Yellow fennel seeds in a pile on white background.

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 chrisanthemum, 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?

spinach can lower blood pressure and improve measures of arterial stiffness

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, non-perishable 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.

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NutritionFacts.org's picture

NUTRITIONFACTS.ORG is a strictly non-commercial, science-based public service provided by Dr. Michael Greger, providing free updates on the latest in nutrition research via bite-sized videos. There are more than a thousand videos on nearly every aspect of healthy eating, with new videos and articles uploaded every day. NutritionFacts.org was launched with seed money and support by the Jesse & Julie Rasch Foundation. Incorporated as a 501c3 nonprofit charity, NutritionFacts.org now relies on individual donors to keep the site alive.

Dr. Greger is a physician, New York Times bestselling author, and internationally recognized speaker on nutrition, food safety, and public health issues. A founding member and Fellow of the American College of Lifestyle Medicine, Dr. Greger is licensed as a general practitioner specializing in clinical nutrition. Currently he proudly serves as the public health director at the Humane Society of the United States. Dr. Greger is a graduate of the Cornell University School of Agriculture and the Tufts University School of Medicine.

His latest book, How Not to Die, became an instant New York Times Best Seller. 100% of all proceeds he has ever received from his books, DVDs, and speaking engagements has always and will always be donated to charity. Dr. Greger receives no compensation for his work on NutritionFacts.org.

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