Harvard Medical School Adviser by the Faculty of Harvard Medical School

Q:

I gave up coffee years ago because it made me too jittery, but my husband still drinks coffee -- sometimes four or more cups -- every day. I've always been under the impression that too much caffeine is bad for your health. Is there harm in drinking so much coffee? Are there any health benefits from drinking it?

A:

It's true that coffee has its good qualities and not-so-good qualities. In excess, coffee, and more particularly, caffeine, can cause problems for some people; nervousness, rapid heart action, heartburn and excessive urination head the list. But study results keep coming that suggest coffee does have some health benefits.

Recently, researchers reported that coffee drinking is associated with lower risks of depression, lethal prostate cancer and stroke. There are also reports of possible protective effects against illnesses ranging from Parkinson's disease to diabetes to some types of cancer.

Caffeine has been studied more than any other ingredient in coffee. But coffee contains literally a thousand different substances. And some of these lesser-known substances may be responsible for healthful effects in various parts of the body. In fact, some studies show caffeinated and decaffeinated coffee have similar effects, which suggests that something besides caffeine is involved.

It gets complicated, though. Caffeine and some of the other substances in coffee seem to have their good and bad sides. Coffee's overall effect may depend on how much the positive and negative effects balance out.

Here's a rundown of how coffee is thought to affect various medical conditions:

-- Alzheimer's disease: Some evidence suggests protection against beta-amyloid plaque that may have a role in causing Alzheimer's.

-- Cancer: Studies suggest a lower risk for some tumors (endometrial, aggressive prostate and estrogen-negative breast cancers). Antioxidant and anti-inflammatory substances could be responsible.

-- Diabetes: Regular use is associated with lower risk, and high intake (three to six cups a day) seems to have a greater effect. Protection may come from factors that affect insulin and blood sugar levels.

-- Heart attack: Coffee drinking increases some factors (homocysteine) associated with higher risk. But one to three cups a day has been linked to a small decrease in risk.

-- Liver disease: Coffee drinking is associated with lower levels of enzymes that indicate liver damage and inflammation.

-- Parkinson's disease: Studies show a 25 percent decrease in risk for coffee drinkers. The effect is smaller in women.

-- Stroke: Three to four cups a day is associated with lower risk. But chance of a stroke may increase immediately after intake, particularly among infrequent drinkers.

-- Migraine headache: The caffeine in coffee helps ease migraine headache in some people by narrowing the expanded blood vessels in the brain that cause migraine pain.

Caffeine can also be a performance enhancer, strengthening muscle contraction and offsetting effects of physical exertion. But, especially in the short term, it also has negative effects, which include raising blood pressure and increasing levels of homocysteine, insulin and possibly cholesterol.

Coffee drinkers concerned about cholesterol weren't happy about some early study results showing that coffee seemed to increase cholesterol levels. But the bad news turns out to be not so bad, because the cholesterol-raising effect seems to be limited to unfiltered coffee.

There is a twist, however. Cafestol and kahweol, the two cholesterol-raising ingredients in coffee, may also have some health benefits. The research is in the preliminary stages, but they could have some anti-cancer effects and be good for the liver.

Switching gears to the antioxidant benefits of coffee, explanations for the connection between coffee and lower rates of heart disease and diabetes point to chlorogenic acid and other antioxidant substances.

But chlorogenic acid might be another coffee ingredient with a split personality. Along with caffeine, it seems to push up levels of homocysteine, which has been associated with atherosclerosis.

Coffee isn't a great source of vitamins and minerals, but it contains some, and a few that we should be getting more of.

A cup of coffee contains about 7 mg of magnesium, which is a drop in the bucket, but because we don't eat enough fruit, vegetables and whole grains, a cup of coffee or two can help.

Potassium, another component of coffee, helps the body by offsetting some of the harmful effects of sodium. At about 116 mg per cup, coffee's contribution toward the 4,700 mg that we're supposed to get daily is small but still helpful.

Research has debunked many early concerns about coffee, but more studies are needed to nail down possible health benefits. For now, it's a matter of personal preference. If you're someone who feels jittery or experiences other side effects from coffee, switch to another beverage. But if coffee goes down well, there is no medical reason to cut down. It's a message many of us will drink to.

(Submit questions to harvard_adviser@hms.harvard.edu.)

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