Does dairy block antioxidants in fruit?
Some claim that a protein in milk inactivates antioxidants in fruit. Are blueberries and yogurt a bad idea?
Nutrition Diva is hosted by Monica Reinagel. A transcript is available at Simplecast.
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I got an email this week from Reggie, who wrote:
“I have heard that dairy has a molecule in it that blocks the absorption of certain polyphenols—in particular blueberries and green tea. Meaning that if you have blueberries on your yogurt, you don’t get the amazing benefits of the blueberries.
But I remember reading elsewhere that when going through the digestive tract, those two types of foods are separated and two completely different enzymes are responsible for each of their absorption into our cells. So the message was that dairy does not block the nutrients of polyphenols.
What do you think? Do you believe that dairy blocks nutrients?”
Let’s start with this interesting idea that different foods or nutrients are separated as they go through the digestive tract. Sort of the way you might sort your laundry. You have your whites, which you wash in hot water with bleach. The colors are done in warm water and color-safe bleach. And the darks in cold water. If you’re in a laundromat, you can do them all at once in three different machines. If you’re at home, you do them sequentially, one at a time.
But the digestive tract is really just a single tube. It’s a pretty elaborate and functional tube, but it can’t sort foods or nutrients and send them down separate chutes to be processed separately. And there’s not really any place for things to be stored so that they can be separated and processed sequentially.
It’s more like a car wash. When you drive your car through the car wash, a sequence of different chemicals squirts out of a series of nozzles, aimed at various parts of your car. But your entire car goes through the car wash together. And if you eat blueberries and dairy together, they will not be physically separated from one another during digestion.
In the stomach, food is mixed together and treated to an acid bath, which serves a couple of purposes. First, the acid kills bacteria and other pathogens that could otherwise make you sick. Secondly, stomach acid begins to break down any proteins and prepare them for later phases of digestion.
Next stop in the digestive car wash is the small intestine. Here the stomach acid is neutralized by pancreatic juices, and this allows a variety of enzymes to go to work. (Most enzymes don’t work too well in acidic environments.)
There aren’t enzymes for blueberries and enzymes for milk, but there are lots of different enzymes for different kinds of proteins, carbohydrates, fats, and other molecules. But again, everything in your meal is being processed at more or less the same time, in more or less the same space.
With this in mind, let’s consider this idea that something in dairy blocks the absorption of polyphenols in blueberries—and that if you put blueberries on your yogurt, you won’t get any of the amazing benefits of blueberries.
The “something in milk” is the protein, casein. And it is true that casein has a propensity to bind with a variety of different antioxidants, including those in blueberries, coffee, tea, and chocolate.
Some research has found that casein reduces the antioxidant activity of certain polyphenols by 10 to 30%. Which means they retain 70 to 90% of their antioxidant capacity. But that’s in a test tube. The practical impact of this on human health and nutrition is unclear.
There’s not a ton of research on this and what we have is somewhat inconclusive. For example, a tiny study involving 11 subjects found that subjects who consumed a serving of blueberries with a cup of whole milk had less antioxidant activity afterward in their blood compared with those who consumed blueberries with a cup of plain water. On the other hand, scientists had subjects drink tea both with and without milk and found that it had absolutely no impact on the level of antioxidant activity in their blood.
Indeed, coffee and tea contribute far more antioxidants to the typical diet than blueberries (or any other fruit or vegetable). Coffee is the #1 source of antioxidants in the American diet, for example. And the health benefits of consuming tea and coffee are observed in people who drink them both with and without milk.
In my opinion, the small amount of research on this issue doesn’t provide enough data to draw any firm conclusions—or to justify dietary recommendations such as “never eat blueberries with dairy.” That one small study on blueberries and milk might not even apply to eating blueberries and yogurt. Yogurt has a much lower pH than milk, and higher microbial activity… both factors that could plausibly enhance antioxidant activity and absorption.
There are so many ways that nutrients or compounds from foods can enhance or inhibit each other’s absorption or activity. In general, I think we worry about this way more than we need to.
For example, I got a rather panicked email a few weeks back from Anne. She wrote:
“Help! I'm being treated for osteoporosis. For at least the past year, I have been eating non-fat Greek yogurt with a teaspoon of strawberry jam and topping it with walnuts for lunch. I try to eat whole grains, and a lot of nuts and seeds.
Now I've read that nuts and grains block the absorption of calcium. And that the combination of alcohol and salty-snacks (which I sometimes enjoy as a treat) is a disaster and flushes the calcium right out of your body. Neither my doctor nor my dietitian ever mentioned this!
Can you help? What can I eat with calcium-containing foods? Are there other calcium blocking foods or calcium-flushing ‘disasters’ I should know about?”
Anne seems to have stumbled on to a website trading in some rather sensational and fear-mongering misinformation.
Concerns about the mineral-blocking potential of foods containing phytates (such as nuts) have been vastly over-exaggerated. If her diet were very low in calcium and very high in phytates, and she only consumed calcium in combination with nuts, mayyyybbeee Anne would have something to worry about. But yogurt is probably not the only source of calcium in her diet. And the nuts are likely having only a minor effect. In fact, people whose diets are higher in phytates actually have a lower risk of osteoporosis.
Furthermore, although salty snacks and alcohol may transiently increase calcium excretion, they are not going to flush all the calcium out of your body—assuming you're consuming them in moderation. I think the reason Anne’s doctor and dietitian didn't mention it is because it's not worth mentioning.
The ways in which whole foods interact—both to enhance and inhibit various activities—is enormously complex and very incompletely understood. I think it’s good enough to just focus on eating a lot of nutrient-rich foods—in whatever combinations you find enjoyable or convenient: whether that’s putting nuts or blueberries on your yogurt, milk in your tea, or asparagus in your coffee. (Just kidding, don’t put asparagus in your coffee. That sounds neither enjoyable nor particularly convenient.)
Keep those nutrition questions coming. Together, we can do our little part to dispel confusion and ratchet down anxiety. You can email me at nurition@quickanddirtytips.com or record me a voicemail at 443-961-6206.