Japanese Knotweed (Polygonum cuspidatum): The Source of Resveratrol

When a resveratrol supplement carries the note “from Polygonum cuspidatum,” it almost always refers to Japanese knotweed. Today, this plant is the most important botanical source of resveratrol in food supplements – ahead of even the grape. This guide explains what's behind the name, which compounds the root provides, and how to recognize a good extract.

What Is Japanese Knotweed?

Japanese knotweed is a perennial, herbaceous plant from the knotweed family (Polygonaceae). Native to East Asia – primarily Japan, China, and Korea – it grows there on embankments, along streams, and near volcanic soils. Botanically, it carries two common names: Polygonum cuspidatum is the older designation, while Fallopia japonica is the more recent classification. Both names refer to the same species; they appear side by side in the scientific literature and on labels.

In Europe and North America, the plant is best known as a persistent invasive species: it spreads rapidly, forms dense stands, and is difficult to displace. This very robustness is linked to its metabolism – for its own defense, knotweed produces a whole range of polyphenols, which is exactly what makes it so interesting for extract production. Resveratrol belongs to the so-called phytoalexins, defense compounds that a plant produces in greater amounts under stress, for example when attacked by fungi. This explains why a wild plant growing under harsh conditions has become such a productive source of raw material.

Traditional Use in East Asia

In its native countries, knotweed has long been known as both a useful and a wild plant. In China, the dried root is called “Hu Zhang”; in Japan, the plant is known as “Itadori,” and the young shoots are also regionally prepared as a vegetable there. This long-standing familiarity with the plant is one reason why its root, in particular, became a focus early on as a raw material for standardized extracts. One important distinction: historical use is a cultural classification and does not constitute a health-related claim for a modern food supplement.

Why the Root and Not the Leaf?

Extracts are made almost exclusively from the root and rhizome (the underground root stock). The reason is simple: resveratrol and related compounds accumulate primarily in the roots, while the stems and leaves show markedly lower levels. Studies examining the quality of Fallopia japonica as a resveratrol raw material also show that content varies seasonally, typically reaching its highest level in the roots during autumn.

The roots are harvested, cleaned, dried, and then processed into a powder that serves as the starting material for extraction. The valuable polyphenols are then drawn out of this raw material and concentrated. Various methods can be used for extraction – from classic extraction with water or ethanol to gentle extraction using supercritical CO₂. The method chosen also influences how pure the final product turns out and how well unwanted accompanying substances can be separated out.

The Key Compounds at a Glance

Knotweed root extract is not a single pure substance but a mixture of various secondary plant compounds. The components relevant to food supplements are:

• trans-Resveratrol: the namesake stilbene polyphenol and the marker compound to which high-quality extracts are standardized. The trans form is considered the more stable variant and the one most thoroughly studied in research.
• Polydatin (Piceid): a “precursor” molecule of resveratrol bound to a sugar unit. It is often present in the root in considerable amounts and is partly converted into resveratrol in the body.
• Emodin: an anthraquinone that naturally adheres to the root material. Emodin is a point that reputable manufacturers keep an eye on (see the Quality section).
• Other Polyphenols: including anthraquinone derivatives and flavonoids that round out the plant's natural profile of compounds.

The term polydatin deserves a brief explanation: chemically, it is the glucoside of resveratrol – in other words, resveratrol with an attached sugar unit. This bond makes the molecule more water-soluble and more stable. In the root, a significant part of the “resveratrol potential” actually exists as polydatin, which is why some analyses report the combined content of both substances together. When comparing products, it helps to know whether a percentage figure refers to pure trans-resveratrol or to the sum of the stilbenes.

To learn how resveratrol fits into the broader group of polyphenols and what sets it apart from the polyphenols found in grapes, see our guide Resveratrol – The Polyphenol of the Grape.

Knotweed versus Grape as a Resveratrol Source

Resveratrol occurs naturally in many places – in the skin of red grapes, in peanuts, in certain berries. The practical drawback of these sources: content is low and highly variable. Obtaining a standardized amount of resveratrol would require large quantities of grape material.

The root of knotweed, by contrast, naturally contains a significantly higher concentration. This is why Polygonum cuspidatum has become the standard raw material for concentrated resveratrol supplements, while grape extracts tend to be used where the broader polyphenol spectrum of the grape is desired. Both approaches have their merits – what matters is what is actually declared and standardized on the label.

Understanding Extract Ratio and Standardization

Two terms help in assessing the quality of an extract:

The Extract Ratio (DER)

The drug-extract ratio indicates how much dried root material is contained in one unit of extract – for example, 20:1. It describes the degree of concentration but, on its own, doesn't yet reveal the exact resveratrol content.

Standardization

More informative is the stated standardized active-compound content, for example “standardized to 50% trans-resveratrol.” Such extracts are available across a wide range, often between about 50 and 98 percent. From this percentage and the capsule amount, the actual resveratrol content per serving can be calculated – the number that really matters.

We've summarized exactly what to look for when buying – from labeling to purity – in our guide Buying Resveratrol – What to Look For.

Quality: How to Recognize a Good Root Extract

Because the starting material is a wild plant with a natural profile of accompanying substances, purification is a key quality marker. Look out for the following points:

• Clear Standardization: The trans-resveratrol content should be stated both as a percentage and in milligrams per daily serving.
• Trans Form Declared: Good products explicitly name trans-resveratrol, not just an unspecific “resveratrol.”
• Emodin Reduction and Contaminant Testing: Reputable manufacturers monitor the natural emodin content as well as possible residues (heavy metals, pesticides, solvents) through laboratory analysis.
• Origin and Batch Testing: Traceable origin of the root material and certificates of analysis for each batch speak for transparency.

A high percentage alone is not a seal of quality – only combined with thorough purification and verified purity does a high-quality extract result. Another aspect is stability: trans-resveratrol is sensitive to light and oxygen and can convert into the less-studied cis form under unfavorable conditions. Good products account for this with light-protected, tightly sealed packaging. Storage at home – cool, dry, and dark – also helps ensure that the declared content is maintained throughout its shelf life.

Intake and Classification as a Food Supplement

Extracts from Japanese knotweed are sold in Germany as food in the form of food supplements – usually as capsules or powder. They do not replace a varied diet and are not a medicinal product. There is currently no EU-approved health claim for resveratrol from this source; accordingly, such a product may not and should not be advertised with promises of effect.

For guidance on dosage, taking it with a meal, and sensible combinations – for example with fat-containing food – see our guide Resveratrol – Intake and Combination. Anyone taking medication, who is pregnant, breastfeeding, or has pre-existing conditions should always clarify use with a doctor beforehand. This is especially true because polyphenols like resveratrol can interact with the metabolism of certain medications – an individual assessment by a physician is the right approach here. A food supplement is intended as a complement, not as a replacement for a balanced diet or medical treatment.

Frequently Asked Questions (FAQ)

Is Polygonum cuspidatum the Same as Fallopia japonica?

Yes. These are two botanical names for the same plant, Japanese knotweed. Polygonum cuspidatum is the older name, Fallopia japonica the more recent one. Both names are used on labels and in studies.

Which Plant Part Provides the Resveratrol?

The dried root, or rhizome, is used. There, resveratrol, polydatin, and other polyphenols accumulate at significantly higher concentrations than in the stems and leaves, which play almost no role in extract production.

Why Is Knotweed Used Instead of Grapes?

The root naturally contains a considerably higher resveratrol content than the skin of red grapes. This makes it possible to produce standardized, concentrated extracts more efficiently and with more consistent content. Grape extracts tend to be used instead for their broader polyphenol spectrum.

What Does “Standardized to 50% Resveratrol” Mean?

This figure describes what proportion of the extract consists of trans-resveratrol. At 50%, one gram of extract mathematically provides 500 milligrams of resveratrol. Combined with the capsule amount, this makes it possible to determine the actual content per serving – the most meaningful figure when comparing products.

Is Emodin Content a Concern?

Emodin is a natural accompanying substance in the root. Good-quality extracts are purified so that the emodin content is kept low and verified through laboratory testing. Look for manufacturers who transparently disclose purity and accompanying substances through a certificate of analysis.