Walnuts

Health Benefits

Walnuts are round, single-seeded stone fruits that grow from the walnut tree. Walnut trees are native to eastern North America but are now commonly grown in China, Iran, and within the United States in California and Arizona.

Beneath the husk of the walnut fruit is a wrinkly, globe-shaped nut. The walnut is split into two flat segments to be sold commercially.

Walnuts are available both raw or roasted, and salted or unsalted.

This MNT Knowledge Center feature is part of a collection of articles on the health benefits of popular foods. It provides a nutritional breakdown of walnuts and an in-depth look at its possible health benefits, how to incorporate more walnuts into your diet and any potential health risks of consuming walnuts.

One ounce of walnuts also provides the following percentages of recommended daily nutrient intake:

48 percent of manganese  

22 percent of Copper

11 percent of magnesium

10 percent of phosphorus

8 percent of vitamin B6

5 percent of iron

Walnuts are high in monounsaturated and polyunsaturated fats and a good source of protein. Nuts have a reputation for being a high-calorie and high-fat food. However, they are dense in nutrients and provide heart-healthy fats.

The combination of healthy fats, protein, and fiber in walnuts increase satisfaction and fullness, which makes them a great snack compared to simple carbohydrate foods like chips or crackers.

Possible health benefits of consuming walnuts

Consuming plant-based foods of all kinds has long been associated with a reduced risk of many lifestyle-related health conditions.

Many studies suggest that eating more plant-based foods like walnuts decreases the risk of obesity, diabetes, heartdisease, and overall mortality. Eating these foods may also promote a healthy complexion and hair, increased energy, and overall lower weight.

Heart health

A study found that regularly eating walnuts could reduce the risk of coronary heart disease.

The monounsaturated and polyunsaturated fatty acids found in walnuts have been shown to decrease LDL (harmful) cholesterol  and triglyceride levels, in turn reducing the risk of cardiovascular disease, stock and heart attack.

A study published in the British Journal of Nutrition showed that the risk of coronary heart disease is 37 percent lower for those consuming nuts more than four times per week compared to those who never or rarely consumed nuts.

The U.S. Food and Drug Administration (FDA) have approved the claim for food labels that "eating 1.5 ounces per day of most nuts as part of a diet low in saturated fat and cholesterol may reduce the risk of heart disease."

Weight management

According to research published in the Asia Pacific Journal of Clinical Nutrition,  routine nut consumption is associated with elevated resting energy expenditure.

In trials that compare weight loss using diets that include or exclude nuts, the diets that included nuts in moderation showed greater weight loss.

A study published in the American Journal of Clinical Nutrition also found that women who reported rarely eating nuts had a greater incidence of weight gain over an 8-year period than those who consumed nuts two times a week or more.5

Gallstone disease

According to another study in the American Journal of Clinical Nutrition, frequent nut consumption is associated with a reduced risk of cholecystectomy - an operation to remove the gallbladder. In over a million people documented over 20 years, women who consumed more than 5 ounces of nuts a week had a significantly lower risk of cholecystectomy than women who ate less than 1 ounce of nuts each week.

Bone health

Walnuts are a good source of the mineral copper. Severe copper deficiency is associated with lower bone mineral density and an increased risk of osteoporosis. Osteoporosis is a condition where bones become thinner and less dense, making them easier to fracture and break.

More research is needed on the effects of marginal copper deficiency and on the potential benefits of copper supplements to prevent and manage osteoporosis.

Copper also plays an important role in the maintenance of collagen and elastin, major structural components of the body. Without sufficient copper, the body cannot replace damaged connective tissue or the collagen that makes up the building blocks for bone. This can lead to a range of issues including joint dysfunction.

Walnuts contain a high amount of manganese. Manganese has been shown to prevent osteoporosis in combination with the minerals calcium and copper.

Magnesium, another mineral in walnuts, is important for bone formation as it helps with the absorption of calcium into the bone.

Epilepsy

Rats with manganese deficiency have been shown to be more susceptible to seizures. People with epilepsy have also been shown to have lower whole blood manganese levels than those without epilepsy.

It is uncertain if there is a possible genetic relationship between manganese deficiency and epilepsy, and whether manganese supplementation would help. More research is needed.

How to incorporate more walnuts into your diet

Nuts have a high fat content, and so they are prone to becoming rancid. Keep walnuts in a cool, dark, and dry place to improve shelf life. If properly stored, walnuts will keep for a few months at room temperature, a year in the refrigerator, or 2 years in the freezer.

Risks and precautions for eating walnuts

Walnuts are dense in calories and should be consumed in moderation. A 1 ounce serving of walnuts contains about 14 half-walnut pieces.

It is the total diet or overall eating pattern that is most important in disease prevention and achieving good health. It is better to eat a diet with variety than to concentrate on individual foods as the key to good health.

What's other New and Beneficial about Walnuts

Researchers are convinced—more than ever before—about the nutritional benefits of walnuts when consumed in whole form, including the skin. We now know that approximately 90% of the phenols in walnuts are found in the skin, including key phenolic acids, tannins, and flavonoids. Some websites will encourage you to remove the walnut skin—that whitish, sometimes waxy, sometimes flaky, outermost part of shelled walnuts. There can be slight bitterness to this skin, and that's often the reason that websites give for removing it. However, we encourage you not to remove this phenol-rich portion.

The form of vitamin E found in walnuts is somewhat unusual, and particularly beneficial. Instead of having most of its vitamin E present in the alpha-tocopherol form, walnuts provide an unusually high level of vitamin E in the form of gamma-tocopherol. Particularly in studies on the cardiovascular health of men, this gamma-tocopherol form of vitamin E has been found to provide significant protection from heart problems.

Most U.S. adults have yet to discover the benefits of walnuts. A recent study has determined that only 5.5% of all adults (ages 19-50) consume tree nuts of any kind! This small percentage of people actually do a pretty good job of integrating tree nuts (including walnuts) into their diet, and average about 1.25 ounces of tree nuts per day. But the other 94.5% of us report no consumption of tree nuts whatsoever. In a recent look at the nutritional differences between tree nut eaters and non-eaters, researchers have reported some pretty notable findings: on a daily average, tree nut eaters take in 5 grams more fiber, 260 milligrams more potassium, 73 more milligrams of calcium, 95 more milligrams of magnesium, 3.7 milligrams more vitamin E, and 157 milligrams less sodium!

Many of us can go local for our supply of walnuts. According to the latest trade statistics, 38% of all walnuts are grown in the U.S. Of that 38%, the vast majority (almost 90%) come from California, and particularly from the San Joaquin and Sacramento Valleys. Buying walnuts closer to home can provide great benefits from the standpoint of sustainability.

Phytonutrient research on the antioxidant and anti-inflammatory benefits of walnuts has moved this food further and further up the ladder of foods that are protective against metabolic syndrome, cardiovascular problems, and type 2 diabetes. Some phytonutrients found in walnuts—for example, the quinone juglone—are found in virtually no other commonly-eaten foods. Other phytonutrients—like the tannin tellimagrandin or the flavonol morin—are also rare and valuable as antioxidants and anti-inflammatory nutrients. These anti-inflammatory and antioxidant phytonutrients also help explain the decreased risk of certain cancers—including prostate cancer and breast cancer—in relationship to walnut consumption.

Recommendations

Walnuts are part of the tree nut family. This food family includes Brazil nuts, cashews, hazelnuts (filberts), macadamia nuts, pecans, pine nuts, pistachios and walnuts. It would be difficult to overestimate the potential health benefits associated with this food family! In the majority of dietary studies, approximately one ounce of tree nuts per day is the minimal amount needed to provide statistically significant benefits, and that's the amount we recommend that you incorporate into your daily diet. In the case of walnuts, one ounce means about 7 shelled walnuts, or 14 walnut halves. Of course, since tree nuts (including walnuts) are a high-calorie food, it's important to incorporate tree nuts into an overall healthy diet that remains on target in terms of calories. Luckily, research has shown that many people are able to take this step with good success in terms of overall caloric intake.

Walnuts not only taste great but are a rich source of heart-healthy monounsaturated fats and an excellent source of those hard to find omega-3 fatty acids. Like most nuts, they can easily be added to your Healthiest Way of Eating. Just chop and add to your favorite salad, vegetable dish, fruit, or dessert. 

Health Benefits

Cardiovascular Benefits

No aspect of walnuts has been better evaluated in the research than their benefits for the heart and circulatory system. Some review studies have emphasized the very favorable impact of walnuts on "vascular reactivity," namely, the ability of our blood vessels to respond to various stimuli in a healthy manner. In order to respond to different stimuli in a healthy way, many aspects of our cardiovascular system must be functioning optimally. These aspects include: ample presence of antioxidant and anti-inflammatory nutrients, proper blood composition, correct balance in inflammation-regulating molecules, and proper composition and flexibility in our blood vessel walls. Researchers have determined the ability of walnuts to have a favorable impact on all of these aspects. The chart below summarizes some key research findings about walnuts and heart health:

Cardiovascular Aspect	Walnut Benefit
Blood Quality	decreased LDL cholesterol; decreased total cholesterol; increased gamma-tocopherol; increased omega-3 fatty acids in red blood cells (alpha-linolenic acid)
Vasomotor Tone	decreased aortic endothelin; improved endothelial cell function
Risk of Excessive Clotting	decreased maximum platelet aggregation rate; decreased platelet activation
Risk of Excessive Inflammation	decreased C reactive protein (CRP); decreased tumor necrosis factor alpha (TNF-a)

Research on the blood pressure benefits of walnuts has been mixed. We suspect that these mixed results are related to the surprising differences in mineral composition amongst different varieties of walnuts. Researchers have long been aware of the relationship between healthy blood pressure and intake of specific minerals, including potassium, calcium, and magnesium. In multiple studies, these minerals have a much greater impact on blood pressure than the mineral sodium (familiar to most people in its sodium chloride form, i.e., everyday table salt). We've seen studies showing the following ranges for key blood pressure-regulating minerals in walnuts:

Mineral	Natural Range Found Amongst Different Walnut Varieties (milligrams per 100 grams)
Potassium	375-500
Calcium	13-91
Magnesium	189-278

Even though there are valuable amounts of these blood pressure-regulating minerals in virtually all varieties of walnuts, the ranges above may help explain why some studies have shown statistically significant benefits from walnuts on blood pressure while others have not.

Not in question with respect to walnuts and cardiovascular support is their reliable omega-3 content. Adequate intake of omega-3s, including the alpha-linolenic acid (ALA) present in walnuts, has repeatedly been shown to help improve a wide variety of cardiovascular functions, including blood pressure. In at least one research study, adults have been able to significantly increase their blood level of ALA with as few as 4 walnuts per day.

Walnuts Help Reduce Problems in Metabolic Syndrome

In the United States, as many as 1 in 4 adults may be eligible for diagnosis with Metabolic Syndrome (MetS). MetS isn't so much a "disease" as a constellation of problematic and 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 shown that approximately one ounce of walnuts daily over a period of 2-3 months can help reduce several of these MetS-related problems. In addition, addition of walnuts to participant diets has also been shown to decrease "abdominal adiposity"—the technical term for the depositing of fat around the mid-section. Importantly, the MetS benefits of added walnuts have been achieved without causing weight gain in any the studies we've seen to date.

Benefits in Treatment of Type 2 Diabetes

Although we think about type 2 diabetes as a problem primarily related to blood sugar control and insulin metabolism, persons diagnosed with type 2 diabetes typically have health problems in other related systems, and are at special risk for cardiovascular problems. An important part of the goal in designing a diet plan for persons with type 2 diabetes is lowering the risk of future cardiovascular problems. In this context, consumption of walnuts is establishing a more and more impressive research track record. Increased 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) show a relatively small amount of daily walnut intake (1-2 ounces) to provide significant benefits in this area for persons with type 2 diabetes. Better blood fat composition (including less LDL cholesterol and less total cholesterol) has also been demonstrated in persons with type 2 diabetes.

Anti-Cancer Benefits

Given the wide variety antioxidant and anti-inflammatory nutrients found in walnuts, it's not surprising to see research on this tree nut showing measurable anti-cancer benefits. The antioxidant properties of walnuts help lower risk of chronic oxidative stress, and the anti-inflammatory properties help lower risk of chronic inflammation, and it is precisely these two types of risk, that, when combined, pose the greatest threat for cancer development. Prostate cancer and breast cancer are the best-studied types of cancer with respect to walnut intake, and their risk has been found to be reduced by fairly large amounts of walnut consumption. (Large in this case means approximately 3 ounces per day.) For prostate cancer, the evidence is somewhat stronger, and more studies have involved human subjects. For breast cancer, most of the evidence has been based on studies of rats and mice.

Other Health Benefits

The anti-inflammatory nutrients in walnuts may play a special role in support of bone health. A recent study has shown that large amounts of walnuts decrease blood levels of N-telopeptides of type 1 collagen (NTx). These collagen components provide a good indicator of bone turnover, and their decreased blood level in response to walnut intake is an indication of better bone stability and less mineral loss from the bone. "Large amounts" of walnuts (in this study, actually raw walnuts plus walnut oil) translated into 50% of total dietary fat. In an everyday diet that provided 2,000 calories and 30% of those calories from fat, this 50% standard for walnuts would mean about 67 grams of fat from walnuts or 4 ounces of this tree nut on a daily basis. While this amount is more than would most people would ordinarily consume, we expect the health benefits of walnuts for bone health to be demonstrated in future studies at substantially lower levels of intake.

Walnuts have also produced a good track record in the research as a desirable food for support of weight loss and for prevention of obesity. That finding often surprises people because they think of high-fat, high-calorie foods as a primary contributing factor to obesity and to weight gain. In general, overconsumption of high-fat, high-calorie foods is a primary contributing factor to obesity and weight gain. However, obesity has also been clearly identified by researchers as involving chronic, unwanted inflammation. As discussed earlier in this Health Benefits section and throughout this walnuts' profile, walnuts are unique in their collection of anti-inflammatory nutrients. These nutrients include omega-3 fatty acids; phytonutrients including tannins, phenolic acids, and flavonoids; quinones like juglone; and other anti-inflammatory phytonutrients. These anti-inflammatory benefits can overshadow the high-calorie and high-fat risk posed by walnuts, and that's exactly what they have done in an increasing number of research studies involving risk and/or treatment of obesity. While it is definitely possible to overconsume walnuts, most everyday diets could remain correctly balanced in terms of calories and fat while still including fairly generous amounts of walnuts (in the range of 1-3 ounces).

A limited (but increasing) number of studies have shown potential health benefits for walnuts in the area of memory and general thought processes (often referred to as "cognitive" processes). Thus far, most of the initial research in this area has involved rats and mice, but we expect to see cognitive benefits of walnuts for humans becoming a topic of increasing research interest.

A final fascinating aspect of walnuts and their potential health benefits involves melatonin (MLT). MLT is a widely-active messaging molecule in our nervous system, and very hormone-like in its regulatory properties. MLT is critical in the regulation of sleep, daily (circadian) rhythms, light-dark adjustment, and other processes. It has also been found to be naturally occurring within walnuts. Average melatonin (MLT) content of walnuts is approximately 3.6 nanograms (ng) per gram (g), or 102ng/ounce. Other commonly eaten foods—for example, cherries—have also been found to measurable amounts of MLT. Researchers are not yet sure how everyday intake of MLT from walnuts is involved in our health, but several study authors have hypothesized about the MLT in walnuts as playing an important role (along with other walnut nutrients) in the anti-cancer benefits of this unusual food.

Description

Walnuts are a delicious way to add extra nutrition, flavor and crunch to a meal. While walnuts are harvested in December, they are available year round and a great source of those all-important omega-3 fatty acids.

It is no surprise that the regal and delicious walnut comes from an ornamental tree that is highly prized for its beauty. The walnut kernel consists of two bumpy lobes that look like abstract butterflies. The lobes are off white in color and covered by a thin, light brown skin. They are partially attached to each other. The kernels are enclosed in round or oblong shells that are brown in color and very hard.

While there are numerous species of walnut trees, three of the main types of walnuts consumed are the English (or Persian) walnut, Juglans regia; the black walnut, Juglans nigra; and the white (or butternut) walnut, Juglans cinerea. The English walnut is the most popular type in the United States and features a thinner shell that is easily broken with a nutcracker. The black walnut has thicker shells that are harder to crack and a much more pungent distinctive flavor. The white walnut features a sweeter and oilier taste than the other two types, although it is not as widely available and therefore may be more difficult to find in the marketplace. Within these basic types of walnuts, there are dozens of different varieties (also called cultivars). It's not uncommon to see research studies that evaluate several dozen different cultivars of English or black walnuts. All types and varieties of walnuts can have unique nutrient composition. Sometimes within a particular type of walnut—for example, English walnut—there is a surprising amount of nutritional variety. The bottom line here is to not to get caught up in thinking that one main type of walnut (for example, English versus black) is best, but to take advantage of the nutritional variety offered by walnuts overall.

History

While walnut trees have been cultivated for thousands of years, the different types have varying origins. The English walnut originated in India and the regions surrounding the Caspian Sea, hence it is known as the Persian walnut. In the 4th century AD, the ancient Romans introduced the walnut into many European countries where it has been grown since. Throughout its history, the walnut tree has been highly revered; not only does it have a life span that is several times that of humans, but its uses include food, medicine, shelter, dye and lamp oil. It is thought that the walnuts grown in North America gained the moniker "English walnuts," since they were introduced into America via English merchant ships.

Black walnuts and white walnuts are native to North America, specifically the Central Mississippi Valley and Appalachian area. They played an important role in the diets and lifestyles of both the Native American Indians and the early colonial settlers.

China is presently the largest commercial producer of walnuts in the world, with about 360,000 metric tons produced per year. The United States is second, with about 294,000 metric tons of production. Within the U.S., about 90% of all walnuts are grown in California, particularly within the San Joaquin and Sacramento Valleys The annual combined walnut output of Iran and Turkey is approximately the same as the United States, and the Ukraine and Romania are next in line in terms of total walnut production.

How to Select and Store

When purchasing whole walnuts that have not been shelled choose those that feel heavy for their size. Their shells should not be cracked, pierced or stained, as this is oftentimes a sign of mold development on the nutmeat, which renders it unsafe for consumption.

Shelled walnuts are generally available in prepackaged containers as well as bulk bins. Just as with any other food that you may purchase in the bulk section, make sure that the bins containing the walnuts are covered and that the store has a good product turnover so as to ensure its maximal freshness. Whether purchasing walnuts in bulk or in a packaged container avoid those that look rubbery or shriveled. If it is possible to smell the walnuts, do so in order to ensure that they are not rancid.

Due to their high polyunsaturated fat content, walnuts are extremely perishable and care should be taken in their storage. Shelled walnuts should be stored in an airtight container and placed in the refrigerator, where they will keep for six months, or the freezer, where they will last for one year. Unshelled walnuts should preferably be stored in the refrigerator, although as long as you keep them in a cool, dry, dark place they will stay fresh for up to six months.

Individual Concerns

Walnuts and Food Allergies

Tree nuts, such as walnuts, are among the eight food types considered to be major food allergens in the U.S., requiring identification on food labels.

Nutritional Profile

Walnuts are an excellent source of anti-inflammatory omega-3 essential fatty acids, in the form of alpha-linolenic acid (ALA). Walnuts are also rich in antioxidants, including being a very good source of manganese and copper. They are also a good source of molybdenum and the B vitamin biotin. Many other minerals are provided by walnuts in valuable amounts. These minerals include calcium, chromium, iron, magnesium, phosphorus, potassium, selenium, vanadium and zinc. Vitamin B6, while not especially concentrated in walnuts, may be more bioavailable in this food. In terms of phytonutrients, walnuts contain antioxidant and anti-inflammatory compounds, including more than a dozen phenolic acids, numerous tannins (especially ellagitannins, including tellimagrandins), and a wide variety of flavonoids. The vitamin E composition of walnuts is also of special mention, since there is an unusual concentration of the gamma-tocopherol form of vitamin E in this tree nut.