s low saturated fat and high MUFA content, the micronutrients and bioactive compounds found in hazelnuts — specifically magnesium, copper, vitamin E, tocopherols, and phytosterols — may also be involved in reducing LDL cholesterol levels.

An eight-week randomized, single blind, controlled, three arm, parallel-group intervention study of 66 children and adolescents with hyperlipidemia investigated the effects of hazelnut intake on lipid profiles and fatty acid composition of red blood cells (RBCs).¹⁴ Participants were divided into three groups: group 1 consumed unpeeled hazelnuts, group 2 consumed peeled hazelnuts, and group 3 (the control group) received only dietary advice and no hazelnut supplementation. Study results indicated that both hazelnut groups showed significantly increased levels of MUFAs and oleic acid and a favorable MUFAs/saturated fatty acids ratio, while no such effects were observed in the control group. It appears that the high oleic acid content in hazelnuts modulated the fatty acid composition of RBC membranes.

Metabolic-Related Benefits

In the United States, 25-30% of the population is considered obese, whereas in Europe, 12-14% of the population is obese.⁹ According to recent estimates, by 2030, the incidence of DM2 will increase from 285 million people worldwide to 439 million.⁶ DM2 accounts for 90-95% of diabetic cases and is a major risk factor for CVD. Individuals who have an increased waist circumference, dyslipidemia (high triglycerides, high LDL-cholesterol and low HDL cholesterols), or are diagnosed with metabolic syndrome are at significantly greater risk for CVD and DM2.⁹ Fortunately, there is mounting epidemiological evidence that shows an inverse, dose-dependent response between nut consumption and incidence of CVD.⁶

A 2013 randomized, controlled trial investigated the effects of hazelnut consumption on fasting blood sugar (FBS) and lipid profiles in patients with DM2.⁶ The 50 eligible participants were assigned to a control group or intervention group. In the intervention group, 10% of total daily calorie intake was replaced with an average of 29 grams of hazelnuts (depending on the energy intake of each participant) for eight weeks. Blood samples were collected from fasting patients at the end of the study. Results indicated that the most significant benefit experienced by the hazelnut intervention group was stabilization of HDL levels. In this study, replacing 10% of daily calories with hazelnuts had no significant effects on FBS, TC, TGs, or LDL concentrations.

A 2017 two-week randomized dietary intervention trial explored the potential effects of flavanols from cocoa powder (from cacao, Theobroma cacao, Malvaceae), hazelnut, and combinations of both on blood pressure and metabolic profile. Sixty-one healthy participants were divided into six groups and received one of six breakfasts which included varying amounts of unpeeled hazelnut paste, cocoa powder, both, or a breakfast with neither active ingredient.⁹ Results indicated that the intake of unpeeled hazelnut paste in healthy individuals improved blood lipid profiles by reducing LDL cholesterol and increasing HDL cholesterol. When hazelnut paste and cocoa were combined, there was evidence of a reduction in vascular reactivity as well as improved blood lipid profiles. The effects of hazelnut and/or cocoa breakfast intervention improved vascular flow and vascular reactivity more so than affecting lipid profiles.

Antioxidant Benefits

As stated previously, hazelnuts are a major food source of vitamin E, an important fat-soluble antioxidant that strengthen and protects cell membranes. Other antioxidant compounds present in hazelnuts are phenolic compounds including gallic acid, epicatechin, caffeic acid, p-hydroxybenzoic acid, and quercetin.¹³ Additionally, the copper content in hazelnuts is crucial for the production of superoxide dismutase (SOD), an important antioxidant enzyme responsible for neutralizing free radicals that can damage healthy cell membranes and tissues.^2,8

Vitamin and nutrient deficits become more common after the age of 70. This can be problematic since lower vitamin E and magnesium levels are associated with increased frailty and incidence of age-related diseases, such as Alzheimer’s disease.⁷ In an effort to explore ways in which foods may contribute to improvements in vitamin E and magnesium status, a 16-week dietary intervention study by Michels et al. (2018) investigated the effects of 57 grams of hazelnuts daily to older adults. Blood and urine samples were taken at the beginning and end of the study to assess blood levels of magnesium, lipids, glucose, insulin, C-reactive protein, and urinary vitamin E metabolites. Upon conclusion of the clinical study, blood level of magnesium had increased by 10% while blood levels of α-tocopherol did not significantly increase. These results differ from those of a clinical trial by Tey et al. in 2011 of 46 healthy participants who completed a randomized dietary intervention of consuming 30 grams daily of ground, sliced, or whole hazelnuts to assess absorption differences.¹⁵ Not only were there significant improvements in blood lipid profiles but there was also a significant increase in blood levels of α-tocopherol concentrations. Further studies are needed to confirm the extent to which hazelnuts improve vitamin E status.

Other small but significant positive outcomes from the 2018 study included an improvement in participants in the ideal range of LDL cholesterol (< 100 mg/dL) from 28% at study onset to 44% at study conclusion, as well as an increase in the number of participants (31% to 51%) who reached and maintained healthier blood glucose concentration levels (< 100 mg/dL) at the conclusion of the 16-week hazelnut dietary intervention.⁷

Consumer Considerations

Despite the continued demand for high-quality and healthy vegetable oils, there is an increased incidence of refining lower-quality products in order to produce cheap consumer goods.¹⁶ Lower-quality hazelnuts often are used to produce hazelnut oil and are subject to refining, bleaching, and deodorizing.^5,17 During the edible oil refining process, undesirable compounds, such as free fatty acids, waxes, pigments, metal ions and oxidation products are removed but there is an unfortunate caveat: The process of refining cooking oils alters levels of bioactive compounds and can also lower the overall oxidative stability of the refined oil.⁵ For example, lutein and zeaxanthin, the main carotenoids present in hazelnut, are lost completely during the bleaching and deodorization steps of refining hazelnut oil with only a partial loss of antioxidant-rich phenolic compounds and tocopherols.

The process of refining foods to create cooking oils not only results in a loss of valuable phytochemicals but also causes the formation of chloropropanol and its glycidyl esters, and trans fatty acids — all potentially harmful substances that may have a negative, cumulative impact on health.⁵ Among the varieties of commercially produced cooking oils, cold-pressed oils retain more of their nutritional properties than expeller-pressed oils, which expose beneficial compounds to further degradation due to excessive heat during processing. With the frequency in which edible oils are consumed, cleaner, more innovative technologies and strategies are needed to enhance consumer health.

Hazelnuts are available commercially raw or roasted, packaged whole with skins removed, finely ground into hazelnut meal, or in the form of hazelnut butter or paste.² Due to their high content of delicate MUFAs, it is best to store shelled nuts in air-tight bags in a cool, dark place or in the refrigerator or freezer and consume them within eight months.^2,18 Consumers should avoid purchasing pre-chopped nuts that are not vacuum-sealed to avoid accidentally ingesting rancid nuts. Hazelnut oil should be stored in the refrigerator for optimal freshness and shelf life.

The moisture content of nuts can be an indicator of shelf life. The low moisture content of hazelnuts (4-6% moisture in raw; 2-3% moisture in roasted) combined with its high vitamin E content provides an innate resistance to oxidation and bacterial contamination.² When stored as in-shell nuts at ambient temperatures, hazelnut quality is maintained for approximately eight months after harvest.¹⁸

Food-induced allergies, which produce a specific immunological response in sensitized individuals, are on the rise in adult and pediatric populations.¹⁹ Tree nuts are included on a list of 14 groups of specific substances or products associated with allergic reactions or intolerances that are required in the United States to be listed when present in commercially prepared products. Hazelnut is the most studied tree nut regarding potential allergic impact in sensitized individuals. Since the primary means of preventing allergic reactions is avoidance of the offending substance, proper food labeling is crucial in safeguarding allergic individuals.

Nutrient Profile²⁰

Macronutrient Profile: (Per 1 ounce hazelnuts [approx. 21 whole nuts])

178 calories
4.24 g protein
4.73 g carbohydrate
17.22 g fat

Secondary Metabolites: (Per 1 ounce hazelnuts [approx. 21 whole nuts])

Excellent source of:

Manganese: 1.75 mg (76.1% DV)
Vitamin E: 4.26 mg (28.4% DV)

Very good source of:

Thiamin: 0.18 mg (15% DV)
Magnesium: 46 mg (11% DV)

Good source of:

Vitamin B₆: 0.16 mg (9.4% DV)
Dietary Fiber: 2.7 g (9% DV)
Folate: 32 mcg (8% DV)
Iron: 1.33 mg (7.4% DV)
Phosphorus: 82 mg (6.6% DV)

Also provides:

Potassium: 193 mg (4.1% DV)
Vitamin K: 4 mcg (3.3% DV)
Niacin: 0.51 mg (3.2% DV)
Calcium: 32 mg (2.5% DV)
Riboflavin: 0.03 mg (2.3% DV)
Vitamin C: 1.8 mg (2% DV)

DV = Daily Value as established by the US Food and Drug Administration, based on a 2,000-calorie diet.

Image credits (top to bottom):

Hazelnuts image courtesy of Agnieszka Kwiecien.
Illustration of hazelnut plant parts from Flora von Deutschland, Österreich und der Schweiz by Otto Wilhelm Thomé; Germany, 1885.
Corylus americana immature hazelnut on the branch. ©2019 Steven Foster.
Corylus avellana hazel branch. ©2019 Steven Foster.
Corylus americana hazelnuts in husks. ©2019 Steven Foster.

References

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