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Herb Drug Mix: Effects of Fibers, Tannins and Phytates

The absorption mystery
By Robert Rountree, M.D.
March/April 2001
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Flaxseeds are rich in fiber, so they may reduce the absorption of certain drugs.
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Have you ever stopped in the middle of a meal and thought about what you’re putting into your body? If you were to perform a nutritional analysis of all of the different carbohydrates, proteins, fats, vitamins, minerals, and other chemicals contained in the food you’re eating, you might find yourself wondering how it would ever be possible to digest and assimilate such a complex mixture. Somehow, we’re able to take in an astounding variety of foods, grind them up, blend them together into a slurry, add in some enzymes, and extract from them the nutrients we need to stay healthy.

It’s precisely this background information that comes to mind when a patient asks me whether it’s safe to take more than one herb, vitamin, or mineral supplement at a time and whether any herbs or supplements could potentially interfere with prescription medications.

On one hand, the possibilities for a potential interaction are enormous—entire chapters of nutritional textbooks are devoted to the subject. And more recently, medical journals are also addressing the subject of herb-drug interactions in some detail. It’s impossible to read such texts and not end up paranoid about what could go wrong. On the other hand, our digestive tracts are used to dealing with a high level of complexity. The nutrients that we ingest are always interacting with each other. This interaction may be synergistic, neutral, or antagonistic. Consequently, the only thing that’s important is the result—is it beneficial or not?

A potential for interference

Adding an herb or vitamin supplement to the mix is not that different from adding a food. It’s difficult to predict what the outcome might be based on analyzing the potential interactions of individual constituents. Again, there’s only one significant question: What is the final result?

One area of potential interaction that’s particularly complex has to do with so-called “non-nutrient” substances in foods and herbs that could potentially interfere with absorption of nutrients or medications. These substances act by either binding to the nutrient or by making it precipitate (so that it’s no longer dissolved in the slurry and therefore unable to be absorbed). When this happens, some or all of the nutrient or medication gets excreted without making it into the bloodstream. There are three important groups of these non-nutrient substances: fibers, tannins, and phytates (or phytic acid).

Fibers include gums, resins, and mucilages. They are abundant in certain seeds such as flax (Linum usitatissimum), fenugreek (Trigonella foenum-graecum), and psyllium (Plantago spp.). Oat and barley bran are also good sources. So are konjac root (Armophophallus konjac) and guar gum (Cyamopsis tetragonolobus). Marsh-mallow root (Althaea officinalis) and slippery elm bark (Ulmus rubra) are well known for their mucilages.

Medical research shows that fibers have clearly beneficial effects—they can lower cholesterol, stabilize blood sugar in diabetics, improve bowel function, and soothe inflamed mucous membranes. However, they also can decrease absorption of numerous drugs such as antibiotics, digoxin, and coumadin; retard absorption of minerals such as calcium, iron, and zinc; and impair absorption of vitamin B12 as well as the fat-soluble vitamins A, D, and E. Obviously, the important question here is exactly how much they affect absorption. What quantity does one need to take and for how long before the fibers interfere with the action of the drug? Unfortunately, there are no good answers to these questions.

Tannins and phytic acid

The definition of plant tannins—members of the polyphenol chemical family—is based on their ability to make proteins precipitate. Tannins are found in a wide variety of plants. They’re produced as a natural insecticide and as protection against the damaging effects of ultraviolet radiation from the sun. Medicinal and food plants rich in tannins include tea (Camellia sinensis), oak (Quercus spp.), black walnut (Juglans nigra), uva ursi (Arctostaphylos uva-ursi), red raspberry (Rubus idaeus), bilberry (Vaccinium myrtillus), and purple grapes (especially the varieties used to make cabernet and merlot).

Tannins have astringent and anti- inflammatory properties that make them useful for treating the swollen mucous membranes resulting from infection, allergies, or inflammatory bowel disease. However, they also inhibit iron absorption, especially when the iron is in the inorganic form (meaning not the iron in meat). Tannins also may inhibit other minerals as well as B-complex vitamins, although the effect doesn’t appear to be as potent. In addition, published reports indicate that they can impair absorption of certain drugs, including atropine, codeine, theophylline, ephedrine, and pseudoephedrine. The same questions still apply: How much and for what duration is necessary for this to be a problem? Again, the available information is unclear.

Phytic acid—also called inositol hexaphosphate (IP-6)—is a substance found in soybeans and the bran of cereal grains. For many years, it was considered to be an “anti-nutrient” because of its perceived tendency to bind calcium, zinc, and other minerals and increase their excretion. However, perceptions changed when medical studies showed that phytic acid had cancer-preventive properties. Along with these studies came new research that cast doubt on the significance of the calcium-binding effect. Nevertheless, soybeans and other plants containing phytates are still listed in nutrition texts for their potential to induce mineral deficiency.

My recommendations

Given the complexity of this situation, how can one apply some common sense?

I would start with a bit of wisdom from the book Principles and Practice of Phytotherapy (Churchill Livingstone, 1999): “Any substantial consumption of tea, coffee, alcohol, or tobacco will have more effects on drug activity than most herbal preparations.” In other words, it’s a lot easier to overdo it by drinking tea and coffee than it is to take too much psyllium or eat too many blueberries. Certainly, if you’re taking a medication that is difficult to absorb (especially if the label indicates taking it on an empty stomach), common sense would suggest not chasing it with a cup of tea or mixing it with oat bran.

I would also consider the following observation from Foundations of Nutritional Medicine (Third Line, 1997): “Absorption of supplemental calcium is about 10 percent higher when it is co-ingested with food. Even though food may contain oxalates, phytates, and other anti-absorbers, they are irrelevant as they are already fully complexed with the calcium in the food source.” Most of the available data appears to show that it’s preferable to take vitamins and minerals with foods to achieve better absorption. As for medicinal herbs, very little information is available about the effect of foods on absorption. Most of the time, you will probably do just fine taking them with meals and letting your digestive tract figure it out—just like it usually does.


Robert Rountree, M.D., is a physician at the Helios Health Center in Boulder, Colorado, where he practices integrative medicine. He is coauthor of both Immunotics (Putnam, 2000) and Smart Medicine for a Healthier Child (Avery, 1994), and is an Herb Research Foundation advisory board member.


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