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Ep. 70 – Research on flavonols

Do you ever start your day with a cup of black tea? How about closing the day with a beer? You just might be surprised at the flavonols they contain. On this episode, Dr. Chet Zelasko explains the latest research on flavonols and their relationship with mortality

Welcome to STOH. I’m your host Dr. Chet Zelasko. STOH is a joint production with WGVU in Grand Rapids MI. I examine the world of health. Nutrition. Exercise. Diet. Supplementation. If there’s something new, I look at the science behind them, and let you know whether it’s real or not. You can check out other things that I do on my website Drchet.com and sign up for my free emails.

Observational studies are just what the title indicates: observing something over time. In the case of the National Health and Nutrition Examination Survey, called NHANES for short, one of the objectives is to observe the relationship between diet and whether it impacts how long we live. I recently came across a paper in a journal that examined the relationship between flavanol intake and mortality. Observational studies cannot give cause and effect; they can only suggest a relationship which would then be followed up with randomized controlled trials. That doesn’t happen often enough in nutrition studies. Why not? How do you blind subjects to the foods they are eating? There are a few studies with small numbers of subjects that have tried to compare high fat with low fat diet with some success. But it involves preparing all meals for the subjects and trusting them not to exceed any limitations on what they can eat at home. Very difficult. Therefore, we take what we can get in observational studies.

Researchers wanted to find out the relationship between one class of flavonoids called flavanols and mortality. Before I go further, let me define and identify all the flavonoids for your information as well as to keep in mind we’re talking about a single class of flavonoids in this STOH.

Using the Linus Pauling website for defining flavonoids, which by the way, is an excellent resource for finding out basic information on any nutrient and they have a phenomenal glossary of nutrition terminology, flavonoids are a large family of polyphenolic plant compounds. Okay, what’s a polyphenol? It describes the structure of the phytonutrient which have a basic ring structure in common. What phenols attach to it determines what flavonoid it becomes. Riveting, isn’t it?

But so important because there are six major subclasses of flavonoids, namely anthocyanidins, flavan-3-ols, flavonols (our topic of this podcast), flavanones, flavones, and isoflavones. Of all the subclasses, flavonols are the most widespread in the human diet, even a poor one. Dietary flavonoids are naturally occurring in fruit, vegetables, chocolate, and beverages like wine, tea, and beer.

Getting back to the study. They only used data from the volunteers in three different NHANES data collection periods who completed a 24 hour dietary recall with an in person interview and telephone follow-up. This is about as good as it gets when collecting dietary data in such large studies—over 11,000 subjects. If you’ve listened long enough, I cringe every time studies use Food Frequency Questionnaires because of the reliance on correlations in the development of those questionnaires. But that’s a topic for another day.

After accounting for age, gender, and other factors, there was a clear relationship between flavanol intake and mortality. Total flavanol intake was associated with an overall decrease in all-cause, cancer-specific, and CVD-specific mortality risks. The decrease in hazard ratio was as much as cutting the risk in half (cancer-specific) and by one-third from CVD and all-cause mortality. They did not find the same benefit for Alzheimer Disease and diabetes. However, most people die from related conditions, not from the diagnosed disease itself in those cases.

The analyses of the NHANES study on flavanols and mortality did more than just look at a “class” of phytonutrients. They also looked at the individual flavanols and how they impacted mortality as well. A little background first.

Remember the 6 classes of flavonoids I mentioned earlier? Each of those classes are made up of individual phytonutrients. Flavanols have 4 primary phytonutrients in its class: quercetin, kaempferol, myricetin, and isorhamnetin. I’m going to skip the rest of the individual phytonutrients in the other classes as they were not part of the study.

One of the interesting things the researchers did was to examine the reduction in mortality for each flavanol phytonutrient. When comparing the first quartile (lowest) with the fourth quartile (highest) intake, not every phytonutrient reduced the mortality from all conditions. In other words, the overall reduction in mortality was greatest when looking at total flavanol intake, not in any single phytonutrient. The reason I mentioned it was because the focus of research is often on single phytonutrients as potential treatments of conditions and diseases. Quercetin is one of those phytonutrients as it has anti-inflammatory benefits. Doesn’t make it bad; just more medicine than nutrient.

As mentioned, flavanols are one sub-class of the flavonoids. One characteristic is that they seem to be poorly absorbed. Or are they? It could be that there are unknown genetic factors that impact absorption and/or utilization of those nutrients. It may be that when isolated from a plant, the absorption is interfered with in some way that’s not apparent. It may be that the microbiome has a role to play in absorption.

What is most likely is that when eaten or drunk in its natural or prepared state, it is the interaction of all the flavanols that help absorption. Or it may be that combinations of flavonoids found in specific foods that work together for absorption.

What foods have the highest flavanol content? When looking at mg/100 g, raw onions, cooked onions, apples with the peel, brewed black decaf tea, and brewed black tea top the list. When looking at the top three consumed? Onions, black tea, and apples top the list but beer comes in fourth place. While the amount per 100 g is low, we consume a lot of it. The rest of the top 10 consumed is iceberg lettuce, coffee brewed from grounds, canned tomatoes and red ripe tomatoes. You know how people say that iceberg lettuce has no nutritional value? Romaine and kale are not on that list. Of course, there are other phytonutrients found in those two greens but iceberg lettuce holds is own.

I feel compelled to mention that the list of flavonols and all flavonoid containing foods are limited. We find a way to get samples of produce from all over the country to test for hundreds of potential chemicals every year. Yet, we don’t do the same for phytonutrient content of the foods that those pesticides and herbicides are designed to protect. We have to take any research, observational in nature as this study was, and keep it in perspective. In the 15 years that covered the three NHANES data collection periods and follow-up to determine mortality rates, the types of produce have changed, the weather has changed, and the amount of sunlight in some areas which directly impacts the development of flavonoids and all phytonutrients has changed. Some levels have most likely declined while others may have improved. We don’t know because it hasn’t been tested.

Still, using the data we do have provides us with research that shows benefits of eating and drinking vegetables, fruits, and drinks that come from them. At the end of the day, the most important thing is that we have to consume flavanols to get the benefits. In reality, your momma was right: just eat your fruits and vegetables. They’re good for you. And if you chase them down with a beer, that’s good for you as well. As for me, I’m all out of time. Remember that health is a choice. Choose wisely today and every day. I'm Dr. Chet Zelasko.


1. Nature Reports. 2024. https://doi.org/10.1038/s41598-024-55145-y

Dr. Chet Zelasko is a scientist, speaker, and author. Dr. Chet has a Ph.D. and MA in Exercise Physiology and Health Education from Michigan State University and a BS in Physical Education from Canisius College. He’s certified by the American College of Sports Medicine as a Health and Fitness Specialist, belongs to the American Society of Nutrition, and has conducted research and been published in peer-reviewed journals. You can find him online at drchet.com.
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