According to the United States Department of Agriculture (USDA) National Nutrient Database, one raw fennel bulb weighing contains:

• 73 calories
• 0.47 g of fat
• 2.9 g of protein
• 17 g of carbohydrate
• 7.3 g of dietary fiber – Fiber decreases the risk of heart disease as it helps reduce the total amount of cholesterol in the blood.The fiber content in fennel helps to prevent constipation and promotes regularity for a healthy digestive tract.
• no cholesterol

A cup of fennel also provides:

• 360 milligrams (mg) of potassium
• 45 mg of sodium
• 838 international units (IU) of vitamin A
• 43 mg of calcium
• 10.4 mg of vitamin C  – Raw fennel is an excellent source of vitamin C. Vitamin C is essential to collagen, the support system of the skin, and also works as an antioxidant to help prevent damage caused by the sun, pollution, and smoke. Vitamin C also promotes the ability of collagen to smooth wrinkles and improve the overall texture of the skin.
• 0.64 mg of iron
• 0.041 mg of vitamin B-6 – Vitamin B-6 and folate prevent the build-up of a compound called homocysteine by converting it into a different compound, methionine. When excessive amounts of homocysteine build up, it can damage blood vessels and lead to heart problems.
• 15 mg of magnesium

Fennel also contains:

• phosphorous
• zinc
• copper
• manganese – Both the bulb and seeds contain the mineral manganese, which is important for enzyme activation, metabolism, cellular protection, bone development, blood sugar regulation, and wound healing.
• selenium – Selenium is a mineral in fennel but not most other fruits and vegetables (as it is primarily found in Brazil nuts and animal proteins). It contributes to liver enzyme function and helps detoxify some cancer-causing compounds in the body. Selenium can also prevent inflammation and decrease tumor growth rates. The selenium found in fennel appears to stimulate production of killer T-cells. This suggests that it can improve the immune response to infection.
• niacin
• pantothenic acid
• folate – Plays a role in DNA synthesis and repair. This might help prevent cancer cells from forming because of mutations in the DNA.
• choline – Choline is a very important and versatile nutrient in fennel that helps with sleep, muscle movement, learning, and memory.
• beta-carotene
• lutein
• zeaxanthin
• vitamin E
• vitamin K
• Estrogen – Fennel is a natural source of estrogen. Estrogen plays a central role in regulating the female reproductive cycle, and it can also determine fertility. Menopausal women have lower estrogen levels which are associated with more abdominal weight gain. Some research has suggested that fennel extract may reduce the effects of premenstrual syndrome (PMS).
• Phosphate and calcium are both important in bone structure.
• Iron and zinc are crucial for the production and maturation of collagen. Pairing high-vitamin-C foods, such as fennel, with iron-rich foods can improve the ability of the body to absorb iron.
• Bone formation requires the mineral manganese.
• Low intakes of vitamin K have been associated with a higher risk for bone fracture.
• The nitrates in fennel can help moderate blood pressure – Nitrates have vasodilatory and vasoprotective properties. Because of this, they help lower blood pressure and protect the heart.

The fiber, potassium, folate, vitamin C, vitamin B-6, and phytonutrient content in fennel, coupled with its lack of cholesterol, all support heart health.

Perhaps the most impressive benefits of fennel and fennel seeds come from the antioxidants and potent plant compounds they contain. Essential oil of the plant has been shown to contain more than 87 volatile compounds, including the polyphenol antioxidants rosmarinic acid, chlorogenic acid, quercetin, and apigenin.

There are over 28 compounds have been identified in fennel seeds, including anethole, fenchone, methyl chavicol, and limonene.

Animal and test-tube studies note that the organic compound anethole has anticancer, antimicrobial, antiviral, and anti-inflammatory properties. The plant compound limonene helps combat free radicals and has been shown to protect rat cells from damage caused by certain chronic diseases.

Fennel seeds may curb appetite.

A study in 9 healthy women demonstrated that those who drank 8.5 ounces of tea made with 2 grams of fennel seeds before eating lunch felt significantly less hungry and consumed fewer calories during the meal than those who drank a placebo tea. Anethole, a major component of fennel essential oil, may be behind the appetite-suppressing qualities of the plant.

Fennel has been shown to have galactogenic properties, meaning it helps increase milk secretion. Research suggests that specific substances found in anethole, such as dianethole and photoanethole, are responsible for the galactogenic effects of the plant.

Fennel may increase milk secretion and blood levels of prolactin, a hormone that signals the body to produce breast milk.

Though fennel and its seeds are likely safe when eaten in moderation, there are some safety concerns over more concentrated sources of fennel, such as extracts and supplements. Due to its estrogen-like activity, there is concern over the plant’s potential teratogenicity, the potential to disturb fetal growth and development. A study that evaluated the teratogenicity of fennel essential oil showed that high doses may have toxic effects on fetal cells.

Although eating fennel and its seeds is likely safe, pregnant women should avoid taking supplements or ingesting the essential oil of this plant.

Fennel may also interact with certain medications, including estrogen pills and certain cancer medications, so always consult your healthcare provider before using high doses in supplement, essential oil, or extract form.