What is Watercress?
Watercress (Nasturtium officinale) is a fast-growing aquatic member of the cabbage family, native to Europe and Asia and now naturalized in cold, clean streams across most of the world. It has been eaten as a food and used as a medicine for at least two thousand years, and it holds an unusual distinction in modern nutrition science: when the CDC ranked 47 fruits and vegetables by nutrient density per calorie, watercress finished first with a perfect score. Its modern supplement reputation rests on a class of sulfur compounds it shares with broccoli and kale, and on a series of studies looking at detoxification enzymes and DNA damage.
Watercress is a cruciferous vegetable first and a supplement second. The leaves and stems are used, eaten fresh or juiced in traditional practice, and sold in capsule and powder form as a concentrated green.
Common Names
Watercress, water cress
Yellowcress, brown cress
Nasturtium officinale (botanical name, not related to garden nasturtium)
Primary Active Compounds
Gluconasturtiin, a glucosinolate that converts to phenethyl isothiocyanate (PEITC)
Vitamin K1 (phylloquinone), in very high concentration
Vitamin C and beta-carotene
Lutein and zeaxanthin
Dietary nitrates
Calcium, potassium, and manganese
Key Nutritional Note
Watercress is roughly 95 percent water and extremely low in calories, which is exactly why it scores so high on nutrient density per calorie. A cup of raw watercress supplies more than 100 percent of the daily value for vitamin K while contributing about four calories. That vitamin K figure is the single most practical fact about this plant for anyone on anticoagulant medication.
Watercress is a nutrient-dense cruciferous green with a specific sulfur chemistry, which is why the interesting claims about it are all downstream of how that chemistry survives processing.
What the Label Won't Tell You
The watercress research that sells capsules was run on fresh watercress. PEITC, the compound behind the DNA-protection and detox-enzyme headlines, does not exist in the intact plant. It forms only when the enzyme myrosinase meets the glucosinolate gluconasturtiin, which happens when you chew, chop, or crush living leaf tissue. Myrosinase is heat-sensitive and degrades during the drying and extraction steps used to make most powders and capsules. A dried watercress supplement can list an accurate glucosinolate content and still deliver almost no PEITC, because the enzyme that unlocks it is gone. Nothing on the label distinguishes the two. The trial protocols usually cited on these bottles fed people 85 grams of raw watercress daily, not a capsule.
Primary Functions & Benefits
Nutrient Density & Micronutrient Support
Exceptionally high vitamin K content supporting normal clotting and bone metabolism
Vitamin C, beta-carotene, and mineral content in a very low calorie package
Lutein and zeaxanthin for macular and retinal support
Cruciferous Sulfur Chemistry
PEITC supports phase II detoxification enzyme activity
Studied for effects on oxidative DNA damage markers in human trials
Part of the broader cruciferous research base alongside broccoli and sulforaphane
Antioxidant & Cellular Protection
Polyphenols and carotenoids counter oxidative stress
Studied for reduced lipid peroxidation in small human trials
Circulatory & Exercise Support
Dietary nitrate content contributes to nitric oxide production, the same mechanism behind beetroot's reputation
Traditionally regarded as a spring tonic and blood cleanser
Who Should Take Watercress
People wanting to raise micronutrient intake without raising calories
Those already eating cruciferous vegetables who want to widen the variety
People interested in the cruciferous sulfur research who are willing to eat it fresh
Those looking for dietary nitrates from a leafy source
Anyone building an eye-health diet around lutein and zeaxanthin
Who Should AVOID or Use Caution
Contraindications
Anyone on warfarin or another vitamin K antagonist without prescriber coordination
Known allergy to cruciferous vegetables
Wild-harvested watercress from streams that livestock access, due to liver fluke risk
Use Caution
People with thyroid conditions, since cruciferous vegetables contain goitrogens that matter mainly at high intake with low iodine
People with a history of kidney stones, given the plant's oxalate content
Pregnant individuals, where large medicinal amounts are traditionally discouraged while normal food amounts are considered fine
People with interstitial cystitis or an irritable bladder, since watercress is mildly diuretic and pungent
Critical Safety Point
The vitamin K content is not a footnote. Watercress can deliver more vitamin K per cup than nearly any other common vegetable, which directly opposes the action of warfarin and can push an INR out of range within days of a diet change. The answer is consistency, not avoidance. Pick an intake and hold it steady, and tell your prescriber before you change it in either direction. Separately, if you are gathering watercress from the wild rather than buying cultivated stock, the fascioliasis risk from sheep and cattle pasture runoff is real and is the reason traditional foragers cooked it.
Recommended Dosages
Fresh Watercress (Preferred)
Research protocols commonly use around 85 grams (roughly 3 ounces, or 2 to 3 cups loosely packed) daily
Culinary amounts of half a cup to a cup daily are a reasonable everyday intake
Juice
Traditionally used in small amounts, around 30 to 60 mL, often diluted, since the flavor is sharp
Capsules & Powders
Follow product labeling, commonly 500 to 1,000 mg of leaf powder daily
Recognize that these deliver the nutrients better than they deliver PEITC
Duration
Nutrient benefits accrue with regular ongoing intake
Study endpoints for oxidative markers were typically measured over 8 weeks
Timing & Administration
Eat It Raw When Possible
Chewing fresh leaves is what triggers the enzymatic conversion to PEITC; cooking deactivates myrosinase
With Fat
The carotenoids, lutein, and vitamin K are fat-soluble, so an oil-based dressing meaningfully improves absorption
Spread Across the Day
If using it as a nitrate source before training, roughly 90 minutes ahead matches the timing used in nitrate research
Rinse Well
Even cultivated watercress benefits from a thorough rinse, given how it is grown
Benefits of Taking Watercress
Among the most nutrient-dense foods per calorie that has been formally measured
Very high vitamin K, vitamin C, and carotenoid content
Supplies lutein and zeaxanthin for eye health
Supports phase II detoxification pathways through PEITC
Contributes dietary nitrates for circulatory support
Adds cruciferous variety without the sulfur load some people dislike in broccoli or cabbage
Effectively calorie-free, so it displaces nothing in the diet
Potential Negatives & Side Effects
Vitamin K interference with anticoagulant medication, the most consequential issue
Digestive irritation or heartburn from the pungent mustard oils in sensitive people
Increased urination from its mild diuretic action
Oxalate contribution for stone formers
Goitrogenic potential at very high intake combined with low iodine
Parasite exposure from wild-harvested plants in grazed watersheds
Deficiency Symptoms
Watercress is a food, not a required nutrient, so there is no watercress deficiency. What it delivers, however, includes nutrients you genuinely can run short on.
What its role addresses
Low vitamin K1 intake, which affects clotting factor production and bone protein carboxylation
Low carotenoid and lutein intake, relevant to macular pigment density
Generally low vegetable intake, which is the actual gap for most people reaching for a greens capsule
Bottom line
Nobody needs watercress specifically. It is one of the more efficient ways to close a vegetable and vitamin K gap, and if you already eat a wide range of greens it is a variety play rather than a fix.
Toxicity Symptoms
Watercress has no established toxic dose as a food and a long history of safe culinary use.
At high intake
Stomach upset, heartburn, or bladder irritation from concentrated juice or very large raw servings
Clinically meaningful shifts in INR for people on warfarin
Added oxalate load in people prone to calcium oxalate stones
Signs to reduce or stop
Burning or irritation in the mouth, stomach, or urinary tract
Unexpected INR movement on anticoagulant therapy
Skin reaction or swelling suggesting cruciferous sensitivity
General note
Overdose in the pharmacological sense is not the concern here. The realistic problems are medication interference, oxalate load, and, for wild-harvested plants, infection.
How Watercress Works
The Myrosinase Reaction
Intact cells store gluconasturtiin and the enzyme myrosinase in separate compartments; mechanical damage from chewing brings them together and produces PEITC
Phase II Enzyme Induction
PEITC upregulates glutathione S-transferase and related conjugating enzymes, the pathway responsible for tagging certain compounds for excretion
Antioxidant Contribution
Vitamin C, carotenoids, and polyphenols reduce oxidative damage to lipids and DNA, which is the endpoint most watercress trials measured
Nitrate to Nitric Oxide
Dietary nitrate is reduced to nitrite by oral bacteria and then to nitric oxide, supporting vasodilation and blood flow
Vitamin K and Carboxylation
Vitamin K1 serves as a cofactor for the carboxylation of clotting factors and of osteocalcin in bone
Synergistic Supplements
For Cruciferous Support
Broccoli sprouts or sulforaphane - the closely related isothiocyanate pathway
Mustard seed powder - supplies exogenous myrosinase, which is why it is added to processed cruciferous supplements
For Eye Health
Lutein and zeaxanthin isolates - reinforce macular pigment
Astaxanthin - complementary carotenoid support
Zinc - supports retinal enzyme function
For Bone Health
Vitamin D and calcium - pair with vitamin K1 for bone matrix support
Magnesium - broad bone mineralization support
For Circulation
Beetroot - stacks the dietary nitrate contribution
Vitamin C - supports nitric oxide availability
Interactions & What NOT to Take
Warfarin and Vitamin K Antagonists
This is the headline interaction. Sudden increases or decreases in watercress intake can shift INR. Keep intake consistent and inform your prescriber.
Thyroid Medication
Goitrogens matter mainly at very high intake with insufficient iodine; separate large servings from levothyroxine dosing and monitor if you have thyroid disease
Chlorzoxazone and CYP2E1 Substrates
Watercress has been shown to inhibit CYP2E1 in human studies, which can slow the clearance of drugs metabolized by that enzyme
Diuretics and Lithium
Mild diuretic activity warrants caution alongside prescription diuretics, and by extension with lithium
Special Considerations
Fresh vs Processed
This is the defining consideration. Fresh and raw preserves the enzyme chemistry; dried, heated, or encapsulated generally does not.
Sourcing
Buy cultivated, commercially grown watercress. Wild watercress from streams below livestock pasture is the classic route for liver fluke exposure.
Not Garden Nasturtium
Despite the botanical name, watercress is unrelated to the ornamental garden nasturtium (Tropaeolum majus) sold as an edible flower
Storage
Watercress is highly perishable and loses both nutrients and pungency quickly; use within a few days of purchase
Oxalates
Moderate oxalate content is worth accounting for if you are managing calcium oxalate stones, though it is lower than spinach
Research Status & Evidence Quality
Strong Evidence For
Nutrient density and micronutrient content, which is straightforward compositional analysis
Very high vitamin K1 content and the resulting anticoagulant interaction
Moderate Evidence For
Reduction of oxidative DNA damage markers in human trials, including work in smokers and in healthy adults
Increases in plasma carotenoid levels with regular intake
Inhibition of CYP2E1 activity
Emerging / Preliminary Evidence For
PEITC and cancer-related endpoints, which are largely cell and animal work rather than clinical outcomes
Exercise recovery and oxidative stress after training
Blood pressure effects from its nitrate content
Research Limitations
Human trials are small, often under 100 participants, and short
Nearly all of the meaningful research used fresh watercress rather than supplements, so results do not transfer cleanly to capsules
Outcomes are typically biomarkers rather than disease endpoints
Summary & Key Takeaways
Watercress is one of the few supplements where the honest recommendation is to skip the supplement. It is the most nutrient-dense vegetable ever formally scored, it carries the cruciferous sulfur chemistry that makes broccoli interesting, and it supplies vitamin K, carotenoids, and nitrates in a package with almost no calories. But its most cited benefit depends on an enzyme reaction that processing destroys, and every trial worth quoting used the fresh plant.
Bottom Line
Buy it in the produce section, not the supplement aisle. Two to three cups of fresh watercress daily matches the amount used in the research that supplement labels borrow from. If you do buy a powder, buy it for the nutrients and do not expect the PEITC story to hold.
Key Safety Points
The vitamin K content can move an INR, so anyone on warfarin needs consistent intake and a conversation with their prescriber. Source cultivated rather than wild plants to avoid liver fluke. Use caution with thyroid disease at very high intake, and account for oxalates if you form calcium oxalate stones.
Special Note
Watercress makes a broader point about greens supplements in general. Whenever a capsule's evidence base was built on a fresh, raw, enzymatically active food, ask whether the enzyme survived the manufacturing. In most cases the answer is no, and no label is required to tell you.
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Disclaimer: This content is for informational and educational purposes only and is not medical advice. It is not intended to diagnose, treat, cure, or prevent any disease. Always consult a qualified healthcare professional before starting any supplement, especially if you are pregnant, breastfeeding, taking medication, or managing a health condition.