Sunday, July 31, 2011

Ask Dr. Andro: "Does Adding Milk to My Tea Reduce Its Health Benefits or Destroy the Antioxidants? Is There a Difference For Black, Green and White Tea?"

Image 1: Monkey milk tea by Conchibi. No matter how decorative it may look, according to various online sources, the milky monkey decoration is just about to annihilate the beneficial health effects of tea at the very moment it mingles with the antioxidant brew... or is this just another urban myth that is reaffirmed by 'gurus' all over the Internet?
Question from Samir Banga (via Facebook): "Just wanted to ask you about tea and if adding milk or cream for that matter disturbs or destroys the antioxidants/good stuff in the tea. Also would this be the same with black, green and white tea?"

Answer Dr. Andro: This is one of the typical cases where just typing in a question in Google produces either answer A "tea with milk = no problem" or answer B "tea with milk = worthless swill" - it just depends on how you formulate the question - or maybe who on whether the milk industry advertises with Google? Who knows.... A more thorough investigation, on the other hand, turns up a whole host of studies, the results of which are similarly conflictive. So what can we do to find out the truth? Well, we will do our very own little meta review. Sounds pretty sophisticated, doesn't it? Certainly something only Dr. Andro can do. Well, not really... take my virtual hand and I will show you that, oftentimes, you can easily answer your questions yourself, if you mix PubMed, Excel and some of your own gray matter and simmer that for a few hours.

First we will have to find some real data related to the question

To "google" the answer would be a highly unreliable undertaking. Too many self-proclaimed experts (like me) run fancy looking blogs (like the SuppVersity) where they propagate factoids - something you will of course never find on the SuppVersity ;-) Our first task is thus to find real data, a process that obviously requires
  1. to know what real data is
  2. to know where to find this real data
  3. to know how to find this real data
  4. to have access to this real data
The latter, I assume could unfortunately pose an insurmountable hurdle for some of you, because even scientists, or rather the editors and publishers of the magazines the scientists publish their articles in, need to pay for their tea and milk and tend to charge horrendous sums for a single article (I have seen individual physics papers for >199$), if you do not  have access to the respective journals from a university or public library. So, in case you cannot follow all my next steps minutely do not fret, the number of freely accessible articles is ever increasing and I promise that you will get the answer to Samir's question and even more for free ;-)

'Pubmeding' is 'googling' for smart people

Image 2: By using search tokens, we
were able to limit the no. of results to
138, a simple search for "milk in tea"
would have produced 503 hits and
good (?) old google would have spit
out roughly 96.500.000 predominant-
ly irrelevant if not corrupted results
First thing we will do is go to the medical database of the US National Library of Medicine (www.pubmed.com) - and though I think that much of the dietary advice coming from the US National Institute of Health does more harm than good, the PubMed database comprises almost all medical papers that were published in any serious journal (not only the prestigious ones) from the realms of medicine, exercise, nutrition and beyond. On the PubMed website, we will  type in the following search term "milk AND (coffee OR tea) AND (antioxidant OR polyphenol OR vitamin OR health)". It goes without saying that this will deliver all articles which deal with milk in relation to coffee or tea and their antioxidant, polyphenol, vitamin and caffeine content, as well as diverse health effects. 
Note: You may have noticed that, out of personal curiosity, I have expanded our complex search term beyond Samir's original question to another often overlooked yet similarly healthy caffeinated beverage, of which the average American drinks 3.1 cups each day - coffee! And since I have heard rumors that milk could also influence the pharmacodynamics, i.e. the absorption and effect, of caffeine, when it is added to caffeinated beverages, I also included the term "caffeine" in our complex request. Notwithstanding the complexity of the latter, the use of multiple key words and tokens for their logical connection limits the number of "hits", i.e. studies that match our criteria to 503 (cf. image 1).
From the initial 503 published articles the database has to offer, those papers which actually have our keywords in their title are obviously most likely to contain the information we are looking for. A mouse-click on the "see more" link below the "titles with your search terms" heading in the right column of the webpage (highlighted in image 2) reduces the results by -99.4% and leaves a manageable amount of three studies for us to 'review':
Illustration 1: The way to a manageable
amount of reliable data for a meta-analysis
1: Ryan L, Petit S. Addition of whole, semiskimmed, and skimmed bovine milk reduces the total antioxidant capacity of black tea. Nutr Res. 2010 Jan;30(1):14-20. PubMed PMID: 20116655.

2: Reddy VC, Vidya Sagar GV, Sreeramulu D, Venu L, Raghunath M. Addition of milk does not alter the antioxidant activity of black tea. Ann Nutr Metab. 2005 May-Jun;49(3):189-95. Epub 2005 Jul 13. PubMed PMID: 16020939.

3: Leenen R, Roodenburg AJ, Tijburg LB, Wiseman SA. A single dose of tea with or without milk increases plasma antioxidant activity in humans. Eur J Clin Nutr. 2000 Jan;54(1):87-92. PubMed PMID: 10694777.
And, lo and behold, all three are actually relevant to the question we want to answer! Heureka! Our initial euphoria begins to fade, however, when we have a closer look at the titles. While the most recent study (Ryan. 2010) claims that the addition of milk reduces the total antioxidant capacity of black tea, Reddy et al. (Reddy. 2005) and Leenen et al. (Leenen. 2000) in their studies, five and ten years before, found no reduction in the antioxidant activity of (black) tea.... well, I guess the question would not have been worth being included in the famous *hem* Ask Dr. Andro column if typing in a few words into a search engine would have delivered the correct answer right away*.
* We will have to postpone the effects on caffeine absorbtion and differences between the effects of milk in coffee vs. tea on another Ask Dr. Andro column, though, because the titles of our search results suggest that there won't be any information on these secondary issues in the respective fulltexts.

Its good not to have to rely
on pubmed for the links to
the fulltexts of papers you
are interested in; learn how
to find the FTs right at the
source, the webpage of the
respective journal
How to find a full text, if pubmed does not link to it? Usually the pubmed guys make it pretty easy for you to find the fulltexts to the studies their database contains. Normally, a single click on a button-like image of the logo of the respective publisher in the upper right corner of the pubmed sub-page with the abstract suffices, yet from time to time, the guy who fed the abstract into the database was to lazy to provide a link... and guess what, in  case of the Leenen study, the button is missing. From the cryptic abbreviation  "Eur J Clin Nutr. 2000 Jan;54(1):87-92" that is given in the line right above the title of the article and in the bibliographical information I provided, you do yet know that the paper has been published in 2000 in the 54th volume, 1st issue of the European Journal of Clinical Nutrition. Now, google will tell you that the "archive of " can be found at http://www.nature.com/ejcn/archive/index.html, and a few clicks later (year 2000 > volume 54 no. 1) you can download the study as PDF document, et voila!

Diging into it: Understanding the research design

Since we already know that the antioxidants in tea are good for us - otherwise we would hardly be worried that the addition of milk could "disturb or destroy" them, as Samir put it - the synopsis of the research, an obligatory part of the introduction to any scientific paper, which, in the case of the Ryan paper, focuses on the health benefits of tea and the tea consumption of the average UK citizen, is not really of interest to us. What is of utmost importance, though, is the research hypothesis, which is usually stated at the end of the introduction. Here, Ryan & Petit state three concrete objectives, which are...
  1. To analyze and compare the antioxidant capacity of five brands of commercially available tea
  2. To determine the effect of infusion time on the antioxidant capacity 
  3. To examine the effect of different volumes of bovine milk (10, 15, and 20 mL) and milk of varying fat content (whole, semiskimmed, and skimmed) on the total antioxidant capacity of the 5 teas
In essence, only objective #3 is relevant in the context of the question at hand; yet, knowing how to vary infusion times to maximize the antioxidant content of our tea, in the first place, would certainly be a nice bonus.

Next, we are going to collect relevant information on the actual experimental procedure
  • the tea was bought at a conventional supermarket
  • content of the tea bags varied slightly (2.7-3.3g) between the 5 commercial brands
  • 200ml of 90° hot water were used for infusion
  • 6 infusions for all 6 infusion-times (1, 2, 3, 4, 5 and 10 min); thus 36 individual samples for each brand
  • individual analysis of concentration effect of squeezing (10s right after removal) the tea bag after 1, 2, 4 and 8min of brewing; another 24 samples for each brand
  • addition of 10, 15 and 20ml whole, skimmed and semi-skimmed milk (after cooling the sample by infusion of the same amount of cool water); another 54 samples for each brand
  • technique used to analyze the 570 samples: ferric iron reducing antioxidant power (FRAP) assay
Now, the penultimate of these 7 points should make you wonder, whether this would not distort the results, because adding milk to a previously diluted tea would of course produce reduced antioxidant values compared to an undiluted version without milk. Ryan & Petit were however conscious of this problem and used accordingly diluted samples as their references, which increased the number of samples the scientists had to analyze to 588.

Finding and analyzing the relevant data

While obviously, the major results of this large scale investigation can be found in the text, the tables and figures the authors provide are of even greater value to independent thinkers like us. Using some sort of spreadsheet software we make good use of the 'raw' data on the influence of infusion times on the FRAP concentration of the freshly brewed tea. Instead of relying on the confusingly overcrowded and not very informative figure the authors provide, we create our own figure (figure 1) which comprises the FRAP values of tea 1 (median FRAP) and tea 2 (maximal FRAP) after 1, 2, 3, 4, 5 and 10min brewing with and without a tea bag. In that, we go beyond the simple graph Ryan and Petit had to offer, and bring to light how meaningful the negative effect of the tea bag in terms of the total antioxidant concentration of the brew actually is.
Figure 1: Although authors usually provide you with graphical illustrations of their data, it is often worth-wile to plot your own graphs from the data they provide, to illustrate and understand things the authors may have considered ir-/less relevant and have thus not included in their graphical analysis - in this case this would be the effect of the tea bag, which is missing from figure 1 in the original study by Ryan & Petit (data from Ryan. 2010)
If we now have a look at our fancy graph, we will immediately notice that convenience, as so often, takes it toll:
Just because we are too lazy to tinker with the loose tea leafs we give away somewhere between 50% and 5% of the total antioxidant capacity (depending on the brand and infusion time of the tea).
In that, it is interesting to note that the overall effect appears to be much greater with Tea 1 - as it can be seen in figure 1, the FRAP values of teas 1 and 2 are almost identical, if they are removed from the tea bag before infusion, which suggests that the quality of the teabag is an unexpected, yet important factor, when it comes to brewing an antioxidant rich tea. Altogether, these observations tell us that the use of teabags is generally not advisable, if you do not like the bitter taste long-infused teas tend to develop.

Doing some basic math to be able to estimate effect sizes

As interesting (and surprising) as they may be, the insights on the effects of tea bags on the amount of antioxidants are not really relevant for our personal research question, which relates to the effect of milk on the antioxidant activity of tea. Now, before we even bother with the data analysis, imagine, we did not even have data on the antioxidant activity of the tea + milk mixture. In that case, the only, yet questionable way to decide whether the addition of milk reduces the anti-oxidant value of tea would be to analyze the antioxidant value of milk, which is something Ryan & Petit actually did, and do some basic volume-based calculations.

Ryan & Petit found that fresh whole milk, semi-skimmed and skimmed milk had about 68%, 66% and 60% of the maximal FRAP value of (unbagged) tea and that these values declined by roughly 37.5% and 13.75% after 7 and 14days, respectively. Now, if we add a liquid with a FRAP value of say 5000µmol/L (fresh whole milk) to a brew of tea with a frap value of >8000µmol/L (Tea 1, leafs only) we will obviously reduce the total antioxidant level of the mixture. If we simply assume that the pre-cooled tea (20ml cold water added) has a FRAP value of 8000µmol/L (the exact reduction due to the addition of water would only matter, if the author had not used the 'cooled' specimen as reference, anyway), the addition of another 20ml of whole milk with a FRAP of 5000µmol/L would create a 11:1 mixture of high vs. low FRAP liquids, which would then have a FRAP value of 7.750µmol/L, which would obviously be a -3.125% reduction in the total antioxidant capacity of our tea by the addition of whole milk.

The smart SuppVersity teacher and students that we are, we are without doubt aware that this basic calculation oversimplifies our "problem". Yet, imagine the scientists found that the addition of 20ml of whole milk did in fact reduce the FRAP value by no more than -3.125%. In that case, we would have a reduction, but that reduction would be meaningless in view of our research question, because overall you would even get more antioxidants from your 240ml of tea with added milk and water than from your 220ml of tea without milk.
Figure 2: Relative changes in FRAP values of Tea 1 due to milk addition compared to the addition of the same amount of water (data calculated based on Ryan. 2010)
Again, we use our spreadsheet skills and create a graph that represents the relative changes in FRAP values that occur due to the addition of whole, skimmed and semi-skimmed milk to the tea (figure 2) and have to realize that our calculation underestimated the reduction by >2.6%. Consequently, there must be more to the effect of milk on the ability of tea to modulate ferric iron reduction than just a dilutive effect. And what's more, it must in one way or the other be related to the fatty acid content of the milk, with skimmed milk producing an effect that is 'only' 3x more pronounced than that of its full fat counterpart, although the individual antioxidant capacity of the latter is only about 8% greater.

Understanding the results of the study

If we think about the composition of whole, skimmed and semi-skimmed milk two things which are closely related to the fat content come to mind: First, low fat milk contains more carbohydrates than full fat milk. And, secondly, the low fat variety also has more protein on a liter per liter base than its full fat precursor. On the other hand, previous studies Ryan & Petit mention in their discussion found a profound interaction of tea polyphenols and protein.
Polyphenols can not only bind to proteins, but that the "interaction between flavonoids and proteins affects their antioxidant capacity in vitro".
If you are interested in optics you will also know that skimmed milk derives its bluish tint from the absence of fat globules, so that only the rayleigh-scattered high frequency blue light from the casein proteins hits the retina of your eye. If you combine that knowledge with the findings of Luck et al. (Luck 1994) who found that proline-rich proteins such as casein have a very high binding affinity for polyphenols, the answer to the influence of the macronutrient composition of the different types of milk is apparent: The casein fraction which is particularly high in skimmed milk binds to the beneficial tea polyphenols and thus decreases its antioxidant capacity. Well, I guess this would be our answer, then: Adding milk to your tea is not advisable, because its protein content will bind the beneficial polyphenols. And yes, it would be, if there were not the two other studies our initial PubMed search had brought up, which report no effect of milk on the total antioxidant capacity of black tea?

Is the newer study always right? Not one study is like another.

The easiest way to tackle the conflicting data would be to assume that the 2010 study is likely to be more accurate than the 5 and 10 years older studies by Reddy and Leenen, but would that be scientifically valid? I don't think so. So what do we do? We take a closer look at the methodology of the Reddy and Leenen studies... it won't take us long to find that in contrast to the Ryan & Petit, both Reddy and Leenen analyzed serum samples of subjects after ingestion of the respective tea+water / tea+milk mixtures. This major, yet easily overlooked difference is of great importance, because it does not take a rocket scientist to know that one of the fundamental functions of our digestive tract is to disassemble the food we consume so that the cells in the endothelial lining of our intestines can absorb the nutrients and deliver them to our bloodstream. Analyzing the ferric reducing ability of human plasma after the subjects consumed a tea + water / tea + milk mixture is thus completely different from putting the mixtures themselves to test.

With the influence of digestion, it is quite obvious that both the ingestion time, or rather the status of the digestive tract at that moment the subjects consume the samples, as well as the time at which the blood for the analysis is drawn post ingestion of the teas is of paramount importance. In both the Reddy and the Leenen study, the subjects reported to the laboratory fasted, and the studies also share some problems in regard of the tea preparation.
  • Reddy, for example used only 280ml of water for the preparation of the tea to which then 70ml milk would be added. Since there is however a saturation level for the maximal amount of antioxidants a unit volume of water is able to hold, the 'no milk reference sample', which had been boiled in 350ml of water, probably had an (unfortunately unquantifiable) 'antioxidant advantage' over the 'milk sample'. 
  • In the Leenen study, on the other hand, the milk was just added to an accordingly reduced amount of tea (60ml milk to 240ml tea vs. 300ml tea), which does also reduce the overall amount of tea polyphenols, the subjects in the tea + milk groups were consuming. 
Both of these minor flaws could explain - to a certain degree - why the subjects exhibited slightly lower increases in serum FRAP (-17% Reddy; -23% Leenen smaller area under the FRAP curve sampled in four and five 30 minute intervals after ingestion of the sample, in Reddy and Leenen, respectively) when they consumed their tea with milk.

So what? Does milk destroy the antioxidant value of tea?

Unfortunately, we cannot be sure that the aforementioned methodological crudities in the studies by Reddy and Leenen can adequately explain the milk-induced reductions in total antioxidant uptake. The -23% reduction in total ferric to ferric ion reduction capacity Leenen et al. measured in their study does yet match the reduced intake of tea polyphenols in the 240ml tea + 60ml milk vs. the 300ml tea group (-20% less polyphenols) pretty well. In view of the fact that Leenen et al. also used 2.5 times more whole milk (20% milk in tea vs. 8% milk in tea) than Ryan & Petit in their in-vitro study, one could do another ignorant calculation in order to estimate how effective our digestive system cleaves the proteins from the polyphenols:
5.73% of the total polyphenol content of 220ml tea are bound by the addition of 20ml whole milk; thus 20ml whole milk can bind the polyphenols from 12.606 ml of tea and 60ml of whole milk should be able to bind the polyphenols from 37.818ml of tea, so that the subjects  in the Leenen study who consumed their tea with milk should be missing an additional 15.7575% of the antioxidants, not just meager 3%, which would ultimately mean that their digestive tract recovered >80% of the protein bound antioxidants from the milky brew. [note - this calculation ignores that the milk would have an antioxidant value by itself, as well]
No matter how "scientific" these funky figures with 4 decimal places may appear - in essence the whole calculation did not only oversimplify the matter to a degree bordering bloggerish pseudoscience. It assumes, for example, a linear binding affinity, i.e. more milk = more binding, which obviously goes against what Ryan and Petit have found (cf. fig. 2). More importantly, though, it was also completely unnecessary in view of the fact that the 'missing' 3% we tried to explain are statistically non-significant, because standard error of the mean AUC (area under the FRAP curve) we based our calculations on is 19.64% and thus more than 6x above the loss we were going to lose our sleep over.

Before we go into even more speculative, or should I say outragous (?) calculations on the influence of the ratio of tea to water on the total amount of antioxidants present in tea after 3 minutes of infusion, in order to do similarly meaningless calculations for the Reddy study, we better bring this highly educative issue of our (almost) weekly Ask Dr. Andro column to a close and summarize our results as follows:
  • the protein fraction in milk can bind antioxidant molecules in tea
  • the antioxidant value of the tea + milk mix is reduced by up to -18% depending on the type and amount of milk you are using
    • skimmed, i.e. low fat, 'high' protein milk reduces binds more antioxidants than whole milk
    • the effect of milk on polyphenol binding in tea is non-linear or, in other words, if you double the amount of milk you won't lose twice as much of the antioxidants
  • in in-vivo studies, i.e. studies that analyze the sera of living human beings after consumption of tea and tea + milk samples, the increase in serum FRAP (Ferric reducing ability of plasma) has been found slightly reduced by the addition of relatively high amounts (20%) of milk, but...
    • this did not compromise the desired positive downstream effects on health markers measured in the Reddy study and
    • the lower FRAP values after ingestion of the tea + milk mixtures could be reducible to the individual study design
And last but not least, there are the results of the Reddy study, which suggest that the effect is similar in both green and black tea. The effects discussed in this article are thus very likely occuring for the all forms of "tea" (a brew created from camellia sinensis leavey), regardless of how the leaves were treated before you put them into hot water.
Bottom line: No matter which "color" your tea has, be it green, black or white, if you enjoy drinking your tea with milk, continue doing so, but do not use a tea bag! The magic of drinking tea goes way beyond the mere physiological effects of its ingredients, anyway (eg. Keenan. 1996); and having to force your tea down without milk, because you want to "waste" those additional 3% or whatever percent of antioxidants, would certainly do more harm than good. Well... when, I am thinking about it, it would go well with the fashionable dietary self-chastisement that is so popular among the followers of certain self-proclaimed Internet gurus (suggested read Sean Casey's "The Guru I am Not"), these days... whatever, I guess I better shut up and get some tea (lprepared from loose laves, with whole milk!), before anybody feels offended ;-)

11 comments:

  1. Thank you very much for the HIGHLY informative and very useful write-up!
    Much appreciated :)

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  2. Dr Andro, thanks for this entertaining read, the real point of which I think is to expose us to the experience of exhaustive logic, a very uncommon one for most of us, it seems, including "scientists"! Bottom line is, the question of tea with or without milk will lead nowhere unless we first ask what it means to ask the question.

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  3. great comment, anynomous... and a comment that stands in line with my repeated recommendation not to worry about making everything 101% right. Even if this does not lead to doing nothing right at all (which it often does), it will certainly lead to chronically elevated cortisol levels ;-)

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  4. great analysis, but what about two spoons of instant coffee with 50ml of milk but no sugar on an empty stomach ?
    That's what I do each morning but I'm wondering now if it's a good idea...

    ReplyDelete
    Replies
    1. Fresher coffee would most likely be "better", but if you look at Andro's comment above, Andro is a lot about not stressing out over minutiae. Optimization of lifestyle is good sometimes, but I think a lot of us stress about it so much that we often end up making things worse and harder on us, not easier.

      Depends on your goals.
      Want to just be healthy? You are probably fine with your coffee.

      Professional athlete like Adelfo or you have the time to spare to make some fresh coffee? Might be worth it for some of the "whole food" benefits.

      There is another post on here about the differences between instant and regular brewed:
      http://suppversity.blogspot.com/2012/03/only-real-not-decaffeinated-instant.html

      Still have some nice antioxidant activity going on there, albeit not statistically significant.

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    2. Erik did a pretty good job. I have for quite some time used instant coffee, as well. How "effective" those are will yet depend on many factors - while all expect the de-caffeinated ones will give you a boost, the "mild" ones are - at least in many cases - processed in ways that reduce the acids, and among them the beneficial ones such as chlorogenic acid (see previous posts on cholorogenic acid)

      but since simply picking the "wrong" variety of normal coffee beans could leave you with less of those than in many instant coffees, you better follow Erik's sound advice not to overstress it.

      If you just want to try something different, get yourself one of those > http://www.living-quality.de/images/product_images/info_images/101439_300x300_300x300.jpg some roasted fresh coffee powder and a coffee filter 2min. and your coffee to go is ready

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    3. Dear friends thanks for your clear answers. Gotta love this website !

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  5. You can also find lots of antioxidants in beverages like green tea, real grape juice and in many different wines. For years medical professionals have been researching the health benefits of antioxidants and why they are

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  6. Thank you Dr. Andro! You have not only answered my question about adding milk to tea (or, perhaps, coffee and cocoa), but you have provided a great example of how to access and analyze the primary literature to search beyond the headlines. I think I'll show it to my college students.

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  7. Great write-up and entertaining tone. Glad someone else is willing to do the leg work. It would be interesting to know how the scientists who wrote the original articles would respond.

    I think there is an omission of the words "do not" in the bottom line conclusion, which makes it seemingly contradictory out of context.

    [...having to force your tea down without milk, because you DO NOT want to "waste" those additional 3% or whatever percent of antioxidants, would certainly do more harm than good.]

    Dr. Andro does not dislike his double negatives, doesn't he? :P

    Another ex: "Even if this does not lead to doing nothing right at all..."

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    Replies
    1. thanks GrandMaster bronx. You are obviously right

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