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Fennel Seeds for a Nitrate Boost

Dozens of studies now suggest that the nitrates in vegetables, such as beets and green leafy vegetables, may help not only sick people “as a low-cost prevention and treatment intervention for patients suffering from blood flow disorders” like high blood pressure and peripheral vascular disease, but also healthy people as an effective, natural performance-enhancing aid for athletes. Most of the studies were done with beet juice, though, which is why I was so delighted to see a study on whole beets, which showed the same benefit. But what about studies on whole green leafy vegetables? That’s one of the topics I cover in my video Fennel Seeds to Improve Athletic Performance.

There was a study a while ago suggesting that one of the reasons the Okinawans in Japan looked forward to many more years of good health at the same age at which many Americans and Europeans were dying is all the nitrate in their green leafy vegetables, which tends to bring down blood pressures. The reason I didn’t report on this at the time is because I had never heard of the vegetables in the study. I know what chrysanthemum flowers are, but I didn’t think most of my viewers (or I) would be able to find garland chrisantemum, ta cai, chin gin cai, Osaka shirona, nozavana (or nozawana) pickles, or water dropwort at the local store.

What about less exotic greens, like frozen spinach? Researchers wanted to test the immediate effects on our arteries of a single meal containing a cooked box of frozen spinach, for both arterial stiffness and blood pressure. First, they needed a meal to increase artery stiffness and pressure, so they gave people a chicken and cheese sandwich, which lowered the elasticity of their arteries within hours of eating. But, when they added the spinach, the opposite happened. After chicken and cheese, the force the heart had to pump went up within minutes, but the spinach kept things level. So, a meal with lots of “spinach can lower blood pressure and improve measures of arterial stiffness.”

That’s great for day-to-day cardiovascular health, but what if you want a whole food source that can improve your performance when you’re out hiking, for example? Beets and spinach aren’t the most convenient of foods when you’re out and about. Is there anything we can add easily to our trail mix? Well, if you look at a list of high-nitrate vegetables, you see celery, endive, lettuce, Swiss chard, and the like—not much you can just stick in your pocket. But what about fennel? That’s on the list. Could fennel seeds (which actually aren’t seeds at all, but the whole little fruits of the fennel plant) be the convenient, high-nitrate source we’re looking for?

Fennel seeds are “often used as mouth fresheners after a meal in both the Indian sub-continent and around the world.” You’ll typically see a bowl of fennel seeds, sometimes candy-coated, as you walk out of Indian restaurants. When you chew them, you can get a significant bump in nitric oxide production, which has the predictable vasodilatory effect of opening up blood vessels. This makes them a cheap and easy way to carry a lightweight, nonperishable source of nitrates. Researchers singled out mountaineers, thinking chewing fennel seeds could help maintain oxygen levels at high altitudes and help prevent HAPE—high altitude pulmonary edema—which is one of the leading killers of mountain climbers once you get more than a mile and a half or so over sea level. Don’t confuse HAPE with HAFE, though, which is caused by the expansion of gas at high altitudes—a condition known as high altitude flatus expulsion or “Rocky Mountain barking spiders.”

Fennel seeds may help with that, too, as they’ve been used traditionally as a carminative, meaning a remedy for intestinal gas. “Fennel has also shown antihirsutism activity,” combatting excessive hair growth in women, the so-called bearded woman syndrome. Indeed, applying a little fennel seed cream can significantly reduce it.

If fennel seeds have such a strong hormonal effect, should we be worried about chewing them? There have been cases reported of premature breast development among young girls drinking fennel seed tea a couple times a day for several months. Their estrogen levels were elevated, but, after stopping the tea, their chests and hormone levels went back to normal.

Current guidelines recommend against prolonged use in vulnerable groups—children under 12 and pregnant and breastfeeding women—and perhaps your pet rat, as rodents metabolize a compound in fennel called estragole into a carcinogen, but our cells appear able to detoxify it.

If you’re interested in learning more about using nitrates to improve athletic performance, check out:

Curious about non-nitrate athletic performance tweaks? See:

And what about sports drinks? See: Are Sports Drinks Safe and Effective? and Coconut Water for Athletic Performance vs. Sports Drinks

In health,
Michael Greger, M.D.

PS: If you haven’t yet, you can subscribe to my free videos here and watch my live, year-in-review presentations:


Michael Greger M.D., FACLM

Michael Greger, M.D. FACLM, is a physician, New York Times bestselling author, and internationally recognized professional speaker on a number of important public health issues. Dr. Greger has lectured at the Conference on World Affairs, the National Institutes of Health, and the International Bird Flu Summit, testified before Congress, appeared on The Dr. Oz Show and The Colbert Report, and was invited as an expert witness in defense of Oprah Winfrey at the infamous "meat defamation" trial.

59 responses to “Fennel Seeds for a Nitrate Boost

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  1. I once met someone who works for a spice company. She told me she’s seen now fennel is processed and it’s not pretty. The words I’ll always remember her saying is “when you see fennel, think fecal.”

        1. Yeah, it looked fairly automated.

          The guy’s hands looked pretty dirty but other than that I didn’t feel the heeby-jeebies or anything. I suspect many companies probably have more updated processes than that one.

          Maybe avoid her company then?

          1. Deb, I noticed his hands too :) The person I knew works for one of the biggest spice companies so not sure if it can be avoided.

            I was told the story of the spice laying on the floor in big mounds. Most spices were OK, it was just fennel that seemed to stick out.

  2. On a different topic,

    Not sure if others noticed this news item about Israeli scientists saying that probiotics are pretty much useless.
    My gastroenterologist was of the same opinion 8 years ago when I asked him. In the recent experiments, I found it interesting that after a course of antibiotics, volunteers who took probiotics did the worst. Those that had a fecal microbiome transfer (of their own gut microbiome) did the best recovering in days. Those that did nothing fared second best.
    To be effective, researchers say that probiotics would have to be crafted on an individual basis.

    On, another note, I was disappointed in my trial of pistachios over the weekend. I suffered strong and unexpected allergic reaction to them! :( I have not noticed allergies to other nuts so far. Is this common to be selectively allergic to one or two kinds of nuts? Be well all.

    1. That those who did nothing fared better than those who took probiotics is interesting.

      Interesting about the nut allergy.

      I don’t know the answer, but I do know that they believe they can desensitize people to nut allergies.

      Was that the first time you ever ate pistachios or was this the first time you had an allergic reaction to them? You might need to get tested for allergies to other nuts to see if you are developing a bigger allergy. Nut allergies are nothing to play with.

      1. Thank you Deb! It sure took me by surprise. I haven’t eaten pistachios since maybe the mid 90’s that I recall. I haved developed a couple of other allergies in the last 10 years, but so far eating a walnut 3 times a week doesnt bother me. Definitely on my list to ask the doctor. Thanks for the link !..
        P.S. I really enjoyed the pistachios until my tongue started swelling and my scalp started itching !

        1. If this is right, it would be cashews to watch out for:

          Peanuts are legumes and are not related to tree nuts (almonds, walnuts, cashews, etc.). … There is a high degree of cross-reactivity between cashew and pistachio and between walnut and pecan. Most people who are allergic to one tree nut are not allergic to all tree nuts.

  3. The blog states, “Researchers singled out mountaineers, thinking chewing fennel seeds could help maintain oxygen levels at high altitudes and help prevent HAPE—high altitude pulmonary edema.” However, the blog does not clarify whether that “thinking” was 1) backed up with specific research, 2) based on a research finding that all high-nitrate foods have that benefit, or 3) based on an assumption that all high nitrate foods have that benefit.

    1. There is a series of NF videos and blogs about nitrites that when reviewed, rounded out the picture for me. Here is an article that, based on the abstract, I trust will provide detailed information about fennel nitrite content, from which we should be able to generate an appropriate “dosage.” My library will take 2 weeks to get me a copy of the article :-(

      J Food Sci. 2012 Dec;77(12):H273-9. doi: 10.1111/j.1750-3841.2012.03000.x.
      Nitrites derived from Foneiculum vulgare (fennel) seeds promotes vascular functions.

    2. I am definitely interested in this question also, and that’s the only reason I came on here to see if this question was already answered. An answer to this question will definitely help. Thanks.

    3. A teaspoon, once a day. High use (a tablespoon multiple times daily) can be associated with estrogen-related side effects. -Dr Anderson, Health Support Volunteer

    1. I’ve reviewed the study Dr G discusses. Approximately a teaspoon of fennel seeds. -Dr Anderson Health Support Volunteer

  4. I did a little googling:in the Japanese study many of the veggies are easily bought here, esp if you have an asian market in your community.
    ta cai aka Tat soi
    Chin gin cai aka Chinese broccoli
    Osaka shirona aka Chinese cabbage
    Sayaingen beans aka green beans
    Chinese cabbage
    Shiitake mushrooms 454 ± 38

  5. The vet called today and my dog’s WBC was normal.

    Yes, normal.

    It genuinely was an infection and Oregabiotic and Olive Leaf Extract and D. Mannose and a tiny bit of Garlic worked.

    Not sure which one worked. They all seemed to make a difference, but it didn’t go all the way away until I added in the little bit of Garlic.

      1. Yes!

        Every morning, I visit my friends who work at Starbucks and they ask if it is a good day and I tell them, “Every morning I wake up and my dog is still alive and it is such a good day every single day.”

        Life is precious. Life is good.

  6. Beets and spinach are also on my list of foods not to eat as they can cause kidney stones. I love both of these veggies. Would fennel also contribute to the formation of kidney stones?

    1. I assume you are concerned about oxalates. Try Bibb lettuce aka Boston lettuce AKA Butterhead AKA Butter lettuce. This kind of lettuce is low oxalate but high nitrate according to Gregor’s list of high nitrate veggies. I’ve been eating this (12 oz) blended in smoothies for months with no apparent kidney trouble. It does lower my blood pressure.

    2. Linda,

      I thought I just posted this for you but it didn’t post.

      From Dr. Greger’s video on treating Kidney stones:

      “Most stones are calcium oxalate–formed like rock candy when the urine becomes supersaturated–so doctors just assumed if they’re made out of calcium, we just have to tell people to reduce their calcium intake. So that was like the dietary gospel for kidney stone sufferers until this study was published in the New England Journal of Medicine, pitting the two diets against one another—low calcium versus low animal protein and salt. And it was the restriction of animal protein and salt that provided greater protection: cutting the risk of having another kidney stone within five years in half.

      What about cutting down on oxalates, which are concentrated in certain vegetables? A recent study found there was no increased risk of stone formation with higher vegetable intake. In fact, greater dietary intake of whole plant foods, fruits, and vegetables were each associated with a reduced risk independent of other known risk factors for kidney stones–meaning one may get additional benefits bulking up on plant foods, in addition to just restricting animal foods.”

      Looks like lowering salt and animal products and upping your hydration would be the way to go.

      Another site said to include foods rich in calcium at the meal which you eat oxalates. “The calcium and oxalate bind together in the intestines, reducing the formation of stones.”

      Hope that helps!

    3. Hello Linda, thanks for your comment.

      I did a search on PubMed about any possible link between fennel consumption and kidney stones, but couldn’t find any. In fact, several sites claim that is an herbal treatment for kidney stones. Not so sure how much that is true as there’s no scientific evidence on that, but I guess is based on case reports or experience. However, it’s an apparently safe herb.

      There’s more info about fennel use here:

      Hope it helps!

  7. Nothing really scientifically points to this, but apparently I am not singular in having tried it myself, and with others for difficulty at high altitude…vitamin E 400 or so.. And yes it seemed to work.

    Severe cases, of course that is a medical issue and must have immediate treatment such as immediately going down lower. But with moderate discomfort which is suggested by this article.
    Dexemethazone the steroid type compound used to help as well those with head injury is a common medical treatment.

    This gives a pretty objective look at it and
    Fennel seeds..never really tried them, but expect the taste would be off putting.

    1. Sorry…. A steroid called dexamethasone (Decadron®).
      Curiously it is known in this field of endeavor those with lasik eye surgery may have extreme vision difficulty at altitude of this type. Though it is to my knowledge readily solved with a lower altitude return.

    2. Those of us who have tried fainting are open to fennel for my next mountainous climb.

      This body climbed Mt. Washington and Mt. Whitney.

      The friend I went with for both of those hiked the Himalayas without me and I may not do many more mountains in my life, but if I ever go, I will have some fennel seeds in my pocket.

      So, is HAFE what my car got when it climbed mountains and stalled out?


  8. Dear Deb said, “Those of us who have tried fainting …”

    Huh? Tried? How does one “try” to faint? *says YR, who never fainted in her life*

    Color me confused. ;-)


    1. Shirley, have you had your blood oxygen tested? When I investigate supplements like those, I often see things arginine and citrulline. Just wondering if you need NO booster supplements?

    2. Just to be clear, there is no nitric oxide in nitric oxide supplements.
      They contain arginine (which is what the NO producing foods contain) which the body converts to NO.

    1. B’Healthy- It’s frustrating when headlines come out that seem to contradict solid previous research such as those that have come out recently claiming saturated fat is not harmful for cardiovascular health. While it’s unreasonable to expect full rebuttal of each and every article such as these, I think the following gives a good perspective about why you might see conflicting research and why certainly headlines and sometimes usually respectable journals can get it wrong by using faulty approaches, often relying on skewed meta-analysis.
      Review what Dr Greger has to say in general about reliable studies then review what Dr. Katz has to say: and

      It can be tiring seeing studies come out that are poorly designed with misleading results but don’t get battered around with headlines and even studies that seem to bounce back and forth, ,Finally, trust your instincts. If the study’s conclusion makes recommendations that aren’t clearly warranted by the results, be skeptical, no matter how persuasively the idea or product is promoted.

      Look for collaboration. Science is based on repeatedly obtaining the same result so studies that are duplicated by other research are more trustworthy than a one-time results.

    1. Studies of cooking a variety of nitrate-rich vegetables finds that nitrate levels to decrease with boiling. But health endpoints weren’t worsened by this. There is likely still plenty of beneficial phytonutrients including nitrates even in cooked beets. The juicing thing is that a serving of juice has a higher amount of nitrates bc it’s made from so many beets, and it’s as juice that performance improvement and recovery have been found for beets. On the other hand, juicing eliminates beneficial fiber. Greens, a critical daily component of the diet, also are high in nitrates. -Dr Anderson, Health Support Volunteer

  10. opinions on whether previous estrogen based breast cancer rules out fennel seed use? though i partake in a few when leaving indian restaurants that offer them. wonder about more regular consumption being too much. thoughts?

    1. susa johnson, I wondered the same thing. “Fennel seems to also affect estrogen levels in the body. Taking fennel along with tamoxifen might decrease the effectiveness of tamoxifen (Nolvadex). Do not take fennel if you are taking tamoxifen (Nolvadex).” Also “Hormone-sensitive condition such as breast cancer, uterine cancer, ovarian cancer, endometriosis, or uterine fibroids: Fennel might act like estrogen. If you have any condition that might be made worse by exposure to estrogen, do not use fennel.”

      I don’t know what the evidence behind these restrictions is.

    2. I’m not sure that it “acts” like estrogen as much as it promotes the production of estrogen in your body. Anecdotally, I am currently pregnant and have extremely low ferritin levels- I was taking an iron supplement for a few days and started to experience uncontrollable crying- I know the stereotype about pregnant women but this was not normal- I found out the supplement also contained fennel. Just a little in that 20ml overall dose of the supplement was enough to effect my hormones radically. It’s real. I discontinued use and was better in a day or so. I ate it in some veggie meatballs I made a month later and was a hormonal freak for most of the rest of the day. I don’t do fennel while I’m pregnant anymore.

      I love cooking with it when I’m not pregnant!! And I definitely have used fennel tea or chewing a few seeds of I have an upset or gassy tummy. But as far as the estrogen- in my humble opinion it can be powerful stuff….!!!

    3. I am a volunteer with NFO and attempted to get more clarification on your question of the safety of using fennel seeds regularly with breast cancer dx and unfortunately there seems to be no studies I can find in PubMed specifically related to this question. (21 studies on fennel and hormone effects but these related more to menopausal effects and other conditions, not cancer) While the precautions in using fennel due to estrogen effect seem prudent I could find no studies verifying this and it could be, like soy, the precautions are based on supplements, or excessive use, not common dosage with whole fennel seeds. Surely indulging in use after going into an Indian restaurant makes sense, deliberate daily use might need more scientific study which, alas, we do not seem to have at this point.

  11. Also wondering about breast cancer survivors and fennel seeds. I enjoy fennel root and would be interested in trying the seeds. Since my cancer was estrogen driven the idea of strong estrogenic effects is worrisome. Any information about that is very welcome.

  12. Very interesting article and the comments to it. Like anything in life there is pro and con in fennel use.
    On a different note, I would like Dr. Greger to comment on a recent article on the American Council on Science and Health website ( in which the author calls for a ‘Radical Reform’ in nutrition research. According to the author (Dr. Alex Berezow) many articles in nutritional sciences are biased, for without “positive results” the research would not get funded. I quote from the article:
    “Dr. Ioannidis blames residual confounding and selective reporting. Confounding means incorrectly concluding that A causes B when, in reality, some other factor X causes B. The trouble for researchers is when A and X are related to each other. Teasing out the true cause can be quite difficult. For instance, eating bacon very well may be associated with a shorter lifespan. But maybe bacon-eaters are also less likely to exercise, and lack of exercise (the confounding factor) is the actual cause of a shorter lifespan.”
    Since Dr. Greger is building his arguments on articles cited from nutrition and related sciences journals it would be interesting to know his opinion on articles such as the above that seem to advocate less reliance on the science behind nutrition research until at least some new rules for “correct reporting” are put in place. How do you distinguish what is and what is not biased reporting? Not to mention the huge interest of the agribusiness industry to fund research that increases sales of certain produce. To be fair, same can be said about the meat and dairy industry, which support research that benefit their sales efforts. Consumers are those who are often caught in the middle..

  13. Hi, Will. I have obseved Indian friends chewing fennel seeds after a meal to freshen breath and help prevent intestinal gas. If you like the taste of fennel, you could try this. I have tried it, and found it pleasant. I have also used ground fennel seeds in a variety of recipes. It is often traditionally used in sausage, along with garlic and sage. I use ground fennel seed, garlic, sage, and smoked paprika whenever I want to impart a sausage-y taste to dishes. I have even made WFPB sausage patties with cooked adzuki beans and soaked buckwheat, flavoring it this way. I like fennel in a variety of soups and stews, as well. Experiment to find what flavors you like with it. I hope that helps!

  14. After reading this article I decided to chew a few fennel seeds after most meals. I bought some on Sunday and by Wednesday evening, I started getting these red itchy clusters of skin eruptions on different parts of my body. They look like clusters of mosquito bites. Are they welts or hives? They must be some kind of allergy. I’ve never had this sort of thing before and have no other known allergy. I don’t know if it’s the fennel or not, but I can’t think of any other change in my diet or environment. I think I’ll stop the fennel for a few days and see if the eruptions go away.

    1. Follow Up: It’s 5 in the morning the next day. I just woke up from a dream about a huge ugly parasite chasing me. I woke up in a sweat with my welts itching me. It all makes sense now. Someone left a comment saying to think fecal when you talk about fennel. Fecal matter can contain parasites and the body responds to parasites by releasing histamine which can cause welts or hives. So my welts may not be from the fennel itself, but from it being contaminated with parasites.

  15. Please help me find where to purchase Fennel cream. I’ve tried to find it on my own, but without successs. I am a boomer female with facial hair growth mainly on and just under my chin.

    Thank you!

  16. Hi, Neal A. Hemmelstein! This article, cited in the blog post and companion video, states that, “Preparation with high fennel content (> 7 g of herbal substance) should not be taken for more than
    two weeks without medical advice.” This suggests that less than 7 g/day is safe for most people. The same article includes a section on “Overdose,” with the notation, “No data available.” I hope that helps!

  17. Is there any research on dose? I’m due to run my first marathon in Paris later this month. I’ll take beetroot as normal before an event, but I’m worried the nitrate effect will have worn off before I finish. Fennel seeds seem ideal to carry, but how much would I need to take with me ? Do they take longer than beetroot to start having an effect?

  18. Hi Nine Stone Cowboy – Thanks for your question!

    Check out this video here ( where 2 cups of beet juice were used for the dose and even at days 4, 5, and 6 of taking beet juice for supplementation, plasma nitric oxide levels were increased by 96% compared to the placebo. Therefore you can see that the nitrate effect did not wear off even after several days of daily beet juice consumption.

    In most research studies, a dose of about 2 cups of beet juice has commonly been used and in this review here ( supplementation up to 15 days has been studied and still found to have benefits.

    An even healthier alternative to drinking beet juice is to eat whole beets for improved athletic performance. In this video here (, 1 1/2 cups of whole beets (equal to 1 can of beets) were given 75 minutes prior to a race and the effects were carried throughout the entire 5K.

    Fennel seeds are certainly another alternative to beet juice that are easy to carry with you and can be used for a boost during a marathon run. I hope this helps!

    Janelle RD – Registered Dietitian & Health Support Volunteer

  19. Thank you.The logistics of preparing in a hotel room mean my beets will be whole (vacuum packed, probably not the most nutritious source, but needs must.)

    What I really wanted to know is whether there is any data on dosage for fennel seeds or on how long they take to become effective.

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