Walnuts might not be glamorous or exotic, but they are nutritious powerhouses, with plenty of evidence to support their superfood status.
But it hasn’t always been that way.
Some years ago, in the “all fats are bad” days, walnuts (along with almonds and avocados) were on the black list.
Today, they’re back in the superfood spotlight – and rightfully so.
Walnuts – The Botany
Juglans regia – also known as the Persian walnut, English walnut or European walnut.
This is the most popular variety of walnut in the US. The shell is thinner and more easily broken.
Juglans nigra – also known as Black walnut.
The shell is much thicker, and more difficult to crack.
The flavour and aroma are more distinctive and pungent.
Juglans cinerea – also known as White Walnut, Butternut, Lemon Walnut or Oilnut.
The taste is sweeter and more oily.
There are many different cultivars within each of these species, and the nutritional content can vary quite dramatically.
European walnut is native to the Black Sea and Caspian region, stretching from the Balkans eastward across Persia to the Himalayas and southwest China.
It spread early to the Mediterranean. Although grown in Britain by the Romans, it appears not to have become widespread there until the Medieval period.
Cultivation in China began up to 2000 years ago. Significant planting in Northern Europe began in the 16th and 17th centuries where it was often also grown for its timber. Today it is cultivated commercially in many regions, China and California being the world’s leading producers.
Terms such as seeds, nuts, and fruits have strict botanical definitions, but in practice are used more broadly. For example, true nuts – one-seeded dry indehiscent fruits (fruits that remain closed upon reaching maturity) – are only found in a few genera: Quercus (acorns), Corylus (hazelnut), and Fagus (beechnut).
Walnuts are part of the tree nut family. This food family also includes Brazil nuts, cashews, hazelnuts (filberts), macadamia nuts, pecans, pine nuts, and pistachios.
Walnuts – The Nutrition
The walnut’s watershed moment came in 2006 when a study published in the American Journal of Clinical Nutrition found that these tasty tree nuts had the highest antioxidant content of any common food, aside from a handful of herbs and spices. When the antioxidant content was adjusted for typical serving sizes, walnuts finished second only to blackberries.
And while we don’t know exactly whether these antioxidants translate into measurable health benefits, it is nonetheless good news for this previously maligned morsel.
Walnuts are high in phytochemicals. About 90 % of the phenols (including phenolic acids, tannins and flavonoids) in walnuts are actually located not within the flesh of the nut, but in the skin – so they are best eaten in their whole form, without removing the skin (which can be quite difficult to remove, anyway). The skin tends to taste slightly bitter, which is why some folk like to remove it.
The research on the phytonutrient content of walnuts is ongoing, and gaining momentum with the release of more positive findings.
Walnuts contain certain phytochemicals – such as quinone juglone – which are found in virtually no other commonly consumed foods.
Juglone exhibits antimicrobial, antiparasitic and antineoplastic (inhibiting or preventing the growth and spread of tumors or malignant cells) activities.
Other phytonutrients – such as tannins (especially ellagitannins, including tellimagrandins) and flavols (morin) – are also found in few other foods, and offer valuable antioxidant and anti-inflammatory properties.
Although walnuts are somewhat high in fat at 18 grams per 28 gram serving, they are rich in omega-3 alpha-linolenic acid, or ALA, at 2.5 grams per serving (an amount that is on par with hemp seeds). The rest of their fat content is largely derived from omega-6 polyunsaturated fats (10.8 grams) and monounsaturated fats (2.5 grams).
Walnuts are a good source of manganese and copper. They are also a good source of molybdenum and the B vitamin biotin.
Walnuts contain an uncommon form of Vitamin E – gamma tocopherol, compared to the more common alpha tocopherol – in an unusually high level. Studies have shown that the gamma tocopherol form of Vitamin E can provide significant protection from heart diseases.
Most tree nuts are valuable sources of vitamins, minerals and phytonutrients – but the general population has been slow to become aware of their impressive health benefits. A recent study found that only about 5.5% of Americans (between the ages of 19 -50) include tree nuts in their daily diet.
In a study which compared the differences between tree nut eaters and non-eaters, researchers discovered that tree nut eaters consume – 1. 5 gram more fibre; 2. 260 mg more potassium; 3. 73 mg more calcium; 4. 95 mg more magnesium; 5. 3.7 mg more Vitamin E; 6. 157 mg less sodium.
As with other plants, the nutrient value in walnuts can vary significantly depending on the variety and cultivar – which might explain why the results of studies have been varied.
For example – different studies have measured the potassium content of walnuts at 375 – 500 mg/per 100 grams; the calcium content at 13 – 91 mg/ per 100g; and the magnesium content at 189 – 278 mg/ per 100 grams.
Walnuts – The Evidence
What are the health benefits of walnuts?
Walnuts – Cardiovascular Benefits
No aspect of walnuts has been better evaluated in the research than their benefits for cholesterol levels, the heart and circulatory system, assessing their effects on blood pressure and the risk of heart attacks and strokes.
In one study, a group of researchers examined the effect of eating about 42 – 56 grams (or 21 to 28 halves) of walnuts per day, versus a walnut-free control diet, in 20 young Japanese men and women.
After four weeks on each diet, the walnut eaters had lower total cholesterol, and their ratio of LDL (“bad”) to HDL (“good”) cholesterol improved versus the control group subjects. The women – who were healthy to start the study – also saw a significant drop in their LDL cholesterol.
The male participants didn’t quite reach significance (which could be due to the relatively small size of the study group), and the walnut group also tested with lower levels of apolipoprotein B, or apoB, the marker of small, dense LDL in the bloodstream.
As a source of omega-3 fatty acids (albeit in the less-potent plant form, ALA), walnuts might be assumed to have similar heart health benefits to fish. In reality, the effects of fish versus walnuts, while both seemingly good for our hearts, are actually very different. A 2009 study compared the effects of daily consumption of 42 grams of walnuts versus 113 grams of salmon, eaten twice per week. In 25 people with normal or slightly elevated cholesterol, the walnut eaters tested with lower total and LDL cholesterol (and apoB), while the fish eaters tested with lower triglycerides and higher HDL. The verdict? Eat them both!
Walnuts are also a good source of magnesium, a mineral found to play a significant role in heart health.
Walnuts – Diabetes Benefits
Improved flexibility in the response of the cardiovascular system following meals has been a repeated finding in research on walnuts. A variety of different measurements on blood vessel functioning (including their measurement by ultrasound) demonstrate that a relatively small amount of daily walnut consumption (28-56 grams) can provide significant benefits for people with type 2 diabetes. Improved blood fat composition (including less LDL cholesterol and less total cholesterol) has also been demonstrated in individuals with type 2 diabetes.
Walnuts – Metabolic Syndrome Benefits
It is estimated that in the US, as many as 1 in 4 adults may have the condition known as Metabolic Syndrome (MetS). MetS isn’t so much a “disease” as a combination of overlapping metabolic problems including excessive blood fats (triglycerides), high blood pressure, inadequate HDL cholesterol, and obesity (as measured by waist circumference, and/or body mass index).
Recent studies have demonstrated that eating about 28 grams of walnuts daily over a period of 2-3 months can help reduce several of these MetS-related problems. The addition of walnuts to participants’ diets has also been shown to decrease “abdominal adiposity” – the depositing of fat around the mid-section, which has been shown to play a role in many health conditions. Importantly, the MetS benefits of adding walnuts to the diet have been achieved without causing weight gain in any of the participants.
Walnuts – Anti-Cancer Benefits
Research on the antioxidant and anti-inflammatory effects of walnuts has led to promising results in the fight against cancer.
Daily consumption of about 84 grams of walnuts has been found to potentially reduce the risk of prostate cancer and breast cancer.
The evidence is somewhat stronger for prostate cancer – more of these studies have involved human subjects. For breast cancer, most of the evidence has been based on studies of rats and mice.
More research is required in this area, particularly on human subjects.
Walnuts – Bone Health Benefits
The anti-inflammatory nutrients in walnuts may play a beneficial role in support of bone health. A recent study has demonstrated that large amounts of walnuts decrease blood levels of N-telopeptides of type 1 collagen (NTx). These collagen components provide a reliable indicator of bone turnover. Their decreased blood level in response to walnut consumption is an indication of increased bone stability and decreased mineral loss from the bone.
The study participants consumed about 112 grams of walnuts daily, combined with walnut oil.
It is quite possible that future studies will derive similar benefits, but at lower intake.
Walnuts – Memory Benefits
According to the “doctrine of signatures”, whereby the shape and/or color of certain plants indicates their medical benefit to the human body, walnuts were considered beneficial to the brain (for obvious reasons).
To some degree, these claims might actually have some validity, but as with blueberries, much of the research is in rats.
In one study, walnut consumption was associated with better working memory (remembering tasks, written lists, images, etc) in older adults, though it did not appear to impact verbal memory (remembering instructions or conversations) or overall cognitive impairment.
The study found that in young adults, walnuts seem to have no effect on memory, mood, or non-verbal reasoning, though they may trigger a moderate improvement in inferential reasoning.
Again, more research is required.
Walnuts – Melatonin Benefits
Another of walnuts’ potential health benefits involves melatonin (MLT). Melatonin is a widely-active messaging molecule in our nervous system, and hormone-like in its regulatory properties. Melatonin is a critical factor in the regulation of sleep, daily (circadian) rhythms, and other processes.
Melatonin occurs naturally in walnuts. Average melatonin (MLT) content of walnuts is approximately 3.6 nanograms (ng) per gram (g). Cherries also have measurable amounts of melatonin.
Researchers have yet to establish links between the melatonin content in walnuts, and beneficial effects to human health, but melatonin is known to play a role in immunity, weight control and hormonal balance.
Walnuts – Weight Control Benefits
Despite the high calorie and high fat content of nuts, research has consistently demonstrated that nut consumption does not appear to be linked to weight gain or obesity.
In fact, a number of studies have demonstrated that people who eat nuts regularly tend to weigh less than those who don’t eat nuts.
The fats in walnuts stimulate production of cholecystokinin, a protein that triggers feelings of fullness, and the fibre content could also play a role in appetite control. A handful of studies have demonstrated that eating walnuts before a meal (either at the previous meal or as part of a “preload” snack before a meal) results in increased feelings of fullness at the next meal. The result being that you are less likely to overeat.
Additionally, a link between obesity and excessive inflammation in the body has been identified by researchers.
Walnuts are unique in their concentration of anti-inflammatory nutrients – including omega-3 fatty acids, and various phytochemicals (tannins, phenolic acids, flavonoids, quinones, etc). It is possible that these anti-inflammatory compounds negate the effects of higher calories and fat.
Walnuts – Yes, You Can Eat The Shell!
Although it is the actual inner nut that is most commonly eaten, and been the subject of most studies, walnuts are more versatile than most folk realise – from whole pickled walnuts (green skin, shell and nut), to sweet preserves, syrup from the tree sap and “flour” from the shells.
Walnuts can be eaten raw, roasted, or salted as snacks or used in cooking, confectionary, pastries, cakes, ice cream, etc.
The nuts can be ground into a meal and used as a flavouring in sweet and savoury dishes. Walnuts, when cold pressed, yield a light yellow edible oil which is used in foods as flavouring, as salad dressings or in cooking.
Young green, unripe fruit (with their shell) can be pickled in vinegar and eaten – in England these are called “pickled walnuts”.
Walnuts are preserved in sugar syrup and eaten whole in Armenian cuisine. In Italy, liqueurs called Nocino and Nocello are flavoured with walnuts. Walnuts are ground along with other ingredients to make walnut sauce in Georgia.
Walnut sap is tapped from the tree in spring to make a form of sugar, similar to maple syrup.
The finely ground shells of walnuts are used in the stuffing of ‘agnolotti’ pasta and also used as an adulterant of spices. The dried green husks contain 2.5–5% ascorbic acid (vitamin C) – this can be extracted and used as a vitamin supplement. The leaves when dried and crushed can be used as tea.
Walnuts – Lore & Legend
The walnut tree has a long history of traditional medicinal uses. Please note – this is for informational purposes only, please consult your healthcare practitioner before using any of these remedies.
Various parts of the walnut have been reported in folkloric medicine to treat a wide range of ailments and complaints.
The fruit, when young and unripe, makes a wholesome, anti-scorbutic (anti-scurvy) pickle. The vinegar in which the green fruit has been pickled has been used as a gargle for sore and slightly ulcerated throats.
Walnut catsup (spicy sauce with vinegar) embodies the medicinal virtues of the unripe nuts.
The shell of the walnut is said to be anodyne (pain-relieving) and astringent. It is used to treat diarrhoea and anaemia.
The husk, shell and peel are sudorific (induces sweating). Especially if used when the walnuts are green and unripe, the nut has anthelmintic (anti-parasitic) properties.
Walnuts are considered to be antilithic (acts against the formation of calculi, such as kidney stones), diuretic and stimulant. They are used internally to treat lower back pain, frequent urination, weakness of legs, chronic cough, asthma, constipation due to dryness or anaemia, and stones in the urinary tract.
Externally, the seeds are pulverised into a paste and applied as a poultice to areas of dermatitis and eczema.
The walnut seed oil is anthelmintic (anti-parasitic) and the oils are also used in the treatment of menstrual problems and dry skin conditions.
In traditional Chinese medicine (TCM), walnut seeds are primarily considered a kidney tonic. They are also considered beneficial to the brain, back, and skin, and to relieve constipation caused by dehydration. The cotyledons (leaves of the embryo of the seed nut) are used in the treatment of cancer. Male inflorescences (whole flower heads) are made into a broth and used in the treatment of coughs and vertigo.
The leaves are described as alterative, anthelmintic, anti-inflammatory, astringent and depurative (purgative). They are used internally to treat constipation, chronic coughs, asthma, diarrhoea, dyspepsia. The leaves are also used to treat skin ailments and detoxify the blood.
The bark and leaves have alterative, laxative, astringent and detergent properties, and are used in the treatment of skin disorders – as a remedy for scrofulous (TB bacteria) diseases, herpes, eczema, etc., and for healing indolent ulcers. The bark and root bark are anthelmintic, astringent and detergent. The bark, dried and powdered, and made into a strong infusion, is considered a useful purgative.
Walnut hull preparations are used for skin diseases and abscesses.
From the volatile oil of the leaves terpenoid substances (monoterpenes, sesquiterpenes, diterpene and triterpene derivatives) and eugenol have been isolated. Fatty acids, including germanic acid, alpha-and beta-pinene, cincole, limonene, beta-eudesmol and juglone are also important constituents of the volatile oil.
The leaves contain napthoquinones, mainly juglone. The root bark contains bis-juglone and oligomeric juglones. Unripe fruit husk also contains napthoquinones.
The juice of unripe fruits showed significant thyroid hormone enhancing activity (prolonged use of such extract may cause serious side effect).
The inner bark of the white walnut contains napthoquinones, including juglone, juglandin, juglandic acid, tannins and an essential oil.
Walnuts – The Side Effects
Tree nuts (including walnuts, cashews, almonds, pecans, pistachios, Brazil nuts, hazelnuts and chestnuts) are considered potential allergens. In the case of walnuts, there is also some evidence showing cross-reactivity with cashews, peanuts, and sesame. If you suspect a food allergy to walnuts, you may also want to determine the safety of these other nuts in your diet.
Walnuts – The verdict
Unless you have an allergy to nuts, there seems to be no evidence against moderate consumption of walnuts. There’s nothing to lose, and potentially much to gain in health benefits.
As a snack, walnuts are certainly more healthy than any packaged food.