Flavonols in the Diet: Why “Eat More Vegetables and Fruit” Is Not Always Enough

The journal Food & Function has published a paper that asks a simple but important question — if a person follows the standard recommendations on vegetables and fruit, do they get enough flavonols, the plant bioactive compounds linked to cardiovascular benefit?

The analysis drew on data from two large studies — COSMOS in the United States and EPIC-Norfolk in the United Kingdom. Importantly, flavonol intake was assessed not only from dietary questionnaires, but also from two validated urinary biomarkers. In total, biomarker data were available for 30,663 people.

The main finding. A level of at least 500 mg of flavonols per day was reached by only 19.2% of COSMOS participants and 17.9% of EPIC-Norfolk participants. Even among people with high consumption of vegetables and fruit or with a healthier diet, fewer than a quarter of participants reached this level.

For patients with a cancer diagnosis, the meaning of this publication is not that flavonols treat cancer. The practical conclusion is different — the recommendation to “eat more vegetables and fruit” is useful, but too general; it is simply not enough. At the same time, it is important to remember that any capsules, extracts, and supplements during antitumor treatment require discussion with the treating physician.

In Brief, What Happened

• The authors analyzed data from two large studies: COSMOS in the United States and EPIC-Norfolk in the United Kingdom.
• Flavonols were assessed not only from dietary questionnaires, but also from two validated urinary biomarkers: gVLM_B and SREM_B in urine.
• The target level of 500 mg of flavonols per day was reached by 19.2% of COSMOS participants and 17.9% of EPIC-Norfolk participants.
• Even among people who ate a lot of vegetables and fruit and generally followed a healthy diet, fewer than 25% reached the level of 500 mg per day.
• The authors concluded that standard recommendations on vegetables and fruit on their own do not guarantee flavonol intake at the level that was associated with benefit.

What Are Flavonols?

Flavonols are plant compounds from the large group of flavonoids. They are found in various plant-based products: tea, cocoa, apples, berries, grapes, some legumes, and other vegetables and fruit. In the Food & Function paper, the main focus was not on cancer treatment, but on whether an ordinary diet reaches the level of flavonols previously discussed in connection with cardiovascular effects.

Flavonoids are often described as compounds with antioxidant and anti-inflammatory properties.

Key Figures of the Study

• Publication: Food & Function, 2026, the paper “Adhering to dietary guidelines does not yield flavanol intake levels associated with beneficial cardiovascular effects”.
• Publication date: 8 June 2026.
• Type of work — an analysis of data from two large cohorts using dietary data and urinary biomarkers.
• COSMOS, United States — 6,509 participants with urinary biomarkers.
• EPIC-Norfolk, United Kingdom — 24,154 participants with urinary biomarkers.
• In total: 30,663 participants with biomarker data.
• The threshold that was assessed — at least 500 mg of flavonols per day.
• The proportion of participants who reached this level: COSMOS — 19.2%, EPIC-Norfolk — 17.9%.

Dietary modeling showed that five servings of vegetables and fruit, typical of an American diet, were unlikely to provide 500 mg of flavonols per day. Even when choosing products with a higher flavonol content, the probability of reaching 500 mg remained below 50%.

Why This Is Interesting

Usually the medical recommendation sounds simple — “more vegetables, fruit, wholegrain products, legumes, less ultra-processed food and excess sugar”. This is the right foundation. International dietetic associations recommend wholegrain products, vegetables, fruit, and beans as a meaningful part of the daily diet, including at least 400 g of varied non-starchy vegetables and fruit per day.

But the new work reveals an important nuance — vegetables and fruit are not a uniform group. A banana, an apple, broccoli, berries, tea, and cocoa can differ greatly in their content of specific bioactive compounds. That is, the recommendation to “eat more plant foods” remains correct, but sometimes requires clarification. What matters is not only quantity, but also variety, the choice of products, and their actual composition.

The authors give a telling example: the content of epicatechin even in apples of the same variety can vary by more than 10-fold. Therefore, the number of apples needed to obtain 80 mg of epicatechin can theoretically range from 2 to 29. This clearly shows why counting by a single product is difficult.

What This Means for Patients With a Cancer Diagnosis

First, this paper is not about cancer treatment. Flavonols are not antitumor therapy; they do not replace chemotherapy, targeted therapy, immunotherapy, hormone therapy, radiation therapy, or surgical treatment.

Second, plant-based food remains an important part of a healthy diet. But during cancer treatment, nutrition must address specific tasks — maintaining weight, preserving muscle mass, sufficient protein and energy, prevention of dehydration, management of nausea, diarrhea, mucositis, taste disturbance, and loss of appetite. There is more on this on the supportive care page.

Third, foods and capsules are not the same thing. Berries, apples, tea, or cocoa as part of a normal diet are usually discussed as part of nutrition. But concentrated extracts, high doses of antioxidants, and supplements may interact with treatment.

The National Cancer Institute warns that dietary supplements, herbs, and other complementary medicine products can change how antitumor drugs are absorbed, metabolized, and eliminated. It specifically notes that antioxidant supplements during chemotherapy or radiation therapy may require a safety review.

Therefore, the main practical conclusion for a cancer patient is as follows: dietary sources of flavonols can be discussed as part of a normal diet, while supplements with flavonols, green tea, cocoa extract, or other antioxidants must be agreed upon with the treating oncologist.

What You Can Do With Your Diet Today

• Keep the basis of your diet — colorful vegetables and fruit, wholegrain products, legumes, nuts, and sufficient protein.
• Make plant-based food more varied, rather than limiting yourself to two familiar products.
• Add sources of flavonols to your normal diet (tea, berries, apples, grapes, cocoa products without excess sugar, if they are well tolerated).
• Do not start concentrated extracts and antioxidant supplements during chemotherapy, radiation therapy, targeted therapy, or immunotherapy without agreement with a physician.
• In case of weight loss, poor appetite, diarrhea, stomatitis, or nausea, do not place flavonols above the main nutritional tasks — namely protein, calories, fluids, and food tolerance.

When to Discuss Supplements and Nutrition With a Doctor

• If you are already receiving chemotherapy, immunotherapy, targeted therapy, hormone therapy, or radiation treatment.
• If you take green tea in capsules, cocoa extract, curcumin, resveratrol, high doses of vitamins C, E, A, selenium, or other antioxidant supplements.
• If you have diseases of the liver, kidneys, stomach, or intestines, blood clotting disorders, or you take anticoagulants.
• If during treatment you develop diarrhea, nausea, stomatitis, weight loss, marked weakness, or decreased appetite.
• If you want to understand whether a specific supplement can be combined with your treatment regimen.
• If you have completed treatment and want to build a diet for recovery, weight control, and reduction of cardiovascular risks.

Read more about when a second opinion is useful and how it works.

What to Prepare for the Consultation

• The diagnosis, the stage of the disease, and the histology report.
• Records of treatment already completed and ongoing.
• A list of all antitumor drugs.
• A list of all regular medications.
• A list of supplements, vitamins, herbs, extracts, and sports nutrition with dosages.
• Photos of supplement packaging or links to the composition.
• Recent blood tests, including liver and kidney parameters.
• Weight trends over the past 1–3 months.
• Nutrition-related complaints: appetite, nausea, diarrhea, constipation, stomatitis, change in taste.
• A list of questions you want to discuss with the oncologist or a dietitian-nutritionist.

Frequently Asked Questions

More answers on the FAQ page.