Walnut

Walnut is the name for several trees of the family Juglandaceae, primarily Juglans regia, and their hard-shelled fruits. It is considered native to a region encompassing Southeast Europe and western Central Asia; its natural range is uncertain due to its long history of cultivation as a crop, dating back many centuries. Juglans regia is also known as “English walnut” or “Acem walnut”. It is cultivated as a commercial crop in temperate regions worldwide and has hundreds of different cultivars. Both the nutritious walnut kernel and the walnut tree, used in fields ranging from furniture to paint, have been valued throughout history as a significant plant.

History

The use of walnuts by humans extends back to ancient times. Archaeological evidence indicates that walnut trees have been cultivated since around 7000 BCE, particularly in Iran and surrounding areas. Because of its origin in Persia (Iran), the walnut has also been called “Acem walnut” or “Persian walnut”. From around 1000 BCE, walnuts were cultivated in Europe and spread during Roman times to the Mediterranean and northern Europe.

• The Romans highly valued the walnut and named it Jovis glans (the nut of Jupiter); from this, the genus name Juglans derives, meaning “nut of the gods” in Latin.
• Throughout the Middle Ages, walnuts were widely used in Europe and Asia both as food and as a medicinal product. In the 17th century, European colonists introduced walnuts to the Americas, where cultivation began.
• Today, walnut is a global agricultural commodity produced commercially in many countries, including China, the United States, and Türkiye.

Botanical Characteristics

• The walnut is a large, broad-crowned, deciduous tree. A mature walnut tree can reach 10–25 metres in height and a trunk diameter approaching 2 metres.
• Its leaves are typically 20–45 cm long, compound with 5–9 leaflets arranged oppositely.
• The leaflets of some species have toothed margins, but in Juglans regia they are generally entire. The young branches of the walnut tree have pith with a hollow, segmented structure, and the leaves emit a distinctive odor when crushed.

The walnut tree is monoecious; that is, it bears separate male and female flowers on the same tree. Male flowers appear as pendulous catkins, arranged side by side on branches from the previous year, while female flowers occur in small clusters at the tips of current-year shoots. Pollination occurs by wind. The mature walnut fruit, ripening in autumn, has a thick, fleshy outer husk and an inner hard, woody shell (the walnut shell). Botanically, the walnut fruit is a drupe (stone fruit); when the green husk dries and splits open, the hard-shelled seed inside is revealed. This seed, known as the walnut kernel, consists of two characteristic brain-shaped lobes. The wood of the walnut tree is also valuable; the heartwood is dark coloured, the sapwood is lighter and denser, and it takes a fine polish, making it a preferred wood for furniture and carving.

Nutritional Value

Shelled walnut and extracted walnut kernel. Walnut is a highly nutritious food. The edible kernel consists of approximately 65% fat, 15% protein, 14% carbohydrates, and 6–7% fibre. One hundred grams of walnut kernel provides an average of 654 calories. Walnut oil stands out among plant oils for its high content of polyunsaturated fatty acids; about 72% of the total fat is polyunsaturated (58–59% linoleic acid, 13–14% alpha-linolenic acid), 18% is monounsaturated (oleic acid), and only 10% is saturated fatty acids. In this regard, walnut is one of the rare plant foods rich in both omega-6 (linoleic acid) and omega-3 (alpha-linolenic acid) fatty acids.

Walnut is also a high-quality plant-based protein source. Its protein profile is balanced in essential amino acids and contributes significantly to vegetarian diets. For example, when consumed with legumes, walnut can provide a large portion of the body’s required protein. More than half of its fibre content comes from dietary fibre, which benefits the digestive system and supports gut health.

Walnut kernel is rich in vitamins and minerals. It contains fat-soluble vitamin E and water-soluble vitamin C, B1 (thiamin), B2 (riboflavin), B6 (pyridoxine), folic acid (B9), pantothenic acid (B5), and niacin (B3). Among minerals, it is particularly high in manganese and copper; 100 grams of walnut provides approximately 160% of the daily manganese requirement and is a good source of copper. It also contains important minerals such as iron, zinc, magnesium, and phosphorus. Thanks to this rich nutritional profile, walnut is considered one of the “superfoods” and offers valuable contributions to balanced nutrition.

Health Effects

The health benefits of walnut consumption are supported by scientific research. Its high content of antioxidant compounds (e.g., vitamin E, polyphenols, and phytosterols) neutralizes free radicals that cause oxidative damage in the body, offering protection against various diseases. Regular walnut consumption is particularly known for its positive effects on cardiovascular health. Epidemiological studies have observed lower rates of cardiovascular disease in populations that frequently consume nuts such as walnuts. It has been reported that vitamin E in walnuts prevents oxidation of LDL cholesterol, thereby reducing the risk of arterial blockage. Additionally, walnut is a plant-based source of melatonin; its antioxidant effects have been found to benefit heart health and immunity.

The omega-3 fatty acids (ALA) in walnuts are recognized for their ability to regulate heart rhythm and reduce triglycerides. Therefore, walnut consumption may help improve risk factors for heart disease such as high cholesterol and high blood pressure. For instance, various studies have reported that diets including one handful (approximately 30–40 grams) of walnuts per day lead to reductions in total and LDL cholesterol levels. Based on this evidence, in 2016 the USA Food and Drug Administration (FDA) granted a qualified health claim that a diet containing 1.5 that (42 grams) of walnuts may reduce the risk of heart disease.

The effects of walnut on brain health and cognitive function are also under investigation. Walnut is believed to enhance mental function due to its resemblance to the shape of the brain. Some Modern studies partially support this belief: due to its richness in antioxidants and polyunsaturated fatty acids, walnut may slow age-related cognitive decline and show beneficial effects on memory and learning. In particular, regular walnut consumption in older age may have a protective effect against neurodegenerative diseases, as explored in animal experiments and limited human studies.

The metabolic health on benefits of walnut should also be noted. Its fibre and healthy fat content may increase satiety and aid in weight control. Studies have shown that individuals who regularly consume nuts such as walnuts may have a lower risk of obesity. Furthermore, walnut consumption is being studied for its potential to reduce type 2 diabetes risk and improve insulin sensitivity. However, walnut is a high-calorie food; excessive consumption may lead to weight gain, so portion control is essential.

On the other hand, walnut may not be safe for everyone. Like other tree nuts, it can cause allergic reactions in some individuals. Walnut allergy arises when the immune system overreacts to walnut proteins, leading to symptoms ranging from skin itching to life-threatening anaphylaxis. People with tree nut allergies should avoid consuming walnut. Otherwise, walnut is generally well tolerated and safe for most people; it has no toxic effects when fresh and stored under appropriate conditions. However, mouldy or long-stored walnuts may develop aflatoxins, and bitter-tasting walnuts should not be consumed.

Role in Traditional Medicine

The walnut tree and its fruit have been used for centuries in folk medicine for various purposes. Different cultures have believed in the healing properties of walnut leaves, husks, and oil. Due to their anthelmintic (against intestinal worms), antiseptic, and antidiarrheal effects, walnut leaves and especially yet unripe green walnut husks have been traditionally used. For example, in Central East, extracts made from walnut husks are commonly believed to expel intestinal parasites. Similarly, decoctions of walnut leaves have been applied topically to clean wounds and treat skin conditions such as eczema and acne.

In traditional Anatolia medicine, walnut was also used for chronic conditions such as diabetes and asthma. Ottoman physicians classified walnut leaves among herbs that “alleviate sugar disease”. It is believed that walnut leaf tea may help regulate blood sugar levels. In Central Asian and Iranian medicine, mixtures of walnut husk and leaves have been used as supportive treatments for stomach pain and ulcerative intestinal diseases.

In addition, walnut has been used for respiratory ailments. For instance, syrups made from walnut husks have been recorded as treatments for cough and bronchitis. There are beliefs that walnut consumption is beneficial for Asthma patients. In traditional Chinese medicine, walnut (Hútáo 胡桃) is regarded as a tonic that strengthens the kidneys and reduces asthma symptoms.

Walnut tree has also been used in Tooth health; in some cultures, branches of the walnut tree were cleaned and used as a tooth-cleaning stick similar to a miswak. Grinding walnut husk and adding it to toothpowders was also a method used to whiten and strengthen teeth. These examples illustrate the wide range of uses walnut has had in folk medicine. However, from a modern medical perspective, the efficacy of these traditional practices lacks substantial scientific evidence.

Research Findings in Modern Medicine

In recent years, modern scientific studies on walnut have confirmed some traditional knowledge while uncovering new areas of effect. Particularly, the benefits of walnut consumption on heart health have been extensively studied in clinical trials. For example, in controlled nutrition studies, individuals who regularly consumed walnuts showed significant reductions in total cholesterol and “bad” LDL cholesterol levels. Diets rich in walnuts have been reported to improve vascular function and enhance endothelial activity, thereby reducing the risk of atherosclerosis. Based on these findings, walnut has become an indispensable component of Mediterranean diet-like heart-healthy diets.

The anti-cancer potential of walnut is also being investigated through laboratory and animal studies. Polyphenolic compounds and naphthoquinone derivatives such as juglone in walnut have been observed to inhibit the growth of certain cancer cell lines. For instance, ellagic acid derivatives and melatonin found in walnut are considered agents that may reduce cancer risk by reducing oxidative damage through their antioxidant effects. However, whether walnut consumption directly reduces cancer risk in humans remains unclear, and long-term epidemiological studies are ongoing.

Neurological and cognitive studies suggest that the omega-3 fatty acids and polyphenols in walnut may have protective effects on the brain. In animal experiments, aged rats fed a walnut-supplemented diet showed improvements in memory and motor function. In humans, a large-scale study found a positive association between walnut consumption and cognitive test performance in older age groups. However, some other studies have failed to detect direct, significant cognitive benefit in young people and healthy adults. Research in this area continues, with clinical trials underway to better understand the connection between walnut and brain health.

The antimicrobial effects of walnut leaves and husks have also attracted modern scientific interest. Laboratory experiments have demonstrated that Juglans regia leaf extracts exhibit inhibitory effects against various bacteria and fungal strains. In particular, extracts from walnut leaves have been reported to be effective against certain microorganisms causing skin infections. This may explain the scientific basis for the traditional use of walnut leaves in wound cleansing. Additionally, intensive research is being conducted on juglone, a compound derived from walnut husks, with emphasis on its potential antitumour, antiviral, and antifungal properties.

Modern medical research supports the recommendation of walnut consumption as part of a healthy diet. The FDA’s qualified health claim is an example, demonstrating scientific evidence that diets containing walnuts may reduce the risk of heart disease.

Nevertheless, researchers continue to investigate which specific compounds in walnut contribute to these beneficial effects and through what mechanisms. In particular, walnut’s high antioxidant capacity is being studied as a potential food with anti-ageing (geroprotective) effects. In conclusion, modern science research indicates that walnut consumption may be beneficial for cardiovascular health, metabolic health, and potentially neurological health. However, it is important to consume walnut in moderation as part of a balanced diet and alongside other healthy lifestyle habits. Ongoing clinical research on walnut will further clarify its health impacts.

Applications

Walnut is a versatile product with applications in various fields, primarily food. In the food industry, the role of walnut is undeniably important. Walnut kernels are consumed as snacks, either raw or roasted, and are also ingredients in many sweet and salty dishes. In Turkish cuisine, walnut is widely used in desserts such as baklava, kadayıf, walnut sausage, and sweets. It is added to salads, mezes, and dough dishes. For example, walnut is a key ingredient in dishes such as Cherkess chicken (walnut-stuffed chicken), Georgian walnut chicken (satsivi), and fesenjan. In World kitchens, walnut is used extensively, from pestos and sauces to breads and cakes. Green walnuts are pickled in some countries and consumed as “pickled walnut”, offering a distinct flavour. Walnut oil, produced from walnuts, is an aromatic oil commonly used in salad dressings; due to its low smoke point, it is rarely used for frying but is prized in cold applications for its value flavour.

In the fields of pharmaceuticals and cosmetics, walnut and its by-products are utilized in various ways. In traditional pharmacy, ointments prepared from walnut leaves and husks were used for skin conditions due to their antiseptic and anti-inflammatory properties. Today, walnut extracts and oils are incorporated into some botanical cosmetics. Walnut oil, due to its vitamin E and fatty acid content, is included in some skin care products for its nourishing and softening properties. Walnut husk, ground into powder, is used in the cosmetic industry as a natural exfoliant; walnut husk powder is commonly found in facial and body scrubs to cleanse the skin. Additionally, tannins and juglone pigments in walnut husk serve as a natural dye. The dark brown dye obtained from green walnut husks was historically used to darken hair and dye wool/fabric. For example, it is known that Romans and medieval Era Europeans used water boiled from walnut husks as a dye. Walnut-based inks also held an important place in history; Leonardo da Vinci is reputed to have used walnut ink in his drawings.

Walnut also has industrial and other uses. The wood of the walnut tree is hard, durable, and beautifully grained, and has been preferred for centuries in furniture making, carving, and even rifle gunstock production. In particular, walnut timber is highly valued for high-quality furniture and decorative veneers. The hard outer shell of the walnut fruit (after the kernel is removed) is not waste and can be used for various purposes. Ground walnut shells serve as a low-abrasive material in cleaning and polishing applications; walnut shell granules can be used as an alternative to silica sand for cleaning delicate machine parts or in blasting operations. However, in the aviation industry, cleaning with walnut shell powder has been found to cause engine blockages, leading to restrictions on this use. Nevertheless, walnut shell-based abrasives are popular in automotive and metal cleaning industries due to their eco-friendly nature.

Extracts obtained from walnut husks and leaves are also being studied for use in agriculture as natural pesticides or fungicides. For example, it is thought that walnut leaf water may have repellent effects against certain agricultural pests. In this way, walnut is a multifaceted plant that adds value not only through its fruit but also through its other parts.

Production and Trade

Walnut is an important agricultural commodity worldwide, and its production has increased in recent years. According to 2017 data, global walnut production (in-shell weight) reached approximately 3.8 million tonnes.

• China alone accounted for more than half of this production, producing 1.92 million tonnes and clearly ranking as the world’s largest walnut producer.

• The United States of America (particularly California), with a 15% global share and 571,000 tonnes of production, and Iran, with a 9% share and 349,000 tonnes, followed China.

• Türkiye ranked fourth globally in the same year, producing approximately 210,000 tonnes of walnut.

Other significant producing countries include Mexico, Ukraine, Chile, and India. In recent years, walnut cultivation has rapidly expanded in countries such as Chile and Ukraine, extending global walnut production to broader geographical areas.

In international walnut trade, countries such as the United States (California) and Chile are major exporters. Walnuts harvested in the Northern Hemisphere are exported either fresh or after storage and processing. Türkiye, despite high domestic consumption, has increased its walnut production in recent years and has begun to focus on exports. Walnuts are traded either in-shell or shelled. In-shell walnuts, due to their durability, can be transported over long distances, but consumer demand is primarily for ready-to-eat shelled walnuts. Therefore, walnut processing facilities have gained importance in cracking, sorting, and packaging. Properly stored dried walnut kernels can retain their quality for up to a year, enhancing their commercial value.

With increasing global health awareness, demand for walnut is rising, encouraging production in many countries. Although walnut prices are generally slightly higher than those of other nuts, its health benefits and flavour have secured it a lasting place in the market. Commercially, walnut, along with products such as almond and pistachio, forms an important part of the global dried fruit market. The continued growth in walnut production and the development of new varieties are expected to expand the walnut sector for both producers and consumers.

Environmental Impacts

Walnut trees and their cultivation have certain original environmental impacts. First, the walnut tree is a naturally allelopathic species (inhibiting the growth of surrounding plants through chemical means). Walnut leaves, roots, and especially green fruit husks contain a compound called juglone. When this chemical accumulates in the soil, it can inhibit the germination and growth of sensitive plants. For example, it is widely known that plants such as tomato and eggplant do not thrive when planted beneath walnut trees; this is due to juglone’s natural herbicidal effect. Therefore, when planning October in walnut orchards, care is taken to keep plants with low juglone tolerance away from walnut trees.

The ecological impacts of walnut cultivation, like those of other fruit trees, depend on good agricultural practices. Walnut trees are long-lived and large, helping reduce soil erosion in agricultural areas. Their extensive root systems hold soil, and as a perennial plant, they do not require annual replanting, preserving soil structure. Additionally, when properly managed, walnut orchards can contribute to carbon sequestration; growing walnut trees store carbon dioxide in their woody tissues.

On the other hand, intensive walnut cultivation carries risks associated with monoculture. Uniform walnut orchards can facilitate the spread of certain harmful insects and diseases. Pests such as the walnut husk fly and walnut anthracnose can rapidly problem in large walnut plantations. Excessive pesticide use to combat these pests has the potential to harm the environment. Therefore, implementing integrated pest management and biological control methods is essential for environmental sustainability in walnut cultivation.

The management of walnut processing waste is another environmental concern. After harvest, green husks and hard shells from cracking are not waste if properly utilized. Green walnut husks, due to their organic content, can be used in composting, but the composting process must be carefully managed to allow juglone to be neutralized. It is also recommended to collect and remove fallen walnut leaves in autumn to reduce juglone accumulation.

Positively, the industrial use of waste materials such as walnut shells—for example, as natural abrasives or fuel pellets—contributes to waste management and reduces environmental burden. Activated carbon derived from walnut shells can also be used in environmental applications such as water filtration. Moreover, walnut trees, in addition to fruit production, are valuable as habitat providers, offering shelter and food for birds and small mammals.

In conclusion, when proper cultivation and waste management practices are applied, walnut can be an environmentally compatible product. Its long-lived structure, similar to that of forest trees, and its multifaceted applications enable it to occupy a place in sustainable agricultural systems. Combining traditional information methods with modern ecological approaches in walnut cultivation will increase productivity while minimizing environmental impacts.