Understanding Resveratrol in Grape Components
Resveratrol is a natural polyphenol produced by certain plants, including grapes, in response to external stressors such as fungal infections, UV radiation, and mechanical damage. This compound, best known for its presence in red wine, is not distributed evenly throughout the grape berry. The concentration varies significantly between the skin, seeds, and pulp, and is heavily influenced by a range of factors.
For many years, the grape skin was celebrated as the primary source of resveratrol. Early research confirmed its presence there, particularly in varieties used for red wine. However, as analytical methods have become more sophisticated, scientists have been able to provide a more detailed and nuanced picture of the compound's distribution.
The Role of Grape Seeds
While the skins typically contain a higher concentration of resveratrol per gram, this does not mean the seeds are insignificant. Research from the American Journal of Enology and Viticulture on muscadine grapes, for instance, showed that while the seeds had a higher concentration than other parts, they only contributed around 30% of the total resveratrol in the berry. This highlights a key distinction between concentration (amount per unit of weight) and total contribution (amount in the overall portion). Other studies have noted that some cultivars may have lower levels of resveratrol in their seeds compared to their skin and leaves.
The composition of grape seeds is different from the skin. Grape seed extract (GSE) is especially rich in a different, but equally powerful, class of antioxidants known as oligomeric proanthocyanidins (OPCs). These OPCs contribute to many of the established health benefits of GSE, such as cardiovascular support, rather than the resveratrol content.
Factors Influencing Resveratrol Content
The amount of resveratrol in grape seeds and other parts is not static. It is a biological response, and as such, it changes based on numerous factors.
- Grape Variety (Genetics): Different grape varieties produce varying amounts of resveratrol. Muscadine grapes (Vitis rotundifolia) are known for having particularly high levels of stilbenes compared to European grapes (Vitis vinifera). Some cultivars may even contain negligible or zero resveratrol in their seeds.
- Environmental Stressors: Biotic factors, such as fungal infections like Botrytis cinerea, can trigger the plant to produce more resveratrol as a defense mechanism. Abiotic stressors, including UV irradiation and water stress, also play a crucial role in enhancing resveratrol synthesis.
- Processing and Extraction: The methods used to process grapes significantly impact the final resveratrol yield. For example, the skin-contact time during winemaking is vital for extracting polyphenols, including resveratrol, into the wine. For supplements, the extraction technique can also dramatically affect how much resveratrol is obtained from seeds versus skins.
- Maturation Stage: The ripeness of the grapes affects resveratrol levels. Research shows that concentrations of trans-resveratrol in grape skins can be highest during the unripeness stage, with levels potentially decreasing as the fruit ripens.
Resveratrol Content Comparison
To better understand the distribution of resveratrol, here is a comparison of typical concentrations in different grape components and products, noting that these figures are approximate and highly variable.
| Component | Typical Resveratrol Content | Primary Polyphenols | Notes |
|---|---|---|---|
| Grape Skin | 50–100 µg/g | Resveratrol, Anthocyanins, Flavonols | High concentration per gram; acts as a plant defense mechanism. |
| Grape Seeds | Variable, but often lower than skin | Oligomeric Proanthocyanidins (OPCs) | Lower concentration of resveratrol, but contributes to total amount in some varieties. |
| Grape Seed Extract | Low, unless fortified | OPCs (up to 85%), Flavonoids | Standardized for OPCs, not typically for resveratrol. |
| Red Wine | Up to 15 µg/mL | Resveratrol, Quercetin, Catechins | Resveratrol is extracted during fermentation from grape skins. |
Practical Implications for Supplements
If you are seeking resveratrol in a dietary supplement, relying on grape seed extract as the sole source is not the most reliable approach. As the table illustrates, grape seed extract is primarily standardized for its high content of proanthocyanidins, not resveratrol. Many resveratrol supplements on the market, such as those combining it with grape seed extract, often derive their resveratrol from Japanese knotweed (Fallopia japonica), which is a much more concentrated and reliable source for this specific compound.
Conclusion
In summary, while grape seeds do contain some resveratrol, their contribution is generally lower in concentration compared to grape skins in most varieties. Furthermore, the overall resveratrol content is heavily dependent on several factors, making it highly variable. The most significant phytochemicals in grape seeds are not stilbenes like resveratrol, but rather oligomeric proanthocyanidins, which are also powerful antioxidants. Therefore, for those interested in a standardized dose of resveratrol, supplements often combine grape seed extract with other, more potent sources of resveratrol, such as Japanese knotweed extract, to ensure efficacy. It is a misconception that grape seed extract alone is a primary or reliable source for a significant amount of resveratrol.
Frequently Asked Questions
Is grape seed extract a good source of resveratrol?
No, grape seed extract is not a primary source of resveratrol. While grape seeds do contain trace amounts, the extract is primarily valued and standardized for its high concentration of other beneficial polyphenols known as oligomeric proanthocyanidins (OPCs).
Which part of the grape has the most resveratrol?
The grape skin is the richest source of resveratrol, containing significantly higher concentrations than the seeds or pulp in most varieties.
Can I get enough resveratrol from just eating grapes?
While eating grapes provides some resveratrol, the amount is relatively low compared to concentrated supplements. The content also varies depending on the grape variety and growing conditions, making it an unreliable source for specific therapeutic doses.
Why do supplements combine grape seed extract and resveratrol?
Supplements combine these two ingredients to leverage the benefits of both. Grape seed extract provides powerful proanthocyanidins, while the resveratrol is typically sourced separately from a more concentrated plant like Japanese knotweed to ensure a potent dose.
Are the seeds of muscadine grapes a good source of resveratrol?
Research indicates that muscadine grape seeds can have a higher concentration of resveratrol than some other varieties and contribute a notable portion of the berry's total resveratrol. However, the skin remains the primary source for the whole berry.
What factors cause the resveratrol content to vary?
Several factors influence resveratrol levels, including the grape variety, environmental conditions like UV radiation exposure, and stresses such as fungal infections. The ripeness of the grape at harvest also plays a role.
What are the main antioxidants in grape seed extract?
The main antioxidants in grape seed extract are oligomeric proanthocyanidins (OPCs), a different class of polyphenols from resveratrol. These are potent antioxidants and contribute significantly to the extract's health benefits.
