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Harvard Commentaries
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Reviewed by the Faculty of Harvard Medical School

Food for Thought Food for Thought

The Coffee Bean -- Not a Fiend

October 23, 2014

By Beth Klos, R.D., L.D.N.
Brigham and Women's Hospital

As that little shiver in the air reminds you that fall is turning into winter, you wrap your fingers around a warm ceramic mug filled with your cold-weather friend: coffee. Even the aroma — rich and spicy with a hint of citrus — makes you feel warmer, not to mention more awake. With every sip, you feel a lift. And as the morning brain fog lifts, you wonder, is there a downside to this tasty brown stimulant? Is it a feast of antioxidants or a nasty stew of chemicals?

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A Complicated Brew

Coffee is probably the number one source of antioxidants in the American diet. That distinction is due to the amounts of coffee Americans drink — 400 million cups a day according to a trade group — rather than coffee's unique nature.

Portion for portion, many other healthful foods have as much or more antioxidants, and are rich with other nutrients that coffee doesn't have. For example, strawberries have twice the amount of antioxidants, and walnuts have about 13 times as much! If you are looking to maximize the antioxidants and others nutrients in your diet, don't drink more coffee. Your antioxidant intake will skyrocket if you include more fruits, nuts, vegetables and whole grains.

Where Do Coffee Beans Come From?

Coffee "beans" are actually the seeds of the coffee cherry fruit. Most coffee trees grow near the equator in what is called the "bean belt." The cherries are handpicked and the outer layer of fruit is stripped away. The inner seed, the "green coffee" is sometimes aged, which may change the flavor. Then the green coffee is roasted, changing it to the lovely familiar brown color. As with wine, cocoa products and other fruits of the earth, growing conditions and processing methods strongly affect the quality and flavor of the final product.

Coffee has more than a thousand different chemicals. In addition to that familiar ingredient, caffeine, coffee contains:

  • Potassium
  • Magnesium (in small amounts)
  • Antioxidants

Before processing, coffee contains a significant amount of phenolic antioxidants. Surprisingly, roasting coffee beans to enhance the flavor also increases the overall antioxidant content, though it destroys some phenols in the process. Certain brown antioxidants, called melanoidins, form while the beans are roasting and may more than make up for the losses.

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Coffee and Caffeine - The Dark Side

Caffeine has some possible negative effects on the body. These include:

  • Anxiety
  • Nervousness
  • Frequent urination
  • Caffeine withdrawal
  • Interactions with some medications

Coffee drinkers may not have the healthiest lifestyles either. According to two large, long-term studies, 30% of heavy coffee drinkers are smokers. They are also more likely to drink alcohol and less likely to exercise and take multivitamin supplements. Also, coffee may worsen acid reflux.

Over the years, researchers have studied many facets of coffee's effects on health, including a possible connection between coffee consumption and three major health threats: heart disease, cancer and diabetes. There's no conclusive evidence at this time that coffee hurts or helps our health. But here's how the evidence stacks up so far.

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Heart Disease

We've known for a while that espresso, French press coffee, and other coffee made without a paper filter raise blood cholesterol levels and are likely to increase the risk of heart disease. That's because, without a filter, two compounds — cafestol and kahweol — wind up in the coffee. Most coffee, however, is made with a paper filter that catches the compounds. Therefore the drink doesn't pose the same threat. Fortunately espresso is served in small portions or used sparingly in cappuccinos and lattes, which lessens the harm. But people should avoid regularly drinking boiled or French press coffee.

We also know that caffeine can temporarily raise blood pressure in some people.

But when it comes to whether caffeine increases the risk for heart disease and heart attacks, the evidence is less clear-cut. Reassuring results from several large studies didn't show any more heart disease among long-time coffee drinkers. Even among those who drank 6 cups a day, researchers didn't find any increased risk. We don't know, however, if something else about coffee drinkers was protecting them.

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In the past, it was feared that coffee increased the risk of cancer. But more recent, well-designed research has challenged this idea. When cigarette smoking is factored out, coffee does not seem to raise the risk of any type of cancer. In fact, several European and Japanese studies showed decreased rates of liver cancer among coffee drinkers, though why this occured is unclear. It's possible that coffee drinkers share another trait that reduced their risk.

There have been numerous studies looking at the association between colorectal cancer and coffee. The results are mixed: some show coffee reduced the risk while others show it had no effect on risk. Until more studies are done, it's hard to say either way what effect coffee has on cancer.


According to short-term studies of coffee's effects on the body, it appeared to lower insulin sensitivity, which means that the body's cells don't recognize or respond to insulin very well. Because insulin's job is to prompt cells to take in glucose, over time this effect could increase the amount of glucose in the blood and the risk for type 2 diabetes among heavy coffee drinkers.

But a 2004 study by Harvard researchers that followed more than 125,000 men and women over many years, found that those who drank four to five cups of coffee a day had lower rates of type 2 diabetes compared with people who didn't drink coffee. The risk was even lower for people who drank six cups a day.

A study published this month in the journal Diabetes Care also found a protective effect of coffee for certain groups of people. This study of mostly middle-class white men and women in their 60s was unique because some of the participants had higher-than-normal glucose levels, which increases the risk for diabetes. The study found the clearest positive effect for people in this group who drank one to four cups of coffee per day but not for those who drank five or more cups a day. These results conflict with the Harvard study, leaving some remaining questions for researchers to answer.

It's difficult to say why coffee has a positive effect on diabetes risk. Is it the caffeine or another component in coffee at work? Or is there something else about coffee-drinkers that protect them? (And unfortunately, not all large studies have found this effect, adding to the uncertainty.) Experts think that over years, caffeine may increase insulin sensitivity, help the body mobilize glucose, and decrease body fat, which may account for its positive effect on diabetes.

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The Bottom Line

It's difficult to draw hard and fast conclusions about coffee's effect on health. For now it seems safe for most healthy adults to drink paper-filtered coffee in moderation. Drinking more coffee to prevent disease isn't recommended because we don't know why coffee drinkers seem less likely to have certain illnesses. The best approach is to be cautious about how much espresso and boiled coffee you drink and to eat plenty of antioxidant-rich foods. Also, pay attention to other lifestyle factors, such as exercise, smoking cigarettes or feasting on empty-calorie foods.

Beth Klos, R.D., L.D.N., is a Senior Nutritionist at Brigham and Women's Hospital. She completed her undergraduate degree at University of Rhode Island and her dietetic internship at Brigham and Women's Hospital. She counsels patients with a wide variety of conditions and enjoys teaching counseling to Brigham and Women's class of 12 dietetic interns. Joycelin Leong, Dietetic Intern, assisted with research for this article.

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