by Marion Nestle

Currently browsing posts about: Coffee

Sep 7 2021

Nutrition research basics: Association does not equal causation

I subscribe to a newsletter, Obesity and Energetics Offerings, that often includes a section on “Headline versus Study.”  This demonstrates how  headline writers jump actual findings and draw attention-getting conclusions that the studies don’t always support.

Here are four examples, all from the UK Biobank study, an enormous epidemiologic investigation of half a million participants who filled out a dietary questionnaire, and then were followed and tested to see what diseases they developed.  These data enable investigators to link specific dietary factors to disease risk.

Such links are associations. 

This kind of study is not designed to determine whether a dietary factor causes (or prevents) a disease.  For that, other kinds of research are required.

These four examples examine associations of coffee intake with disease risk.  That’s all they do.

The headlines, however, assume causation—an epidemiologic fallacy.

I.  Headline: A Daily Dose of Coffee May Protect You from COVID-19.

Study: Self-Reported Dietary Behaviors and Incident COVID-19 in the UK Biobank.

Results: After multivariable adjustment, the odds (95% CI) of COVID-19 positivity was 0.90 (0.83, 0.96) when consuming 2–3 cups of coffee/day (vs. <1 cup/day), 0.88 (0.80, 0.98) when consuming vegetables in the third quartile of servings/day (vs. lowest quartile), 1.14 (1.01, 1.29) when consuming fourth quartile servings of processed meats (vs. lowest quartile), and 0.91 (0.85, 0.98) when having been breastfed (vs. not breastfed). Associations were attenuated when further adjusted for COVID-19 exposure, but patterns of associations remained.

Conclusions: In the UK Biobank, consumption of coffee, vegetables, and being breastfed as a baby were favorably associated with incident COVID-19; intake of processed meat was adversely associated.

Caveats:  This study looked at associations of several dietary factors to Covid risk: breastfed as baby, and consumption of coffee, tea, oily fish, processed meat, red meat, fruit, and vegetables.  Based on the results, the fallacious headlines could just as easily have read:

  • Vegetables may protect you from COVID-19
  • Being breastfed may protect you from COVID-19
  • Processed meats may increase your risk of COVID-19

The operative word is “may.”  This means that “may not” might also be appropriate.

2.  Headline: Too Much Coffee Can Cause Your Brain to Shrink, Raise Risk of Dementia by 53 Percent, Study Finds.

Study: Associations Among High Coffee Consumption, Brain Volume and Risk of Dementia and Stroke in UK Biobank.

Results: There were inverse linear associations between habitual coffee consumption and total brain (fully adjusted β per cup −1.42, 95% CI −1.89, −0.94), grey matter (β −0.91, 95% CI −1.20, −0.62), white matter (β −0.51, 95% CI −0.83, −0.19) and hippocampal volumes (β −0.01, 95% CI −0.02, −0.003), but no evidence to support an association with white matter hyperintensity (WMH) volume (β −0.01, 95% CI −0.07, 0.05). The association between coffee consumption and dementia was non-linear (Pnon-linearity= 0.0001), with evidence for higher odds for non-coffee and decaffeinated coffee drinkers and those drinking >6 cups/day, compared to light coffee drinkers. After full covariate adjustment, consumption of >6 cups/day was associated with 53% higher odds of dementia compared to consumption of 1–2 cups/day (fully adjusted OR 1.53, 95% CI 1.28, 1.83), with less evidence for an association with stroke (OR 1.17, 95% CI 1.00, 1.37, p = 0.055).

Conclusion: High coffee consumption was associated with smaller total brain volumes and increased odds of dementia.

Caveat: But why non-linear?  If coffee had these effects, shouldn’t more of it should have more of an effect?  Association, alas, does not equal causation.

3.  Headline: Your Morning Cup of Coffee May Lower Your Risk of Liver Disease.

Study: All Coffee Types are Associated with Lower Risk of Adverse Clinical Outcomes in Chronic Liver Disease: A UK Biobank Study.

Results: Compared to non-coffee drinkers, coffee drinkers had lower adjusted HRs of CLD (HR 0.79, 95% CI 0.72–0.86), CLD or steatosis (HR 0.80, 95% CI 0.75–0.86), death from CLD (HR 0.51, 95% CI 0.39–0.67) and HCC (HR 0.80, 95% CI 0.54–1.19). The associations for decaffeinated, instant and ground coffee individually were similar to all types combined.

Conclusion: The finding that all types of coffee are protective against CLD is significant given the increasing incidence of CLD worldwide and the potential of coffee as an intervention to prevent CLD onset or progression.

4.  Headline: Coffee Drinking Won’t Harm Your Health, New Study Reveals Caffeine Lowers Arrhythmia Risk.

Study: Coffee Consumption and Incident Tachyarrhythmias with Mendelian Randomization for Caffeine Metabolism in the UK Biobank.

Findings  In this large, prospective, population-based community cohort study of more than 300 000 participants, each additional daily cup of coffee was associated with a 3% reduced risk of developing an arrhythmia; these associations were not significantly modified by genetic variants that affect caffeine metabolism.

Meaning  Neither habitual coffee consumption nor genetically mediated differences in caffeine metabolism was associated with a heightened risk of cardiac arrhythmias.

Comment: Whether coffee drinking is good, bad, or indifferent to health has long been hotly debated.  For a long time, studies seemed aimed at proving coffee bad for health.  Now, studies seem aimed at showing the opposite.  The UK Biobank is expensive; its funders are largely private foundations and government agencies.  As far as I can tell, the coffee industry is not involved.  My take on all of this?  Take these epidemiological associations with a pinch of caffeine, and enjoy coffee if you like it.

Jul 2 2021

Weekend reading: Michael Pollan’s “Your Mind on Plants”

Michael Pollan.  This is Your Mind on Plants.  Penguin, 2021.

This book is a great read: informative, smart, hilariously funny on occasion, and wonderfully written, as is only to be expected from anything Pollan produces.

The book is about three plants that are sources of mind-altering drugs, poppies (opium), tea and coffee (caffeine), and peyote cactus (mescaline).

The tea and coffee bring it into the realm of food politics, and I’ll stick to them for the moment (but the poppies chapter is particularly riveting, tough, and timely).

An excerpt beginning on page 99:

Most of the various plant chemicals, or alkaloids, that people have used to alter the textures of consciousness are chemicals originally selected for defense. Yet even in the insect world, the dose makes the poison, and if the dose is low enough, a chemical made for defense can serve a very different purpose: to attract, and secure the enduring loyalty of pollinators.  This appears to be what’s going on between bees and certain caffeine-producing plants, in a symbiotic relationship that may have something important to tell us about our own relationship to caffeine…[in an experiment] even at concentrations too small for the bees to taste, the presence of caffeine helped them to quickly learn and recall a particular scent and to favor it…Actually we don’t know whethe the bees feel anything when they ingest caffeine, only that the chemical helps them to remembe–which, as we will see, caffeine appears to do for us, too.

Another from page 145:

Would people have ever discovered coffee or tea, let alone continued to drink them for hundreds of years, if not for caffeine?  There are countless other seeds and leaves that can be steeped in hot water to make a beverage, and some number of them surely taste better than coffee or tea, but where are the shrines to those plants in our homes and offices and shops?  Let’s face it: The rococo structures of meaning we’ve erected atop those psychoactive molecules are just culture’s way of dressing up our desire to change consciousness in the finery of metaphor and association.  Indeed, what really commends these beverages to us is their association not with wood smoke or stone fruit or biscuits, but with the experience of well-being—of euphoria—they reliably give us.

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Jan 31 2019

Industry-funded study of the week: coffee protects DNA

I love coffee (preferably not over-roasted) and have long been convinced that it is not a health risk (more on that in the comment below).  But a miracle drug?

As I discuss in Unsavory Truth: How Food Companies Skew the Science of What We Eat, industry-funded research almost invariably produces conclusions that favor the sponsor’s marketing interests, as this one does.

Title: Consumption of a dark roast coffee blend reduces DNA damage in humans: results from a 4-week randomised controlled study  Dorothea Schipp, Jana Tulinska, Maria Sustrova, Aurelia Liskova, and 10 others.  European Journal of Nutrition, published online 17 November 2018.

Purpose: “To determine the DNA protective effects of a standard coffee beverage in comparison to water consumption.”

Conclusions: “Our results indicate that regular consumption of a dark roast coffee blend has a beneficial protective effect on human DNA integrity in both, men and women.”

Funding: “This study has been supported by Tchibo GmbH, Hamburg, Germany” [Tchibo is a German coffee company].

Conflict of interest: “D. Schipp is a self-employed statistician, who has been appointed and financed by Tchibo GmbH for this and other projects.”

Comment: this study was designed to demonstrate coffee’s protective effects, and did so.  Here’s what I wrote about coffee in What to Eat:

There is something abut coffee–perhaps just the caffeine–that makes researchers try hard to find something wrong with it.  My files are full of papers claiming that coffee raises the risk for heart disease, heartburn, cancer, infertility, fetal growth retardation, spontaneous abortion, breast lumps, osteoporosis, ulcers, and any number of other health problems, but the observed effects are so small and so inconsistent that the studies are not very convincing.  Instead, well-designed studies tend to show no harmful effects…Complicating an overall assessment of the health effects of coffee are studies showing the benefits of drinking it…As with so many studies of foods and health, research on coffee and health is hard to do.

Bottom line: If you like drinking coffee, enjoy.

Thanks to Daniel Skavén Ruben for sending this one.



Dec 18 2015

Weekend Reading: Mark Pendergrast’s Fair Trade

Mark Pendergrast: Beyond Fair Trade: How One Small Coffee Company Helped Transform a Hillside Village in Thailand.  Greystone Books, 2015.

fair trade

Mark Pendergrast is the author of the definitive history of Coca-Cola, For God, Love, and Coca-Cola, about which I have warmly appreciative things to say in my own contribution to that genre, Soda Politics.  

He writes a “semi regular”column on coffee for the Wine Spectator, and this is his second book on coffee.  The first was Uncommon Grounds: The History of Coffee and How it Transformed the World.

Here, he focuses on the Doi Chaang Coffee Company, the result of a business partnership between a Canadian coffee company and a coffee-growing hill tribe in Thailand.  This is an inspiring story of social entrepreneurship at its best. Sometimes these things work.  It’s worth reading about how this one did.

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Apr 10 2012

Nutritionist’s Notebook: Caffeine Cravings

On Tuesdays, I answer questions about nutrition in NYU’s student newspaper, the Washington Square News.   These appear intermittently on the newspaper’s website.  Today’s is about caffeine.

Question: What kind of effect does caffeine have on our metabolism and general health? What is an appropriate amount of caffeine to have? And are certain sources of caffeine better than others? 

Answer: Caffeine is a mild upper. It perks up your central nervous system and makes you feel more alert, energetic and cheery. Caffeine is common in plants, but coffee, chocolate and tea have the most. The amount of caffeine depends on the type, amount used and brewing time, from 30 milligrams for a small cup of weak tea to more than 300 milligrams for some of the larger and stronger Starbucks drinks. When caffeine appears on the labels, you know exactly how much you are getting.

Energy drinks made for adults, like Red Bull, contain about 80 milligrams in an eight-ounce can. Coca-Cola, Pepsi-Cola and other soft drinks marketed to children have much less — 30 to 40 milligrams in 12 ounces.

People react to caffeine in different ways and, by this stage in your life, you undoubtedly know how much of it you can handle and at what time of the day you can handle it. If you take in more than your personal limit, you may feel nervous, shaky and sleepless. The more caffeine you drink, the more you become accustomed to it and the harder it is to give up. Some researchers think that the mix of sugar with caffeine is what makes some people feel addicted to soft drinks.

Perhaps it’s the caffeine in coffee that makes researchers want to find something wrong with it. I have a thick file of papers claiming that coffee raises the risk for heartburn, cancer, heart disease, infertility, ulcers and many other health problems, but the observed effects are small, inconsistent and unconvincing. When given as a drug, caffeine stimulates urine production and suppresses appetite, but the amounts in all but the strongest coffees are too low to produce such effects. If you get shaky when you drink caffeinated beverages, it’s time to stop.

—A version of this article appeared in the Tuesday, April 10 print edition. Marion Nestle is a contributing columnist. Email her questions at

Additional note on the food politics of caffeineSenatorDick Durbin (Dem-IL) has just asked the FDA to enforce its own rules on drink labeling.  Some makers of high-caffeine “sports” drinks are marketing them as dietary supplements to avoid having to adhere to FDA rules on how much can go into soft drinks.

Aug 8 2011

It’s time for some Q and A’s

I’ve just turned in the copy-edited manuscript of Why Calories Count: From Science to Politics (pub date March 2012) and now have time to catch up on some questions:

Q. I was recently given to read a book titled “The China Study” which is based on research conducted in 1970’s in China by Dr. Colin Campbell. His main conclusion is that eating dairy and meat causes cancer. His resolution is that a plant-based diet (i.e. vegan) is the (only?) healthy diet for humans. This book has made strong enough of a point to convince several of my friends to “convert” to a vegan diet in order to save their health. Could you share some comments on the validity of the research and conclusions this book presents with regards to detrimental effects of dairy and meat on human health?

A. Campbell makes a forceful argument based on his interpretation of the research and on case studies of people whose diseases resolved when they became vegans. And yes I’ve seen Dr. Campbell’s new movie, Forks over Knives. The first half is a terrific introduction to how the current food environment promotes unhealthy eating.  The second half promotes Dr. Campbell’s ideas about the hazards of meat and dairy foods.

Whether you agree with these ideas or not, the film is well done and worth a look.

Some scientists, however, interpret the research as demonstrating that people are healthier when they eat dairy foods.  For example, the enormous consensus report on diet and cancer risk from the American Institute for Cancer Research and the World Cancer Research Fund concluded in 2007 that eating lots of red meat and processed meat is convincingly associated with an increased risk of colorectal cancer (but no others).

On the other hand, they found dairy foods to be associated with a decrease in the risk of colorectal cancer.  They found limited and less convincing evidence that dairy foods might decrease the risk of bladder cancer but increase the risk of prostate cancer.

How to make sense of this?  These are two food groups in the diets of people who consume many kinds of foods and who do many things that might increase or decrease cancer risk.  Given this complexity, one food or food group seems unlikely to have that much influence on cancer when considered in the context of everything else people eat and do.

Nutrition research, as I am fond of saying, is difficult to do and requires interpretation. Intelligent people can interpret the studies differently depending on their point of view.

The new Dietary Guidelines say to cut down on saturated fats. Those are most plentiful in meat and dairy foods (plant foods have them, but in smaller amounts). Pretty much everyone agrees that plant-based diets promote health/  But whether they have to be 100% plant-based is highly debatable.

The new USDA MyPlate food guide suggests piling plant foods—fruit, vegetables, and grains—on 75% of your plate so the argument is really about what goes on the remaining 25%, what USDA calls the  “Protein” section. You can put beans in that quarter if you don’t want to eat red meat, poultry, or fish.

Q. I’d love to hear your take on the recent walnut flap [accusations that the FDA now considers walnuts to be drugs].  I suspect walnuts got caught with such offenders as Pom, Froot Loops, and Juicy-Juice, but I’d love to find out what the FDA actually said about this. For some odd reason I don’t believe the article is presenting the whole truth.

A. This is a health claims issue. The FDA is not saying walnuts are drugs. It is saying that Diamond Walnut is claiming walnuts as drugs on package labels. How so?

The labels say the omega-3 fatty acids in walnuts may help lower cholesterol; protect against heart disease, stroke and some cancers (e.g. breast cancer); inhibit tumor growth; ease arthritis and other inflammatory diseases; and even fight depression and other mental illnesses. These are disease claims for which the FDA requires scientific substantiation.

The company’s petition did not provide that substantiation so the FDA issued a warning letter. In general, you should be skeptical any time you see a nutritional factor advertised for its ability to prevent or treat such a broad range of problems.

Q. A question about sugar and how it is counted: My books say: 4 g = 1 teaspoon = 15 calories. My Illy Caffe says 10 g of sugar, but 50 calories. Ingredients: coffee, sugar, potassium bicarbonate, potassium citrate. If the drink is 50 calories, shouldn’t it say 12 g or more for the sugar listing?

A. Sugar should be the only ingredient that has calories in this coffee but I’ve seen calorie lists that say 5 calories per gram for sugars. Food companies have some leeway in the way they compute calories. Illy may be using a method that gives 5 rather than 4. But the difference between 40 and 50 is hardly measurable and I wouldn’t worry about amounts this small, annoying as imprecise figures may seem.